Tyre Pressures

[Ilikeearlgrey]

I'm a bit confused about this. I always thought that if it was cold you would up the tyre pressures to allow the tyres to heat up quicker & i know alot of other are doing it this way. But, i met a team that insist that it's the other way around, the theory being the more tyre in contact the more friction there is & thus more heat, which sort of makes sense.

Which is correct?

[Chris Y]

Er, both I think!

I'm no expert at tyre pressures, so I stand to be corrected on this, but I think that a higher cold pressure will get you to operating temperature and pressure quicker, simply because there's less degrees/PSI to gain. Conversely though, having a lower initial pressure will make the cacass and/or tread move around more, generating more heat, so that also gets the tyre up to operating pressure/temperature...

I'm sure there's a few other factors involved, but generally you want a higher initial pressure, otherwise with the lower pressure, you may end up generating too much heat too quickly and the tyres overheat.

That's what I thought anyway. Probably wrong.

[zefarelly]

our tyres go up between 4 & 6 PSI, I run 30/28 PSI in the dry and 32/30 in the wet

the idea being to help open up the treads a little with higher pressures, bt if it dries they do overheat. as we run vintage Dunlops and theyre crossplies I wouldnt have thought we're a very good example to use though.

incidentally . . .they've never gone higher than 35/36psi regardless of where you start

[MikeM]

With FF tyres I was told that you need to run 21 psi hot so you reduce by the amount needed for warm up. The hotter the day means you start lower as tyres will heat up more. On a normal day I would run 17/18 to get 21. Does this make sense? When wet, tyres don't heat up as much so you start higher but the higher pressure also gives a "crown" on the tread which breaks through water better. I hope this is right!

[topwelshman]

I've heard this as well, I have a mechanic mate that does it just that way but we've always run between 20-26 psi, 20 being for a very hot day upto 26 for a very cold day.

[Tim Falce]

Here's my tuppence worth.
When rubber moves around, the molecules inside get excited and generate heat. So I would go with the theory that having a lower pressure will get the tyres moving a bit and generate more heat rather than a higher pressure which would keep them more rigid.

[j'bar]

If you want to check your tyre pressures when the tyres are cold, the rule is : take the recommended pressure at 20°C, then add 0.1 bar for each increase of 10°C of outside temperature, and remove 10 for each decrease. For example : your tyres should be at 2.5 Bar (car manufacturer's spec), then if its 30°C outside, they should be set at 2.6 Bar, if it's 0°C they should be set at 2.3 Bar. But remember it's for a cold tyre.

[silver bullet]

I think a lot of it is trial and error to find out what works best for your own situation.
On a hot dry day we go out with 18psi after the race it can be up to 24-26. On a cold wet day we go out with 22ish and it normally stays at that.
I'm talking yoko's in rallycross by the way.

[THR]

to verify the pressure are right the only way u can do it is to check the temps at the end of the race... should be slightly hotter in the inside of the tyre. will check the camber and the pressure at once! but of course only at the end of the race.

after speaking to the nice spring people at the autosport show this year, they said, its important to understand that tyres grow and shrink during a race, and corner weights become greatly important. in a short 20 min race you usually set up a car to be corner weight perfect with say 22psi front and 20 psi rear
and you aim to get these pressures HOT while testing.
if you want the car to be better at the start you need to set the corner weights to allow for the offset, this may give u a better chance over the cars in front on the beginning laps... it all depends where u want the car to be perfect..

like in f1 at the mo ferrari are pefect the last half of the race. this could be down to the pressures and temps being dead right during the last half of the race.

you can only set the pressures right by looking at the tyre and taking the temps. ... im talking slicks here tho.
camber tow and pressure are all interlinked and most important!! you dnot even need driver input if u konw the pressure and temps after a session.

id say if u dont know wot pressure to run, spend £100 on a temp gauge and test um. youll find out then!

[haggispeed]

THR is spot on, I was never completely clear on tyre temps apart from taking the (tyre) manufacturers recommended operating pressure and aiming for that. This year have started using a pyrometer and have been slowly reducing pressures, started at 15/17 now down to 11/14 and we finally have even temps across the tyres, have also had to make camber changes as well. Now the procedure is to come straight in from practice, take pressures and temps (3 across each tyre) if necessary set the correct tyre pressure hot then you're done for the day, no guessing what they might go up from when cold. If you haven't got a pyrometer set the correct pressures immediately you finish practice to what they should be hot and leave them there.
I was recommended not to get one of these infrared pyrometers as they measure surface temp, which starts dropping as soon as you come in, get one with a needle that you can push in to the tread of the tyre and get real temps.

[THR]

u dont really want the temps even, you want a higher temp on the side of the tyre. only a bit tho!!
easy to adjust tho, take 0.25deg out the camber.
i think this is so that you always have the same part of the tyre touching the ground at all times so corner wieghts dont change when u load it up (if u know wot i mean i aint explained it very well).

checking tyre temps are a lot cheaper than buying a better engine!!

[Eric Falce]

I think the best way is to spend a few bob on a track day,go out and get the car to handle as you want it by changing tyre pressures.Avon said that the best pressure for my Escort saloon was 28 lbs at the end of the race,start with a lower pressure and see how close you can get,me, I'm going to do the track day thing,lot more fun and gives you more track time!!

[maddogf3]

Originally Posted by MikeM

With FF tyres I was told that you need to run 21 psi hot so you reduce by the amount needed for warm up. The hotter the day means you start lower as tyres will heat up more. On a normal day I would run 17/18 to get 21. Does this make sense? When wet, tyres don't heat up as much so you start higher but the higher pressure also gives a "crown" on the tread which breaks through water better. I hope this is right!

All absolutly correct . For FF we tested at Anglesey a while back one warm day . Found anything from 21 -- 24 worked but 25 made car handle strange ( drivers words not mine ) 24 actually gave fastest time at around 1. 05 .We usualy finished on the podium . In the wet you need them to balloon and go up to 25 -26 .all Fact. Avon recommend 22 hot .Work out cold pressure's yourself . Never saw anything over about 48-50 deg on temps ( in FF) where as slicks go up 60 deg . not got the guts to drop tyre press on F3 to 12lbs yet to see if theory works Most Race engineers go with a certain theory and stick to it Make sure temps are even across the tread. Proves Camber and set up correct .( New Tyres make all the difference as well )

[haggispeed]

Originally Posted by THR

u dont really want the temps even, you want a higher temp on the side of the tyre. only a bit tho!!
easy to adjust tho, take 0.25deg out the camber.
i think this is so that you always have the same part of the tyre touching the ground at all times so corner wieghts dont change when u load it up (if u know wot i mean i aint explained it very well).

checking tyre temps are a lot cheaper than buying a better engine!!

Not sure I agree THR - surely you want all the tyre in contact all the time ?, if you have higher temp on the inside then you are not using all the contact patch efficiently ?, how does this effect the corner weights ? - which of course are changing dynamically once you are driving in anything other than a straight line. My experience is only running crossply slicks not radials where you obviously need more camber to make them work properly

[maddogf3]

Mr Haggispeed if you dont mind me asking? What happened to lap times when you dropped pressure that low ? might save us a few wasted laps testing . I run wheels bolt upright sometimes with a small amount of possitive camber -- Yes I did say possitive camber (with crossply slicks )on the inside wheels . to get wear correct across the tyre contact patch Castle Combe Snetterton and the like which have some very quick r/h bends. optimise high speed bends to sacrifice low speed bends go down the next straight quicker , but everybody knows that. l Know of some who have run upto 0.4 positive it does work but look around the paddock how many cars do you see running pos camber Radials being a totaly different keetle of fish. Just read Skip Barber article He says " Driver who has most grip has tires at their optimum range,running them hard enough to create temp but not so hard that they overheat and go off " I believe that's opposite of what theory says -- run press lower tyres get hotter quicker . Think I will just keep taking the tablets and run around in circles until I disapear up my own Arse again

[maddogf3]

Forgot to say we use Nitrogen and have done so, for a few years now .Saves all the hassle with pumps and things and there is no way that you forget to take a ruddy big bottle with you . The other thing is tyres dont loose pressure over a few weeks and quickly drop to cold pressure settings from hot.They appear to be a lot more stable, the way they heat up and cool off as a bonus.
The only disadvantage is the bottle is heavy and you have to roll the airline up after you have used it

[THR]

haggis speed, why have the whole tyre touch the ground when you go in a straight line? you dont need it, when u corner the tyre deforms and then the whole lot touchs the ground

[maddogf3]

Originally Posted by THR

haggis speed, why have the whole tyre touch the ground when you go in a straight line? you dont need it, when u corner the tyre deforms and then the whole lot touchs the ground

This one could open up a can of worms -- Front dont have to touch rears do . Could write a book on suspension geometry ( Plenty Have ) No one has even mentioned Castor angle yet or why we dont use Ackerman angle or do use ackerman . I've tried both on same chassis and know the effects .That cost us £ 250 testing on a Blackdog day ( which I would recommend ) testing before a Race is a total waste of time most days you spend in the pits under a red Flag . Blackdog days are superb .

[goforit500]

Originally Posted by falcemob

Here's my tuppence worth.
When rubber moves around, the molecules inside get excited and generate heat. So I would go with the theory that having a lower pressure will get the tyres moving a bit and generate more heat rather than a higher pressure which would keep them more rigid.

Don't lots of things get excited when rubber moves around ??

K...Thats me - Back in me box :-)