# Planning 1980 Mini conversion



## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Since the A-series engine and gearbox are combined into a single housing, I would be considering opting for direct drive to the front wheels; either through a differential or by using two motors directly coupled to the original driveshafts; unless there is an overwhelmingly good reason to couple an electric motor to a transmission.


The torque output of an electric motor is related to the size of the motor, so a motor with enough torque to drive the wheels directly is very large and heavy. The overwhelmingly good reason to use some sort of reduction drive (a gearbox, although people have used chains and sprockets or toothed belts and pulleys) is that it allows you to use a much smaller motor to get the same torque to the wheels. Another aspect of this is that the big high-torque motor requires a lot of current, so you can use a controller of much lower current capacity to run the smaller motor.

In first gear, the 65 lb⋅ft (88 N⋅m) output of the 1275 GT's engine would be multiplied by 3.32 by the first gear, then 3.105 by the final drive, becoming 670 lb⋅ft (907 N⋅m); to match that would require an enormous motor. Even in top gear, the engine plus transmission can put 202 lb⋅ft (273 N⋅m) to the wheels around 2500 rpm engine speed.

That means (usually) a gearbox, but doesn't necessarily mean a multi-speed gearbox (transmission). Production EVs use a single-speed transmission (gearbox), but they also use high-voltage AC motors which can produce their rated power over a very wide speed range, and nearly up to their maximum rotational speed. Some DIY EV conversion builders find it helpful to have more than one transmission speed (more than one gearbox ratio), because low-voltage motors have a relatively narrow powerband, so it helps to be able to shift to keep the motor at a suitable speed to produce enough power.

The pinion and ring gear set of a typical final drive unit is one stage of gear reduction; the differential itself doesn't provide any gear reduction, but when people say "differential" they often mean the whole final drive unit.

I agree that the classic Mini gearbox is a poor choice for an EV, although it has been used.


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## Duncan (Dec 8, 2008)

Hi TechGeekNZ

Where are you? - I assume NZ!

I'm an old mini fan - I was tempted to convert a mini - but I made a Locost type roadster instead

Where abouts?


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## brian_ (Feb 7, 2017)

Anyone interested in a classic Mini conversion should be aware of the types of conversions already done... here are a few I found in a quick search (I was curious about the transmissions used):

Ian Motion

appears to use single motor on single-ratio transaxle



> Range 150 km*
> Max speed 120 km/h
> Charging time 7 hours
> 
> ...




dbrive.ch

induction motor rated at 22 kW continuous
belt drive to original Mini transaxle
10 kWh battery of 32 CALB 100 Ah LFP cells

evmini.ca

HPEV AC50
Honda Civic transaxle
32x ThunderSky Lithium Ion 100AH cells (so 10 kWh) filling the trunk - this is terrible for weight distribution
given how completely clueless seemed to be about both cars and this specific project at the beginning, I'm really impressed that it was completed

The Lynch Atlantis

Motor: Lynch type LM200 7.5R, 8.5Kw on 48V
original Mini transaxle
Batteries: 8 x 6V 225 AH lead-acid
claimed performance:


> Max. Speed: 60 mph
> Max. Gradient: 1 : 4
> Range: 50 miles



this is a golf-cart level of conversion; the performance claim seems implausible to me

1972 Mini Cooper E.

D&D 6.7" brushed DC motor 
chain drive to original Mini transaxle
30x 100AH CALB LFP cells (10kWh) in the trunk
may not have been completed

Classic Mini A.C. Electric Conversion

likely aborted project

... and more links from another thread:


gottdi said:


> Here are some links just so you know you are not alone converting a MINI.
> 
> 
> http://www.killawatt-electric-car-conversions.com/electric-mini.html
> ...


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## brian_ (Feb 7, 2017)

Although I haven't seen anyone do this, there is a transmission approach that might not need much custom fabrication: use a motorcycle engine conversion kit for the Mini (example: Lynx).

They typically use a chain drive (from the output of the motorcycle transmission, but from the electric motor in this case) to a differential, with a subframe and bearings to hold the differential and shafts. The fabrication would be mounting the motor instead of the motorcycle engine... plus of course all of the usual mounting of controller, charger, and battery. This would be a single-ratio reduction, and would be noisy compared to a gearbox but maybe not bad. It would require the same chain maintenance as a chain-driven motorcycle.


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## TechGeekNZ (Feb 18, 2019)

Hello everyone. Thanks for the warm welcome and very useful info!



brian_ said:


> The torque output of an electric motor is related to the size of the motor, so a motor with enough torque to drive the wheels directly is very large and heavy. The overwhelmingly good reason to use some sort of reduction drive (a gearbox, although people have used chains and sprockets or toothed belts and pulleys) is that it allows you to use a much smaller motor to get the same torque to the wheels. Another aspect of this is that the big high-torque motor requires a lot of current, so you can use a controller of much lower current capacity to run the smaller motor.
> 
> In first gear, the 65 lb⋅ft (88 N⋅m) output of the 1275 GT's engine would be multiplied by 3.32 by the first gear, then 3.105 by the final drive, becoming 670 lb⋅ft (907 N⋅m); to match that would require an enormous motor. Even in top gear, the engine plus transmission can put 202 lb⋅ft (273 N⋅m) to the wheels around 2500 rpm engine speed.
> 
> That means (usually) a gearbox, but doesn't necessarily mean a multi-speed gearbox (transmission).


That's very useful, Brian. I haven't yet done any serious number crunching (this idea is still very much in its formative stages); and that gives a very good starting point.

The thing about learning theory is that it often doesn't give a sense of the scale of things in practice; a 40 kW electric motor that could develop 900 Nm without the mechanical advantage offered by gearing sounds like it would be a very big machine indeed . It does gets me thinking, though; what would be the drive ratio of the gearbox built into a typical induction motor? Could that be sufficient to drive the wheels, either directly or through a typical final drive (and differential) unit?

I admit I don't know quite as much as I'd like to about the pure mechanical aspects of cars (that's very much my brother's domain; although my own mechanical skills aren't too bad, simply by virtue of owning a car manufactured by BLMC); however, upon reflection, it's obvious that the torque figure of interest is the one where the rubber meets the road, after the engine's torque has been multiplied by the transmission. So, to match the performance of the original vehicle (prior to many of its original horses escaping), that would be the figure I'd need to replicate with an electric drivetrain?




brian_ said:


> Production EVs use a single-speed transmission (gearbox), but they also use high-voltage AC motors which can produce their rated power over a very wide speed range, and nearly up to their maximum rotational speed. Some DIY EV conversion builders find it helpful to have more than one transmission speed (more than one gearbox ratio), because low-voltage motors have a relatively narrow powerband, so it helps to be able to shift to keep the motor at a suitable speed to produce enough power.


That is a brilliant explanation for why many of the EV conversions I read about retain the manual transmission from the original car, even though theory would suggest it is unnecessary. Not only is it a convenient way to obtain the required drive ratio; but it is also an effective way to use a lower-powered motor.

Given my background theory in electrical machines (induction motors, mostly), and given the widespread availability of all manner of readily-available induction motors (in everything from domestic appliances to industrial machines), that would seem the natural choice. I also like the sound of a machine that produces (close to) maximum torque right across its speed range; the acceleration (and deceleration) of the resulting vehicle would be phenomenal .

Now, controlling the thing (and managing the batteries) would be an entirely different matter; and that, I believe, is where the real engineering challenge will be.

Given that they have no permanent magnetic field, I was somewhat surprised to learn that induction motors can also function as generators, by operating in the negative slip region. Since, in practical terms, this simply means the magnetic field induced in the rotor is rotating faster than the supplied stator field; it suggests an effective means of governing the vehicle's speed, i.e. set the frequency of the motor drive based upon the position of the accelerator pedal. A faster stator field causes the motor to speed up (motoring), while a slower field causes it to slow down (regenerating).

Since this would be a function of the controller, it should also simplify the implementation of regenerative braking to assist the Mini's stock (non-servo-assisted) front disc and rear drum brakes (I may tweak things to favor regenerative braking over mechanical braking, but there's no way I'd even consider completely ditching the hydraulic brakes).



brian_ said:


> Anyone interested in a classic Mini conversion should be aware of the types of conversions already done... here are a few I found in a quick search (I was curious about the transmissions used).





brian_ said:


> Although I haven't seen anyone do this, there is a transmission approach that might not need much custom fabrication: use a motorcycle engine conversion kit for the Mini.
> 
> They typically use a chain drive (from the output of the motorcycle transmission, but from the electric motor in this case) to a differential.


Thanks for all those useful links; I will likewise be interested to learn what replacement transmissions were used. Interestingly, the A-series engine actually uses a timing chain to connect the camshaft to the crankshaft; so I wonder just how much a chain drive would compare to the original A-series power unit (along with its distinctive gearbox whine) in terms of noise .




Duncan said:


> Where are you? - I assume NZ!


Hi, Duncan. I live in the Manawatu, so I'm rather spoiled for choice of excellent driving roads, and of course Manfeild Motorsport Park.

It looks like I have quite a lot to think about, and plenty of research to do; but thanks very much for pointing me in the right direction. Let's see if we can get this project rolling .


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## brian_ (Feb 7, 2017)

Another, more recent discussion of Mini conversion possibilities... but mixed up with MGB ideas:
Smart Fortwo Electric Drive as Mini/MGB donor?
Most of the earlier Mini project links are from the era in which common practices were to assemble LFP cells to build a battery pack, and to use aftermarket motors. Recent trends are to use salvaged EV battery packs, and (to a lesser extent) to use salvaged EV motors or complete drive units.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> ... what would be the drive ratio of the gearbox built into a typical induction motor? Could that be sufficient to drive the wheels, either directly or through a typical final drive (and differential) unit?


A motor doesn't typically have a gearbox built into it. If you're referring to the complete drive unit (motor + gearbox + differential) of a typical modern production EV... the only common EV using an induction motor is the Tesla Model S/X, and it has a roughly 10:1 ratio of motor speed to axle speed. Since the motor can run at least 12,000 rpm and the tires are tall, top speed can be high. Most other EV motors are configured to run a bit more slowly, so (for instance) the Nissan Leaf overall ratio is about 8:1.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> I admit I don't know quite as much as I'd like to about the pure mechanical aspects of cars (that's very much my brother's domain; although my own mechanical skills aren't too bad, simply by virtue of owning a car manufactured by BLMC)...


This forum's membership is an interesting mix, with long time auto enthusiasts who can rebuild an entire vehicle but are completely new to electronics (beyond the "plug in the black box" level), to electronics experts who can build anything from scratch but have no idea how anything in a car works, to computer programmers who can create complex logic in code but have never really touched the actual physical devices that do the work, and every combination in between.

Fortunately, everyone can fill in the gaps for everyone else. 



TechGeekNZ said:


> ... upon reflection, it's obvious that the torque figure of interest is the one where the rubber meets the road, after the engine's torque has been multiplied by the transmission. So, to match the performance of the original vehicle (prior to many of its original horses escaping), that would be the figure I'd need to replicate with an electric drivetrain?


Yes, but since that changes with each gear and depends on the shape of the engine or motor's torque-versus-speed curve, it's hard to pick a single value.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Given my background theory in electrical machines (induction motors, mostly), and given the widespread availability of all manner of readily-available induction motors (in everything from domestic appliances to industrial machines), that would seem the natural choice.


Induction machines have become nearly ubiquitous since AC power distribution was adopted. Familiarity and availability are significant factors... they're the only reasons that the cars now branded "Tesla" started with induction motors, and why the company used that name



TechGeekNZ said:


> I also like the sound of a machine that produces (close to) maximum torque right across its speed range; the acceleration (and deceleration) of the resulting vehicle would be phenomenal .


The same characteristics can be true to some extent of almost any type of motor, but especially brushless motors (anything other than a brushed DC motor) which are less limited in voltage. 

Almost any motor produces relatively constant torque up to some transition speed, limited by the current which can be supplied by the battery, handled by the controller/inverter, and tolerated (in heating terms) by the motor. Above that transition, if there's enough voltage available then they are generally limited to constant power battery capability and cooling of the motor and electronics; that's constant power, not constant torque, so torque drops off with speed (power = torque x rotational speed = force x linear speed = voltage x current).

In modern EVs, the transition point is only 1/4 to 1/3 of the way up the speed range from stall, so for most of the motor speed range the full rated power is available; below the transition speed torque is constant so power drops off as the speed drops, but acceleration is limited by traction anyway so that's okay. Providing enough voltage for high motor speed at full power is the reason that typical modern EVs run 360 V to 400 V battery packs. Even Toyota's non-plug-in hybrids, which have small NiCd batteries running between 200 and 300 volts, use a voltage doubler before the inverter so the motors can run with higher voltage.



TechGeekNZ said:


> Now, controlling the thing (and managing the batteries) would be an entirely different matter; and that, I believe, is where the real engineering challenge will be.


Yes, but the challenge has already been handled to a large extent by the designers of the motor controller (and the battery management system), whether that is salvaged from a production EV or part of the design of home-built hardware using someone else's plan.



TechGeekNZ said:


> Given that they have no permanent magnetic field, I was somewhat surprised to learn that induction motors can also function as generators, by operating in the negative slip region. Since, in practical terms, this simply means the magnetic field induced in the rotor is rotating faster than the supplied stator field; it suggests an effective means of governing the vehicle's speed, i.e. set the frequency of the motor drive based upon the position of the accelerator pedal. A faster stator field causes the motor to speed up (motoring), while a slower field causes it to slow down (regenerating).
> 
> Since this would be a function of the controller, it should also simplify the implementation of regenerative braking to assist the Mini's stock (non-servo-assisted) front disc and rear drum brakes (I may tweak things to favor regenerative braking over mechanical braking, but there's no way I'd even consider completely ditching the hydraulic brakes).


Almost all of the electrical power supplied to the utility grids is from induction generators in power plants, whether hydro or thermal or wind powered. 

I get the speed-controlling theory, and that's the sort of reasoning that works with fixed-frequency industrial power supplies (to provide a relatively constant machine speed) or with a variable-frequency drive without speed feedback... but in practice it doesn't work well in a vehicle. The normal approach in an EV is for the accelerator pedal position to be interpreted as a torque or power request (the two are the same, except for the speed multiplier). Speed limiting or cruise control is handled the same way as with an engine, using a secondary control loop: if the vehicle goes faster than intended, power is reduced, and vice versa.

It is possible to drive without friction brakes, entirely using the motor(s); however...

this generally isn't legal for a road vehicle (friction brakes mechanically/hydraulically linked to the pedal are required)
for reliability, it would be unwise
when near zero speed actively powering the motor in reverse would be required to stop, and using power to apply holding torque with the motor would be required to remain stationary
brakes are needed on all wheels, so friction brakes are still required on a two-wheel-drive vehicle

Although regenerative braking doesn't return a huge amount of energy, it is certainly desirable. The big challenge is coordinating it with friction braking, so that braking is smoothly controlled, sensible to the driver, balanced (braking just one end of the car is bad for handling and stability), and doesn't use more friction braking than necessary. Anti-lock braking system and stability control system behaviour must be considered as well, although I think most DIY conversions give up on these features even if the original vehicle had them. In an older vehicle with a simple hydraulic braking system (assisted or not), one problem is how to keep the friction brakes from applying on the powered wheels when regenerative braking is active; I haven't seen anyone address this in a DIY project.

It is now popular to have "one pedal" driving, in which zero accelerator pedal application is treated as a moderate degree of braking. It is a challenge to make this feel right to the driver, especially since the right amount depends on driver and driving conditions.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Interestingly, the A-series engine actually uses a timing chain to connect the camshaft to the crankshaft; so I wonder just how much a chain drive would compare to the original A-series power unit (along with its distinctive gearbox whine) in terms of noise .


Timing chains don't transmit much power compared to the engine (or electric motor) output, but noise was one reason that the automotive industry moved from timing chains to timing belts for overhead-cam engines a few decades ago; however, they have almost entirely moved back to timing chains to avoid the need for occasional belt replacement. Most pushrod engines have always used a timing chain, although heavy-duty and racing engines tend to use gears instead.

Many transmissions have used a drive chain (particularly in transverse automatics, but also in the early Toyota Prius hybrid and some oddball manuals), and most traditionally configured transfer cases in production 4WD vehicles use a drive chain for transmission to the front axle. The noise isn't generally noticeable, even in relatively quiet vehicles.

Properly installed, a timing chain or internal transmission chain doesn't make much noise, but these homebuilt transmission chains might be another matter.


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## TechGeekNZ (Feb 18, 2019)

I neglected to mention that I found this on the subject of Mini aerodynamics. Although I would like to preserve the basic appearance of the car, I will certainly consider making some aerodynamic mods such as de-seaming (especially given that the roof gutter and side seams are rotting anyway) and boxing in the underside with a metal plate. I might also consider a bodykit, so long as it's done tastefully .

Internally, the gauges will likely be replaced with modern, electronic instruments; mostly for the convenience of working with them, but also because I have reason to believe the car's original speedometer no longer works properly.

After reading the story about the ForkenSwift, I like the idea of rigging up the choke cable as a quick disconnect, and I could certainly utilize some of their ideas about working on a shoe-string budget.

Something that hasn't yet been mentioned is heat dissipation. How much cooling is likely to be needed to keep the electric systems in serviceable order? What approaches have been used to solve this problem?

Somewhat related to this is New Zealand's temperate climate. If I intend to use the car in any season other than early autumn or late spring, it will need a heater (for winter) and (preferably) an air conditioner for all seasons (the car will get hot in summer and, as I recall; the Mini's stock window de-mistor was barely adequate to clear the front window, never mind the side ones, in winter ). I have heard of some cars using a small heat pump for this purpose; does this seem plausible?


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## TechGeekNZ (Feb 18, 2019)

The bulletin board software doesn't support nested quotes? Pity .



brian_ said:


> A motor doesn't typically have a gearbox built into it. If you're referring to the complete drive unit (motor + gearbox + differential) of a typical modern production EV... the only common EV using an induction motor is the Tesla Model S/X, and it has a roughly 10:1 ratio of motor speed to axle speed. Since the motor can run at least 12,000 rpm and the tires are tall, top speed can be high. Most other EV motors are configured to run a bit more slowly, so (for instance) the Nissan Leaf overall ratio is about 8:1.


This is terrific information. You obviously know a thing or two about engineering yourself .




brian_ said:


> This forum's membership is an interesting mix, with long time auto enthusiasts who can rebuild an entire vehicle but are completely new to electronics (beyond the "plug in the black box" level), to electronics experts who can build anything from scratch but have no idea how anything in a car works, to computer programmers who can create complex logic in code but have never really touched the actual physical devices that do the work, and every combination in between.
> 
> Fortunately, everyone can fill in the gaps for everyone else. .


I tend to think of myself as a programmer with a screwdriver and soldering iron. If I can't fix it in software, I'll fix it in hardware; or vice versa .




brian_ said:


> Almost any motor produces relatively constant torque up to some transition speed, limited by the current which can be supplied by the battery, handled by the controller/inverter, and tolerated (in heating terms) by the motor. Above that transition, if there's enough voltage available then they are generally limited to constant power battery capability and cooling of the motor and electronics; that's constant power, not constant torque, so torque drops off with speed (power = torque x rotational speed = force x linear speed = voltage x current).


That seems like perfectly reasonable and solid engineering to me.




brian_ said:


> Providing enough voltage for high motor speed at full power is the reason that typical modern EVs run 360 V to 400 V battery packs.


I like this; we aren't making any toy power drill here. More volts equals less amps for the same power; although I am going to have to give some serious thoughts to where I'm going to put all those cells, and how to protect them if the worst-case scenario occurs .




brian_ said:


> Yes, but the challenge has already been handled to a large extent by the designers of the motor controller (and the battery management system), whether that is salvaged from a production EV or part of the design of home-built hardware using someone else's plan.


 Even if I do take the sensible approach of using someone else's controller design, I will most likely want to make a few customizations to it; that's just the way I think.




brian_ said:


> Almost all of the electrical power supplied to the utility grids is from induction generators in power plants, whether hydro or thermal or wind powered.


It's quite interesting that this wasn't mentioned in my electrical engineering curriculum; but I can certainly see the logic here, not least of which is that an induction machine would be able to accept minor variations in the input frequency without affecting the synchronous frequency.




brian_ said:


> I get the speed-controlling theory, and that's the sort of reasoning that works with fixed-frequency industrial power supplies (to provide a relatively constant machine speed) or with a variable-frequency drive without speed feedback... but in practice it doesn't work well in a vehicle. The normal approach in an EV is for the accelerator pedal position to be interpreted as a torque or power request (the two are the same, except for the speed multiplier). Speed limiting or cruise control is handled the same way as with an engine, using a secondary control loop: if the vehicle goes faster than intended, power is reduced, and vice versa.


That was one of my thoughts, too. It would also make the vehicle behave similar to the direct mechanical linkage from the accelerator pedal to the carburettor; i.e. more pedal = more fuel = bigger bang = more torque.




brian_ said:


> It is possible to drive without friction brakes, entirely using the motor(s); however...
> 
> this generally isn't legal for a road vehicle (friction brakes mechanically/hydraulically linked to the pedal are required)
> for reliability, it would be unwise
> ...


I never could figure out how modern vehicles do away with the simplicity and redundancy of a cable-operated handbrake. If the vehicle's service brakes fail, you're basically screwed.




brian_ said:


> Although regenerative braking doesn't return a huge amount of energy, it is certainly desirable. The big challenge is coordinating it with friction braking, so that braking is smoothly controlled, sensible to the driver, balanced (braking just one end of the car is bad for handling and stability), and doesn't use more friction braking than necessary.


 That does sound like a challenge. However, since the Mini's original equipment 8.4" front discs are not servo-assisted (that didn't happen until about 1989), augmenting it with regeneration doesn't sound like a bad idea. The Mini really needs all the braking assistance it can get .

The only time I've ever seen the front wheels of a Mini out-brake the rear is when the car has been significantly shortened (a "shorty") and the original car had front discs .




brian_ said:


> Anti-lock braking system and stability control system behaviour must be considered as well. In an older vehicle with a simple hydraulic braking system (assisted or not), one problem is how to keep the friction brakes from applying on the powered wheels when regenerative braking is active; I haven't seen anyone address this in a DIY project.


Needless to say, I will not be messing around with adding anti-lock braking or stability control (the Mini is, quite accidentally, perfectly stable on its own; provided I don't mess up the weight distribution). I fully intend to keep the brake pedal firmly connected to the hydraulic brakes, regenerative or not. The only change I might make there is to increase the travel of the brake pedal so that the hydraulic brakes kick in later in the pedal's travel; leaving a dead zone at the top of travel where electric-only braking can occur. Regenerative braking would then occur over the complete travel of the pedal, augmenting rather than replacing the function of the original brakes.




brian_ said:


> It is now popular to have "one pedal" driving, in which zero accelerator pedal application is treated as a moderate degree of braking. It is a challenge to make this feel right to the driver, especially since the right amount depends on driver and driving conditions.


The intention would be to have mild regeneration occurring with no accelerator input; mimicing the deceleration and engine braking of the original engine and (manual) transmission.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> The bulletin board software doesn't support nested quotes? Pity .


The forum software automatically removes the inner quote blocks in a reply, but they can be pasted back in if desired; I've done that occasionally when it was important for context.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> I never could figure out how modern vehicles do away with the simplicity and redundancy of a cable-operated handbrake. If the vehicle's service brakes fail, you're basically screwed.


Long ago, the handbrake was both for parking and emergency use, but that was before the service brakes were required to have dual hydraulic circuits. Since then, the service brakes have had their own redundancy, so a separate hand- (or foot-) operated brake system is no longer for emergency use; it is just for parking.

This is a good thing, since those parking brakes were never much good for emergency braking.

One issue is that they act on the *wrong wheels for traction*...

When brakes were all drum, it was easier to fit the cable-actuated mechanism in the single-cylinder drum brakes on the back than the more effective but more complex dual-cylinder (two leading shoe) drum brakes on the front. When front disks came in, it was much easier to run the remaining drum brakes on the rear with a cable than the disks in front. Even with the four wheel disk brakes that are now normal, it's still better to add the weight and complication of a parking mechanism to the rear brakes (which typically have smaller calipers) than to the front brakes.

Regardless of mechanism, it is preferable to run the parking brake cables to the rear wheels than to run them to the front and have them bending with steering action. The end result is that the lighter end of the car (in most cases) has the parking brakes. Even if the vehicle has equal front-rear weight distribution, under braking the load shifts to the front, so the front tires are more effective for braking.

Saab actually put the parking brake on the front wheels for a while, but that was back when they were gloriously weird. 

So if you really need braking and all you've got is a typical handbrake, you're still screwed.

A related issue is that they act on the *wrong wheels for stability*...

The most common use of a handbrake, other than parking, is breaking the rear tire traction to spin the car around - it's actually a valid corner entry technique in some circumstances in rally competition. Pulling the handbrake to stop the car in an emergency is bad idea, as the likely result is hitting something while sliding sideways or backward, instead of hitting the same thing going forward, so you're even more screwed.

Those Saabs were popular in rally, and had to add a hydraulic handbrake lever to brake the rear wheels. 

And they are difficult to use anyway because the *lock on*:

To be practical as a parking brake, they're always arranged to latch on; so you lose control if used in an emergency because they don't release when the driver stops pulling on the handle.

With the classic handbrake lever the driver can hold the button in, but that's awkward and most people will forget to do that in a panic situation. The handle that sprouted from the dash in the first pickup that I drove was turned to release - that would work better, but is still likely to be confused in an emergency... and it's almost impossible to maintain steering control while reaching for that thing. Then there's the foot pedal with separate release - try steering with your left knee up around the steering wheel to push the pedal while reaching down to the release handle with your left hand... I think you're screwed. The worst is probably the foot pedal which automatically releases when you push it again, leading to an on-off dance that would probably be amusing if it wasn't happening in the lead-up to a collision.


So I say forget the parking brake as an emergency brake. In the very rare event of a service brake failure, use engine braking or regenerative braking to slow down, and use the parking brake or jam it in park (or stall it in gear if you have an engine) for stop the final bit of rolling.


While the cable-operated parking brake is largely gone, the mechanical function is still there. It is just operated by a tiny electric motor on the brake caliper now. That means it is needs power to operate, and can be affected by computer problems, but it will probably still work... and is operated by an easy-to-reach switch. Some even have quite sophisticated control, so if you apply them while moving the system kills the drive power and gradually applies the parking brake - this may work better than the classic hand lever and cable system.

Many cable-operated parking brakes are not functional anyway, because they are not used (in automatic transmission vehicles) and the cables rust in the off position. If applied by someone other than the regular driver, they can stick on. Compare ten-year-old vehicles with electric and cable parking brakes, and I wouldn't be surprised if the electric ones are more likely to be functioning properly.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> ... since the Mini's original equipment 8.4" front discs are not servo-assisted (that didn't happen until about 1989), augmenting it with regeneration doesn't sound like a bad idea. The Mini really needs all the braking assistance it can get .
> 
> The only time I've ever seen the front wheels of a Mini out-brake the rear is when the car has been significantly shortened (a "shorty") and the original car had front discs .
> 
> Needless to say, I will not be messing around with adding anti-lock braking or stability control (the Mini is, quite accidentally, perfectly stable on its own; provided I don't mess up the weight distribution). I fully intend to keep the brake pedal firmly connected to the hydraulic brakes, regenerative or not. The only change I might make there is to increase the travel of the brake pedal so that the hydraulic brakes kick in later in the pedal's travel; leaving a dead zone at the top of travel where electric-only braking can occur. Regenerative braking would then occur over the complete travel of the pedal, augmenting rather than replacing the function of the original brakes.


That makes sense, but there are two issues:

it's going to feel weird
if the regen is not full-on by the time the hydraulic brakes hit, there will be less regen than might be possible, but if regen is full-on before there is any hydraulic braking, the front-rear bias will always be wrong. At least this is more tolerable with regen on the front than it would be with regen on the rear only.
Production EVs all seem to use some sort of manual mode control to allow the driver to manage the balance of regenerative and friction braking, whether it's a switch or shifter position or steering column paddle. With a manual transmission and clutch (which no production EV has) you could set a moderate level of regen and shift to change the degree of effect (just like downshifting for engine braking or to change the effectiveness of a diesel compression brake).

In comparison tests the Tesla Model 3 clearly has less off-accelerator regen than competitors such as the Chevrolet Bolt. I believe that this is deliberate, because while using regen without any friction braking Tesla wanted to avoid excessive rear bias; the Model 3 is rear-drive, while the others are front-drive.

Of course AWD EVs have a big advantage in managing regenerative braking and balancing it with friction brakes (because it can regeneratively brake all four wheels), but that's not something that many DIY builders are attempting.


----------



## TechGeekNZ (Feb 18, 2019)

I am editing this on my phone. Apologies in advance...



brian_ said:


> TechGeekNZ said:
> 
> 
> > I never could figure out how modern vehicles do away with the simplicity and redundancy of a cable-operated handbrake. If the vehicle's service brakes fail, you're basically screwed.
> ...





brian_ said:


> The most common use of a handbrake, other than parking, is breaking the rear tire traction to spin the car around - it's actually a valid corner entry technique in some circumstances in rally competition. Pulling the handbrake to stop the car in an emergency is bad idea, as the likely result is hitting something while sliding sideways or backward, instead of hitting the same thing going forward, so you're even more screwed.


Ah, yes; the good ol' handbrake turn. Pretty much the only thing the handbrake was ever useful for in the Mini; as, even when the handbrake cable and rear brake shoes were properly adjusted, they never did seem to be particularly effective, even for parking .



brian_ said:


> ... those parking brakes were never much good for emergency braking.
> 
> One issue is that they act on the *wrong wheels for traction*...


I actually did have a sudden and total failure of the hydraulic brakes on my 1989 Toyota Corolla once. It turned out that one of the rear brake shoes had been installed incorrectly and caused brake fluid to leak out of the system until there was insufficient pressure for operation. Fortunately, the failure occurred on a quiet city street and I was able to yank up on the handbrake to stop the car in time. I'm not sure who was the most surprised at the roundabout that day; me, or the driver I was attempting to give way to .




brian_ said:


> So if you really need braking and all you've got is a typical handbrake, you're still screwed.
> 
> ...
> 
> ...


Despite my personal anecdote, I would tend to agree; although with an ineffective brake, you're still less screwed than with no brakes. Obviously, it's very important to know how the vehicle is going to respond in situations like that before attempting such a maneuver. Also, if I hadn't had so much experience pulling handbrake turns on grassy fields in the Mini, I probably would have found a way to screw up my emergency stopping maneuver and hit the oncoming car instead of stopping short of it.



brian_ said:


> So I say forget the parking brake as an emergency brake. In the very rare event of a service brake failure, use engine braking or regenerative braking to slow down, and use the parking brake or jam it in park (or stall it in gear if you have an engine) for stop the final bit of rolling.


In situations where you actually have time to think about it, using the engine and gears to slow down is by far the better way to do it. May sentient beings have mercy on you if you happen to be driving an automatic without knowing how to force a downshift in such a situation.




brian_ said:


> While the cable-operated parking brake is largely gone, the mechanical function is still there. It is just operated by a tiny electric motor on the brake caliper now. That means it is needs power to operate, and can be affected by computer problems, but it will probably still work... and is operated by an easy-to-reach switch. Some even have quite sophisticated control, so if you apply them while moving the system kills the drive power and gradually applies the parking brake - this may work better than the classic hand lever and cable system.


 It's somewhat reassuring that there is still _some_ redundancy in the system. Although, as you say, it is rather unlikely that a modern dual-circuit hydraulic system will fail completely; so there is still another level of redundancy there.



brian_ said:


> Many cable-operated parking brakes are not functional anyway, because they are not used (in automatic transmission vehicles) and the cables rust in the off position.


That's a good reason in and of itself to actually use the parking brake when parking an automatic transmission. I wonder why more people don't do that ?




brian_ said:


> TechGeekNZ said:
> 
> 
> > The only change I might make there is to increase the travel of the brake pedal so that the hydraulic brakes kick in later in the pedal's travel; leaving a dead zone at the top of travel where electric-only braking can occur. Regenerative braking would then occur over the complete travel of the pedal, augmenting rather than replacing the function of the original brakes.
> ...


More very useful things to think about. Thank you!



brian_ said:


> Production EVs all seem to use some sort of manual mode control to allow the driver to manage the balance of regenerative and friction braking, whether it's a switch or shifter position or steering column paddle. With a manual transmission and clutch (which no production EV has) you could set a moderate level of regen and shift to change the degree of effect (just like downshifting for engine braking or to change the effectiveness of a diesel compression brake).


Another great idea; and a potential use for the clutch pedal?



brian_ said:


> Of course AWD EVs have a big advantage in managing regenerative braking and balancing it with friction brakes (because it can regeneratively brake all four wheels), but that's not something that many DIY builders are attempting.


I have actually seen a hybrid car (a Toyota Oddysey, perhaps?) that installed electric motors to the rear wheels for the express purpose of providing regenerative braking; so it's not outside the realm of possibility.


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## Duncan (Dec 8, 2008)

Hi
My mini through it's incarnations

From a 1430 A series to a 170 hp two litre twin cam

To my current electric roadster


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> I have actually seen a hybrid car (a Toyota Oddysey, perhaps?) that installed electric motors to the rear wheels for the express purpose of providing regenerative braking; so it's not outside the realm of possibility.


The Odyssey is a non-hybrid Honda minivan... you're presumably thinking of the Toyota Highlander (or RAV4, or Lexus RX, or Lexus NX) hybrid version. The Honda Pilot / Acura MDX hybrids are similar. In those cases, the rear drive unit will regeneratively brake, but that's not its primary purpose: it's there to provide all wheel drive when required. The front has comparable or greater regenerative braking capability, in the hybrid drive system (in fact, one of the motor-generators in a Toyota Synergy Hybrid Drive transaxle - called "MG-2" - is typically identical to the motor-generator in the rear, called "MG-R").


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## TechGeekNZ (Feb 18, 2019)

Duncan said:


> Hi
> My mini through it's incarnations
> 
> From a 1430 A series to a 170 hp two litre twin cam
> ...


Wow, that's an impressive array of mods. Well done with the open-top conversion, and very well done stretching the engine bay around that Lancia motor without converting it to the (slightly roomier) Clubman front-end .

That roadster is cool; what did you use as a base vehicle?


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> I actually did have a sudden and total failure of the hydraulic brakes on my 1989 Toyota Corolla once. It turned out that one of the rear brake shoes had been installed incorrectly and caused brake fluid to leak out of the system until there was insufficient pressure for operation. Fortunately, the failure occurred on a quiet city street and I was able to yank up on the handbrake to stop the car in time.


Any 1989 car would have had a dual hydraulic brake system. While the brake pedal dropping nearly to the floor due to the rear circuit being empty would have been alarming (and might make the average driver think there were no brakes), the other half of the service brakes would have still been working (unless something killed that half, too ). That could have been both front brakes, or one front and one rear, depending on whether it was a front-back split or a diagonal split system.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> That's a good reason in and of itself to actually use the parking brake when parking an automatic transmission. I wonder why more people don't do that?



They don't understand this reasoning.
They are not concerned when the vehicle rolls slightly against the parking feature of the transmission so they don't apply the brake to avoid that.
They're lazy.


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## TechGeekNZ (Feb 18, 2019)

brian_ said:


> The Odyssey is a non-hybrid Honda minivan...



Yeah, a Toyota Odyssey would be quite the oddity . It was a Japanese import, most likely one of these Honda Odyssey hybrid minivans. It was quite entertaining (from an engineering perspective) to watch it managing the two different propulsion systems on the real-time status monitor—every ripple in the road was an opportunity for regeneration. The front wheels were driven by both the ICE and electric motor, the rear wheels were electric-only. I'm not sure if it actually used the rear wheels for propulsion on that journey (presumably, it would if it needed to), but it sure liked using them for decelerating and braking.

Obviously, the design of the Mini's rear suspension makes it difficult to attach systems to the rear wheels; however, it does make me think. I wonder how much of a braking effect could be achieved by attaching a pair of old alternators to the rear wheels of the Mini..?

Another possibility; since the drum brakes never changed in size over the lifetime of the Mini, there is some space between the wheel hub and the 12" wheel rim that could be used to build the stator of an inside-out induction motor/generator.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> It was a Japanese import, most likely one of these Honda Odyssey hybrid minivans. It was quite entertaining (from an engineering perspective) to watch it managing the two different propulsion systems on the real-time status monitor—every ripple in the road was an opportunity for regeneration. The front wheels were driven by both the ICE and electric motor, the rear wheels were electric-only. I'm not sure if it actually used the rear wheels for propulsion on that journey (presumably, it would if it needed to), but it sure liked using them for decelerating and braking.


That sounds like the same approach as the other hybrids with electric-only drive to one axle such as Honda's Acura MDX, although the hybrid system in this Odyssey is quite different from the hybrid system in the MDX.


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## TechGeekNZ (Feb 18, 2019)

TechGeekNZ said:


> ... it is rather unlikely that a modern dual-circuit hydraulic system will fail completely; so there is still another level of redundancy there.


It still strikes me as odd that the Corolla's brakes failed in the way they did. On the previous occasion that I used them, they were working normally and effectively; but on the next, all that was available was a very minor braking effect on one circuit and (obviously) nothing on the failed circuit. Somehow, the total loss of pressure in one circuit must have allowed air to leak into the other; which seems to totally defeat the point of having a split system in the first place .


----------



## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Obviously, the design of the Mini's rear suspension makes it difficult to attach systems to the rear wheels; however, it does make me think. I wonder how much of a braking effect could be achieved by attaching a pair of old alternators to the rear wheels of the Mini..?


I can't imagine going to the effort of connecting generators to the rear wheels without using them as motors to provide all wheel drive.

To get a handle on the power level involved in regenerative braking, multiply the braking force by the road speed. For example, if you have a one-tonne (1000 kg) vehicle, and you decelerate it at only one-tenth of the acceleration of gravity (0.1 G), while moving at 70 km/h (44 mph, 20 m/s), that's
Power = 1000 kg * 0.1*9.81 N/kg * 20 m/s = 20 kW

(Amusing note: if you put "1000 kg * 0.1*9.81 N/kg * 20 m/s" into the Chromer browser address/search box, it actually does the calculation complete with units of measure and reports "= 19620 watts")

You can do the calculation with antiquated units of measure (pounds, feet per second squared, miles per hour, horsepower), but you'll need some conversion factors.

So to do a useful amount of braking at even urban speeds, a substantial amount of power is involved - adapted engine alternators won't do it. Those "mild hybrid" systems (which typically run at 48 volts) such as the eTorque system on Ram pickups use a much more powerful alternator than typically used in a vehicle.



TechGeekNZ said:


> Another possibility; since the drum brakes never changed in size over the lifetime of the Mini, there is some space between the wheel hub and the 12" wheel rim that could be used to build the stator of an inside-out induction motor/generator.


Ah, the hub motor... black hole for time and resources. As far as I can tell, no one has ever made this worthwhile on anything larger than a scooter, although it is often used to fish for capital investment because it just sounds so cool. The primary problem is that a motor (or generator) running at the wheel speed is too heavy for a useful power level.


----------



## brian_ (Feb 7, 2017)

TechGeekNZ said:


> It still strikes me as odd that the Corolla's brakes failed in the way they did. On the previous occasion that I used them, they were working normally and effectively; but on the next, all that was available was a very minor braking effect on one circuit and (obviously) nothing on the failed circuit. Somehow, the total loss of pressure in one circuit must have allowed air to leak into the other; which seems to totally defeat the point of having a split system in the first place .


Right - the apparent behaviour doesn't make physical sense, because there is no fluid connection between the circuits. Even if all of the fluid is drained from one circuit, the divided reservoir on top of the master cylinder will still hold fluid for the other circuit. Most likely, the master cylinder seals were shot, allowing the good circuit's fluid to leak into the bad circuit. That's a problem allowed to get worse for a long time, although the apparent failure may seem sudden; this is a bit like the person who works for many years and finally makes it as a "overnight success"... but in a bad way.


----------



## TechGeekNZ (Feb 18, 2019)

brian_ said:


> Saab actually put the parking brake on the front wheels for a while, but that was back when they were gloriously weird.


Ah yes, Saab; the true pioneers of automotive safety. I've heard it said that nobody could ever figure out why a Saab was so expensive until they crashed one:





brian_ said:


> ... Then there's the foot pedal with separate release - try steering with your left knee up around the steering wheel ...


For someone who is 6-foot tall, that is basically the standard driving position for a Mini .




brian_ said:


> The worst is probably the foot pedal which automatically releases when you push it again, leading to an on-off dance that would probably be amusing if it wasn't happening in the lead-up to a collision.


It's amazing just how much "innovation" has gone into parking brake systems. I just imagined accidentally hitting one of those while driving normally—it's just like all those traction control disengagement buttons that, in modern cars, are conveniently located right next to the driver's kneecap – an accident waiting to happen .


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## TechGeekNZ (Feb 18, 2019)

brian_ said:


> I can't imagine going to the effort of connecting generators to the rear wheels without using them as motors to provide all wheel drive.


Most likely, if I did go to that kind of effort, I would consider the same. Other than attempting to preserve the car's original handling characteristics, there isn't much reason not to.




brian_ said:


> To get a handle on the power level involved in regenerative braking, multiply the braking force by the road speed. For example, if you have a one-tonne (1000 kg) vehicle, and you decelerate it at only one-tenth of the acceleration of gravity (0.1 G), while moving at 70 km/h (44 mph, 20 m/s), that's
> Power = 1000 kg * 0.1*9.81 N/kg * 20 m/s = 20 kW
> 
> (Amusing note: if you put "1000 kg * 0.1*9.81 N/kg * 20 m/s" into the Chromer browser address/search box, it actually does the calculation complete with units of measure and reports "= 19620 watts")


Google calculator supports all kinds of weird and wonderful units. For instance, you could do (using the Mini as an example):
"3/4 imperial ton * 0.1*(gravity on earth) * 3800 ft/min" and still get a sane answer. Wolfram|Alpha is even worse .




brian_ said:


> You can do the calculation with antiquated units of measure (pounds, feet per second squared, miles per hour, horsepower), but you'll need some conversion factors.


I use metric all the way; except of course when discussing people's dimensions (it's a Kiwi thing) and replacing the Mini's bolts .




brian_ said:


> So to do a useful amount of braking at even urban speeds, a substantial amount of power is involved - adapted engine alternators won't do it.


Certainly not a standard 16A or 18A Lucas unit; when the interior ventilation fan is on, those things barely manage to keep the battery charged; or indeed supply sufficient voltage at idle for the bimetallic direction indicator relay to do its job. I asked the question purely out of curiosity, not expecting it to be practical.



brian_ said:


> TechGeekNZ said:
> 
> 
> > Another possibility; since the drum brakes never changed in size over the lifetime of the Mini, there is some space between the wheel hub and the 12" wheel rim that could be used to build the stator of an inside-out induction motor/generator.
> ...


Sadly, I agree. I don't think that technology will ever become practical; because, if it was, someone would have already done it.




brian_ said:


> Right - the apparent behaviour doesn't make physical sense, because there is no fluid connection between the circuits. Even if all of the fluid is drained from one circuit, the divided reservoir on top of the master cylinder will still hold fluid for the other circuit. Most likely, the master cylinder seals were shot, allowing the good circuit's fluid to leak into the bad circuit. That's a problem allowed to get worse for a long time, although the apparent failure may seem sudden; this is a bit like the person who works for many years and finally makes it as a "overnight success"... but in a bad way.


Although the car was regularly maintained and serviced, that would not surprise me at all. The car would have been over 20 years old when it happened, so the combination of a faulty master cylinder and a leaky drum brake cylinder slowly bleeding the system of brake fluid over the course of several months (leading up to its next scheduled service) is entirely feasible.

Even just knowing that such a failure mode can occur (even if unlikely) gives me plenty of reason to ensure the Mini's hydraulics get a complete overhaul before it goes anywhere near the road again; even more so since it has now been sitting unused for more than a decade.


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## TechGeekNZ (Feb 18, 2019)

brian_ said:


> Any 1989 car would have had a dual hydraulic brake system. While the brake pedal dropping nearly to the floor due to the rear circuit being empty would have been alarming (and might make the average driver think there were no brakes), the other half of the service brakes would have still been working (unless something killed that half, too ). That could have been both front brakes, or one front and one rear, depending on whether it was a front-back split or a diagonal split system.


Yeah, it was definitely a diagonal split. After the incident occurred, I drove slowly around the residential block I was in, repeatedly pumping the brakes to try to regain some pressure. With the very little pressure there was in there, I could feel the car twisting to one side. I quickly determined the car was undriveable, parked it, and sought alternative transportation.

Although I'd have to double-check, I'm fairly certain that the Mini 1275 GT also has a diagonal-split system—I'm pretty sure I recall seeing the rear brake lines crossing over each other near the rear subframe.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> brian_ said:
> 
> 
> > The worst is probably the foot pedal which automatically releases when you push it again, leading to an on-off dance that would probably be amusing if it wasn't happening in the lead-up to a collision.
> ...


Fortunately, this type is only (as far as I know) only used in vehicles with an automatic transmission, in which the left foot isn't used for anything else and it's very unlikely that anyone would hit the parking brake pedal while trying for the brake pedal. The risk, of course, is a manual transmission driver hitting it as a clutch pedal... although it's usually the brake pedal that gets hit this way.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Google calculator supports all kinds of weird and wonderful units. For instance, you could do (using the Mini as an example):
> "3/4 imperial ton * 0.1*(gravity on earth) * 3800 ft/min" and still get a sane answer. Wolfram|Alpha is even worse .



That's an excellent example of the brilliance of xkcd.


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Certainly not a standard 16A or 18A Lucas unit; when the interior ventilation fan is on, those things barely manage to keep the battery charged; or indeed supply sufficient voltage at idle for the bimetallic direction indicator relay to do its job. I asked the question purely out of curiosity, not expecting it to be practical.


There have been quite a few completely serious alternator schemes. They can even be converted to motors. One "interesting" member of this forum piled something like 16 alternators on a trailer inspired by Rube Goldberg and tried to collect "free kinetic energy" while moving at constant speed.


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## TechGeekNZ (Feb 18, 2019)

TechGeekNZ said:


> brian_ said:
> 
> 
> > Ah, the hub motor... black hole for time and resources. As far as I can tell, no one has ever made this worthwhile on anything larger than a scooter, although it is often used to fish for capital investment because it just sounds so cool. The primary problem is that a motor (or generator) running at the wheel speed is too heavy for a useful power level.
> ...


At first, I assumed you were talking specifically about an inside-out induction motor rather than the general concept of a motor built directly into a wheel. In either case, you are correct that a motor of those dimensions would likely be too heavy, too underpowered, or both.

To have any chance of developing this concept, I would need to determine exactly how much torque is generated by good condition, correctly-functioning rear drum brakes on the Mini. Obviously, they do have _some_ effect, even if it doesn't always seem apparent from the driver's seat .


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## TechGeekNZ (Feb 18, 2019)

brian_ said:


> There have been quite a few completely serious alternator schemes. They can even be converted to motors. One "interesting" member of this forum piled something like 16 alternators on a trailer inspired by Rube Goldberg and tried to collect "free kinetic energy" while moving at constant speed.


Don't laugh; there is a time I might have considered doing that. Not the "free energy" bit, of course; I've known since childhood that nothing comes for free .


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Don't laugh; there is a time I might have considered doing that.


Sure, it's rational to go through thoughts of generators on trailers before realizing

how little energy is available from regenerative braking, compared to the energy cost of towing a trailer (in rolling drag and aero drag)
how inefficient "through the road" charging (driving one set of tires with an engine while running a generator from a different set of tires) is as a feature of a hybrid powertrain


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> To have any chance of developing this concept, I would need to determine exactly how much torque is generated by good condition, correctly-functioning rear drum brakes on the Mini. Obviously, they do have _some_ effect, even if it doesn't always seem apparent from the driver's seat .


The static load distribution is probably 60/40 front/rear, and braking force balance will be more front-biased due to load transfer under braking, so for a wild guess assume 30% of the braking effort at the rear, or maybe 25% for a low estimate. To know the total braking force, you just need the vehicle mass and the rate of deceleration (less than 1 g max, and a small fraction of that normally). You can work out torque from the force with tire radius.

My earlier example was 20 kW of braking, but of course some of that comes from rolling and aero drag, so less needs to be absorbed by the braking system (whether friction or regenerative). So maybe only a couple of kilowatts per rear wheel in a moderate stop.


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## Duncan (Dec 8, 2008)

Given that one person can push a mini with it's back wheels locked by the handbrake the effective braking and power from the rear wheels is bugger all

IMHO you should simply ignore all thoughts about messing with the back end until you have the front end all working


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## TechGeekNZ (Feb 18, 2019)

Edited on a handheld device. Apologies in advance.



brian_ said:


> TechGeekNZ said:
> 
> 
> > Obviously, (the Mini's rear brakes) do have _some_ effect, even if it doesn't always seem apparent from the driver's seat 🆒.
> ...


Just for fun, let me try to put some numbers on this.

The outer diameter of the Mini's standard 145/70R12 tyres is 20", and the curb weight of a 1275 GT is 655 kg, or about half that of an average compact car. Under heavy braking in dry conditions, the brakes of most older cars should stop the car dead within 1.5 seconds (factoring in 0.5s of driver reaction time, for the highway code's recommended 2 second rule for estimating following distances); so at the New Zealand highway speed limit of 100 km/h, that will be 18.5 ms^-2 average deceleration, or a total braking force of 12.5 kN (which I'll round up to 12.75 kN, just because I can 😉), or a total (average) torque of 3230 Nm and average power of 180 kW. Under heavy braking, 100% of the Mini's weight transfers to the front axle, so I'll assume no more than 25% of the braking effort is done by the rear wheels, so as to avoid locking them into a skid. So that's 45 kW, or 22.5 kW per rear wheel, in a worst-case scenario. So it seems you're right on the money with your estimate of 2 kW per rear wheel for a moderate braking scenario.


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## TechGeekNZ (Feb 18, 2019)

Duncan said:


> Given that one person can push a Mini with it's back wheels locked by the handbrake the effective braking and power from the rear wheels is bugger all
> 
> IMHO you should simply ignore all thoughts about messing with the back end until you have the front end all working


Hi, Duncan.

Given that I have actually done that, several times, I would say it's actually two tenths of bugger all :wink:.

One of the best features I liked about my Mini was how incredibly easy it was to push. Well, that and the fact that, being 6' tall, I could stand on one side of the car while washing the roof on the _other_ side .

On a more serious note, what kinds of things should I be looking out for when it comes time to get the car re-certified after its "conversion to alternative fuel"?


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## brian_ (Feb 7, 2017)

TechGeekNZ said:


> Edited on a handheld device. Apologies in advance.


For some reason, the URL tags were A tags (whatever those are supposed to do), so they didn't work. I fixed them...



TechGeekNZ said:


> Just for fun, let me try to put some numbers on this.
> 
> The outer diameter of the Mini's standard 145/70R12 tyres is 20", and the curb weight of a 1275 GT is 655 kg, or about half that of an average compact car. Under heavy braking in dry conditions, the brakes of most older cars should stop the car dead within 1.5 seconds (factoring in 0.5s of driver reaction time, for the highway code's recommended 2 second rule for estimating following distances); so at the New Zealand highway speed limit of 100 km/h, that will be 18.5 ms^-2 average deceleration, or a total braking force of 12.5 kN (which I'll round up to 12.75 kN, just because I can 😉), or a total (average) torque of 3230 Nm and average power of 180 kW. Under heavy braking, 100% of the Mini's weight transfers to the front axle, so I'll assume no more than 25% of the braking effort is done by the rear wheels, so as to avoid locking them into a skid. So that's 45 kW, or 22.5 kW per rear wheel, in a worst-case scenario. So it seems you're right on the money with your estimate of 2 kW per rear wheel for a moderate braking scenario.


That's almost 2 g braking, which is wildly unreasonable for an ordinary car; it's about double what you can reasonably expect on street tires and without aero downforce. The 1.5 second requirement would be unreasonable for highway speed - you really don't need to be able to stop within highway following distance. So divide the numbers which follow by two, and you have more realistic torque and power values.

All of the load transferring to the front under braking is an exaggeration, but works for the linked discussion of brake balance. Occasionally someone will demonstrate that a forward-cab vehicle with no cargo and two heavy occupants can lift the rear wheels under braking, but even then they usually need to pump the brakes to bounce the vehicle on its suspension - a Mini will not do this, because even the short wheelbase is so much longer than the centre of mass height, and the static weight distribution just isn't that front-heavy.

I agree with the end result - not much braking by the rear in a hard stop, and a small fraction of a smaller amount of total braking in a normal stop, so there's not much to be gained in the rear. While driving, on the other hand, load transfer means that AWD would help acceleration.


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## Emyr (Oct 27, 2016)

TechGeekNZ said:


> Under heavy braking in dry conditions, the brakes of most older cars should stop the car dead within 1.5 seconds (factoring in 0.5s of driver reaction time, for the highway code's recommended 2 second rule for estimating following distances);


That's 2 seconds following distance, not two seconds stopping time. The vehicle you're following won't stop instantly. Even if it hits an immovable object, the rear of that vehicle will take longer to stop than the front.


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## Duncan (Dec 8, 2008)

TechGeekNZ said:


> Hi, Duncan.
> 
> Given that I have actually done that, several times, I would say it's actually two tenths of bugger all :wink:.
> 
> ...


 Hi
You need to go to the 
https://www.lvvta.org.nz/

And download the "Hobby Car Manual" - some of it is not applicable to an EV but it was written by keen Hot Rodders and is quite easy to read

Then you need the LVV standards from the same web page

Then you need to find a certifier (also same page) - only a few certifiers can do EV's - There was only one on the South Island when I did mine

And talk to him

One thing that I would say is that unless you are keeping the same weight distribution front/rear then it's worth using a dual master cylinder system with an adjustable balance bar - and if you don't have a servo that is something else not to go wrong

The brake test is multiple brake downs from 100 kph to ensure that your brakes will not fade


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## clsund (Jan 18, 2019)

ev4u.com


they teach a class on how to convert old minis.

Let me know how it goes.


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## TechGeekNZ (Feb 18, 2019)

brian_ said:


> TechGeekNZ said:
> 
> 
> > Edited on a handheld device. Apologies in advance.
> ...


Yeah, I got the BBcode syntax wrong; I assumed it followed HTML syntax. I've fixed it now.



brian_ said:


> TechGeekNZ said:
> 
> 
> > Just for fun, let me try to put some numbers on this.
> ...


That's a pity; all that math for nothing. Perhaps I should have asked Wolfram|Alpha how long it takes to stop a car 🙂.



brian_ said:


> All of the load transferring to the front under braking is an exaggeration, but works for the linked discussion of brake balance.


It actually isn't terribly difficult to make a Mini stand on two wheels; just cut the middle out, sit in the back, and kick the brakes 😉.



brian_ said:


> While driving, on the other hand, load transfer means that AWD would help acceleration.


That, I think we can all agree with. Whether it will be worthwhile from an engineering standpoint remains to be seen.


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