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      03-08-2012, 11:11 PM   #10
Pete_vB's Avatar

Drives: '69 GT3, GT4, 1M, 912
Join Date: Nov 2010
Location: SF Bay Area, Shenzhen, Oman

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Originally Posted by M3 Adjuster View Post
just not sure which I would rather a-x with.. 18s or 19s .
Yea, that's a fair question. I'd put my money pretty heavily on the 18s for the following reasons:

Generally, when I can't change gearing, I inform tire OD by the speed in gear (2nd) for the courses I run (65-70 mph). With the 1M's wheelspin issues there is no real advantage to go shorter, however, so I'd want to stick with something relatively tall.

After gearing I generally look at the issues with the car I'm trying to correct, be it turn in, precision, etc. With the 1M the issue clearly seems wheelspin and oversteer.

I'd make the argument that taller sidewalls have advantages putting power down in general. Obviously construction has an awful lot to do with it, but a taller sidewall is softer by 10-15% per every 5 points of sidewall height (so a 265/40/18 will be ~14% softer than our 265/35/19s). The softer sidewall allows more flex, bad for precision but good for putting power down and less abrupt breakaway, two areas where the 1M needs help. Lower mass and rotating inertia are bonuses. Consider that many race cars deliberately engineer for taller sidewalls to put power down- a GT3 cup car uses a 9.7 cm sidewall while a more powerful RSR uses a taller 12.7 cm sidewall to help put power down. Obviously drag cars make use of the concept too (with tires engineered for the purpose, but still).

I run a set of 17s as well as 18s (front) and 19s (rear) on my 911, BTW, so I have some back to back experience. I generally think the 17s are the better solution for the car, though of course it's a special case. Given that I can't get the same types of tires on both rims I can't do an apples to apples, but the 17s are far more progressive in breakaway with any tires I've used, which seems to support the theory.

Originally Posted by M3 Adjuster View Post
Hmmm.. this really could be an issue of switching from Michelins to Brand X. I would not be surprised to find that Michelin Supersports and the OE PS2 have a similar ride...
Having had both on the same car, I can say the PSS doesn't have as stiff a sidewall as the PS2, but it's still stiffer than something like a Goodyear F1 Asymmetric or Continental. So I agree you could get some of the feel of an 18 like he's talking about with a different tire, but all else being equal I'd rather use the PSS, and the soft sidewalls do tend to encourage pinch-flats. What continentals did you AX on, and how did they change the feel?
1M, GT4, 1969 Porsche 911 w/ 997 GT3 Cup Motor (435hp & 2,100 lbs)