Most cold mornings I am running 33-34 front tires and 39 on rear. During a drive they rarely increase more than 2 or 3 PSI, however, the story on the track is different for heat reasons.

I am wondering if there a number that is too high and dangerous where pressure should be relieved?

 

No. Never 'relieve' tire pressures. Set them before you drive and leave them alone even if they reach what sounds like 'high' pressures to you.

Gary

 

The max pressure readings on the sidewall are at least what they will hold. It's unlikely that unless you drive them 125mph for 100 miles on an August day in Dubai that you will ever build enough pressure to see them pop.

Remember, tires build/lose 1 lb. for every 10 degrees in temp. difference.

 

Just run the cold pressures...even at the track. Most street tires have max pressure of 51lbs....so if you track them don't let them get that high of a pressure.

 

Gary, sorry but never relieving pressure is inadequate advice at least as it pertains to track pressures. It's very common, as I'm sure you know, for many tires to increase 8-10 lbs on a summer's day and running over 40 lbs in a street tire on the track is simply inviting them to become greasy and to "go away". Most drivers will want to run significantly less than that depending on the tire and it's difficult, at least for me, to start them low enough to hit those targets so bleeding after the first DE session or two is almost mandatory.

With respect to street driving, I agree with you completely and perhaps that was what your post was specifically addressing. Best,

 

No, I meant all driving. We set pressures on race cars by monitoring the on-track temperature of the tread and carcass and you can't realistically do that with road cars, but the principle is the same. The tire engineers design the tires knowing that pressures increase when they work harder. They specify the 'cold' pressures taking into account the rise in pressure as the tire goes to work for a living and that rise in pressure is essential to the tire's functioning and ultimately its life. When you see a race engineer adjusting tire pressures at a track, it is to find the optimum pressure for that track and that day's temps and the driving style of the driver. It isn't because of the temperature rise with use. The engineer is just doing a more refined version of what the tire designers do when they specify one pressure lightly loaded and one with full load, but in the end they both are putting the tire at a rest condition, a garage or pit pressure, that will let the tire self-adjust from there according to its workload.

If you set the tires to their spec'd pressure at their standard temperature of... 70F I think, but I'm too lazy to cross the room to check... and then you find they increase to perhaps +10 lb after the warm-up laps on a hot day, do not reduce the pressure for your competitive laps. You don't have the data to pick a better resting pressure than the one you began with. (Well, you may have it with the latest and greatest TPMS systems, but that isn't what we're discussing right now.)

I'm not sure what to say about your experience of tires getting greasy if their pressure is too high. Diagnosing what's going on with tires on a track is a full time job for an engineer in race events where the money involved permits that. I can only talk about it a little bit and refer you to the best book I know. I will say the bottom line is still: never bleed air unless you have a tire engineer calculating the exact amount based on telemetry. No kidding. This isn't a whimsical or "Granny go to market" piece of advice. Let me discuss.

Tire temps rise and fall as the car goes around the track. As they enter a corner, the temp rises first from braking at the entry, then as the cornering loads rise, the outer half of the tread climbs. As you start to accelerate out of a slow corner, the powered tires start to warm across the whole width while the outer tread continues to stay warmer than the average. On straightaways and fast bends, even though you're accelerating the actual g-load isn't great on anything but the most powerful cars, and the rising air mass crossing the tires means more efficient cooling. The tire tread and even the body begins to fall in temperature sharply. At the big course of Willow, the highest temps are reached in that 3-4-5 complex, but without telemetry you have to guess what they reach because the tires cool constantly from there to the pits. The turns 6, 7, and 8 do nothing significant to warm them back up.

Now that's going on continuously around the track and the tire engineers take all that into account. Road cars do not stress their tires that way on a continuous basis, but the cycle of braking, cornering, accelerating still occurs. I haven't kept up with modern design trends, but I know they are using multiple compounds to allow for one compound handling the high temps of the extreme performance intervals while another dominates in cool operations like normal driving, but especially in wet weather.

All those operating scenarios generate a range of temps from about 100 F up to 450 F, and the compound is more or less efficient at different parts of its design operating temperature range. One characteristic of tires that varies widely across brands and models is the load vs slip curve. Tire slip is what generates cornering force, and... (...and this conversation would be easier using vector analysis but we'll struggle along in English. So never mind what I was about to type.) As the slip angle increases, the cornering force increases as well. For a while. After the optimum angle of slip is reached, the cornering force begins to fall off with further increases in slip angle. From here the issue of chassis set-up gets beyond what I can express well without mathematics, but basically you have different slip angles at each end of the car at different cornering speeds. You can't maintain the same slip angle at both ends for long, so having a very peaky slip v load curve is a pain in the aspirations of the chassis setup engineer. What you want is a broad flattish top to the curve, so front and rear can have the same efficiency in producing cornering force even when they're working at somewhat different slip angles.

One exciting thing about race tires in ye olden times was the slip v load curves were peaky as hell. At the peak they would produce half again as much cornering force as a road tire, but the fall off was scary. (Scary because letting the car move into oversteer or understeer too quickly meant you passed that peak cornering force before you could catch it. In turn, that meant that you could go from mild oversteer to wild oversteer in a heartbeat. Since we had no protective rails and the next stop was the basement 800 feet below, that behavior especially got your attention in hill climbs.)

Now we get back to pressures. One aspect of compound design that influences that slip v load curve is the temperature response. 'Hard' compounds build heat very slowly, in addition to their other traits. 'Soft' compounds heat rapidly, so by mid-turn the tread portion generating the cornering force may be 150 F warmer than it was during braking. The tire designers take all that into account, and part of their design technique is using the tire body as a heat sink. The rising heat from the tread blocks is drawn into the tire body.

Now what happens with pressures? Well, the sources of heat for the tire body are:

1. Ambient temperatures of the track surface and the air mass flowing across the carcass;

2. Radiated heat from the brakes and other components like the exhaust system;

3. Tread heat generated by the traction role it plays, the cornering, braking and accelerating forces;

4. Tire flexing in several parts.

That last one is where the pressure fluctuations helps the tire designer most. The tire must flex in several directions as the contact patch moves around the tire. (That's looking at from a tire's point of view. To us, the patch stays still and the tire rolls.) When the car is driven faster, the contact patch moves across a particular bit of the tire at a higher frequency. That lowers the time it has to cool between each flexing event, and to some extent affects how much heat is generated by its response. Now when you're driving conservatively, the total of cooling effects is enough to keep the temperature from rising overall in the tire. The area of the contact patch rises in temperature from the front to the back of the patch, but the net temperature gain is less than is radiated away or conducted away by the time the patch comes around again. When you're not driving slowly, the heat is not shed before that patch returns to add more heat by flexing. All of this is independent of the other sources of heat remember. We'll get back to those later.

So the tire begins heating at whatever speed was reached to create that condition. What keeps it from continuing to heat? The pressure rises in response to the temperature rise. As the pressure rises, the contact patch grows smaller. When the pressure goes up ten percent, the contact patch becomes ten percent smaller. The smaller the patch, the less the tire components must flex as the patch crosses them, so they generate less heat from flexing.

Now the pressure went up and the tire patch went down. That does indeed reduce the cornering, braking and accelerating traction in some situations. The 'grip' as we collectively describe it. But that is by no means a universal rule because it depends on average behavior of average tires in average driving. In fact, we have so many effects going on at once that it takes a computer simulation of the most sophisticated type to design a modern tire and it is never obvious ahead of time whether a pressure rise will increase the grip or lower it. That's what we use test days for in racing.

I doubt that's what you're seeing though. I greatly suspect that when you feel the pressures are too high at a track event, what is actually happening is the tread temperature is too high. You may not make that better by increasing the pressures (within the spec'd range of the tire) but I can't see any way you could make it better by lowering them. [Afterthought. That is another dangerous thing to suggest. When I speak of moving the rest pressure higher at the track, I am referring to moving toward the high pressure recommendation provided by the manufacturer. So in my 997.2 the light load pressures are 34/40 when cold. If I thought it might help, I might move toward the 38/44 cold pressure recommendation. Not higher.

When you say that running more than forty psi on a track is an invitation to them "going away" I can't disagree if you mean the tire is designed for the older 30 to 35 range of pressures. (Mine are spec'd as I described above though, so we'll skip discussing those very low profile examples.) The trouble is I think we're considering different numbers. I set my pressures at the specified temperature. If I have to adjust them later, I compute the difference in that specified pressure according to the temperature of the tire body. If you don't know how to compute that, you need a latest generation TPMS that measures the tire's temperature and computes it for you. Or you need test days at the track and a lot more time to talk about it then we have right now.

Basically, the tire that starts out at... let's say 36 psi, not the pressures mine run... that tire at 36 psi cold is bound to reach 40 psi working hard on a track in the California desert. But lowering those pressures is a terrible thing to do to an innocent piece of rubber. You absolutely need to work with the designer here. The higher the ambient temps, the less the heat burden must be from flexing with the contact patch. It is absolutely necessary to let the normal course of events protect the tire. The pressures must rise as it works harder.

I can't point to anything but professional experience (and a shelf full of trophies from my youth), but I strongly recommend you set your tires cold and let them reach whatever pressures result from track conditions. To read more about this and some specific car set-up guidance from a guy who is pretty darn good, see "Race Car Engineering & Mechanics" by Paul Van Valkenburgh.

Gotta run, so I hope this is reasonable lucid. It's already long I know, but I couldn't cover all the issues in tire management in four times this much space. I'll read it again later and proof better, but my wife needs me right now.

Gary

 

Gary,
Thanks for the detailed response and I will consider it carefully. You obviously have a great deal of information and experience and I have little to offer in the way of a productive counterpoint. That said, I will look forward to other track guys weighing in on the matter. You may recall a similar thread on Rennlist a few months ago where the "more pressure" argument was taken by many drivers in South America while a number of other well-credentialed posters thought that was just nuts. Anyway, I hope this generates some discussion because I think real-world practices are all over the map and it would be interesting to review how others approach it. Best,

 

if you set cold pressures to what car manual says and wil run it on track where it goes 10+ psi up you will end up with center tire section worn out in a couple of sessions.

every tire has its own optimal pressure/temperature curve. on kumho v700 i start with 28psi front 30psi rear and it goes up to 35psi front 38psi rear or so hot on an average day and works best this way. hoosiers warm up more and get more pressure so they have to be monitored more carefully.

idea of driving track tires at 50psi makes no sense to me.

 

I'm guessing the OP means he has increased the cold tire pressure for winter since the tires are so cold for street driving that they seem low (although probably would be correct accounting for temp if set according the manual or door). Then at the track, they'll seem too high by a couple of lbs.

At least that is how I read it and I have been wondering the same thing as my "waking" tire pressure, while being perfectly fine according to the TPMS and spec, starts out really low. 31 in front for example - the difference between 38F and 68F. However, they just don't heat up enough on the road to feel right, so I am thinking to bump them about 2lbs today and see how that does - well within the limits even if they get warm.

If the OP is starting out with a "winter cold pressure" of 33-34, it means he is 2-3 lbs above what a 68F cold pressure setting would have given him...for partial load summer tires and speeds under 100mph. Full load and/or high speeds call for +3 lbs front and +4 to +6 rear depending on wheel size. Whether or not that applies to tracking, I don't know - all the owner's manual info I'm guessing is for street driving on street tires.

As for Gary's advice - that's a lot of info and seems like he has given this issue more than just casual attention.

And the OP's main question is also interesting - it would be cool if TPMS would also warn against over-inflation:

"...if there a number that is too high and dangerous where pressure should be relieved?"

 

Is anyone running nitrogen in their tires?? I have it on my other cars and like it.

 

Yes, I run a custom blend gas in my tires using nearly 80% nitrogen - works incredibly well - I definitely recommend it.

 

Most of us do. It's a great formulation.

 

I start with the 78% mixture of nitrogen. I add some every month because some the 21% oxygen escapes through the rubber & the nitrogen stays behind. After a while I figure I should get up to 99% nitrogen after all the oxygen is purged from the system.

 

Not quite. The issue is you also get some H2O vapor in your mix and that is not so good. Try to thrive for a very dry blend and you are set.

 

ha ha you guys are funny