Tire pressure and their affects on handling

[sinned]

I'm putting this here vs. in the tire topics as it pertains to handling and suspension tuning more than how to fit 20's under a first gen.

Being bored with nothing else to do I was watching a few laps of the Nextel race in Texas. Tire pressure and the effects it has on corner entry/exit came up during commentary and they described that Edwards crew was going to take out .25lbs (not a typo, that is 1/4 lbs) of air from the R/R tire. Now my first thought was that I don't even recall having ever seen an air pressure gauge that would measure down that low, hell mine all have a precision tolerance of +/- 2 lbs.

How many guys actually go through the trouble of setting air pressure "exactly" and monitoring it throughout the day? I check it in the AM and maybe again before going home but I don't think I have ever seen anybody moving it around (some will take temps and record them for future tuning but not making adjustments at the track). Does anyone even a device capable of measuring pressures that low?

I also note that a lot of guys take temps after getting back to the paddock; I wonder how much heat is lost during the drive from the track back to the paddock. Obviously bringing a pit crew to the track for real time temps right at the exit road would be ideal but not many of us have that luxury (not to mention most track officials would get pesisd if guys were jumping the walls to record tire temps).

Obviously tire temps and pressures don’t mean a whole lot if your set-up is junk to begin with but to the guys who are pretty dialed in, how much of an impact does tire pressure tend to make toward the overall feel of the car?

[TitoJones]

I plan on filling my Pilot Cup R compound tires with Nitrogen vs air so the tire pressure will stay consistant no matter what the temps are.

Tyler

[element180]

I don't know about all that but I do know a guy that autocrossed a C4 and he used a length of hose connected to the valve stems of 2 tires to equalized the pressure right to left

[Damn True]

The use of nitrogen in your tires will make your pressures more stable but not completely stable.

The primary reason N2 is used is to eliminate water and water vapor from the tire system. Changes in temp will cause water to condense out inside the tire and the tire is more resistant to changes in external air density.

N2 because it is virtually dry will be less affected by changes in temp than "air". However, the combined gas law (Boyles+Charles+Lussac) still applies. If you heat it, it will excite the molecules and pressure will rise etc etc. However, the rate at which it does will be far more predictable than with random mixtures of gasses and water vapor found in "free air".


Some other benefits:
Nitrogen permeates through the tire 3 to 4 times slower than oxygen - tires maintain pressure longer for better fuel economy, better tire life, and increased safety.
Nitrogen reduces oxidation to tire rubber and rims caused by oxygen in standard compressed air.
Nitrogen filled tires run about 20% cooler than tires filled with standard compressed air - tires last longer.

[element180]

You might want to loot at this if you're looking for an accurate guage

http://www.longacreracing.com/catalog/item.asp?id=210&catid=8


1.5 - 100 psi - Display reads in 1/10th psi increments

[sinned]

That would be why I have never seen a gauge with that type of accuracy. :eek: 300.00 for a damn tire gauge.

Not to derail this too much already but the use of Nitrogen is a common practice and its benefits are fairly well known. I'm more curious about how much minute changes in pressure affect feel and if anyone here has a chassis set-up in line enough to notice the differences 1 or 2 pounds in a tire would make.

Typo corrected

[Mike Holleman]

299.50 is three hundred dollars.
Ouch!

[chicane67]

Well, from my experience and various types of racing..... it has had quite an effect on how the chassis works..... at the tire level alone.

The pressure set points change the amount of supported load at each corner. So in effect, you can change the spring rate per corner by small (near infinite) amounts. Afterall, the side wall of the tire is part of the suspension.

Per-sey with my truck experience and using lower pressures, it is looked at how stiff the side wall is from the pressure set point. Its pretty much running as low of a pressure you can without peeling the sidewall off of the bead of the rim to increase traction in off pavement usage. This also effects the damper speed which also effects the shock valving.

From my road experience (meaning non-left hand turn only), it was really for tuning a tight or loose condition..... but it has more of an effect in 'left hand turn only' applications due to also having an effect on stagger thrown into the equation. Even a quarter PSIG, changes the stagger in a tire diameter measurement..... which changes a whole bunch of things like rate, turn-in and under/oversteer.

Temperature and pressure do allow a good indication of the available tire contact patch to plant the suspension. In the AutoX arena, I'd be adjusting the pressures until the tire 'read' the way I felt was being utilized the best for the conditions. This is even more relevent in a open course because you are still looking at getting the traction across the entire tread width but also trying to maximize tire life. The thing is, its best to take the temperature right off of the track, as it does shed enough heat to not really give you the true indication of 'running' temperature.

I think the most scrutiny I ever put into it was maybe half a pound.... only due to that I didnt have a digital pressure guage that would go into a range under that. Since then, they now have both analog and digi guages that will do 1/10th psi measurements.

On my truck, I run 28F and 24-25 rear. Ride and tire wear are solid.

On the 67 with the old tire/wheel set-up I ran 34F and 26-28 rear just to set it up for a loose differential and wanting very light initial understeer with light throttlesteer. It was pretty balanced and neutral.

As an FYI, I too have been running Nitrogen in my street tires for about 16+ years.

TRUE's comments on the water vapor density (and its acidic nature), oxidation and reduction in operating temperatures, have been my main reasons for it use..... and that the pit gun just happends to be right there and uses the same stuff.

[shmoov69]

I helped a buddy with his late model asphalt stock car a few years ago and we had to get the pressures on the money and did experiment with it quite a bit. We took temps from 4 points across the tire after laps and such. In roundy round, it can make a huge difference, dunno about anything else though.

[Stu Seitz]

I plan on filling my Pilot Cup R compound tires with Nitrogen vs air so the tire pressure will stay consistant no matter what the temps are.

Tyler

Yay for Coscto!

[LS1_78Bandit]

I'm curious how argon would compare to using N2 to the fill the tires. I have easy access to argon (I use it for scuba diving), but not N2 ...

[Lowend]

I check and adjust tire pressures on every single run when auto-xing
3 lbs can make a huge difference in understeer vs oversteer

[David Pozzi]

Pressure also affects the tires spring rate and contact patch size in addition to getting even heat across the tread. The Nextel Cup tires are radials which are not supposed to vary temps across the tread with as much sensitivity to pressure as bias ply race tires.

But radial race tires are not exactly radials.

I've not played a lot with pressures for handling balance. I tend to set pressures for even temps across the tread, then tune balance with antiroll bars. For autocross I have upped pressures F&R for courses that have very tight or tough turns, then for easier more flowing courses I've lowered pressures on courses where you don't throw the car around, -to get a bit longer contact patch and hopefully get a little more traction.
On a course with severe corners where the car is thrown in and braked hard like a decreasing radius turn at the end of a straight, higher pressures help keep the tire from rolling over on the sidewall.