981 Tire pressures

I've been going through the 981 iManual. I was surprised to see that front and rear tire pressures for 18" wheels @ 29 psi F/R and 20" wheels @ 33 psi F/R are the same for front and back. 19" is 29 psi front, 30 psi back - one more psi for rears. That's quite a change from my 06 18" wheels which are 29 psi front and 37 psi back.

Is that an error in the iManual or has the tire pressure suggestion really changed that much for the rear wheels?



Edited 1 time(s). Last edit at 06/23/2013 12:12AM by Guenter in Ontario. (view changes)

I might try 29 all around, too, even before changing out the rears, which I figure will be due in a couple thousand miles. (Somewhere back there's my post about the rear skittering in the wet-- but on subsequent rainy days that hasn't happened.) When I drove cross country, a lot of it was at high ambient temperatures, and if I recall correctly, I saw pressures as high as 37, maybe 38 rear. There were some fairly cold mornings, so I didn't constantly do pressure adjustments. Also, concerning the winter tires, I ran the specified 32 for all around, but as the Michelin PA4 n specs have a bit of a coarse ride*, next winter I'll try 30. Their responsiveness is quite crisp, almost summer tire-like, so I don't think doing this will be at all dangerous.
* I don't think it's their (and/or wheel) balance or trueness, but we'll see. Someday when they wear out, I might try the now-available Conti n-spec snows, or what ever sota n-spec tire is around then.

Everyone finds out that on the track, the best pressure balance is much closer to 1:1. I typically run (hot) 38R/37F.

My speculation is that the extreme stagger was intended to make the car understeer and thus be safer under normal circumstances.

Possibly the new car is so much more tame that they no longer need to play games?

The higher pressure for lower profile tires is, i believe, obvious. Keep them from collapsing and ruining the rims.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
grant

The higher pressure for lower profile tires is, i believe, obvious. Keep them from collapsing and ruining the rims.

Grant

Intuitively, that's what I would have thought. However,In the April 2012 edition of Excellence, a Porsche engineer said: "One of the best advantages [of 20 inch wheels] turned out to be that the bigger tire patch let us reduce tire pressure because the potential tire load is higher with these than with smaller tires. It was a little step that not only makes the car faster but also more comfortable."
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I know I am running significantly lower pressures than I did in my 987 - per the manuals. I recall the 987 with 18 inch wheels called for something like 29/36. The 981 with 20 inch wheels calls for 33/33 but the manual also talks about the "comfort setting" at 30/30.

Lawdevil
2013 Boxster S - Agate Grey,
2016 Macan Turbo - jet black
Cashiers, NC & Atlanta

So, rather than doing a bunch of math, the easiest way to get a close approximation of the pneumatic capacity is to look at load rating - but NOT XL ratings (which raise reference pressure and stiffen the sidewall). That way we can move beyond speculation on my part.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
Lawdevil & CURVN8R

Intuitively, that's what I would have thought. However,In the April 2012 edition of Excellence, a Porsche engineer said: "One of the best advantages [of 20 inch wheels] turned out to be that the bigger tire patch let us reduce tire pressure because the potential tire load is higher with these than with smaller tires. It was a little step that not only makes the car faster but also more comfortable."
.
I know I am running significantly lower pressures than I did in my 987 - per the manuals. I recall the 987 with 18 inch wheels called for something like 29/36. The 981 with 20 inch wheels calls for 33/33 but the manual also talks about the "comfort setting" at 30/30.

Thanks Mike. Interesting info from the engineer. I recall talking to Pedro at BRBS. He had a chance to drive a Cayman R, Boxster Spyder and 981 at, I believe Sebring. There were two if each model. One with 19" wheels and one with 20". In each case, the car with the 20" wheels turned a faster track time than the 19" equipped cars.

I just found it interesting that the 981 uses same pressure front and rear. I'm wondering how much that has to do with the new chassis//wheel base/body dimensions.