the ongoing tire pressure question

[Mondo]

Well it's usually blazing hot around now but we have some overcast today so went out and had a great time in the foothills this morning. There's this one section that has over a dozen 2nd/3rd gear left-right turns that all about the same amount of degrees and are one after the other with no straight between them... great for trying to go a bit faster each turn. I've noticed that the front sticks like glue and the rear is the first slip, which I assume is a good thing. But I can't help thinking I could pull more Gs if the back was tuned in a bit more.

 

The car is corner balanced with right at 50/50 front to back and only a few pounds diference side to side. Rear alignment has slight negative camber (can't remember off hand) and tire wear shows all across the tire (more than the front actually).

I have 20lbs tire pressure all around in my 1450lbs car (plus my 200lbs). Even with the 275 width, the rear tires seem a bit loose...

Don't get me wrong, the thing pulls some good Gs... maybe I'm just getting used to it and need to work more on smoothness and throttle control. But I think I'm getting a pretty good feel for the car lately. From my sportbike days, I know tire pressure can make a huge difference.

 

My question is: with tire wear showing all across the tread pattern, should I still try lowering the pressure a pound or two? Will that be ok out in the foothills as I'd hate to roll the tire off the rim?

Edited August 22, 2009 by Mondo

[WestTexasS2K]

I know a few folks that have run down to 15lbs on the race track. You are going to have a hard time rolling it off the bead unless you get down to 5 psi or so.

[Mondo]

Thanks Loren,

I'll try dropping the pressure a pound or two next time out.... really having fun with the car now:)

[Davemk1]

Thanks Loren,

I'll try dropping the pressure a pound or two next time out.... really having fun with the car now:)

 

Street tires, unlike a road race slick, don't change foot print size (and therefore traction) that much with small changes in air pressure. The thick belts under the tread have more to do with your contact patch size than the pressure does. Sure if you go way up or down you'll get the contact patch size to change but a few pounds won't make much difference.

 

What will change a good bit with air pressure is how stiff the sidewall of the tire is. At low pressures the tire will tend to roll over during hard cornering and you will reduce your momentary contact patch. If you increase air pressure you will stiffen the sidewall and keep more of the thread on the ground and have better traction.

 

There is another thing that increasing the tire pressure does and that is reduce the slip angle. This will help the tire roll in the direction it's pointed. It's too nice out to write a bunch about slip angle but I'll bet if you google it you get more than you want.

 

If it were me I'd bump the rear pressures up some. The car might feel a bit more edgey and have a more narrow window but I'll bet it hooks up better.

 

This is only for DOT tires. Once you move to slicks you can play a good bit more with pressures and contact patch size vs sidewall stiffness.

 

Dave

[WestTexasS2K]

Street tires, unlike a road race slick, don't change foot print size (and therefore traction) that much with small changes in air pressure. The thick belts under the tread have more to do with your contact patch size than the pressure does. Sure if you go way up or down you'll get the contact patch size to change but a few pounds won't make much difference.

 

What will change a good bit with air pressure is how stiff the sidewall of the tire is. At low pressures the tire will tend to roll over during hard cornering and you will reduce your momentary contact patch. If you increase air pressure you will stiffen the sidewall and keep more of the thread on the ground and have better traction.

 

There is another thing that increasing the tire pressure does and that is reduce the slip angle. This will help the tire roll in the direction it's pointed. It's too nice out to write a bunch about slip angle but I'll bet if you google it you get more than you want.

 

If it were me I'd bump the rear pressures up some. The car might feel a bit more edgey and have a more narrow window but I'll bet it hooks up better.

 

I dont know that I agree with you. I have experimented alot with tire pressures on street tires and 1-2 pounds makes a huge difference in the way the car handles. Example when I ran my front tires at 21 psi I had severe understeer issues. Drop the pressures to 18 psi and it turned like it was on rails. Same track same temps same corners. I shaved 2 seconds off lap times with the lower pressures. I had the same experience on the street. I suppose this can change from one manufacturer to another. I think Monda and I are using the same tires.

[Davemk1]

Results never lie so if it works go with it. I suspect you might be seeing that the pressure you were running was too high for the amount of camber/caster in your set up. Run lots of camber and high pressure and you'll ride around on the inside edges and see big push. The if you lower the pressures you will soften the side walls and allow the tire to distort to make a larger/flatter contact patch.

 

I think you are right in that different tires react differently........... and suspension set ups will favor different pressures. Experiment and find out what works best for your tire, your car and your driving style.

 

Dave

 

 

 

I dont know that I agree with you. I have experimented alot with tire pressures on street tires and 1-2 pounds makes a huge difference in the way the car handles. Example when I ran my front tires at 21 psi I had severe understeer issues. Drop the pressures to 18 psi and it turned like it was on rails. Same track same temps same corners. I shaved 2 seconds off lap times with the lower pressures. I had the same experience on the street. I suppose this can change from one manufacturer to another. I think Monda and I are using the same tires.

[slngsht]

Well it's usually blazing hot around now but we have some overcast today so went out and had a great time in the foothills this morning. There's this one section that has over a dozen 2nd/3rd gear left-right turns that all about the same amount of degrees and are one after the other with no straight between them... great for trying to go a bit faster each turn. I've noticed that the front sticks like glue and the rear is the first slip, which I assume is a good thing. But I can't help thinking I could pull more Gs if the back was tuned in a bit more.

 

The car is corner balanced with right at 50/50 front to back and only a few pounds diference side to side. Rear alignment has slight negative camber (can't remember off hand) and tire wear shows all across the tire (more than the front actually).

I have 20lbs tire pressure all around in my 1450lbs car (plus my 200lbs). Even with the 275 width, the rear tires seem a bit loose...

Don't get me wrong, the thing pulls some good Gs... maybe I'm just getting used to it and need to work more on smoothness and throttle control. But I think I'm getting a pretty good feel for the car lately. From my sportbike days, I know tire pressure can make a huge difference.

 

My question is: with tire wear showing all across the tread pattern, should I still try lowering the pressure a pound or two? Will that be ok out in the foothills as I'd hate to roll the tire off the rim?

 

Not answering your question here, but I think the car will be safer to drive if it has a SLIGHT understeer at the limit in corners. You can get rid of the understeer by either slightly lifting off or slightly accelerating to control the car. If the car oversteers when driven through a corner, it'll be a handful as it'll be difficult to change its attitude with the throttle, unless you have a stiff limited slip unit which could make the car push when torque is applied.

[powderbrake]

I am not sure what tires you are running, but on my Ultralite, I have Falken Azenis'. I run about 20 psi on the street, as I want sufficient pressure to protect against road hazards and potholes.

My car acts like yours when pushed hard, i.e., a bit of sliding around of the rear, and the front always sticking as it should.

 

At my first track day event, I had the tires at 20 psi and the rear was too loose. On Loren's advice, I lowered all the tires to about 16, then went down to 15 , then to 14 psi. It made a big difference in control. Of course, this was on a track day lappping session, so the tires were hot and sticky. I now drop the pressure to 14 when I want to stick.

 

Bear in mind the Falkens are sticky tires, particularly when they are warm, and that typical blats don't really get them hot enough. Also, depending on how old the tires are, and how much heat cycling they have received, they tend to get harder with age.

[catracer]

Mondo, these cars can feel a bit loose for a several reasons. But little recognized is the fact that we are just about sitting over the real wheels, so any small displacement "feels" like it's very significant, where, in fact, it may be little. The beauty of these cars is that they really "telegraph" whats happening in the tires. Good thing, but a bit hard to get used to coming from the typical 3000# + daily driver/grocerygetter/commuter type of vehicle. You mentioned you are feeling better about the car, that's seat time.

 

As others, it's really hard to predict optimum tire pressures because of all the different comounds, constructions, sizes, corner loads, suspension setups, dynamic weight transfer, etc. etc. Just keep at it and you will find a point where you and the car kinda "meld" togther. Keep accurate notes so you can always come back to some baseline. When you change tire types, it will be a bit easier the next time.

 

Another thing to explore is the rake angle of the laden chassis. If the rear is too high, you may gain significant benefit from lowering rear ride height and lessening lateral weight transfer in cornering. I'd recommend no more that about 1/4" overall rake from front to back measured at the flat portions of the frame front and rear. Not sure what rear suspension you have, but if you change, you may have to realign, but if you are high now, it will be well worth it.

 

Corner weights can have a large effect also. Since you, as driver, represent say 10-15% of the total package, and you need to sit off the midline of the chassis, you need to set up corners maybe a bit different that recommended for a lot of other race cars. Formula car set up really doesn't apply here because of driver position, nor does, say, set up for a large roadracing sedan because of the huge weight differential. Two choices: set up anticipating a passenger, or set up for driver only. I usually recommend that we set up a car for driver only since yuo would expect a bit more driver restraint with screaming, terrified passenger along.... maybe not ...

 

Anyway, I digress. If you need more specifics on corner weight PM me so I don't bore the good people here any more that I have.

 

Chip

[Mondo]

Thanks guys.

 

I have Falken rt615 tires, though a bit wider than normal (185 front & 275 rear), so I think I'll lower the pressure a bit on the rear as our roads aren't too bad. Also agree that the senses are heightened being so close to the rear tires and the backroads aren't the best place to be testing the limits of traction anyway.

 

The cornerweights are based with me in the car and it's got a pretty low ride height, actually raised the car a bit after scraping my oilpan:( I'm way to inexperienced to play with spring rates and imagine Brian (WCM owner) knows much more than me. So, with all that, adjusting tire pressure should allow me to play around without getting things screwed up too much;)

 

Nice to know I'm not off base, thanks again