Tire Pressures
 

I've been paying a lot of attention to tire pressures lately, and thought I'd share this online article which I found useful:

http://www.turnfast.com/tech_handling/handling_pressure

FWIW, with the Cayman R on AD08 street tires, grip seems to drop considerably when pressures get over about 36 psi. Not sure yet how much this is due to tire pressures vs tire temps.

You must be talking about initial pressures? 36 PSI doesn't seem extreme for a high performance street tyre. Pressures go up fast as tyres heat up on track.

It is true that most tyres have a sweet spot for track use. I've found it doesn't really matter that much what the ambient temps (during the normal season) are when warming up tyres. Though 90+ would cause me to lower my initial presures some.

36 psi is the hot pressure. I've tried hot pressures ranging from 33 to 42 psi with these tires, and the grip seems to be noticeably down by about 38 or 39 psi. The car's tire pressure monitors seem to be reasonably accurate, so I've been able to compare the pressures vs grip in real time while on the track, which has been very helpful.

I haven't used the pyrometer yet, but will hopefully be able to do that this coming weekend.

For street use, the recommended cold pressures from Porsche are 31 psi for the Cayman R (much lower than a 997), so that might be part of the reason why the hot pressures need to be somewhat low also.

Attention other Cayman/Boxster drivers: please post your optimum hot tire pressures (and the associated tires).

Good article.

One thing I've discovered after being around autox'ing and racing for 30+ years, is that modern low-profile tires are harder to decipher as far as optimum tire pressure. In the good 'ol days of 70- and 60-series tires being the performance norm, watching for "roll over" wear on the tread's shoulder was a dependable way to manage tire pressure. But it seems like modern super low profile tires makes this a less useful indicator nowadays.

Also seeing nuances in tire design which affect visible indicators. Case in point is the R888. It apparently has a soft tread area design and very stiff sidewalls. Also tuned somewhat to take advantage of negative camber. Early on, people were noticing excessive treadwear in the middle, which is usually indicative of tire pressure being too high; causing tire to buldge in the middle and wearing more. But lowering pressures only seemed to make it worse. I'd read that what was actually happening was the soft tread area of the carcass was actually bulging under cornering loads. Increased tire pressure was the cure, in order to stabilize the tread area.

Another example are Hoosier R6's. The optimum pressure I've found for my 944Cup is around 36-38 PSI hot. However, increasing to 40 PSI will yield a bit more grip, but with a smaller slip angle, making it a bit more challenging to drive with quicker break-aways.

Bottom line, IMHO, is that optimum tire pressures can vary between tire makers, car makers, suspension design/tweaks, air temp, sunlight, etc. Most tires will operate well somewhere around 34-40 PSI (notice I didn't say all...). If peak performance is your goal, best to systematically make your measurements, monitor (even log) your results, and confer with others using similar car & tire combo.

Things like checking pressures on the grid versus in the paddock will make a difference. On the track, I have a cold PSI I set, and expect the tire to reach a hot PSI after a session. Whereas at an autox, where tire temp can't increase as much, I'll start with PSI much closer to optimum hot track PSI. And with modern wide tires, a pyrometer may also be a good investment, to monitor varying temps across the tread's surface. Will be indicative of tire pressure, and suspension settings.

In virtually all our cars, I'll start at 32 PSI cold for the track, looking for around 38 PSI hot. At an autox, I'll usually start around 38 PSI.

FWIW, back in the old days, my wife ran an '83 GTI at autox's. We ran 50 PSI on the front tires, but moreso to keep the sidewalls from flexxing so much. But with today's 40- and 30-series profile tires, probably not as much an issue.

Pressures and temps are linked. If you don't own a pyrometer yet purchase one and learn to use it properly. Used with care it will pay for itself in extended tire life in short order.

There are so many variables in cars, tires, driving methods, tracks, temps, tire condition and so much more that I would tend to say you might be better off testing and tuning the car to the tires used and the tires to the conditions rather than going on cloud intel. Brand X R comp manu will give opt temps as XXX deg. I like to aim at the optimum temps via pressure ajustment and susp tuning (to make it even across the contact patch).

What tire you are droving on is key as this will tell the compound and construction. As to the car type you want to tune so the tire is as even in temp across the contact patch as you can get. some meters have memory for temps so you simply move from tst point to test point and decode the data later. this is nice as it lets you gather the info quickly which counts with temps, the lighter the tire carcass is the faster it will cool.

As Tony said most true street tires fall in a general range. They tend to be made to work well in street driving and at colder temps than seen during a good track workout. the best pressure is one that keeps the contact patch flat to the track. Too much pressure might make the tire round up a bit too much. you will see this in increased center temps too low and you see increased inner and outer edge temps. Too much camber and the inners run hot too little and the outer does.

Test, decode and ajust. I might be inclined to say a better question would be what model pryometer do you like and how do I use it.

I have a pyrometer that you are welcome to borrow this weekend. Look for me.

Larry,l

You gonna make it on the track?

I appreciate the offer. I actually have a pyrometer, but haven't used it yet. Part of the reason is that I wasn't sure how useful the temps would be due to cooling of the tires by the time I checked the temps, but I'll try it and see how it goes. I would have tried it this past weekend (good event with PCA Reisentoter - nice people), but the wife and I were sharing the car with her sessions coming right after mine. She cools the tires down nicely. :lol:

Pyrometer checking is best done right after you exit the track, and somewhere safe/allowable on the hot grid. Perhaps you can solicit someone to take the readings for you, while you and your are switching places if sessions are back to back.

Not the way to earn domestic point-by's........;)

Although certainly true, this is valid only if you are trying to determine camber/toe settings' effect on the tires. How would a pyro help determine optimum tire pressures (not heat distribution across the thread)?