Regen reduced while turning

I've noticed that the regenerative braking is significantly reduced if the steering wheel is turned. I first came across this while exiting a freeway ramp and was using regen to slow down from 65 MPH to a stop at the end. There is a slight curve near the end of the ramp and instead of slowing down to a stop I found that the car just freewheeled and I ended up needing to use the friction brakes to stop.

It's been a week of daily driving the i3 and I finally figured out that the i3 will reduce regen power while turning, and I'm getting used to that. The ActiveE certainly didn't do that, and the Tesla doesn't. Regen is not cut completely (as the ActiveE would do during traction control "events" providing a nice "kick in the butt" feeling as it cut out), but instead seems limited to 2 "bars" on the display. It will return to full when the wheel is straightened out. Once you know it happens its fun to play with and totally anticipatable, but before that I thought regen was on my car was flaky, or I was getting traction control events for no obvious reason. I was using the friction brakes way more often in the i3 than I ever did in the ActiveE due to this.

I am aware that proper driving technique is slow first, then turn, but that's when driving at the limits of grip, where the tires have enough traction to either turn or stop, and not both at the same time. But for normal street driving, no where near the limits, I apparently regen while turning quite a bit, and expect the car to keep slowing. It doesn't, so be careful!

--Woof!

Good info, thank you! Sounds like it is indeed traction control reducing the regenerative braking. As you noted, turning under braking reduces traction via understeer. It seems logical that BMW would opt to reduce full regenerative braking as soon as it detects the wheels turning to ensure maximum traction. In terms of safety, traction > regenerative braking.

elptex said:

Good info, thank you! Sounds like it is indeed traction control reducing the regenerative braking. As you noted, turning under braking reduces traction via understeer. It seems logical that BMW would opt to reduce full regenerative braking as soon as it detects the wheels turning to ensure maximum traction. In terms of safety, traction > regenerative braking.

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I came off dual carriageway with full regeneration and into a 90 degree turn and regeneration suddenly stopped... sure was a surprise... i wasn't even going quick...

I have a long and bumpy/twisty stretch of road near me which goes quite steeply downhill (or uphill depending which way I'm going!)

I'm also a former racing driver...!

Going downhill in my i3 BEV I can feel the regen increasing/decreasing as it goes over bumps, which I'm sure is to prevent possible rear wheel locking. I haven't noticed any instability when going fast into corners, in fact it's a great point and squirt machine on a twisty road with tremendous acceleration out of corners. You're right that the regen is reduced when the front wheels are turned - any race car with too much rear brake bias is very unstable in corners. I also suspect, but don't know for sure, that pressing the brake pedal also stops regeneration so as to provide proper balance front to rear. The brakes are very good.

One thing I have noticed is a tendency for the front wheels to get pulled into a turn if you hit a cats eye or bump side-on - that can be quite scary given how direct the steering is!

There's certainly a lot of complicated calculations being done at high speed - pretty impressive!

DerfSdrawd said:

One thing I have noticed is a tendency for the front wheels to get pulled into a turn if you hit a cats eye or bump side-on - that can be quite scary given how direct the steering is!

There's certainly a lot of complicated calculations being done at high speed - pretty impressive!

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I noticed the same thing on our overnight drive. I assumed it was due to the front tires being so skinny that a cats eye is practically 50% of their footprint, pulling them to the other side of the cats eye.

DerfSdrawd said:

Going downhill in my i3 BEV I can feel the regen increasing/decreasing as it goes over bumps, which I'm sure is to prevent possible rear wheel locking.
...You're right that the regen is reduced when the front wheels are turned - any race car with too much rear brake bias ...

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I don't think so- Remember that the rear (or front) brakes are not applied at all during the regen. It's the electric motor working in reverse, as a generator, that is causing the car to slow. So the rear wheels cannot lock because the disc brakes are not being applied (unless you press the pedal, and certain conditions are met).

Surge said:

DerfSdrawd said:

Going downhill in my i3 BEV I can feel the regen increasing/decreasing as it goes over bumps, which I'm sure is to prevent possible rear wheel locking.
...You're right that the regen is reduced when the front wheels are turned - any race car with too much rear brake bias ...

Click to expand...

I don't think so- Remember that the rear (or front) brakes are not applied at all during the regen. It's the electric motor working in reverse, as a generator, that is causing the car to slow. So the rear wheels cannot lock because the disc brakes are not being applied (unless you press the pedal, and certain conditions are met).

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Doesn't matter whether it's the engine (or motor in the i3) or the brakes applying the braking force, if the braking force exceeds the rotation force being applied to the wheel it will lock up -usually because one of the wheels is unloaded momentarily by a bump or dip in the road :shock: . The differential also plays a part in this effect. That's why you can see the regen reducing on a bumpy road if you look at the display. It also seems to work that way in the wet (very handy for the UK ).