Winter fitments on and getting 'herky-jerky' feel

[GreggT]

I think there is common info out there that for PSM and all-wheel......that the front-rear difference in tire-wheel diameter or circumference should be kept under 3 or 4%.

But.....do newer 997.2s actually need that 3%?

Sounds strange but right now it's all I can think of.

From my math and various tire-wheel calculators on-line it appears that OE tires & wheels (both 18s and 19s) have around a 3% dif in the front-rear number.....the rear being the on up 3%.

OK...so my winter setup actually has identical running diameter or circumfrence front to back.....and that's by my design as it's been a set-up I've previously used on 993C4 and 996C4. Basically I have narrower rears than Porsche recomends since I actually do drive snow.....real snow.

So with the winter 18s on the car.....the car has a feeling I'll describe as a mild 'herky-jerky'.

In low to moderate speed.....or just maintaining speed, as long as I am applying some gas I get a mild pulsing or surging feel. It's almost as if the PSM is going through minor corrections or fighting with itself.

Anyone heard of this?.....or have a better explanation.

Thanks in advance.

Edited November 9, 2011 by GreggT

[wvicary]

If you turn off the PSM does it still display the fault? I haven't heard of a fault like this before.

[GreggT]

Thanks.

OK I should have been more clear.....here is some additional info.

PSM does not display a fault.

But really it wouldn't.......and now, with more thought, I'd take back my comment that PSM is even involved.

PSM intervenes when the line of travel of the car is out of sorts (slides, etc), and it's brain takes into acount 4 or 5 factors involving lateral acceleration, steering position, speed, etc.

Straight line 'herky-jerky' (2nd gear 3500 RPM) would not be the PSM's area......and having it's switch on/off makes no dif. for the issue I am experiencing.

I'm back to the 3%.

I wonder what cars have felt like (especially C4s & Turbos...997.2s) when the front-rear 3% has not been observed at all.

[wwest]

PSM is not simply stability control, it also includes traction control and "virtual" LSD functionality. Trac detects rear wheelspin/slip and INSTANTLY dethrottles the engine and will moderately apply brakes, ABS like brake modulation, if the condition persists. Winter tires often have a higher level of siping thus have less CSA for roadbed contact, additionally you mention they are narrower tires.

All that adds up, for me, that Trac is activating.

Edited December 31, 2011 by wwest