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Road racing tire pressures for an FD?

Old 03-30-03, 06:41 PM
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Road racing tire pressures for an FD?

Anybody got good ideas on tire pressures for an FD for Road Racing. I'm running stock rims/sizes with Bridgestone RE 730's.

I've heard a variety of opinions on this. Some guys say that a good rule of thumb is that the difference between the cold and hot temp should jump about 6 degrees.

Based on that I've started at 32 and it does jump to about 38 when hot.

Any ideas?

Thanks,

David
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Old 03-30-03, 07:42 PM
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Nobody can tell you what pressures to run, you learn from experience. Things like weather, track conditions, setup, etc. determine what pressure you should run. At 32, did it take a long time to heat the tires up or did they heat up too quick? Once they heated up, did they grip too little or too much? What was the air temp when you ran them at 32?

I generally start pressures at 32, and go up or down as conditions permit.
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Old 03-30-03, 09:02 PM
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well, you can try to do it subjectively by how the car feels with different pressures.....but truthfully that just doesn't work very well. you really need to use a tire pyrometer to do it right.

there's lots of books and sources on how to do this.....but briefly you want your tire temp to be the same all across the tire. high temp in the middle of the tread compared to the edges means overinflation. underinflation yields higher temps on the edges. pyrometer testing is also the only real way to set camber correctly.

good luck
fabian
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Old 03-31-03, 05:50 AM
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Thanks for the input guys. My buddies got a pyrometer, and I plan to use it the next time out. Figuring this out by my ***-o-meter wasn't working out; probably due to my lack of experience, but I'm glad Foko confirmed that.

My first time out, my instructor tells me, "if you've got 42 lb max tires that's the inflation you want cold; that way they'll never overheat and will only jump up a pound or two." Well, they didn't but I had really lousy grip as well. The next time out I started at 32 and the performance was greatly improved.

I just wanted to know a better way to dial in that 32 starting pressure.

Btw Foko I enjoyed that video that Cossie put up on Laguna Seca. His video is probably the same as all of mine; it never gives the impression of your true speed. I'm forever telling people, "yeah, right here I'm doing 130mph, it just doesn't look like it!", etc. So far I've only gotten Sebring (several times; 1 1/2 hours away) and Road Atlanta under my belt. What a blast and thanks again for the help.

David
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Old 03-31-03, 05:57 AM
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Oh yeah, a couple more things; just realized my Avatar is my Sebring "proof", and I had a question on camber.

I run mine "neutral" I guess, because I also drive my car on the street. How much does it make a difference, or does it really come into play at ***** to the wall (and hopefully not your car to the wall) speeds.

That's something that no one's ever mentioned to me about adjusting. Maybe because they realize mines a streetable car, street tires, etc. and thus it's too much of a pain for too little return?

Thanks,

David
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Old 03-31-03, 07:12 AM
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In "***** to the wall" camber makes a big difference. Its all about the contact patch the tire makes with the track - the better the contact in the turn, the more grip, and the faster you'll go. We slot the strut towers in the Spec7's to get 1.5* of neg. camber, and we'd like more.

I don't know much about 3rd gen suspension, but I'd imagine there are plenty of mods to get as much or as little camber as you need.
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Old 03-31-03, 08:10 AM
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My first time out, my instructor tells me, "if you've got 42 lb max tires that's the inflation you want cold; that way they'll never overheat and will only jump up a pound or two." Well, they didn't but I had really lousy grip as well. The next time out I started at 32 and the performance was greatly improved.
your instructor was trying to help you. street tires are not built(compound wise) for track use. running these tires at the temps needed to melt the rubber tends to make street tires chunk and delaminate. by running a higher pressure in the tire you will not gain as much heat and the tire will be preserved. tire temps and setup for street tires will be different than for R-compound tires. the R compound tires are designed for the high temps and lower pressures needed for extended track use. by lowering the pressure you will get more performance but make sure to monitor these tires and look at getting a set of R compound tires for the track
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Old 03-31-03, 09:13 AM
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Camber pays off big dividends. I know other FD's that have -2.2 degrees of camber all-around ... maxed out. During some time trials, my -1.8 degrees on 245/45 was enough to compensate for my lack of straightline speed against a GTS Viper. He'd leave me in the straights, but I'd be on his butt in the corners. It does wear out tires pretty quick though.

On the tire pressure note, different tires will have different requirements for "good" pressure. I notice that the Kumhos typically start around 30-32 psi, but Hoosiers are altogether different ... pressures start around 38-40psi. Of course, I'm also still doing the ***-o-meter adjustments.
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Old 03-31-03, 10:57 PM
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i did some pyrometer testing at sears point and buttonwillow recently which yielded the following camber specs: -0.5 rears was close to perfect. -1.5 front was "hot" on the outside. -2.0 was trending hot on the inside. so for my setup -1.8 in front and -.5 in back is what i'll be using for now.

suspension setup 285/30-18 khumo v700 all around...~32 psi cold. spring rates 14kg/mm front and 12kg/mm rear. racing beat front and rear sway bar. JIC dampers. when i change to the tripoint front sway bar, i'll be starting over on the pyrometer testing for camber.

remember that your suspension setup will alter this data. overall "roll" rate is the key issue.....or individual wheel spring rate + antiroll bar rate. simply speaking, the more body roll you have, the more negative camber you need to optimize the contact patch. while that's just a generalization, it holds true most of the time.

at the limit, appropriate camber makes a huge difference. for everyday street driving, a bunch of neg. camber is just gonna wear your tires out quickly and actually will worsen braking and acceleration traction.

life is full of compromise it turns out.

good luck
fabian
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