Bob Rouleau

John - just a data point. My info about 40 lbs HOT being the approximate correct pressure for STREET tires came from a seminar put on by Michelin. They informed the audience that street tires are designed that way - or were three years ago when I attended their tire-tech seminar. Apparently this is a guide line used by tire designers who have to compromise on the average pressure suggested by vehicle manufacturers. R compound tires are different and can vary more from one brand to another. Michelin likes 32-34 hot (higher figure for 3000 lb cars) Corsa Pirelli likes 36 hot (also on a 3000 lb car) and I believe Hoosier likes more. Back to street tires - my personal experience on track is that street tires like the PS1 PS2, SO2, SO3, AVS S1 etc do work best at very near 40 lbs hot.

I agree that underinflation can cause more heat and therefore pressure buildup. If you recall, I suggested 32 lbs cold and then bleed to 40 hot after a few laps.


Best,

Larry Herman

Ok then, here's a question for John, Bob, et al:

If underinflation causes excessive tire flexing, heat, and pressure buildup, then once the pressure got to the proper range, wouldn't the tire stiffen up, reduce flexing and stop gaining temperature?

Bob Rouleau

Larry - The way I see it, underinflation will either bring the pressure up to where you want it... or not. Using the MPSC for example, during the summer I start with around 28 lbs cold. A few laps and the pressure is up to around 34 lbs which is where I like them on my car. If I started much lower (I have tried) the pressure does not build to what I consider optimum and even after five laps the car feels wrong - squirmy sort of. By the way for those watching this thread, let's say I guessed wrong with my starting pressure and my tires don't get up to my target hot value. I then add air, one pound of cold air for every two pounds I am below my target hot pressure. Example, I start at 26 lbs and for whatever reason fail to get enough heat into the tires to get to my targert 34 lbs. I measure and see I have on 32 lbs - I'll add 1 pound of cold air.

Best,

RedlineMan

Bob -

I think we have arrived at roughly the same place in the end. Glad to know about the Michelin advice on street pressure. I think that 40 hot is a nice round number that will be pretty close to optimal in terms of both chemical and mechanical grip - as well as tire safety - for any tire. Always consult factory performance literature (or better yet, the tire engineers) for specific advice.

Larry -

Of course you are right. A tire will only heat so much. When it gets that hot, it is usually quite greasy. However, it is still that pressure increase that you need to watch to make sure you are not overworking a tire. Getting to 40 hot from 30 cold is much safer than from 25. 25 is just plain too soft, unless you have a very light car. Again, much more than a 10-pound increase is something to take a close look at.

Remember the BFG R1 Groove of Doom? Classic case of underinflation. That was a hard-edged tire with incredible grip... to a point. That point was a huge and immediate breakaway! Guys were running them at 25 and below to "make them feel better," but this underinflation would not hold the tread surface taught enough, causing the groove to wear in.

Bob Rouleau

John - I agree about the old R1. It was a macho thing I thought. Guys would go around saying "I start at 20 lbs cold" - the implication being that they drove so hard and so fast that the tire heated up immensely. Yeah. Sure. They also suffered the groove of doom after a day's use.

I think we can close in agreement. So far we've only talked about cold and hot pressure. The real secret to fine tuning is a pyrometer. For example the Corsa likes tread temps in the 185-200 degree F range for optimum grip. If I see that my tread temps are low, I take out some air, if they are too hot, I add air. That opens up another discussion about the proper use of pyrometers and the impact of alignment settings on tread temperatures - camber and toe especially.

Best,

LVDell

Well.......I just came back to this thread I started to soak up the knowledge again and have a question for Bob et. al.

Re: Pyrometer.

Any pointers, etc. for "proper" use of a pyrometer? I am going to pick one up this week for my track day this weekend and wanted to document my tire pressures, temps, etc.

Thanks again!

Bob Rouleau

LV you thought there were different opinions about tire pressure? Hah, now you have opened a big can of worms. There are two basic types, infrared and probe. The former is very convenient because you just point it (it has a built in laser pointer) at a spot on the tire and read the temp on a digital meter. The probe type is like a meat themometer, it has a needle you stick into the tire. Some like the infrared because it is fast and convenient. Purists say that it promotes false readings because most users are not careful to measure from a consistent distance AND the outside of a tire cools more rapidly than the inner layer which the probe reaches. I see pro race teams use one, the other or both so get ready for strong opinions. I prefer the infrared type since it is useful for many things, I can measure the temp of my heads, headers, wife etc. For purposes of tire pressure and camber settings either type works (for me) since I am looking for relatively even temps across the surface. The tire manufacturer normally supplies optimum temp ranges for the product. A good setup will see temps like 170 deg.(outer 1/3) 170 middle third and 180 inside third.

Hope this helps!

jrgordonsenior

Bob I've just went through another set of MPSC's rears with only 4 track days, probably 18 heat cycles on them. This is the second set that has corded on the extreme outside edges on the rears. This last set (315's), ran at 40 lbs. hot as I increased pressure to try an alleviate the outside cording from the previous set. I'm at 2.5 neg camber, and the inside tread was definitely wearing. Oh and it does handle better at the increased temp. Pyrometer test by myself were around 190,190,180,inside to out. This is a heavy car at 3300 lbs.
So my question would be to stop the premature outside cording would you go up another pound or two, or would you add a few degrees more neg camber maybe up to 2.7 (fronts are 3.0), or ?????

RedlineMan

Yes, Bob is right;

My strong opinion is that the probe type (at least 3/8") is the only one for TRULY accurate tire info. As Bob mentioned, the tread surface cools quickly. Also, the outside edge of a tire's surface cools quicker than the inside because it is in more open air. Lastly, the danger zone for excess heat is between the cap ply (tread) and the first reinforcement ply. This is where the tire will start to delaminate if it overheats. Obviously a probe type will reach this critical area where an IR will not. No-brainer!

If you see a pro team using an IR, I think they are more interested in a general idea of whether they are in range for compound utilization. They likely already know what their true core temps are and are just checking to make sure nothing is amiss.

jgordon - You don't say what pressure you started at, or what the increase was. I'd probably bump the negative up a couple tenths and see if I could get inner and outer to chord simultanteously. It's my guess that more air will simply lead to a loose condition. I'd also think you would want a touch more understeer to take the load off the rear tires a bit

LVDell

Anybody that can point me in the direction of rec'd tire temps for the Bridgestone S02? I checked the Bridgestone site (what a joke that was).

Thanks!

JayM

Just came back to this thread because I have new shoes and alignment and a track day tomorrow. I have a Ruf RTurbo (think 996 TT) with a new set of 19 inch Michelin PS2s. Alignment is set at -.75 degress front/-1.75 degrees rear. I have some track experience, but am not an advanced driver by any means (and don't intend to try to drive like one). It's 100 miles or so to the track, so by the time I get there there should be close to 200 miles on the tires. Here is what I plan on doing; all suggested changes warmly welcomed.

I plan on shooting for 38/42-3 hot, as long as the increase in temperature from cold is less than 10 pounds. My reasoning based on 40 pounds hot being optimal for street tires, wanting to have a F/R differential so I get some understeer, and my belief that the front pressure on a TT has more of an impact on handling than the rear balanced by the fact that the overinflation slope is steeper than the underinflation slope. I will also take it easy the first time out to make sure the tires have a chance to cycle.

I have both a digital pressure gauge and the onboard SmartTire system, which gives average temperature readings inside each tire, but no pyrometer. Ambient temperature tomorrow is supposed to be 71 degrees at 9am, rising to 85 degrees in mid-afternoon. Is there a "hot" temperature range I should be shooting for for these tires?