Regen braking and hard bumps on road

I believe this was discussed before...

It sure is scary having the regen. braking disconnect/shut down over a hard bump on the road.

Caught me a few times leading to hard braking. I thought I could slow the car down gradually in front of the intersection.

Does anyone know what's the reason for this?
Is it a design flaw or do other EVs have similar issues?

ASUN said:

I believe this was discussed before...

It sure is scary having the regen. braking disconnect/shut down over a hard bump on the road.

Caught me a few times leading to hard braking. I thought I could slow the car down gradually in front of the intersection.

Does anyone know what's the reason for this?
Is it a design flaw or do other EVs have similar issues?

Click to expand...

I can tell you the Nissan Leaf had the same exact issue. It probably makes design sense in that once the resistance on the wheels from the weight is reduced the circuit probably senses a rapid change in current (in or out) and has to do something. It may not be a design flaw, but obviously firmware could theoretically replace the regen breaking when cutting it out with some amount of mechanical breaking. For example other cars I have that have regen breaking will smoothly transition/replace between the two (almost undetectably).

The bigger issue for me on the i3 is when regen breaking completely cuts off as you turn the front wheels, which when decelerating into a turn all of a sudden results in accelerating into the curve. This, to me, is a design flaw and very unsafe.

It's not a design flaw, it does that to prevent jarring shocks through to the drivetrain as early i3's were breaking motor mount bolts and driveshafts.

As said, a spinning wheel when it resumes contact with the road, or one that has stopped and needs to then rapidly accelerate, creates HUGE stresses on the system. The rotational sensors are only so sensitive, and by far the safest way to prevent those shocks is to let the wheel freewheel (i.e., keep the same rotational rate), so when it does regain contact, the stress is minimal. FWIW, the car will adjust the acceleration if you hit a bump, too which keeps the wheels from spinning, then rapidly slowing down when they hit the road.

The Gen-3 Prius had a similar problem although it was reduced with a software patch. But it was obscure because it was hard to reproduce:

• slippery road - AND
• modest braking - AND
• ~18 mph - AND
• speed-bump or sized pot hole

The tan line shows how stopping paused for about 700-800 milliseconds:


This is the pot hole I used to test the car before and after the patch:


I still have an impressive collection of bumps that induced the problem:
http://hiwaay.net/~bzwilson/prius/Prius_bump_010.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_020.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_030.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_040.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_050.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_060.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_070.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_080.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_090.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_100.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_110.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_120.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_130.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_140.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_150.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_160.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_170.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_180.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_190.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_200.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_210.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_220.jpg
http://hiwaay.net/~bzwilson/prius/Prius_bump_240.jpg

Bob Wilson

A difference between the Prius and an i3 is the amount of torque and power available...the i3 has the energy to spin the unloaded wheel MUCH easier than the both heavier and lower-powered Prius. You always need to be ready to brake, as regen may not provide what's needed.

I regret stating it as a design flaw...poor choice of word...

From what I am hearing now, this is actually normal and is to prevent "injuries" to the vehicle.

Yes, we must always be ready to apply the manual brakes, can't just rely on the regen braking.

But still...that moment of "freewheeling" is truly scary to experience.

Hopefully a software patch can at least minimize this effect.