I haven't yet experienced anything like this. Does it also do it during regen in L without brakes? I'm wondering if the blending system (brakes+regen+TC) is causing some of this.

 

It should not do this.
Regen is happening "in the centre" from the drivetrain. It should not be directional unless there is an issue with the system. (You should also experience disproportionate power to the wheels on accel if so)

freshcut has the likely issue - one side is blending more brakes than the other. Since it doesn't do it in neutral, it's probably not pads or caliper issue, but electronic brake unit issue.

Does it do it if you put it in L and put gentle pressure on the pedal? (Should force max regen and no friction brakes)
Only takes about an inch or so of pedal travel.

 

I would take it in to the dealership and have them check the front brake pads and calipers. It's possible that one is not installed correctly (or was dislodged) and is now dragging against the brake rotor during regenerative braking.

 

If it was the pads and calipers, why wouldn't it do it when I brake in Neutral?

 

Possibly drag from a hub going?

 

If it's not doing it in neutral, we may have ruled brakes out, but perhaps not entirely. What does it do when coasting in L? ANY pull? When braking while coasting in gear, it's going to regen for about the first inch of pedal travel with the regen force increasing the harder to press the brake before the physical brakes begin to engage. It could still be a combination as I suggested in my first post since it will brake proportionally on each individual wheel based on speed sensors in each wheel that are controlled by the traction control system as well. It's not entirely impossible that its not a programming or even a physical issue with the regen function, but unless it's done it from day 1 or if you've had programming done or work that resulted in this occuring it's almost certainly not a programming concern that just showed up as a sudden glitch and brakes and their related components tend to lend themselves to being an automatic culprit. If it seems to be a dangerous condition with the amount of pull you're experiencing then I would absolutely have it looked at by a VT.

 

I assume this characteristic is a result of the regenerative braking. ... Does anybody else's Volt do this?

Nope. There's something wrong.

If it's not pulling under friction braking, the brakes seem not to be it. That should also rule out alignment.
Regen happens in the transmission, so it SHOULD be symmetrical regardless. But clearly there's something there that's not apparent. It's possible something's messed up in the drivetrain or something else.

The important thing is to put the car in low and see if the same thing happens. If it does NOT pull in Low, but it DOES pull under braking, then there's something wrong in the brake blending system.

 

Nope. There's something wrong.

If it's not pulling under friction braking, the brakes seem not to be it. That should also rule out alignment.
Regen happens in the transmission, so it SHOULD be symmetrical regardless. But clearly there's something there that's not apparent. It's possible something's messed up in the drivetrain or something else.

*nod* This is beginning to feel like there's something weird happening in whatever the Volt uses for a differential and it's not locking between the wheels properly during the reverse "acceleration". so there's kind of a reverse torque steer happening.

 

I will try to get some data under braking with the car in Low.

Everyone: Please remember that the amount of pulling under braking is extremely minor. I suspect that most people would not even notice it. But I do, and I'd like to understand it.

 

Using brakes applies braking to all four wheels. Regen only applies braking to the fronts. Somehow 4-wheel braking is masking the issue.

I would look at a loose/broken motor/tyranny mount or suspension component. The area to look at would be anything where reverse torque applies different directional force on these components than friction brakes.

It could also be a bad tire on the front such as internal damage with cord separation. Hitting a pothole could cause this. Try rotating them and see if the issue changes.

Tough one to diagnose since we don't know the car's history.

One other thought: it could be normal operation. Regen sometimes feels 'weird' to a new driver. Volt does have some torque steer. Which would act in the opposite direction during regen.

 

try rotating the tires first. You would be surprised how much a separated cord internally will cause a little bit more drag on one side...or at least when the front end gets loaded down during light dynamic braking. You probably cant tell during harder braking...just thinking out loud. I doubt it is the differential.

 

You win the prize! I swapped the LF and RF wheels and now the car pulls slightly to the right. (This condition will also appear if I lft on the throttle while in L. I don't have to press the brake pedal to get it to pull.

The front tire pressures are the same (40 psi). The tread depth is the same (6/32")

This car has no vibration at highway speeds. How could there be a cord separation without vibration?

 

FIXED!

I rotated the offending front tire (the one that doesn't match the other 3) to the LR position. I also rotated the other front tire to the RR position.

No more pulling under regenerative braking.

I'm surprised that the car is so sensitive to the tires on the front of the car.

Thanks to everyone who helped.