First off, this is intended as a fact sharing thread. I don't wish to argue the veracity of my opinions or certainly not whether what I share is factual or fictional. I have better things to do than that.

The reason I'm posting this is because yet again a manufacturer's tire engineer gave information to me (more on this below) that validated what I've shared before that I believe has been overlooked because it is not the most popular suggestion.

There have been several posts that deal with tire pressures on these cars and performance driving. For whatever reason, many of you folks have become somewhat fixated/convinced that you should drive to a certain hot pressure. In fact, as I've read many of you start removing air from your tires to keep at that pressure when hot. I've tried to diplomatically comment on why I thought that was not the best thing to do (it then alters the balance of the car). Generally I think my input was ignored. That's fine. What you do with your car is what you do - my only feedback was to try and give you information based on my gazillion track miles of experience AND manufacturer shared information to help you. If you choose to do otherwise, I fully respect your choice and will not engage in arguments - I'm sure all of you are big boys and girls.

Having said that...

First off, ideally if you have a tire pyrometer, generally speaking for performance and wear you adjust tire pressures such that you get reasonably consistent tire pressures across the tread. I purport that you will take the most wearing tire as the control tire (usually a front), and then adjust the rear pressures based on car balance. This is because on a street car, we don't have the luxury of changing spring rates or sway bar stiffness to balance a car, so the tire pressure becomes the only adjustable parameter you have to balance your car. I don't have a pyrometer, and my track events are infrequent, so I've chosen to look at the sidewalls and use rollover as my measure of cold pressure.

Since I'm running Daytona next week on a track event, I thought I'd reach out to Michelin for guidance (I'm running PS4s). Why did I reach out to Michelin? Because even though I run 34 psi front/36 or 37 psi rear on my F type at my local track (PBIR) - I was confident that was not a safe pressure to run a high speed track like Daytona. I learned long ago that that pressure is key to tire structural limits at speed - more pressure gives you higher speed capability. Michelin first referred me to their outsourced Motorsports vendor. Once I was able to communicate my request to them - they actually went back to Michelin to speak to the Michelin engineer responsible for the PS4s. The first feedback (as I expected) was to run the Cup2 tires, because they are designed for exactly what us track guys do - run street cars on race tracks (for which normal street tires are NOT designed). The second feedback was - to my prior points - TAKE MY REFERENCE VALUES AND ADD 10 PSI PER RIGHT TIRE AND 5 PSI PER LEFT TIRE NOT TO EXCEED TO THE MAXIMUM COLD PRESSURE. Keep whatever bias front/rear I wish, however at Daytona where the speeds are great and even on the infield there's a good moderate speed sweeper - WE NEED TO RUN HIGH PRESSURE. Note that the tire HOT pressure was never mentioned. This is consistent with when we road raced A Showroom Stock (i.e. Street legal) Mustang on Bridgestone RE-71R tires, and the Lietzinger Motorsports reps we dealt with affirmed way back then - at speed - pressure is your friend - it's what keeps tire integrity. They also shared with us not to worry when cord starts peeking through a touch - you've got another 30 minutes of life. Never back then were hot pressures discussed and now again with a different tire manufacturer hot pressures were not brought up as relevant (hence why the word cold is in front of the max pressure number on the sidewall).

SUMMARY:

1) As I've tried to suggest before, don't worry about your hot tire pressures. Once you've established a bias front and rear that gives you the car balance you like - then if you adjust pressures at all when hot to better balance the car, then remember that adjustment and revise your cold pressure bias the next time accordingly. Tweak as needed.
2) For high speed tracks - tire pressure is your guardian angel. Per Michelin - run up to the max COLD rating in the tire for a track like Daytona. in the case of these tires, the max cold rating is 50 psi each. As such, I'll probably run 45 RF, 48 RR, 40 LF, 43LR. I may tweak the bias if I need to correct for under/over steer tendencies.

Regards, and safe driving. I'll try and post video of the event once I get a chance a couple of weeks from now on the track session thread.

 

I appreciate effort you put into this and discussion you started.

I disagree with what you say and think your suggestion is dangerous for a few reasons:

Reason 1: It is quite possible to exceed 50PSI hot if you don't lower cold tire pressure before starting your track run. It happened to me once one a hot day as a result of personal negligence and not checking and noticing that dealer pumped it to 41 PSI cold after service, but fortunately I noticed this in time to avoid a potential disaster. My understanding, and please correct me if I am wrong, that driving at above 50PSI substantially increases chances of blow-out.

Reason 2: Higher pressure translates in less cornering traction. Subjectively, I feel that tires get slippery when pressure get too high. Again, my understanding to optimize lateral grip you want least tire pressure that would allow tire side wall to avoid collapsing.

I track my MP4S-equipped F-type with 34 PSI cold, with goal of reaching about 44 PSI hot. Outside of track, I keep tires at about 39 PSI cold again with a goal of getting about 44 PSI hot.

 

The track I usually run is not very fast, 15 turns over about 5 km and 3 straights that you can realistically hit 200 km/h (unless you are Miata driver) but not much faster than that.

 

Try a couple of hot laps at 38 psi cold. You'll find they won't heat up past 44 psi either and you'll get more lateral Gs. Going to 50 pounds cold is a bit much in my opinion as the center of the tread will start bulging and you'll loose tire patch. At 32-34 psi (cold) on the track, I've rolled the tire right off the rim. Not good. Listen to the Michelin Man.

 

Thank you for the effort but I disagree. You are missing many other variables in your discussion and yes I have discussed this with a number of manufacturers, done track testing with those manufacturers and I track my cars as well as support my clients that track their cars.

Unhinged, you can easily get over 44psi after 10 hot laps... starting at 37psi. It all depends on ambient and surface temperature, turns, average track speed etc.

I start at 34psi and my goal is to stay at no more than 41 hot, I run nitrogen in my tires.

Anyway, without counting so many of the other important variables this discussion is pointless. The Pyro helps greatly in making sure that the tires, pressures and chosen alignment are a good fit for the track. Used purely for tire pressure can be deceiving.

 

Remember these are street cars, we have limited methods available to properly set up the suspension. As I stated, the pyrometer is the primary means for tire temperatures, however you need to take readings quickly after getting to temperature (i.e. immediately come into the pits and have folks take readings). Those readings on a street car will will likely not be optimum (i.e. uniform temperature distribution across the tread) as we know from when we road raced. All sorts of factors, especially street car suspension geometry which is NOT designed to optimize tire loading on a race track. What a pyrometer does is get you to the nominal pressure that will have the best grip (and we were always advised to record and run with cold pressures as reference). I assure you from actual racing experience that desired track pressure is well above the desired street pressures.

As explained to me years ago by the Yokohama tire engineers - blowouts occur because the higher the speed, the higher the standing wave of the belt construct in the tire. It is not round as you drive down the road - at the pavement contact point there is an upward flex of the tire, then a downward flex again. This builds a wave height as you increase speed. As the tire rotates, the belts flex one way then reverse themselves, generating friction. The higher the wave height, the greater the belt flexure, greater the friction, then voila - greater heat input (friction from the flexure). All elements in the tire construct (most important are the the belt materials) reduce their strength at higher temperature (go study material science). Higher speed, higher heat, weaker belts. Lower pressures then allow the greater flexure, exacerbating the situation. Ultimately the tire loads exceed the belt strength and you have failure. Pressure is your mitigator, because it tends to oppose the standing wave height (making the tire more round); reducing the magnitude of belt flexure.

I have provided facts as based on my personal experience and input from Yokohama tire engineers, Bridgestone tire engineers/racing partners, and now Michelin tire engineers. Use that as you see fit. I will not enjoin in discussions that are not fact based.

As a minimum please for your safety take away from this if you're running a high speed track, you are potentially dangerously low in pressure (and really risk a blowout) if you are driving the pressures you enjoy on the street (or lower!). If you have doubts - do as I did, reach out to the racing arm of the tire manufacturers and take their advice. I chose to do so since I am a professional engineer and I well understand those are the folks in the know.

 

I politely disagreed and you keep insisting...

Here's a fact, I have been professionally racing for over 20 years, including for team the likes of Porsche, BMW, Alfa Romeo and your explanation never applied. I knew exactly the tire pressure and the suspension settings every time I pulled on the track to fine tune the car for the race and never have I heard that. The people that set the TRSD are the very best engineers in the field... Maybe Pirelli and Continental are so special that they have their own ways of doing things.

I race almost every weekend in the summer... From AutoX to GT Club Racing, 34PSI has worked nearly every time. These are FACTS and not what you think you were told.

 

We can certainly agree to disagree. I never doubted your racing experience, nor did I attempt to. I was simply trying to discuss fact based failure modes for this car at Daytona and potentially other very high speed tracks. I certainly agree that mid 30 pressures for a Z rated tire are totally fine on many cars on many tracks and certainly in autocross (which is no comparison regarding tire loading - very short low speed turns, albeit at high Gs). For the vast majority of the amateur drivers out they won't come close to the Z tire speed rating, thereby maintaining a high margin of safety to the failure point regarding tire pressure temperature. I am simply trying to say that folk's perception of the risks at very high speeds (which the vast majority of amateur racers never achieve) may be incorrect for this particular car (186 mph) on any very high speed track (Daytona). I stress amateur drivers because we're talking putting a street tire on a race track with a suspension that is not optimized for racing (aka tire loading) - I call that a far cry from real race car setups that you speak to.

I would be curious as to your basis of observing a hot tire pressure as a limit somehow corresponding that to the max cold pressure rating as published on the tire. If hot pressure is the limit, then why is the word 'cold' embossed on the tire? I am highly confident it's because the hot temperature/pressure that the tire achieves when it is pressurized cold to the max rating and then loaded to the tire's embossed max load rating has been tested over and over again. I'm sure that no matter how many times they run the the tire samples off of the assembly line (which I bet they do as part of their QC program) - when pressurized and loaded as such the tire reaches an equilibrium pressure well above the cold pressure (a guess might be another 10 psi) then stabilizes at a point where the pressure and temperature is in equilibrium (heat generated from the belts equals heat dissipated to the environment) without failure.

So - your point is heard. You disagree with the data and science based explanations I was shared from manufacturers and reps. You've disagreed as I understand based on your experience. I did not mean disrespect; experience is great! That same experience part is how I've learned my entire adult life to apply the science I was taught in every endeavor. I simply wished to share with others for this car at this track what the manufacturer shared with me. If you have conflicting tire manufacturer data that the manufacturers have shared with you - by all means post it for our benefit.

 

I don't want to drag this on forever but here are some answers for you...

Why is the max pressure expressed as COLD INFLATION? Very simple, how often have you seen people warming up their tires and pulling over to check the pressure... or pulling over when it's very hot to make sure they don't exceed the max pressure. Obviously COLD inflation has a predictable and acceptable resulting hot temperature pressure. Quite frankly I fail to understand why you would even mention this but....

What I am assuming that you fail to realize is the the tire pressure affects the geometry of the tire, the contact patch and the stiffness of the walls. The temperature affects those a bit, that's true but pressure is king. Rest assures that I push the (Y)... not Z rated tires to the limit on the track and picking the right pressure is a delicate balance involving grip, responsiveness, wear pattern, tire temperature (safety) and alignment. You oversimplified this in a way that can be rather dangerous.

Those that have gone over 150Mph on the F-Type will notice a nag screen coming up if the tire pressure is below 41psi... the reason for this is that at lower pressures the tires have more rolling resistance and will generate more heat. This can cause a tire failure.

 

There is such thing as tire pressure to low, and it is also dangerous for the reasons FType17 noted. Unhingd also correct in pointing you can roll tire off rim if pressure is too low.

I typically drive my car to the track, so I don't start my run with completely cold pressure. I only have to drop few PSI once I do few hot laps to get temps up. It is only the next day I can get completely cold tire temps and they are usually 34 to 36 PSI that on track corresponds to 44 to 45 PSI.

Outside temperature makes a lot of difference on actual tire pressures during hard driving - 100 F day with sun heating asphalt will be about 3-4 PSI higher than 75 F cloudy day.

Another point to consider - if you let your tires get too hot you will greatly reduce their longevity. I normally get upward of 20 hours of track time, plus regular commute, on a set of MP4S. This is miraculously fantastic in my book.

 

Just some ponderings from me. (not taking sides, just trying to learn)


NASCAR teams tweak handling with one-PSI tire pressure increments. Just sayin', it seems to be that important.

Is tire-rollover an accurate measure of needed pressure in these days of rubber-band thickness tires on 20 inch rims?

 

Carbuff2 - I started racing in AutoCross in 1979 - tire pressures were the way we adjusted in my stock Turbo Mustang. I autocrossed for a good 15 years competitively (mostly Fox Mustangs) and even taught my boys to drive my Cobra replica in Autocross. Generally speaking once I got pressures dialed in, it worked well at all venues unless suspension modifications made - then you needed to readjust. I also used 1 psi as my smallest adjustment for balance.

Later when I started road racing, we did the same. After a couple of years my partner acquired a nicely setup old 240Z to run in SCCA IT - that car was previously owned by another Engineer who swore he had it dialed in. Our first track was Daytona, the previous owner agreed to meet us there for consult. My partner wanted me to run the Z since he was points leader in our Mustang. After my first practice session in the car, I had them tweak all 4 corners (previous owner silent in my presence but verbally doubtful to my partner when I was back out running the qualifying session). My bud simply told the previous owner he's learned by experience to follow my suggestions on tire pressure setup. After the qualifying laps - I came in and said it was great but asked for a 1/2 psi adjustment on one corner. Note only twice behind the wheel maybe a total of 12 laps. In the first race I then set the best lap that this car had ever seen at Daytona. The previous owner was pleasantly surprised. None of us that we road raced with ever checked or worried about hot tire pressures. We would keep track of any adjustments, then note that adjustment for the next cold setup and record.

To your statement - that's the only time I ever tweaked to 1/2 psi. I always adjusted 2 psi if car is notably under or over-sterring, 1 psi to tweak. I generally only did that as front to rear bias - Daytona being the exception (with the feedback from Michelin, my greying brain cells did recall we had a side to side stagger there as well - higher on the right due to the banking).

Lacking a pyrometer, I use edge wear (i.e. rollover) to set the base pressures. Like you question - I wonder the same - since these tires are so short in sidewall, I struggle to find the right baseline visually. I think I'll be going up a little in the future or maybe find a pyrometer and do it right.

 

Well, these days you can buy a Laser Pyrometer from Harbor Freight in the USA for ~$20. Even if it's not absolutely accurate in its readings, you could still get a good idea of across-the-tread temps.

Additionally, your housecat will enjoy chasing the laser beam! (Ours does, LOL)

 

hmmm - I think I'll run down and get one! With the perennial 20% off coupon, it's be right at $21 or so. thanks!

 

Longacre 52-50642 -used by budget conscious folks at the track. Good accuracy and probe type. $130-145

 

There are way too many variables to simply state "start higher cold pressures" or "start lower cold pressures". There is a reason race teams spend millions of dollars in understanding their tires; it's just not simple. A lot also depends on the tire construction. Some tires like higher pressures for performance driving, some do not (which mainly comes down to sidewall construction).

However, take what the manufacturers tell you with a gain of salt. They are going to give you "safe" information unless you are a professional race team as they don't want a lawsuit on their hands if they give you information that puts you into a wall.