So I have been noticing this for a little while on my eTron, but was finally able to collect more data about the phenomenon due to another post explaining how to turn of brake regeneration in the eTron. However, I don't quite fully understand the results. When lightly braking -- IE regeneration only, either through the pedal or either of the 2 regeneration levels on the steering wheel paddles -- and going down a straight flat road, I have noticed a slight drift or pull to the right. This can happen even if I hold the steering wheel perfectly straight and not allow it to move left or right. This effect is more pronounced when slowing from higher speeds (>60MPH) and less so at lower speeds (<30MPH). At first I thought this pull had to be in the friction brake system, however, if I perform the same tests with the car in neutral, which disengages the brake regeneration system entirely, the car will stop smooth and straight on the same roads, regardless of speed, no pulling at all. Next, if I perform a harder stop, where the car goes initially through the regeneration system but then engages the friction brakes I note the car will have a slight drift to the right initially and then will immediately straighten out as the friction brakes start grabbing. The effect, under this harder stop scenario, lasts a very short time as the vehicle is slowing down rapidly.

All of this information leads me to some weird conclusions about the brake regeneration system in my eTron. When I think about the forces involved while braking on pure friction brakes, those forces are between the road, tire, brake caliper and rotor and connecting equipment between those things. However, with pure regeneration braking, there are no forces being applied to the rotors or calipers. Instead, those forces go from the tire, through the wheel, through the drive axles (front and rear), through the differential and ultimate into the motor rotor where the opposing magnetic fields between the motor stator and rotor generate electric current that ultimate flows back into the battery. The pulling effect seems to behave like reverse torque-steer, suggesting there is an unequal length path between the left and right drive axles (I am assuming the front, but could be both front and rear).

Just curious if anyone else has seen this behavior before or if they could perform some tests when otherwise out for other activities and let me know.

Other thoughts to explain the phenomenon?

 

I do not think I ever noticed anything like that. I was on an off-ramp where I had to brake fairly hard because of traffic in front of me. This was all still in the regenerative braking area and it did not pull to the left or right. Based on your theory, this should then also happen when you accelerate. Does it pull to the right when accelerating?

It looks like the motor sits centered between the wheels: https://fourtitude.com/wp-content/up...940_medium.jpg

So all the rods and connections would be mirrored on each side which is different from an ICE with front wheel drive. This then leaves the differential as one spot where things could go wrong. I do not know how the quatro system works exactly and whether it can partially lock the differential.

 

Two possible scenarios:

1) The Crown of the Road. Most all roads are not truly flat, and slope to the right for drainage.

2) Your wheel alignment is out of spec on the car.

 

The problem with these explanations is that the friction brakes (car in neutral) do not exhibit the same right pull on the same spot on the same roads at the same speeds.

 

Hard to be sure, but perhaps very slightly. I'd have to test this more specifically.

 

Car pulling right or left drives me crazy, and I’m super sensitive about it. At the same time I have regen braking on manual and it happens I have it on no regen while traveling at different speeds.
Having these in mind, I have never experienced what you have in terms of pulling to sides, at any speed and on any regen setting.

 

While I was at the dealership yesterday waiting for the 10k service to be done, I decided to test drive a new eTron. I was able to confirm that the new eTron, which was made on 11/19, also had this same brake pull to the right only during regenerative braking. Tried it multiple times on different roads and speeds and always the same result: slight pull to the right. Friction brakes alone never pulled.

 

My 11/19 does not and never has pulled to the right under regen. Tracks straight and true whether I brake hard enough to engage the friction brakes, or not.

 

I don't know what to tell you. I am reporting my observations. I have tried it on many different roads, from highways down to surface streets. I have even tried driving on the wrong side of the road and the vehicle will actually pull left in that case when using regenerative brakes. It's obviously following the crown of the road, but here's the problem: it does none of this when using the friction brakes alone on those same roads by putting the car in neutral and braking. So the difference might be that the eTron's regenerative brake system only engages the front motor and not both motors. And because of this imbalance the front wheels are just following the crown of the road.

 

Your explanation sounds plausible. I hadn't really thought about it before but I wouldn't expect the car to be using both motors for regen.