# DC motor folks - do you worry about lack of secondary braking?



## vocalnick (Aug 5, 2013)

Hey folks,

I'd been leaning toward AC for my upcoming build, but I'm starting to have some second thoughts. Price/performance ratio seems a lot better with DC, and while regen is nice, I can just use some of that saved money, buy another few cells and increase my range that way. 

But I do worry a little about not having engine-braking. I live in a pretty hilly area, and while the chances are fairly slim, a brake/hydraulic failure on a slope here would put me in a pretty dangerous position (particularly with the added weight of an EV)

Have any of you guys factored that in to your builds? Or am I just being paranoid?


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## Duncan (Dec 8, 2008)

Hi
(1) What added weight?
(2) I have two totally independent hydraulic brake circuits + the mechanical "hand brake"

I believe most/all cars since the 1980's have dual circuit hydraulics,
They have a single master cylinder but it's split internally and from there on everything is dual

If you lost hydraulics you would at most lose front right, rear left (or opposite)

My car being a scratch built has two separate master cylinders (for front and rear) coupled by a balance bar (so I can set front/rear balance)
If I lost a circuit the balance bar would hit a stop and still allow me to brake on the other cylinder


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## vocalnick (Aug 5, 2013)

Duncan said:


> Hi
> (1) What added weight?


From reading several build articles, it seems that a lot of them end up weighing over stock once all the batteries go in. 



> I believe most/all cars since the 1980's have dual circuit hydraulics,
> They have a single master cylinder but it's split internally and from there on everything is dual


Innnnteresting! I'll take a look at the specs on my donor vehicle. That would allay my concern a great deal.


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## steelneck (Apr 19, 2013)

vocalnick said:


> But I do worry a little about not having engine-braking.


Think of all cars with auto transmissions, they have a very weak engine brake, especially the older ones without converter lock-up. But yes, they do wear out brake pads quicker and so will an EV without generative braking.

In my conversion i chose an auto trans partly because of this very reason. With an auto trans it feels quite natural to drive without engine brake (another of my reasons was the Park function).


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## vocalnick (Aug 5, 2013)

steelneck said:


> Think of all cars with auto transmissions, they have a very weak engine brake, especially the older ones without converter lock-up. But yes, they do wear out brake pads quicker and so will an EV without generative braking.


That's a good point.

I think perhaps I'm overcautious on this one - I have a memory as a kid of Dad's car's drum brakes overheating on a long, steep, winding road and him having to use the engine to slow down (while Mum screamed)

Must've left an emotional scar or two 

But that's drums vs 4-wheel discs too...


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## Duncan (Dec 8, 2008)

You can overheat discs as well - even the vented ones

But this is mostly a result of incorrect driving method - trailing the brakes on the downhill

I found it was better to brake harder - and let the brakes cool between braking events,
When you do brake slow down further - it will give a longer cool down period before the next braking event


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## dragonsgate (May 19, 2012)

vocalnick said:


> Hey folks,
> 
> I'd been leaning toward AC for my upcoming build, but I'm starting to have some second thoughts. Price/performance ratio seems a lot better with DC, and while regen is nice, I can just use some of that saved money, buy another few cells and increase my range that way.
> 
> ...


I live in a very hilly area and the braking subject has played heavy in my mind for a long time. I considered plug break or electric reverse as a last ditch method of emergency stopping but when I addressed the subject on this forum I got a big negative. Things about blowing up motors and stuff. I did not go that route because of the extra complicated wiring. Dual reservoir brake cylinder is handy but in my experience rear breaks only do not work well if the front goes but is better than nothing. I have had a couple of cars that the emergency will lock the back wheels to a screeching halt but the POS I am driving now barely holds when the vehicle is already stopped. I faded the brakes once going down a series of hair pin turns in a souped up Fiat and the compression of the engine and gearing down with a lot of over revving helped me get slowed down enough to stop but my wifes Dodge intrepid barely feels like there is any compression braking even when the auto trans in manually shifted to low. If you shave enough nonessential stuff off your car and use a small enough lithium pack it is possible to come in at the same or even a bit less than stock weight. I am a couple hundred pounds over curb and the stock brakes have handled it satisfactorily for several years now. I just inspect the brake system often to keep it in top shape and am on constant look out for a place to ditch the car in the event of a total brake failure.If the AC motors that are on the market now was available when I did my conversion I would have went AC.If I had the energy and money now I would switch to AC.


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## Duncan (Dec 8, 2008)

_If the AC motors that are on the market now was available when I did my conversion I would have went AC.If I had the energy and money now I would switch to AC.

_Trouble is it takes a $20,000 AC system to match a $3000 DC system for power_ - _If I was rich...._
_


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## dougingraham (Jul 26, 2011)

I don 't worry about it. Brakes that are maintained are pretty reliable.

For the smaller lithium powered EV's with range less than 50 miles will usually weigh less than the original car. Lithium powered EV's with a range between 50 and 70 miles weigh about the same as the original car. It is only when you decide you want long range that the car gets progressively more porky. My car with a max range of about 80 miles weighs about 120 lbs more than it originally did. With the latest batteries I could have the same range and weigh less than original. With lead acid you would have to limit your range to less than 15 miles with a fresh pack to keep the weight about the same as original if you can even manage that. Your results may vary.

I would not depend on Regen as a replacement for brakes down a long steep grade. Your braking force is only on the driven wheels which leads to potential instability. You also have limitations similar to those it takes to go up the hill with heating in the motor and controller. If you happen to live at the top of a hill and start off with a full pack there is no place to put the energy. That is a pretty specific failure mode however.

Check the brake fluid level regularly, be aware of changes in pedal feel and your chances of a problem are going to be minimized.


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## steelneck (Apr 19, 2013)

Duncan said:


> I found it was better to brake harder - and let the brakes cool between braking events,
> When you do brake slow down further - it will give a longer cool down period before the next braking event


Yes. I work in an underground iron mine, i am based at 1250 meters below surface. The road down there is around 10km almost constant downhill. No modern cars, or trucks for that matter, have no problems what so ever with that. Most personal transports is done by WV buses and various pick ups, these cars usually have a curb weight around 1,8 ton and many of them even have drum brakes at the rear, no problem. We brake just as you do Duncan, brake quite hard and then let the brakes cool down until next brake. Especially when driving a truck downhill.


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## vocalnick (Aug 5, 2013)

dougingraham said:


> It is only when you decide you want long range that the car gets progressively more porky.


I'm planning a LiFePo4 build, but I am aiming for long range - 100 miles is an aspirational target, but I'll be reasonably happy if I can hit 80 



> If you happen to live at the top of a hill and start off with a full pack there is no place to put the energy. That is a pretty specific failure mode however.


It is pretty specific, but it does actually precisely describe my situation, so it's specific in a good way


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## dougingraham (Jul 26, 2011)

Your car will most likely end up a couple of hundred lbs overweight if you build for a 100 mile range.

Because you live at the top of a hill you will want to undercharge your batteries a little bit so you have someplace to put the regen energy first thing. This can be a fine tuning thing once you get the build done.


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## DavidDymaxion (Dec 1, 2008)

FWIW, here near the site of the 2002 Winter Olympics, my wife has overheated and ruined brakes on two different cars with automatics, multiple times on one of the cars. The several manual transmission cars were OK. She refuses to downshift the auto transmissions, insisting it is bad for them. My theory is the manual tranny cars had enough engine braking to save the day, but without downshifting the autos did not. Anyway, the goal is not to diss on my otherwise endearing wife, but to make the comment if you have hills and you want anyone to be able to safely drive the car, it is probably a bad idea with a regular DC motor, as it coasts freely.

I did a calculation once, and riding the brakes down a long, steep hill generates comparable heat energy to driving the car all out on a road racing track! Each stop on the race track generates more heat, but you get cool down breaks between the corners, whereas the hill generates the heat continuously.

My sepex Kostov does regen. I probably don't really need it for my commute, but it sure is satisfying to grab those Amps. I'm putting lithiums in soon and hoping I have enough range to go skiing this winter and put that regen to truly good use!

Another thought is brakes can be improved. Often the problem is the fluid gets too hot and braking suffers (or even goes away). Air ducts can greatly cool the brakes. Switching to a racing brake fluid, like Castrol SRF or Motul RBF 600 makes the fluid much more resistance to overheating and failing. Brake fluid grabs moisture and the water boils very easily, so just flushing the brake fluid is a good idea for any conversion, regen or not. Once brake fluid has been overheated, it is forever ruined and will fail at a lower temperature in the future -- so if you ever get a squishy pedal (or, heavens forbid, a pedal to the floor), the brake fluid needs to be flushed immediately.


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## steelneck (Apr 19, 2013)

DavidDymaxion said:


> FWIW, here near the site of the 2002 Winter Olympics, my wife has overheated and ruined brakes on two different cars with automatics, multiple times on one of the cars. The several manual transmission cars were OK. She refuses to downshift the auto transmissions, insisting it is bad for them. My theory is the manual tranny cars had enough engine braking to save the day, but without downshifting the autos did not. Anyway, the goal is not to diss on my otherwise endearing wife, but to make the comment if you have hills and you want anyone to be able to safely drive the car, it is probably a bad idea with a regular DC motor, as it coasts freely.


That is not an argument for electric regen. because you have just argued that it is the same electric or not. The above argues against automatic transmissions in general.

In reality the "anyone" is not really just anyone since we usually expects some basic knowledge of someone allowed to drive a car, like not constantly riding the brakes. Even without regen. a car has quite some drag depending on speed and downshifted gearboxes also have a lot of friction (thats the main argument for direct drive and wheel motors). The short test drives i have done with my soon to be finished conversion verify this for me, the car retards more than i thought when letting of the foot from the pedal, but OK, it is an old car with a lot more drag than modern ones.



DavidDymaxion said:


> Another thought is brakes can be improved. Often the problem is the fluid gets too hot and braking suffers (or even goes away). Air ducts can greatly cool the brakes. Switching to a racing brake fluid, like Castrol SRF or Motul RBF 600 makes the fluid much more resistance to overheating and failing. Brake fluid grabs moisture and the water boils very easily, so just flushing the brake fluid is a good idea for any conversion, regen or not. Once brake fluid has been overheated, it is forever ruined and will fail at a lower temperature in the future -- so if you ever get a squishy pedal (or, heavens forbid, a pedal to the floor), the brake fluid needs to be flushed immediately.


That was good info, especially for people in places where DOT4 brake fluid is still not the norm (like the USA). The two brands you mentioned are classified as DOT4 wich is much more heat resistant than DOT3. Here in Sweden where i live anything less than DOT4 is very rare and must be specially ordered, and i think it is the same in most of Europe (except the UK i think).

With series DC it can be a good idea to use a better brake fluid and remember to not drive around with old brake fluid. But as you wrote initially, this is also true for cars with automatic transmissions, especially older ones without lockup.


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## Frank (Dec 6, 2008)

A major issue that requires serious consideration imho is the implementation of a properly-sized vacuum pump and reservoir system. Go for the largest "flow" rate pump you can with large tubes, etc. The system on my car (using components generally used by others 5,6 years ago) is marginal going down hills. I have also found that periodic stabbing, rather than riding them, works best. Increasing pump set points helps but is a crutch.

The cost of air-cooled AC systems is coming down and I would consider them. It's hard to beat the performance of a DC system but...


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## PStechPaul (May 1, 2012)

As for the "dual brake systems", I have a 1999 Saturn and a 1989 Toyota Pickup. On two occasions now, I have had a pedal-to-the-floor event on the Saturn, with no warning and no discernible "secondary braking" action. In both cases it has been a rusted rear brake line that ruptured, and after multiple pumps on the master cylinder the reservoir went dry. I consider myself lucky that each time was as I was parking the car.

On the Toyota Pickup, one of the front calipers had become rusty and did not fully release the pads, so they got hot. I had the brakes inspected and they found that it had gotten so hot that the brake fluid was boiling and thus the "vapor lock" eliminated the necessary compression and hydraulic force to operate the piston, but after enough time (about 1/2 hour) it returned to normal. After a couple such incidents I had the caliper replaced and all has been OK. But the point is that the failure of one brake component rendered the entire system ineffective, and the only way I could drive was to stay in low gear and use compression braking, as well as the parking brake, and fortunately they are manual hand-operated rather than ratcheting foot pedal types, or whatever electronic types are on newer vehicles.


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## DavidDymaxion (Dec 1, 2008)

Steelneck, you made good comments, but I'll rant on driver ability:



> In reality the "anyone" is not really just anyone since we usually expects some basic knowledge of someone allowed to drive a car, like not constantly riding the brakes.


I've have driven thousands of kilometers in Europe, and I have to say your drivers are generally much better than here in the U.S.A. I drove down the Alps and didn't even get a whiff of hot brakes. Here, almost every time I drive to a ski resort I can smell someone burning up their brakes. Several deaths in the U.S.A. have been caused by folks that didn't know to bump the shifter into neutral or at least a lower gear, rather than ride the brakes until they burn up in an engine runaway situation. A lady atop a tall mountain in Hawaii asked me why she was smelling a terrible odor when driving down hills. I explained she needed to downshift and got a blank look. I finally had to point to the shifter and say "put that there when going down hills." She then said to her friend, "Oh, maybe that's why our last rental car lost its brakes!" One of my wife's coworkers was furious his car burned up his brakes after coming down the mountain -- how much you want to bet he never downshifted? There is a certainly (hopefully small!) clueless group of drivers here in the U.S.A.

There is some technological help. There is a Mercedes that automatically downshifts to help down hills. My friends Chevy diesel truck downshifts if the brakes are on and the vehicle is not slowing down (like trying to hold back speed down a hill). It's just a matter of programming nowadays, I expect all cars will eventually downshift if it is detected the driver is riding the brakes down a hill. At least an automatic can be slowed by downshifting -- series DC can't, so that's what makes me nervous about series DC and hills. If you live in a flat area it's not as a big a deal.

My conversion speeds up almost like it is in neutral without regen. I can push the car with one hand on level ground, so it rolls pretty freely.


steelneck said:


> That is not an argument for electric regen. because you have just argued that it is the same electric or not. The above argues against automatic transmissions in general.
> 
> In reality the "anyone" is not really just anyone since we usually expects some basic knowledge of someone allowed to drive a car, like not constantly riding the brakes. Even without regen. a car has quite some drag depending on speed and downshifted gearboxes also have a lot of friction (thats the main argument for direct drive and wheel motors). The short test drives i have done with my soon to be finished conversion verify this for me, the car retards more than i thought when letting of the foot from the pedal, but OK, it is an old car with a lot more drag than modern ones.
> 
> ...


[/QUOTE]


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## steelneck (Apr 19, 2013)

DavidDymaxion said:


> There is some technological help. There is a Mercedes that automatically downshifts to help down hills. My friends Chevy diesel truck downshifts if the brakes are on and the vehicle is not slowing down (like trying to hold back speed down a hill). It's just a matter of programming nowadays, I expect all cars will eventually downshift if it is detected the driver is riding the brakes down a hill. At least an automatic can be slowed by downshifting -- series DC can't, so that's what makes me nervous about series DC and hills. If you live in a flat area it's not as a big a deal.


As i wrote, i work in an underground mine, more than a km deep. We have lots of heavy trucks (loading 35 tons) going both up and down fully loaded (10-14% incline). Most modern trucks have automatic speed holder (though they have oil retarder too) and those with auto trannys also downshift automatically when needed and will downshift on higher RPM the more you brake (unless the ABS cuts in).

In my 25 years in the mine i have only experienced one single occasion when a car (no matter size) have lost its brakes, that time it was a truck with 15 tons of wet concrete going down. The driver was maybe a bit unexperienced and the old truck in question had a Fuller gearbox (unsynced) and he got stuck in neutral... He is lucky to be alive today.

A normal car or small 9 person bus with an auto can go all the way down, 10km constant downhill even with unexperienced drivers leaving it in D. (speed is usually around 50km/h). We have to go back more than 30 years to find cars that would fail. But most even back then had no problems as long as the brakes was in order. 30 years ago this mine was a bit of testing ground for Volvo regarding their heavy hauling trucks.

What many people also forget about is that brake fluid has to be replaced. It is based on glycol and will collect moisture, thus getting its boiling point lower and lower. This is especially true in warm and humid climates.


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## Duncan (Dec 8, 2008)

Just to add another bit to the conversation,

In the old days with drum brakes - this was a reasonable worry
Nowadays with huge ventilated discs and split systems
Not so much
These brakes are massively more powerful!

When I was contemplating a DC build one of the worries was the "full on" failure mode

I would (did) design my car to stop with full power to the rear wheels,
If you have that level of brake overkill hills or mountains are not an issue

As far as the "power brakes" are concerned
IMHO these are a "convenience" - to reduce the loads in normal use
You should be able to apply full brakes without the assist,
Normal use in a modern car is very low force levels - 20lbs??
Even a small weak person should be able to apply 100lbs in an emergency


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## steelneck (Apr 19, 2013)

Duncan said:


> Just to add another bit to the conversation,
> 
> In the old days with drum brakes - this was a reasonable worry


If we talk about sheer stopping power, drum brakes could be just as powerful as discs, depending on size (can even be more powerful since all braking is done at a larger diameter). Just look at the biggest and heaviest trucks, they have all drum brakes and it is this diameter thing that is the reason. On the lighter ones, like 15-20 ton or smaller, discs have become more usual the last 5 years. The issue with drum brakes is not stopping power, the problem is overheating when braking for a long time (weight also).


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## bwjunkie (Jul 31, 2013)

steelneck said:


> If we talk about sheer stopping power, drum brakes could be just as powerful as discs,


Yes I just rode in my brothers Ford super duty diesel truck today, and was perplexed when I saw rear drum brakes. It just made me wonder if my understanding of drum brakes vs disc brakes is lacking.

In response to the thread title. I was driving my tiny VW Cabriolet (EV) around with no brake boost in a heavier than stock car, and when exiting the freeway to a quick stop, pressing somehwat hard into my pedal with my foot, I wondered how I would handle it if my brake pedal just folded in half.

My answer was to brake conservatively and also be sure my emergency brake is in good health.

My '65 cortina has a rickety looking brake system and I will be concerned there as well. I will be looking intoo the possibility of wiring up an emergency brake cable to both rear drums.

-josh


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## Duncan (Dec 8, 2008)

_The issue with drum brakes is not stopping power, the problem is overheating when braking for a long time (weight also).

_Exactly what we are talking about in mountains_
A_lso adjustment - that used to be a regular chore before disc brakes with their built in self adjustment came in


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## DavidDymaxion (Dec 1, 2008)

Getting back to the original question, I would be nervous about taking a series DC car down that road!

I think I can see where we are diverging a bit. Thankfully it is rare the brakes entirely give up. For instance, in my wife's car I noticed the brake pedal was hitting the floor, but the car would still stop at about 0.5 g -- good enough many gentle drivers might not have noticed, but bad enough you might get into a wreck that could have been avoided. (BTW the car was repaired immediately!) I consider this "losing the brakes," but realize for many folks that means 0.0 g of stopping deacceleration.

My state has mandatory safety inspections. For fun, I looked up the stats for brakes. Most are tested by putting the car on a dyno that tests the braking force. Insufficient, or unbalanced forces can fail you, as well as the parking brake. I was surprised to find that 25% of cars fail the safety test, and 16% of those fail the brake test, for an overall rate of 4% of cars failing their brake test. I don't know what percent are hand brakes vs. the foot brake, but I was still shocked the number was that high (well, maybe I shouldn't be shocked given how often I smell hot brakes around here!). Source: http://le.utah.gov/audit/01_06ilr.pdf


steelneck said:


> As i wrote, i work in an underground mine, more than a km deep. We have lots of heavy trucks (loading 35 tons) going both up and down fully loaded (10-14% incline). Most modern trucks have automatic speed holder (though they have oil retarder too) and those with auto trannys also downshift automatically when needed and will downshift on higher RPM the more you brake (unless the ABS cuts in).
> 
> In my 25 years in the mine i have only experienced one single occasion when a car (no matter size) have lost its brakes, that time it was a truck with 15 tons of wet concrete going down. The driver was maybe a bit unexperienced and the old truck in question had a Fuller gearbox (unsynced) and he got stuck in neutral... He is lucky to be alive today.
> 
> ...


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## Duncan (Dec 8, 2008)

Re - safety test
The brake dynos here NZ and the UK have a minimum of 50% for passing,
The system measures the axle weight (or it is typed in) and you need to achieve 50% of that in retardation at the tire surface

Most brakes normally achieve about 70 - 80%

So your brakes have to be pretty bad to fail
A more usual problem is not so much weak as in-balanced - right/left

The hand brake (emergency brake) - on single circuit cars had to achieve 25%
When dual circuit brakes came in that was dropped to 15%

In the old days before the brake dynos - the testing stations used a "G Meter"
This was always a problem with the old Mini vans (van version of the mini) because the back wheels would lock up with the hand brake - but without reaching 0.25G
We used to have to put a sack of sand in the back


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## Duncan (Dec 8, 2008)

_Getting back to the original question, I would be nervous about taking a series DC car down that road!_

I would not "Be Nervous" - as long as the car was not overloaded and was in good condition

If it was not in "good condition" - then I would be nervous if it was DC or AC or IC or powered by elastic bands


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## danh818 (Dec 14, 2011)

I have no worries. I put some very large 4 piston Brembo's on the front of my 944 and moved the stock front brakes to the rear. I had no problem locking up the fronts (oops) coming off a 115mph straight at Laguna Seca last week.


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## steelneck (Apr 19, 2013)

danh818 said:


> I have no worries. I put some very large 4 piston Brembo's on the front of my 944 and moved the stock front brakes to the rear. I had no problem locking up the fronts (oops) coming off a 115mph straight at Laguna Seca last week.


Locking up the wheels is no problem even with ancient cars, but try to go down a curvy mountain road, fully loaded with a trailer behind, in neutral.. Yes, most cars will handle this with a driver who knows what he/she are doing, and even old cars have better brakes today than back when they where made, thanks to modern brake brake fluid.


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## onegreenev (May 18, 2012)

New disc brakes all around on my VW. Be sure your brakes are always in good condition. Don't let them wear into the rotor or drum. Change the pads/shoes when needed. Don't skimp on brakes even if you have an AC motor setup. In the event your motor fails you want to be able to stop. Don't overload your vehicle with too many batteries.


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## Rory166 (Jul 6, 2014)

Although regen braking is difficult with serial motors the Zapi company does produce such a setup. This is recognised by having 4 contactors. I will not go into how it works. Quite honestly AC probably does a better job. I have just bought a Zapi controlled car so will probably know More soon.

Rory

PS now realised secondary braking may mean parking brake.


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## dladd (Jun 1, 2011)

The bigger concern I have is no parking pawl or engine compression to hold the car when parked (I have a pretty steep driveway). As a result of this, my parking brake is adjusted very well, and holds the car solid. And is a nice backup in the event of total hydraulic brake failure. It's right there by my hand, easy to do in an emergency.


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## Moltenmetal (Mar 20, 2014)

Excellent point: better get in the habit of parking properly on hills and using the parking brake again! I have almost never used it except when locking the rear wheels when on wheel ramps for repairs etc., in any car I've owned, standard or auto!


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