mbsl98

I'm curious to see a number of comments negative to use of 19 and 20" wheels with snows. I don't have personal experience above 18's, so this is just a theory question. I always viewed the advice to use a smaller wheel as directed at the benefit of a taller sidewall (with the smaller wheel and overall stock sizes). Basically a half inch taller sidewall if you reduce the wheel size an inch, since the overall diameter stays the same. That taller sidewall is much better at defending against harsher winter road conditions for sure. With Porsche that stays true, but many comment on reduced grip as the reason to not use big wheels. I am curious if there is any science to that, so long as you stay with the same tire widths. I believe the Bosxter/Cayman widths are 235/265 regardless of the wheel size. Therefore, the tire patch and pressure per inch is the same on all wheel sizes, so why would the grip be different?

I have reduced wheels (17 instead of 18) on my BMW AWD wagon and BMW RWD convertible with great results, and went from staggered to square on the convertible as well so the rear wheel drive is on narrower tires and greater grip per inch. I stayed with 18" staggered with my Boxster partly because I wasn't sure if 17's clear the S brakes, and partly because I won't be driving it in actual snow (I hope). However, on the Boxster, I am experimenting with taller sidewalls on the 18's by running 45 series snows (the 981 spec versus my 987.2) and that seems to be a very good trade so far. Note that the stock 981 uses the same tire widths as 987, 235 and 265, but goes from 40 to 45 ratio. I suspect that the move to 45 series tires on 981 is a good part of the reason they ride better than stock 987's. The combination of snow tires and 45 series has made a big improvement in 987 ride for me, partly because the snows stay more flexible in cold, and partly from that half inch taller sidewall.

Thoughts?

Dave in Chicago

I follow your logic. I ran one size narrower and taller aspect ratios on my '88 944 NA with Blizzaks. Outside of an old '97 Subaru Legacy Wagon we had, it was hands down the BEST snow car I've ever driven. That car would climb trees.

On the modern P-cars, I've been inclined to follow Porsche's recommendations for tire size and N-spec winter tires, partly because it's just more complicated. Brake clearance issues, chassis dynamics, staggers, ABS and PSM calculations, etc. It's just a lot to adjust for vs the engineering wizardry that a big car company can throw at it.

You may note that the recommended winter tire sizes may be adjusted by Porsche to allow for occasional use of appropriate tire chains as well. They get it. They expect many of us to drive these wonderfully galvanized vehicles all year and in every possible condition, all over the winter wonderland.

Just my 2 cents...

Dave

DBH

Going down 1 or 2 inches in wheel size while increasing sidewall height provides more protection against the inevitable winter pot holes while the narrower cross section allows the tires to bite into the snow and grip the underlying pavement rather than "float" on top of the snow. Sounds like a win/win for winter driving.


If you're just looking for a tire that doesn't lose grip in cooler temps, go for a good high performance all-season tire in the same size as your summer tires.

mbsl98

I'm on the same page with you guys as far as preferring taller sidewalls and same or narrower tread width for winter. My initial question with this thread goes to the concern some raise about wheel diameter separate from other factors. A number of posts warn about use of 19 or 20" wheels due to loss of traction/spin-outs, etc., and I don't really get the reason why a bigger wheel with the same tire width would be much different for grip. Obviously it will have shorter sidewall height and that will expose to more damage from holes, rough pavement. I wonder if the aversion to bigger wheels ties to the good old days when going to a bigger wheel almost always was accompanied by a move to wider tires, Modern Porsche specs use the same tire widths regardless of wheel size, and thus my thought that just the wheel size shouldn't really reduce grip unless I am missing something.