Motor Noise - Possible Cause and Potential Solution

gavine said:

I bought a BMW i3 as our second car a few weeks ago and I noticed that the regen disables when going over bumps in the road. I wonder if BMW engineers realized in advance that regen over bumps is bad for the bearings so they put this protective measure in place.

If this is true, perhaps Tesla can do the same and bring back full regen again!

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That sounds horrible; bumps in the road cause your deceleration to change? Yuck.

gavine said:

I bought a BMW i3 as our second car a few weeks ago and I noticed that the regen disables when going over bumps in the road. I wonder if BMW engineers realized in advance that regen over bumps is bad for the bearings so they put this protective measure in place.

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All cars that have most of the regen on the brakes do this. Tesla avoided the problem by having all the regen on the accelerator pedal where it belongs. It's also makes the braking system much simpler and robust.

jerry33 said:

All cars that have most of the regen on the brakes do this. Tesla avoided the problem by having all the regen on the accelerator pedal where it belongs. It's also makes the braking system much simpler and robust.

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The i3 has all regen on the accelerator pedal ("where it belongs") too. This story is about something else.

How does it work? Because i3 is RWD, it regenerates with the rear wheels. Does it release the regen in the moment when the front wheels hit the bump? If yes, this method wouldn't work with an AWD car (like a D Tesla) because it was already too late for the front motor.

It's the ABS/stability control system. When the rear wheels go over a bump during regen it can cause the wheels to slip, the ABS system detects the speed difference between the front and rear wheels, and reduces regen until the speeds match again.

Normal, but annoying.

Regen forces on the bearings should be no different in character than drive forces, other than being smaller.

Bangor Bob said:

It's the ABS/stability control system. When the rear wheels go over a bump during regen it can cause the wheels to slip, the ABS system detects the speed difference between the front and rear wheels, and reduces regen until the speeds match again.

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This is 100% correct. ABS is has full override of regen. Every EV I have had dating back to my 2000 Ford Ranger EV had this ABS - Regen connection.

Cutting out braking/regen over bumps is probably traction control kicking in.

Our Prius would cut over from regen to friction braking when it went over a bump. The only problem was the cutover wasn't instant -- it would release regen instantly and then it took a moment for the friction to start, so the car felt like it was accelerating (it wasn't, but the lack of deceleration feels like acceleration <g>) which was unnerving. There was a certain offramp in town where it happened every time. My Volt did something similar, but far less frequently. I've never had the Model S do it yet. It does take some getting used to the fact that you get different amount of regen depending on temperature. I wish they could vector the regen into the heater when it's cold instead of giving you nothing (yes, it's cold here).

I have never driven an AWD car so I will limit this to a RWD or a FWD car. Regen is applied to the wheels by the motor acting as a generator. The force is transmitted to the wheels through the differential. If either wheel loses traction, for whatever reason, regen force is no longer possible because of the way a differential works. A limited slip differential would help with this. The other option would be for the braking system to apply the friction brake to the wheel that has no traction but the duration of the wheel bounce appears to be too short for this to happen. I have not yet experienced the problem in my model S but it happens all of the time in my FWD 2013 Ford Fusion Energi during regen braking. It does feel like acceleration but it is actually the instantaneous loss of all braking that makes it feel like acceleration. If you want to experiment with this situation, find a manhole cover that is significantly lower than the surface of the road. Drive over the manhole cover using regen braking. While the wheel is in the air, over the manhole cover, there will be no regen braking at all until it drops down and makes contact with the manhole cover.