# RWD vs FWD for Regen EV



## Guest (Nov 5, 2009)

Depends upon the size of your motor. Most cars can accept up to a 9" motor in the front for the front wheel drive models. I'd say front wheel drive as the front usually takes most of the brunt of breaking anyway. Regen is excellent if you can do it. 

Pete


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## MJ Monterey (Aug 20, 2009)

I would make sure I had good Anti lock brakes (ABS) on the back for a rear wheel drive with regen system. 

Then a question comes to mind. If the rears do begin to lock up can you kill/adjust the regen with the ABS and still modulate the brakes? Vary the amount of regen with an accelerometer?

My gut feel opinion is that a front wheel drive ABS system would better cope with the variables caused by regen.


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## DavidDymaxion (Dec 1, 2008)

Depends on the car, but many FWD cars will put 80+% of the car's weight during a hard stop on the front wheels. My RWD car puts about 70+% of the car's weight on the front wheels during a hard stop. You can definitely get more regen out of a FWD car.

P = F*v = m*a*v

Suppose your car weighs 1400 kg, the wheels start skidding at 1 g of braking force, and you can generate 60 kW of stopping regen. Assume 80% of the car's weight goes on the front wheels during the hard stop for FWD, but only 30% on the rear wheels for a RWD.

v = 60 kW / (1400 kg * 0.8 * 9.8 m/s^2) = 12 mph

So you can regen hard at 1 g down to 12 mph in the FWD before skidding.

v = 60 kW / (1400 kg * 0.3 * 9.8 m/s^2) = 33 mph

So in the RWD car you are already skidding at 33 mph and below.

In practice, you don't want to count on 1 g of traction for regular street tires and street conditions, so you should halve the 1 g number. Anyway, this shows you can get much more regen from the FWD car.

If you are determined to go RWD (and RWD definitely can accelerate faster than FWD) a Porsche 911 is one of the few cars that is designed to be tail heavy, and balances out to close to a 50/50 weight distribution under heavy braking.

You also need to be careful about drawing heavy regen in a corner so you don't skid.



MJ Monterey said:


> I would make sure I had good Anti lock brakes (ABS) on the back for a rear wheel drive with regen system.
> 
> Then a question comes to mind. If the rears do begin to lock up can you kill/adjust the regen with the ABS and still modulate the brakes? Vary the amount of regen with an accelerometer?
> 
> My gut feel opinion is that a front wheel drive ABS system would better cope with the variables caused by regen.


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## MJ Monterey (Aug 20, 2009)

Thanks David,

You got the math and logic in place that I couldn't pull out of my inner fog!

Good point on the regen during cornering!

Jack


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## dragster (Sep 3, 2008)

Hi
I have regen with rear wheel drive on my explorer Front or Rear it makes no diffrerence


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## DavidDymaxion (Dec 1, 2008)

I'll bet you are not doing 60 kW of regen -- how many Volts is your pack and how many amps of regen can your system generate? Even my 48 V car can skid the wheels, but it can also do hundreds of amps of regen.


dragster said:


> Hi
> I have regen with rear wheel drive on my explorer Front or Rear it makes no diffrerence


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## dragster (Sep 3, 2008)

HI
DC 336 volts converted to 336 Volts 3PH AC


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## DavidDymaxion (Dec 1, 2008)

That's half the formula, do you know what your regen Amps are? Also, is your regen variable, or is it a fixed amount? Fixed regen (like off pedal, or when the brake lights go on) is often set pretty low.


dragster said:


> HI
> DC 336 volts converted to 336 Volts 3PH AC


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## dragster (Sep 3, 2008)

The regen is activated when you take your foot off the throttle we do not have any gages in the vehicle as we wood like our customers to drive it just like they did when it had a gas engine.
As for the brake light we disconnected it because any vehicle behind keeps seeing the light come on every time you take your foot off the throttle this is very unsettling to the car behind you. 
P.S we have a 4 speed automatic transmission so the regen in 4 gear is less then 1 gear like I said it drives just like it did with the gas engine.


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## _Basser_ (Sep 3, 2009)

The design of my regen system will be based on a 3PH AC system running in flux vector mode. There will be a potentiometer on the throttle and on the brake. The throttle will of course control torque from 0%-100%. The hydraulic brake setup will be modified so that the first half of the travel will not engage the mechanical brakes. The first half will be regen braking based on the value from the potentiometer. The VFD's IGBT is good to 400Amp and the battery bank will allow a charge voltage upto 270volts without current limit. It is unlikely to regen at such levels but around 200amps seems reasonable. I will have to study my braking habits in town and determine deacceleration levels otherwise full regen capabilities maybe lost on a RWD chassis.


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## DavidDymaxion (Dec 1, 2008)

Dragster: I'd guess your system is doing 10's of Amps rather than 100's. That's a good thing to have ICE-like responses. Do you have a web page for your vehicle?

Basser: Sounds awesome. I'm still deciding how to vary the regen on my car.


_Basser_ said:


> The design of my regen system will be based on a 3PH AC system running in flux vector mode. There will be a potentiometer on the throttle and on the brake. The throttle will of course control torque from 0%-100%. The hydraulic brake setup will be modified so that the first half of the travel will not engage the mechanical brakes. The first half will be regen braking based on the value from the potentiometer. The VFD's IGBT is good to 400Amp and the battery bank will allow a charge voltage upto 270volts without current limit. It is unlikely to regen at such levels but around 200amps seems reasonable. I will have to study my braking habits in town and determine deacceleration levels otherwise full regen capabilities maybe lost on a RWD chassis.


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## david85 (Nov 12, 2007)

Welcome to the forum, Basser. Nice to have another member from BC (I'm on vancouver island)

As far as driving dynamics go, locked front wheels will probably cause the car to track strait ahead. Locked rear wheels often results in swapping ends if conditions are slipperly enough (or speeds/braking high enough). Front wheels will have more traction so your regen can be more powerful if you want it to be.

You aren't planning on racing this are you? 60kw of braking power makes me wonder what you are up to


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## dragster (Sep 3, 2008)

Hi
When your driving and need more regen just drop down to the next lower gear it's that easy.
Here is the web site
http://www.discbrakesrus.com/make/fordtruck/electriccar.htm


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