Tire pressures

[Pitcrew]

Can anyone give me idea on what they do with tire pressures on a tacky track? Thanks

[billetbirdcage]

For a hammer dowm track that would be in the normal range. Everyone varies some on tire pressure some in the slick and tacky.

I would say the normal range would be like:

LF 8 to 10
RF 11 to 13 --10 would be rare but some do.
LR 6 to 7 -- some may run 8
RR 8 to 12 -- I don't agree with the lower numbers but some do.

Probably the most common would be:

LF 9 RF 12 LR 6.5 RR 10 for an average track

I'm sure you get a wide aray of answers on this one.

[Pitcrew]

Thanks Billet! I have my philosophy about air pressures but explain to me yours on Tacky and Dry slick and on when and when not to. I'm trying to get it down and make the right moves accordingly when the car comes in. Along with everything else. Thanks again.

[billetbirdcage]

It will depend on the track hardness, meaning are the tires working "ON" the surface or "IN" the surface.

Generally when the track has traction the track is softer and the tire works off slightly digging in the surface as well as friction with the surface.

Generally when the track is slick you are just working "ON" the surface so it mostly just the friction of the tire on the surface.

Most LM tires are fairly compliant to the surface as compaired to alot of tires, so up to a point reducing the tire pressure will make the contact patch bigger. This will help when the tire is working on the surface. However reducing the pressure and increasing the contact patch may or maynot be the best thing when the tire works "IN" the surface. The slightly smaller contact patch (higher pressure) will have more PSI (pounds per square inch) on the surface which should dig into the surface most then a bigger contact patch that will have less PSI.

If the tire has a 1000# on it and has a contact patch of 20 square inches when at the higher pressure and 25 square inches of contact patch at the lower pressure. Then the following is this fictisious PSI on the surface:

Higher tire pressure: 50 PSI on the surface.
Lower tire pressure: 40 PSI on the surface.

Theory would state that the higher tire pressure would dig into the surface more then the lower one.

The is alot of varibles as tire roll and roll under, going under the pressure and loosing contact patch by running on the outer stiffer outer edges of the tire.

I'm clearly no expert on tires and don't claim to be, but that above makes sense to me as what could be happening. That above is just explaining how I came up with the following general rule that I go off off.

When the tires are working "IN" the surface I would generally raise the tire pressures 1 to 2 PSI.

You can also effect spring rates of the tires with tire pressure. I'm told that 1 PSI of tire pressure is roughtly 28# of spring rate in the tire. I'm sure that is a rough average. But in a jam can be used to adjust the car in a quick yellow or red flag that they don't allow much adjustments, so you can remove air to soften a corner of the car.

So adjusting pressure you are changing a few more things then you think. I don't work with open tires very often anymore and differnt constructions of tires I'm sure plays a big role and were and how some of those points are effected and to what degree.

I also concentrate more on the RR as it take the brunt of the load on a LM and too low and the tire rolls under or wads up and can make the car bounce going into the corner. If you can stand on the outside of the corner about half way to the middle and watch the RR tire you will see if the tire wads up and springs back which makes the car have a bounce to it. If it does that you generally need more tire pressure on the RR. If you have watched gocarts (solid suspensions-specially cage carts-look like sprints- top heavy) you will see this alot as the car hops into and thru the middle of the corner.

More air should also allow the tire to run cooler, again I'm not a tire expert so take it for what its worths.

[billetbirdcage]

I should add I know guys that win alot of races that don't hardly ever change tire presures and some do a pretty fair amount, so how much is really there is debatiable.

[Pitcrew]

Thanks again for the explanation Billet thats helpful!