tire moving on rim at the track

[ianacole]

I have a theory on this, for all it's worth.

Disclaimer: All my racing experience is in my old '02 VW GTI 1.8T. I've only done HPDE and open lapping in the Boxster as I collect parts for my Spec build.

Theory: All rotation happens under braking, but the fronts are rotating further around the rim than the rears. I further theorize that the driver's side is rotating counter-clockwise, with the passenger side going clockwise.

Reasoning: I don't feel like these cars generate as much rotational force under acceleration as braking. The only time I could see a great deal of acceleration force occurring is in standing starts, which I don't think occurs very often. Think about when you hear tires: under braking, or in corners, never under acceleration out of a corner. I haven't had any telemetry hooked up in the Boxster, but in the VW, with ~215HP at the wheels, I could not break 1G under acceleration, but could regularly in braking and cornering.

Anyone have any in-car telemetry they could share showing peak forces during acceleration and braking?

[maytag]

Originally Posted by ianacole

I have a theory on this, for all it's worth.

Disclaimer: All my racing experience is in my old '02 VW GTI 1.8T. I've only done HPDE and open lapping in the Boxster as I collect parts for my Spec build.

Theory: All rotation happens under breaking, but the fronts are rotating further around the rim than the rears. I further theorize that the driver's side is rotating counter-clockwise, with the passenger side going clockwise.

Reasoning: I don't feel like these cars generate as much rotational force under acceleration as breaking. The only time I could see a great deal of acceleration force occurring is in standing starts, which I don't think occurs very often. Think about when you hear tires: under breaking, or in corners, never under acceleration out of a corner. I haven't had any telemetry hooked up in the Boxster, but in the VW, with ~215HP at the wheels, I could not break 1G under acceleration, but could regularly in breaking and cornering.

Anyone have any in-car telemetry they could share showing peak forces during acceleration and breaking?

Okaay..... I'ma be THAT GUY: It's BRAKING. Not Breaking. Though, honestly, I've had plenty of BOTH at the track, haha.

And no, as the images show clearly, the tires are rotating in opposite directions on the wheels, so they are not both happening under braking forces.
Also: I hear tire noise coming out of nearly ALL slower corners. You're suggesting that it's impossible to oversteer on these cars at corner exit, which is of course wrong. It's quite easy to do. So add-into the equation some sticky tires that don't break (not brake) traction as easily, and you've got a perfect environment for this sort of rotational movement under power.

[Starter986]

Originally Posted by maytag

Okaay..... I'ma be THAT GUY: It's BRAKING. Not Breaking

I would have invoiced $1.15 for correcting that error.

[ianacole]

Originally Posted by maytag

Okaay..... I'ma be THAT GUY: It's BRAKING. Not Breaking. Though, honestly, I've had plenty of BOTH at the track, haha.

And no, as the images show clearly, the tires are rotating in opposite directions on the wheels, so they are not both happening under braking forces.
Also: I hear tire noise coming out of nearly ALL slower corners. You're suggesting that it's impossible to oversteer on these cars at corner exit, which is of course wrong. It's quite easy to do. So add-into the equation some sticky tires that don't break (not brake) traction as easily, and you've got a perfect environment for this sort of rotational movement under power.

You go ahead and be that guy ... I do it too. No one is above mistakes. Thank you, corrected my post.

Also, I don't see how it is "clear" that the the tires are rotating in opposite directions. Whether a clock goes one way or the other, both directions will ultimately arrive at 2.

And you hear squealing from your tires, under acceleration, coming out of corners?

[maytag]

Originally Posted by ianacole

You go ahead and be that guy ... I do it too. No one is above mistakes. Thank you, corrected my post.

Also, I don't see how it is "clear" that the the tires are rotating in opposite directions. Whether a clock goes one way or the other, both directions will ultimately arrive at 2.

And you hear squealing from your tires, under acceleration, coming out of corners?

ooohhh... yeah, I see that you're suggesting one of the tires may've slipped almost all the way around, and only LOOKS like it went the other way because of it's final position. I'll admit that didn't occur to me. You're right, of course, that it is possible. But I still think it unlikely.

There is, as I said below, very little force applied to the rear tires under heavy braking. Weight transfers forward and all the work is being done at the front wheels. The very design of the car shows you where the forces are: Smallest brake is in the rear because it doesn't take much braking force to overcome the available traction when the weight is moved forward. But biggest tire is also in the rear because they need to be, to accommodate the torque being applied under acceleration.

And yes, absolutely; If you aren't hearing tires complain when you go to the gas, then you could be going to MORE gas. :-)
I'm talking about mid-corner, when you're trying to get the drive. It's very easy to get too greedy :troll: with the throttle and snap the back end around. You're flexing that sidewall for all it's got laterally, and then you stick forward motion to it as well. The carcass flexes more and you overcome the available traction. Now apply stickier tires to the mix. There's more force being applied to that rear-tire bead under this scenario than there is under heavy braking.