tyre pressures vs tyre temps

[ozracer]

Is there a correlation between temps and pressures. ie will starting with a low pressure result in a higher or lower tyre temperature? I would have thought a higher pressure would result in a higher temp, but then i was thinking that with a lower pressure there is increased sidewall flex which generates more heat.

Another question is for sedan circuit racing, do yuo pump the tyres up or let them down in the wet? I know the go-karters pump them up to make the tyres stand up, but some people have told me that you let them down in the wet as you get more grip and the sidewalls work harder allowing the tyre to get up to its operating temperature.

[Mackmot]

You use the temperatures and camber to get a good temperature distribution. Ideally the tyre should be even temperatures accross which means that you are using the tyre evenly. if you find the middle temperature down a few degrees you increase the pressure if its too high you reduce it. If the inside is too cool then you increase the negative camber etc. By a engineer to win by carroll smith it'll explain it better

[Lemans B]

Higher presure result in a higher tyre temperature .(stiffer springs to!) The temperature generated by sidewall flex is less importend and heats the tyres sidewalls, not the tyre patches on the road surface.
For your second question , yes and no.
If I have really soft wet tyres, I pump them up and gain heat at the normal way, at the tyre patches.Now it is possible to race in the wet with stiff springs.
If I have not soft wet tyres, I let them down to gain a little heat from the sidewalls . More importend is that with lower pressure in the tyres the stiff springs are no longer "un-driveble"

[ozracer]

We have to use to same tyres for both wet and dry, Bridgestone RE540S.

[zefarelly]

same for the CR65 Dunlops for historic saloons, I've overheard talk of upping pressures in the wet to open the tread . . . . .in short theyre **** tyres (by todays standards) wet or dry !

[avsfan733]

you can use the tire pressure to effect the tire temperature by having it change the shape of the tire, but its not the best way to do it unless your out of time

[laser2]

Originally posted by Mackmot
You use the temperatures and camber to get a good temperature distribution. Ideally the tyre should be even temperatures accross which means that you are using the tyre evenly. if you find the middle temperature down a few degrees you increase the pressure if its too high you reduce it. If the inside is too cool then you increase the negative camber etc. By a engineer to win by carroll smith it'll explain it better

what page does carrol smith say this in Engineer to win??????

[graham bahr]

currently you are juggling with 3 variables, tyre temp, tyre pressure, and the change in pressure caused by the change temperature of the air inside the tyre, if you inflate the tyres with nitrogen you will find the pressure wont vary with temp, therefore you only have 2 variables to consider and life gets a bit easier.

[Revracing]

Sorry graham I beg to differ, pressure, temp and volume of the gas within the tyre are all connected, since the pressure and volume can be considered fixed once set then the only variable is temperature (this directly affects pressure etc etc). The pressure within the tyre will rise a given amount for a quantity of heat input, which means if you can limit the quantity of heat that the gas within the tyre can absorb then you can limit the pressure increase and hence gain greater control over tyre performance and consistancy.

Since air is approx. 73% nitrogen the heat absorbtion rate will be no different with pure nitrogen, Right idea but wrong gas. I beleive Helium has the lowest heat absorbtion rate.

[Adam43]

I thought the main reason for using N was to reduce the other gases in the tyre, especially water and CO2. However it doesn't eliminate the fact that the temperature and the pressure are always proportional.

There are separate and related problems here. Al of which have been mentioned above.

Firstly as the temperature of the tyre increases so will the temperature. PV=nRT (this is the ideal gas equation, but apart from a small deviation it works).

The temperature of the tyre is effected how it is used. It increases as it is driven. How it increases is effected by the shape of the tyre. However the pressure of the tyre effects the shape and the contact area! Complex? Yes.

Therefore a general knowledge of what is going on is needed, however in practice the fundamental theory is not that helpful.

One way of checking is seeing if your tyres are working well is checking the temperature difference across them. Making sure that the inner, centre and outer parts are working as much as each other. Having the same across the whole tyre is what a lot of people think is best, but some like a slight difference toward on edge. The pressure and camber required to achieve this will change for each circuit depending on how much straight there is to corner!

There is no wonder that tyres are described as a black art.

For all this talk we run the same pressure everywhere (we don't get a lot of practice to try anything! We have settled on something that is thereabouts). We run slightly lower pressures if it is hot! We might change if we feel the tyres going 'off' towards the end of practice, but track conditions change through the day too!

Experience is the key and it is hard to get quickly!

[Revracing]

I agree with what your saying Adam, That is exactly what we used to do when I raced Karts, However we never managed to correlate an even tyre temp accross the tread to better tyre performance. I suspect that was due to not having any suspension and hence not having the entire contact patch in contact during cornering.

My point was that in limiting the pressure increase of the gas you can produce a more stable tyre condition that is less likely to go "off".

The principle i am discussing is that the pressure within the tyre will only increase if the gas within the tyre becomes hot, not the rubber. It is possible to have a comparatively hot tyre surface with a much cooler gas and hence limit the pressure change and avoid the more heat = more pressure = more heat problem.

[Revracing]

Made some corrections but was out of time, went on to disscuss specific heat of gasses and such but can't be bothered to write it all out again.....

[Mackmot]

Carroll Smith books in general will give people with no idea how their car is working a better idea of how to get the most from it.

On the principles of gases discussion Adam is right about why we use nitrogen. Id like to add that water vapour that you get from ordinary compressors expands 19 times when it turns to steam.

[laser2]

ok...but carol smith doesn't advocate the use of a pyro at all. From what I remember, he prefers to "read" the tyre and let the stop watch tell the story, thats why I was asking. He doesn't like pyros and I don't agree with this reading the tyre temp across the tread because you have one other factor which affects the reading, which is carcass construction. Tread foot prints can distort under load and give a really poor reading across the tread. Changing the tyre pressures doesn't fix a construction problem.

I also have found that the pressure increase for 'air' is about twice that for 'nitrogen' filled tyres i.e. air filled will increase by say 12 psi while the nitrogen filled tyre will increase by 6 psi.

[PeterMorley]

Originally posted by zefarelly
same for the CR65 Dunlops for historic saloons, I've overheard talk of upping pressures in the wet to open the tread . . . . .in short theyre **** tyres (by todays standards) wet or dry !

I've heard the same - seems that it depends where you are from:

American practice is to up the pressure, in the wet, to open out the tread.

European practice is to lower the pressure, in the wet, to get the temperature up.

Having noticed that Americans tend to avoid racing in the wet whenever possible, I would think twice about taking their advice on wet racing.

Lowering the pressure ups the temperature due to side wall flexing. But you are losing the temperature increase you would gain from increasing the pressure (as governed by PV/T = constant gas equation).

The Temperature in question is absolute (e.g. Centigrade plus 273) so you need a fairly large change in pressure to make a large change in temperature, assuming the volume is constant. Which suggests that flexure is more significant than pressure.

Back to the point:

We've found that the simplest solution is to adopt the mid point of both theories - e.g. run with the same pressure in wet & dry.

Just set the pressure so that you get an even contact patch (e.g. maximum contact/grip) across the tyre (where possible, some things like Bugatti suspension geometry makes that difficult!), and keep to that pressure wet or dry.

The advanced version of that is to check the pressures when the tyres are hot and to stick to that one - there will be a tiny variation due to ambient temperature!

In the end with historic racing it doesn't make a lot of difference - changing driver will have a far greater affect on speed than the tyre pressure!

I thought the main reason for using N was to reduce the other gases in the tyre, especially water and CO2. However it doesn't eliminate the fact that the temperature and the pressure are always proportional.

I was told that Nitrogen had less water (none hopefully) and it is in fact the expansion/contraction of the water (moisture) in air that causes tyre pressure to vary with temperature (e.g. the volume increases but since that is fixed the pressure goes up).

The expansion of air or Nitrogen themselves is relatively insignificant in the temperature range we are talking about.

My local tyre place have started offering Nitrogen for road car tyres, should avoid the need to adjust the pressure of your tyres between winter & summer.

[Red Dog]

Actually, all the drivers I race with, up the pressures for wet tyres and they are really fast in the rain. I find the wet difficult but since getting advice and raising the pressures I am going a lot quicker. You do need more compliance which is done by disconnecting or softening the anti roll bars and damper settings.

[Mackmot]

The most basic things to do in the wet are to up the pressure and disconnect/soften the rear bar.

[NCharman]

Personally I use low pressures on a damp track and progressively more pressure if the track us wetter. On a flooded track the tyre needs more ability to "cut through" the water. On a damp track it needs more surface area on the track to reduce overheating.

Sent from my SM-N9005 using Tapatalk

[MGDavid]

does anyone know what the outright record for thread revival is?
12 years 7 months must be getting close

[knighty]

Heres my 2 pence worth on my pet subject.....tyre pressure stability!

Yes, the principle reason that people use Nitrogen is because its free of moisture, the actual gas composition is a secondary issue. When the air temp gets to about 60DegC inside the moisture releases and turns to steam and has the effect of ballooning the tyres.....to use Nitrogen effectively you need to triple fill the tyres and purge out the air and moisture.......un-screw the valves core, fill 3 times with Nitrogen, then as it deflates at the end of the 3rd fill, screw the valve core back in and fill back with Nitrogen. This is whats done in professional motorsport like BTCC, F3 and GT racing, we certainly did it in the BTCC back in 1999-2003 and the car was on the podium and punching above its weight and won class A & B championships........ people also use "dry air" for the reason that its free from moisture.......they both work

I generally find the struggle with tyre temperature balooning is on a warm to hot day (to state the obvious!), I always carry a laser temp gauge in my pocket and measure the floor temp like a hawk, for qualifying it can be 15-20DegC, then my mid-day it can easily be 45DegC in the floor.

no matter what tyre-gas you are using, best advice is get your kart/car on the warm tarmac thats been in the sun-light all day long for about 5-10 minutes before you go on track, and adjust the pressures when you are on the forming-up grid, right up to the last moment that the karts/cars are released on track.......then you will see a lot less balooning during the race and you will be consistently fast......dont be surprised to see the pressures jump up 2-3 psi from when you take the kart/car from the cool shaded garage/awning, onto the warm forming up area.......watch the pressures like a hawk on the forming up area, I also roll the kart forwards to get the other side of the tyre warmed up and stabilized all around the circumference of the tyre......it works for me.

Also, when you set pressures, set them twice, do 2 loops around your kart/car......on the 2nd adjustment I always find our kart tyres have magically increased by 1psi to when I set them 1 minute before.....whereby the tyres appear to relax a bit about 30 seconds after a big deflate down to the required pressure......we are working in the region of 10 to 30psi if that helps

Cheers B-)

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