Why do US sites recommend higher tyre pressures for track???

[gfreeman]

An anorak moment....

Have recently bought a pyrometer as another toy to play with and had a good search around the various sites to see how to use it properly. What I found amongst all the good info on the pyrometer was a large difference in tyre pressure recomendations between us and our cousins over the pond for the GT3/RS.... Generally the UK recommendations are around 30 - 32 psi hot and the US sites go generally for 34 - 36psi???

And quite a few US boys recommend a max of 40psi hot? Carumba! Even in Florida!?!

Has anyone actually done any proper research other than "Go for 32psi, everyone else does"??

Not only was this pressure difference apparent between US/UK, what I also learnt from the pyrometer info is that you are trying to get an even temp across the tread and heat build up is greatly influenced not only by ambient/track temp etc. but also your particular geometry and driving style (camber obviously the biggest influence).

In other words, all things being equal a different camber setting will need a different tyre pressure for a given day/car/driver combo.

In my capacity as a relative novice to trackdays I freely admit it is difficult to know to a finite degree if the last corner fk up was down to my patent lack of ability or the fact I had 0.00462psi too much air in the front offside tyre.....

[flemke]

gfreeman said:

An anorak moment.... Have recently bought a pyrometer as another toy to play with and had a good search around the various sites to see how to use it properly. What I found amongst all the good info on the pyrometer was a large difference in tyre pressure recomendations between us and our cousins over the pond for the GT3/RS.... Generally the UK recommendations are around 30 - 32 psi hot and the US sites go generally for 34 - 36psi??? And quite a few US boys recommend a max of 40psi hot? Carumba! Even in Florida!?! Has anyone actually done any proper research other than "Go for 32psi, everyone else does"?? Not only was this pressure difference apparent between US/UK, what I also learnt from the pyrometer info is that you are trying to get an even temp across the tread and heat build up is greatly influenced not only by ambient/track temp etc. but also your particular geometry and driving style (camber obviously the biggest influence). In other words, all things being equal a different camber setting will need a different tyre pressure for a given day/car/driver combo. In my capacity as a relative novice to trackdays I freely admit it is difficult to know to a finite degree if the last corner fk up was down to my patent lack of ability or the fact I had 0.00462psi too much air in the front offside tyre.....

Try 2.1/2.2 front and 2.4/2.5 rear HOT, and you'll be happy.

Be sure to remember to re-inflate to normal cold road pressures for the journey home.

[gfreeman]

With all due respect I was not after recomendations, I was just wondering why the yanks tend to prefer higher pressures than us and whether anyone had played around with a pyrometer....

[johnny senna]

Tim,

I too have invested in a pyrometer. I found that by running 31-32 PSI hot all round I was achieving equal tamps across the tyres, so therefore I was at the right pressure. If you achieve equal temps across the tyres all round, then you are using the right pressures, surely?

[gfreeman]

Thanks Doc, so it looks like the brits are right as usual

I was told to expect a little more heat on the inner than the outer edge because more of the running will be done in a straight line and on a cool down lap more cooling will be acheived on the outer edges by the time you get to do a reading.

The idea is to get three temp readings across the tread which should be an even spread with the middle temp as an average of the other two. If the centre reading is higher drop some air out - and vice versa. Problem is that you need to take the readings live, rather than after a cool down lap!

Next is to get a huge dial guage, for precise pressure readings. Every anorak should have one....

Andy T has just got one about 4" diameter.... I am looking to get one of those old forcourt guages about the size of a Porsche spare wheel.... Noice!

[Merritt]

Tim -

I use on of these :

http://tinyurl.com/fmzol

Nice and large, and pretty accurate. As for the Pyrometer - can I ask where you got it from, sounds like a gadget I need to play with

Steve

[willr]

gfreeman said:

With all due respect I was not after recomendations, I was just wondering why the yanks tend to prefer higher pressures than us and whether anyone had played around with a pyrometer....

I've noticed that, too. Strange - they don't seem to have heard of the concept of letting air out of their tires
Only reason I could think of is that perhaps many of the US guys do autocross, which isn't the same as normal track driving.

[gfreeman]

willr said:

I've noticed that, too. Strange - they don't seem to have heard of the concept of letting air out of their tires Only reason I could think of is that perhaps many of the US guys do autocross, which isn't the same as normal track driving.

Even the racers/slick users go for 36psi !! Wierd !

Steve - Andy T and me share a probe type one from eBay but it was a bit 'slow' so I also got a Draper NCT infra red one again from eBay (only paid £12.70!).

[fergus]

A good book by Don Alexander, 'High performance handling handbook' has lots of detail on car setup and includes a good section on use of pyrometers to check camber setup, etc. It recommends experimenting with camber so as to achieve roughly consistant temps accross the tyre, then the use of a camber guage to allow repeatable settings. The concept of a garage giving you a 'trackday' setup is only really accurate if your driving style is as per their reference source. Far better to use the information generated when you are actually driving your car to fine tune it, than treat XYZ garage settings as gospel as several people tend to do....

[GreenV8S]

Tyre pressures are complicated because they are a compromise between so many factors. The principle of getting the center of the tyre to the average of the inner/outer edges gets you to a good starting point, but it is only a starting point. (I'd expect to see the outer edge run hotter than the inner, but that's by the by.) Changing the tyre pressure varies the size and shape of the contact patch, and the amount of flex in the sidewall, and these changes can have a big effect on the level of grip and on the car's handling. For example if the tyre is running colder then there is less grip so the loads on the sidewall are less; it may be beneficial to lower the pressure on the contact patch by a couple of psi. It may be that the US settings are specified or a different climate where they expect a different operating temperature. Or maybe they actually have a different tyre compound in that market. Or maybe the US have invented their own 'inch' - hey if they can do it for gallons ...

[gfreeman]

I guess the reason most specialists offer a 'trackday' setup is that so few of us can truly know enough about the handling of a car to accurately define that for a particular track on a particular day for our particular driving style we need 1/4 deg more neg camber on the offside wheel and another 1/2 psi in the nearside rear etc. etc....

So the garages give a good compromise with a road/track setting that should see the 'average' trackday bloke going far better than with stock street settings.

Then you have the track only setting for the more committed, and further fine tuning for the more expert amongst us....

Lets face it we do not have the facilities, nor the time to carry out extensive testing at each trackday to establish the optimum settings for that day.... Not forgetting the cost as well!

I know Parr and Steve Rance spent a fair bit of time setting up the suspension package that Parr market, but even that must be for "General Track Use" and further fine tuning would be needed for a particular customer.

As far as I see it for myself for the time being using my car on road and track the 'No.2' geometry set up suits me fine, any fine tuning will therefore be limited to maximising the days contact patch using my nice new pyrometer, by fiddling with the pressures.

[Harris_I]

If the US guys are setting the pressure according to the requirements of their climate, surely one would expect more US venues to be warmer than UK venues? (In which case all the more reason to drop the cold tyre pressure).

Also willr mentioned autocross. I find autocross pressures are somewhere between normal road and full-on track driving (i.e. drop maybe 4psi out of front and back).

[gfreeman]

Harris_I said:

If the US guys are setting the pressure according to the requirements of their climate, surely one would expect more US venues to be warmer than UK venues? (In which case all the more reason to drop the cold tyre pressure). Also willr mentioned autocross. I find autocross pressures are somewhere between normal road and full-on track driving (i.e. drop maybe 4psi out of front and back).

You are dead right! The mystery deepens.....

[Merritt]

Could it be something to do with the type of tarmac? I know that in hotter countries they use a different compound of tarmac which resists breakup under temperature, maybe this has a knock on effect to the tyres requiring higher pressures?

Also - don't some circuits in the US still use concrete?

Steve

[Harris_I]

Merritt said:

Could it be something to do with the type of tarmac? I know that in hotter countries they use a different compound of tarmac which resists breakup under temperature, maybe this has a knock on effect to the tyres requiring higher pressures? Also - don't some circuits in the US still use concrete? Steve

I wouldn't say a more heat-resistant tarmac would be the reason. In Dubai, we have a searing heat 6 months of the year (and hence expensively laid roads), and I have all the more reason to drop pressures - in fact, I drop the pressure a couple of psi versus manufacturer recommendation just for normal road driving.

Interestingly, though, one of the more gifted drivers I know who races in the Gulf Radical Cup has experimented with raising pressures on a Ferrari 360 to negate the effect of sand blowing across the Dubai Autodrome. Apparently it works, and he swore blind I should try it on my 911! (Not that I did - it sounds completely crazy though the guy does know what he's talking about).

[UK952]

Because they cant drive?

[andy tims]

UK952 said:

Because they cant drive?

Or because they are so huge that lower tyre pressures would see the side walls completely crushed.

More seriously, they are either on different tyre types, or really not pushing that hard. Just imagine driving a GT3 hard with 40PSI in the rear tyres

[ripton]

UK952 said:

Because they cant drive?

Or is it because the strains on tyres driving round in circles is completely different to those imposed by a combination of left and right corners

[gfreeman]

yeah, very funny boys but you may be surprised to learn there is a hardcore of good ole' boys over the pond who's fanaticism in things Pork makes us look like a bunch of garage queens.....

There are threads on lightening their cars which are mind blowing how far they will go and how much they will spend...

Best one was removal of the gun rack and replacement with a lightweight handgun complete with lightweight steel slugs instead of lead.

Seriously there doesn't appear to be a logical difference. I shall post on Rennlist and see what answers pop out

[UK952]

Spend far too long on rennlist myself - long enough to know they often describe what we call a dragstrip as a racetrack. Maybe when they go to a real track with corners they raise the pressure from the dragstrip settings