Torsen diff and rear tire size sensitivity
 

This morning I switched on my winter tires and headed to PA for Thanksgiving. While driving I kept thinking, "Hmmmm, somethings weird. It must be windy or something because I'm making a lot of steering corrections.

Then a few minutes later I realize that every time I stepped on the gas the car was turning slightly left. And when I let off it was turning right. At low speeds you didn't notice it but at 80 on a straight highway it is very noticeable, and actually incredibly annoying.

I hypothesized that the right tire might have more tread, making it a little bigger, thus putting down more torque now that I have an LSD.

I measured the tread depth with a quarter and couldn't see any difference. This left me scratching my head. What the hell else could it be?

I got to my parents and took both wheels off. Sure enough the right tire was just a little taller than the left. But we're talking less than a quarter of an inch (maybe 1/8th?) measured using a level. Could this possibly make that much of a difference??

So I swapped the wheels and sure enough, the symptoms reversed. The car now pulls right when accelerating (at least I think it does.. it was a crappy test on curvy roads.. I'll know for sure tomorrow.)

I think this is a problem that will go away on its own because the larger tire will wear faster. But I wonder if it's bad for the torsen gears to be working this much while it wears down?

I might also put more air in the smaller tire, but not enough to make them completely equal.

Either way... Torque vectoring diff FTW!

If the tread depth is the same, how is one tire taller than the other? Unless they’re different tires or sizes.

It'll behave that way on an open diff.

It wasn't actually the same, I guess. Just that my half-assed quarter measuring attempt was imprecise.

Hmmmm, really? That doesn't make sense to me. Or match my experience.

We explicitly noticed it on Mike’s 335 when he had one new and one half used tire in back, before he put an LSD on.

Interesting. Maybe the effect exists either way, and it's just multiplied with the LSD. I know I've run different depth tires in the rear (much more than this current difference) and I never noticed it.

I noticed this on a 991S that I test drove. Must have been the tires but turned me off that particular car (not the 991S in general). On throttle one direction, off throttle the other... Slightly. I figured it must have been tire related but could not see obvious signs.

This has been informative.

I'm mostly shocked at how extreme the effect was given the small wheel radius difference. Driving the car at 70+ was incredibly annoying. Constantly corrections needed. On and off the gas created a zig-zag effect.

With one tire at 34psi and the other at 38 the effect is still there but to a smaller degree.

The larger wheel is putting down more torque so it will wear down... it's just a question of how long it takes.