I did the Irvine event over the weekend, which included significant amounts of canyon carving. I noticed that while the tires were warm (after driving normally for a few miles), I had a firm grip on tight curves.

However, when the tires got hotter (after a couple of miles of hard braking, accelerating and turning), I could feel the back slip a bit on each curve. It felt as though the car was rotating slightly into the turn; after a series of hard curves, it began to feel like I was on ice...there was never a firm, strong grip. This would go away after 30 or 60 seconds of easy road, until I hit the next tough sequence. I noticed this same effect at the track after a few hard, fast laps.

My understanding is that hot tires should have better grip, not less. I checked my cold tire pressures before and after the drive, and they were at ~34.5lbs all around. I have plenty of tread left on all 4 tires.

My first thought was that I was just drifting the car well through the curves, but the rotation and slipping kills confidence when you are looking at a 300ft drop at the edge of the road. Does anyone have any idea what was going on?

 

Hi Tanq, my understanding about tire pressure versus grip is the opposite of yours. Higher pressure means more responsiveness but lower ultimate grip. Seems to be a tradeoff between grip and crispness. Off-road 4x4's lower their pressures for extreme grip but lousy turn-in. If you push up into 45-50 PSI you'll definitely notice longer stopping distances.

You might have been getting excess temperature (pressure) rise but that couldn't get tooooo bad if you measured 34.5 psi before and after. So maybe you hit 40+ PSI and noticed the grip difference? Maybe your Torsen diff got hot and didn't work as well? To me the comment that you got better handling after 30-60 seconds of straight line driving goes against the over-heating suggestions because things don't cool off that fast.

 

Street tyres will give you about 4-5 good fast & hard laps, maybe 8 street miles of very hard driving before they become greasy and start sliding. It's quite normal. That's why there are race compound tyres which are slippery when cold but maintain their grip much longer when they are hot.

As for pressures, look at the side walls of the S02's. You'll see little arrows - about 1/4" right at the sidewall/ tread interface. Look at the tyre after you've driven it hard. You have the correct pressure when the tyre contact area is just touching the points of these arrows - if it hasn't touched the arrows there's too much hot pressure, too little and the arrows will be scrubbed.

 

Busaboy, Penforhire,

It sounds like my problem is a combination of the two things you mention (tires becoming greasy and higher air pressures). I normally run the car at 32-33lbs; bumping the pressure up to 35lbs was new, an experiment in improving the handling.

Thanks for the help,
Tanq

 

I don't agree that the pressures were too high or that the higher pressures reduce grip.

1) Every tire and car manufacturer states that you should use higher tire pressures for high speed driving. The tread and sidwalls squirm, even in a straight line, and builds heat. Higher pressures reduce the flexing and, thus, heat.

2) Higher pressures, within reasonable limits, don't reduce grip. Part of raising pressures is to stiffen the sidewalls to enhance turn-in as well as to prevent getting into the sidewalls in hard corners. The pressures we're talking about is generally only a few pounds (<10PSI) from stock and won't cause the tire to somehow become more rounded, reducing grip. The belts in modern tires, especially high-performance tires, don't allow the middle to become greatly distended like you might expect on an older and/or softer tire.

Heat is the killer of all things automotive. The various parts require that you stay within the design parameters or you risk affecting performance and even causing damage. Without actually trying a few things or participating in the event and driving the same way, I'd be hard-pressed to actually state that the pressures were too high or too low. It might be as simple as becoming more aggressive and finding the limits of adhesion more easily...

 

True, except one thing. Top speed, hot air is less dense and thus better for high speed runs...

 

[QUOTE]Originally posted by Penforhire
[B]Higher pressure means more responsiveness but lower ultimate grip.

 

Sorry PFB, but the real world is more complicated than what is taught to freshmen in college. That equation is only approximate even in the simplest situations, and car tires are a fairly complicated system.

Since I can reduce my grip by pumping up my tires, I'd say the equation doesn't completely describe what's going on...

 

Yeah, the friction equation is tempting but the rubber conforms to a rough road surface and creates substantially more apparent friction and this traction IS dependent on contact patch size.

Jeff, I agree with your statement of benefits of higher pressure but less roll-over does not mean higher traction. Most OEM tires don't get greasy due to heat alone, unlike most Auto-X tires. I doubt we would ever need to spray water on S02's between Auto-X runs. In my experience with them the-hotter-the-better as far as grip. I would also say, IMO, 45 PSI drives reasonably but 50 PSI is too loose.

 

.While, of course, the friction equation oversimplifies things, the "freshman" friction equation still holds true...

and basic physics CAN give us insight into how more complex systems work. Better than, say, your seat of the pants impressions