I know in theory thats what adding air does however when I used to race karts which have about 6 inch wide tires running 10-15 psi, you could almost see the difference betwen 2-3 psi by looking at the tire, to me, it seems that the amount our tires will distort from a couple psi, especially once at 35psi+ would be minimal if anything. Pressures in tires like ours have more to do with firmness, and handling characteristics associated with keeping tempreatures constant across the tire. If you can find me somewhere online that states or quantifies the relationship between tire pressure and contact patch for car tires, I'd love to check it out. You might want to reference the websites I attached below...

http://www.longacreracing.com/articles/art.asp?ARTID=33

http://www.turnfast.com/tech_handlin...ingtable.shtml


Scorke

 

thats driving, or hpde 101.

taught in every event, every club i've ever run with.

to say it "decreases" contact patch is misleading though, it actually moves it to the other tires.

to quote the link you just listed..

"Foot Print

More air reduces the size of the contact patch and less air increases the size of the contact patch. You can consider contact patch when making adjustments. Lower air pressure puts more rubber on the ground and that corner will have more grip. Too much of a good thing can cause problems but you can think about how more or less contact patch, due to air pressure, can help to adjust the car.
"

 

To say what decreases contact patch is misleading, what moves what to other tires?

Oh, and nice selective reading/quoting, are you aware that the bulk of the tires we use, are radial?

"Radial Tires

Radial tires react differently to air changes as compared to bias ply. Spring rate is more noticeably affected by air pressure changes. Air adjustments affect the contact patch changing the grip characteristics in a dramatic fashion. Radial tires do not grow with tire pressure. The steel belts are rigid and size consistency is built in. "


Scorke

 

sorry for the but in....every one who I have told to run 39/40 front and 41-43 rear (depending on how "loose" you like it) has thanked me for a car that was more predictable under braking and long roll that sidewall under sweepers...so thery be Damnneed...TOYOS engineers say run 40 MINIMUM....ymmv...

 

hey, you asked for a link to a specific text, and i opened your link, scrolled to the right section and BAM, it was what you were asking for

robi, i didn't get that part about the 41-43 rear, must have been a phone issue. I didn't see my understeer being helped a bit by just jacking up the fronts


Do you understand why they say that radials are different?

This all goes back to your comment of "the tire doesn't know what it's attached to"

==============

This is all in a theorecial statement, and after thinking about it overnight, it can't be 100% correct, but it's gotta be close.

the easy way to think about it is say you have 80 sq inches of contact area. (don't have a clue what we really have, but it's a variable. that makes 20 per tire @ 40 psi. (saying the car is 50/50, weighs 3200 lbs)

40 psi says for every 40 pounds of pressure, that tire needs to deflect or bend, touch the road for one square inch.

so 800 lbs on 40 psi = 20 square inches of contact area per corner.

Say you jack that up to 50 psi, now you have 16 square inches, (800/ 50 = 16) and all the other wheels still have 20. That tire now has 80% of the contact patch, and resonably, traction, that the others do.


But now we're saying the belts cause the tire to not flex as much, so that intoduces a variable of "we don't know what." We obviously know the math of it works, because if you dump the tire down to 4 lbs, now you have 200 square inches of contact patch (again, belts won't let it flex), but we all know that as a FLAT.

If this is online in some other, more proper form, i'd love to read it. Having never had to explain something that made sense, and was a given to me, i was at a complete loss for terms.

edit: the only variable, if all of that works out, that i can't figure out, as i have always been told the contact patch doesn't change. However, i guess i don't understand how air pressure could transfer weight around on the car. alas, who knows, was just a thought, even if it doesn't pan out

 

But that's just it. At 4psi (even 0psi) you don't have 200sq.in. of contact patch, though at that point you're resting on the rims! Ever notice how many of the "laity" drive around on 10psi and never notice it? They sure don't have 3 times the contact patch they would have at 30psi. Pressure does have some effect on contact patch but it also affects spring rate, internal damping, rollover, pneumatic trail, etc.

Dave

 

see the above comment about the steel belting not letting a tire flex to the fullest.

Again, i'm not claiming my thoughts as perfect, but it's just a given, you're told something, you see it in action, you believe it. You don't expect to have to explain it.

This is all again, in response to the idea that tire pressure doesn't effect suspension, or contact patch.