# Not connecting rear brake hydraulics with regenerative braking



## poprock (Apr 29, 2010)

On a car with front mounted engine (fwd or rwd ) I would say go for it, as in my young and foolish days it was easier to put a flange rivet to block the rear brake line than to repair a wheel cylinder.Weight transfer in a panic stop almost eliminates rear braking on older cars *without* a pressure equalizing valve. *HOWEVER*, a rear mounted engine would certainly change those shade-tree mechanic ideas. Risky business


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## DavidDymaxion (Dec 1, 2008)

I would hate to be in a wreck and have a lawyer discover I had disabled the rear brakes. Don't forget you don't have regen with a full pack, and have voltage and current limits in any case (in other words, you might not be able to get full braking with just regen). I'd vote for keeping the rear brakes.

Even on my front heavy gasser, one track day can chew through a set of rear pads -- they are working hard, even if not as hard as the front brakes.

Something you might consider is adding a proportioning valve, so you could fine tune the front/rear braking balance with regen.

BTW I used to have a Fiero. It stopped faster than any other car I had with just the handbrake, which does only the rear brakes. As I recall the front and rear disks and pads are the same size. These are both hints the car uses it rear brakes more than a typical front heavy car.


helluvaengineer96 said:


> Hey folks... slowly making progress on my AC50 Fiero conversion. Have the engine and transmission mounted in the chassis and hauled it down to SC.
> 
> As I was installing the nice new braided cables, it occurred to me maybe I don't really need rear brakes.
> 
> ...


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## madderscience (Jun 28, 2008)

Agree with prior poster. It is a very bad idea to intentionally disable the mechanical brakes. Regen isn't going to be strong enough to totally replace them ; it won't work of the batteries are full or near full (noplace for the recovered energy to go) and it can fail if the controller malfunctions or the motor does.

If an accident occurs and you are discovered to have disconnected the brakes, you are guaranteed a lawsuit, fines, and lots of bad press for ev conversions.

Don't go there. 

(but good luck on the build)


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## Duncan (Dec 8, 2008)

Hi Hell

The Fiero is a mid engines sports car, a lot of the braking will be done by the rear brakes,

your rear brakes will be generating 2-300Hp of braking in a high speed stop - don't think an AC50 is anywhere close to that

Run the numbers, calculate the energy going into the rear brakes and the front rear load shift

You are an engineer - should be a doddle


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## Coley (Jul 26, 2007)

Most state vehicle laws require 4 wheel brakes, plus an emergency (parking) brake.
Very bad idea to not hook up all 4 wheels. The drag is not enough to worry about, compared to causing an accident.


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## helluvaengineer96 (Oct 8, 2010)

Hey folks thanks for the input.
Sound advice, the rear brakes stay.. I honestly did not consider law suits, where I live and where I'll be using it, there is little chance. That said, better to be safe than sorry.

The proportioning valve is integrated with the master cylinder assembly... I think. I'll need to check. Not sure on options to change it. 

I have already upgraded the front brakes to larger diameter with vented rotors. 

Worth noting the rear for EV Fiero is a lot lighter than the 6 cyl ICE Fiero. All batteries will be in the gas tank area, squarely in the middle and low. Though I have lightened the front end a lot by removing all radiators/compressors, ect.

DavidDymaxion,
You are correct.. originally the same size..... I am shocked the rear emergency worked so well for you. I've read it rarely worked correctly. I've bought all the parts to rebuild my emergency brakes from front to rear just in case. I've never driven a Fiero that was in good condition.... mine never worked at all 



Thanks again folks.


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## DavidDymaxion (Dec 1, 2008)

I must have been one of the lucky few, I know they were trouble prone. One trick I read and did with mine is to tie one end of the cable up, and dribble oil down it. My area has annual safety inspections, so I was forced to keep it operable.


helluvaengineer96 said:


> ... DavidDymaxion,
> You are correct.. originally the same size..... I am shocked the rear emergency worked so well for you. I've read it rarely worked correctly. I've bought all the parts to rebuild my emergency brakes from front to rear just in case. I've never driven a Fiero that was in good condition.... mine never worked at all  ...


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## helluvaengineer96 (Oct 8, 2010)

DavidDymaxion said:


> Don't forget you don't have regen with a full pack, and have voltage and current limits in any case (in other words, you might not be able to get full braking with just regen).


David.... I was just thinking about this comment. I am not sure how the controller decides when to recover energy.. Does it measure the pack level, wonder if this can be changed? I have the controller but not delved into yet (no batteries).

At any rate I am using significant clamping diodes to protect the pack from inductor generated voltage spike. Wonder if this would allow regen to work all the time. This does give me reason to increase the current capacity of the Diodes.... might need to trigger some FETs as well.

Something I'll need to test.


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## Guest (May 2, 2011)

It is not about suites but of SAFETY. NEVER disable your brakes. All EV's including Factory built, keep the brakes. It is ALL about safety to you and others. NEVER EVER disable your brakes. YOU DO NEED them. Never think you don't. 

Pete


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## jddcircuit (Mar 18, 2010)

Hell96,
I am also on the same line of thinking that you are.

My car is not on the road yet but I am planning to experiment with not connecting my rear hydraulics and use regen for the rear wheels. I know that this is risky so I will need to be careful.

My motor is directly connected to the wheel with a fixed gear ratio through a differential. This means that if one of my rear wheels slips during regen braking then I will lose braking torque on the other wheel. This might be a deal breaker for me. When I build my dual motor dual wheel direct drive then it might be a different story. With independant dual wheel regen now we're talking anti-lock traction control possibilities

My 3 phase motor is permanent magnet so I will also experiment with a resistive load as an electrical emergency brake dump if I lose regen switching. This should give me 3 ways to stop the car (front hydraulic disc brakes, Regen rear, Emergency resistive rear) to build in some redundancy for safety. If I can't demonstrate a safe stopping distance I will reconnect my rear hydraulics.

Since I am not using a clutch I was thinking of using clutch pedal as my negative torque (regen brake) pedal. It might be a little weird to drive with two foot braking but this is a concept vehicle for me so it might work ok.

Jeff


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## DavidDymaxion (Dec 1, 2008)

The EV-1 used electric brakes on the rear. This allowed blending with various amounts of regen, and provided the parking brake.

BTW losing braking on both wheels together could actually be a feature. Various gasser cars do the rear antilock brakes that way.


jddcircuit said:


> Hell96,
> I am also on the same line of thinking that you are.
> 
> My car is not on the road yet but I am planning to experiment with not connecting my rear hydraulics and use regen for the rear wheels. I know that this is risky so I will need to be careful.
> ...


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## JRoque (Mar 9, 2010)

Hi. I'll agree with others than rear brakes are a must have. A compromise might be to use drum brakes on the back wheels and adjust them so they engage after the front disc brakes. That way regen can be used for normal stopping but in an emergency where the brake pedal continues past it's usual range, the back wheels are mechanically stopped as well.

JR


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## helluvaengineer96 (Oct 8, 2010)

JR,
Fiero has unvented rear rotors. However someone mentioned changing the proportioning valve, which would provide the results you are looking for. I need to do some research on options for the Fiero's proportioning valve. Waiting for the next JC catalog to arrive.

All drum brakes going back to at least '65 have auto adjustment, when you are going in reverse and apply the brakes they will tighten up. Therefore backing off the rear brakes as you suggested would be undone. This mechanism can be removed but you could get uneven wear and have one brake applying harder then the others. This truly is dangerous (the car spinning out of control), no rear brakes at all in this case would be safer (more predictable). 

Thanks for input, I am leaning toward a proportioning valve change for the rears.



JRoque said:


> Hi. I'll agree with others than rear brakes are a must have. A compromise might be to use drum brakes on the back wheels and adjust them so they engage after the front disc brakes. That way regen can be used for normal stopping but in an emergency where the brake pedal continues past it's usual range, the back wheels are mechanically stopped as well.
> 
> JR


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## Duncan (Dec 8, 2008)

Hi JRoque,

_adjust them so they engage after the front disc brakes_

Brakes are hydraulic - everything moves until everything reaches the end of its "free" travel and THEN the pressure builds up and the braking starts

The main thing to remember is that even a powerful re-gen setup is puny compared to hydraulic brakes - when you brake hard your brakes are the equivalent of a huge motor

Example - 2 tonne car at 90mph (40m/sec) - max deceleration 0.8G
2000Kg x 40 x 8 = 640Kw = 850Hp


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## DavidDymaxion (Dec 1, 2008)

Duncan, agreed on your calculation for an AWD car, and it's close for a FWD car, too. However, RWD supports significantly less regen due to less weight on its rear wheels during braking.

Let's say 1/4 of the car's weight is on the rear wheels during a heavy stop, the car is 1500 kg (still pretty heavy), 0.8 g max, and let's say you never exceed 65 mph. Then the max regen hp you need to absorb is 85 hp -- doable by a beefy inverter, or a twin motor inverter setup.

(BTW this calculation does not apply to a Fiero as it is mid engined.)

FWIW I can skid the rear wheels of my conversion via regen at low speeds.

Anyway, I would never do just regen without mechanical brakes for backup. This also illustrates you can potentially get about 4x the regen from a FWD or AWD vehicle. That's an interesting thought that an electric FWD car might be faster than RWD on a race track with corners for that reason.


Duncan said:


> ... The main thing to remember is that even a powerful re-gen setup is puny compared to hydraulic brakes - when you brake hard your brakes are the equivalent of a huge motor
> 
> Example - 2 tonne car at 90mph (40m/sec) - max deceleration 0.8G
> 2000Kg x 40 x 8 = 640Kw = 850Hp


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## Duncan (Dec 8, 2008)

Hi David

_Let's say 1/4 of the car's weight is on the rear wheels during a heavy stop, the car is 1500 kg (still pretty heavy), 0.8 g max, and let's say you never exceed 65 mph. Then the max regen hp you need to absorb is 85 hp _

Lets say the car has a wheelbase of 90 inches and a Center of Mass 22 inches off the ground, at .8G the weight transfer will be - 1500Kg x 0.8 x 22 / 90 = 293Kg
So if you start off 50/50 you end up with 750 - 293 = 457Kg on the back and 750 + 293 = 1043 on the front,
Thats about a third - I suppose a rear wheel drive pick-up would start with less on the rear so It could end up with only a quarter of the weight on the rear wheels

so 1500Kg - 65mph - (26m/sec) 0.8G
1500 x 26 x 8 x 0.25 = 78Kw = 104Hp - little bit more than 85hp - but a lot more doable than 850Hp!!

I intend to have a center of mass more like ten or eleven inches off the ground - makes cornering and braking much easier - but it will reduce my weight transfer on the drag strip


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## helluvaengineer96 (Oct 8, 2010)

Jeff,
Keep me posted on your progress. I plan on experimenting as well. 
I have a clutch so I think this might be helpful. 
Think I have a solution for when the batteries are fully charged.

But that being said I do want the parking/emergency brakes to work. 



jddcircuit said:


> Hell96,
> I am also on the same line of thinking that you are.
> 
> My car is not on the road yet but I am planning to experiment with not connecting my rear hydraulics and use regen for the rear wheels. I know that this is risky so I will need to be careful.
> ...


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