Grassroots Motorsports

Register Log in to GRM
- Community

Birthdays
Recently on the Classic Motorsports forums

Subscribe get a full year for only $23.95 Sample Free Test Drive—no strings attached

Login to post Forums » Grassroots Motorsports » Tire Pressures @ the Track

June 16, 2009 9:52 a.m. Dr. Hess PowerDork

OK, so I'm bringing my HF digital IR pyrometer to the track tomorrow. Now, if I recall from the decade of GRM's I've subscribed to and read, the object of fiddling with tire pressures is to create a smooth distribution across the tire, right? I'm not talking camber observations/changes, as I'll stick with what Lotus designed into it for that, but tire pressures.

So, if after 15 minutes on the track, I take a reading across my right tire(s) and get (made up numbers for illustrative purposes):

Inside Middle Outside 100 120 110

then that would mean I should lower the pressure some, right? And: Inside Middle Outside 100 90 110

would mean I should raise the pressure some, right? And Inside Middle Outside 100 105 110

would be about right? How much pressure (psi) should I change at a time? 2? 4? I'm starting out with the factory recommended pressure for sustained speeds under 100MPH, (21/26), even though my rear tires are 10 mm wider than stock.
June 16, 2009 10:03 a.m. Tommy Suddard SonDork
Correct. I start at 32 all around, then work my way up/down from there. You usually run higher than normal tire pressure.
- I'm still young, so I may be wrong.
June 16, 2009 10:20 a.m. iceracer Reader

What tire runs that low pressure ? Even Hoosier with very stiff sidewalls recommends around 40 psi hot. Which mans starting around 33-35 cold. Temps should be as even as possible. On my ZX2/SR I got my left front up to 149-149-148. This was after a cool down lap at Lime Rock.
Some say change pressures 2lbs at a time, others say 5 lbs.
June 16, 2009 10:34 a.m. Dr. Hess PowerDork

That's the spec for the OEM tire, no longer available, Goodyear 'Eagle NCT' 195/60 VR15 and 235/60 VR15.

My mistake on the pressure and speed recommendations. Correct should be: 21/25 PSI for sustained speeds under 120 MPH. "For continuous speeds over 190 km/h (120 mph) pressures should be increased to 2.0 bar (30 lb/in^2) front and rear." I don't think I can get up to 120 at Hallett, much less sustained. I did hit about 95 at the end of the straight clockwise 2 years ago. We go anti-clockwise tomorrow.
June 16, 2009 10:59 a.m. Tommy Suddard SonDork

That's just what I set Kumho Ecsta XSs to cold at an autocross. I'd have to bump them up for track.
June 16, 2009 11:02 a.m. amg_rx7 Reader

iceracer wrote:

What tire runs that low pressure ? Even Hoosier with very stiff sidewalls recommends around 40 psi hot. Which mans starting around 33-35 cold. Temps should be as even as possible. On my ZX2/SR I got my left front up to 149-149-148. This was after a cool down lap at Lime Rock.
Some say change pressures 2lbs at a time, others say 5 lbs.

The cool down lap likely changed the heat spread across the tire. Come in and measure temps right after a hot lap during the session if you want accuracy.
June 16, 2009 11:02 a.m. Shaun New Reader

In reply to Dr. Hess:

21/25 psi is what I consider "Flat". That is the lowest recommended tire pressure I have ever seen. Be careful if you start out down there.
June 16, 2009 11:26 a.m. aeronca65t HalfDork

Tire pressure sort of depends......like many other things in life.

With the DOT radials (Toyo 888s) I run 36 to 40 psi HOT.

Cooler days, I run a bit colder. Try to test tire pressure ASAP after you come in from a session.

But tires like Hoosier TDs (a bias-ply race tire popular with us vintage racers, runs much lower pressure. Anywhere from 20 to 25 psi hot.

(my numbers are for a race-Spridget)
June 16, 2009 12:21 p.m. Dr. Hess PowerDork

When I was last at Hallett, I ran them at 21/26. I had no trouble staying with Spec Miatas on their spec race tires, with my street tires on a 18 year old "English sports car" and super-newbie status. I could, of course, blow by them on anything remotely straight.

What, besides shoe polish on the sidewall, would indicate too low pressure all around? What would indicate too high pressure? I put a bunch of pressure in the MR2 (RIP) tires once at an autocross. The ride was very rough and I suspect it cost me time. Those tires were junk anyway. I hadn't found GRM yet when I bought them or I would have got some Falkens for about the same price.

Anyway, I figger I'll start where Lotus sez. They know more about their cars on a race track than the rest of the world put together, and they don't "dumb down" stuff. It's gonna be hot, so that's bad for my HP. Our group will have 2 Esprits, 2 RX7's and a Spitfire. That FD gives me concern...
June 16, 2009 1:07 p.m. thatsnowinnebago HalfDork

I'm just impressed that the factory has recommended tire pressures for sustained speed above and below 120mph. I guess that's what happens when you buy a Lotus.
June 16, 2009 1:15 p.m. Strizzo Dork

the low recommended pressures might have something to do with the low amount of weight they have to carry, less weight means less pressure required to keep the sidewalls from flexing
June 16, 2009 1:19 p.m. Xceler8x Dork

Where's you track day Hess? I'll be at VIR on Aug 27th and 28th if you're interested in coming down, or up.
June 16, 2009 1:39 p.m. Armitage Reader

I've been running 30 front, 28 rear HOT pressures on my Azenis at the track. This seems to be the sweet spot for me, but everyone else I've ever talked to on this subject seems to be running much higher pressures.
June 16, 2009 3:51 p.m. Dr. Hess PowerDork

Hallett, OK. VIR's a bit far for me. I'll post some pics when we get back.

Thanks for all the tips, guys.
June 16, 2009 5:57 p.m. billy3esq Dork

It depends a lot on the car and the tire. When I track a new to me car that nobody else has data to work from, I start at the factory-recommended (cold) pressures. Sometimes this is right, sometimes it's a touch low. If you think about it, the 30-32 psi cold that most cars ask for is pretty similar to the 35-40psi hot that most people here are recommending.

I use the shoe polish method (chalk actually) to determine whether the pressures are too low or too high. I've also progressed to a point where I can feel if the tire is too low before I completely destroy the shoulder and sidewall. I use the wear patterns of the tire to figure out alignment issues.

I track a car with much lower (cold) pressures than I autocross with. I suspect this is because the tire never gets that hot when I autocross, whereas it gets plenty hot in a 25 minute run session.

I also tend to run slightly lower pressures than a lot of my peers, but my tires wear better than theirs.

If I were you, Doc, I'd start at the 120mph+ pressures on the Lotus. That pressure number is really more about temperature than actual speed, and you'll be getting your tires just as hot as if you were cruising at 120, if not higher.
June 16, 2009 8:56 p.m. Jensenman MegaDork

Chalk, shoe polish etc is not a real good indicator of tire usage. It's better than nothing but it gives very limited information, ie whether the tire is rolling over too far. The problem: if you jack the pressures up to stop the rollover, there's a good chance you will have overinflation which is every bit as bad for handling as underinflation.

The surface temps are better than the chalk, best of all is a memory pyrometer with a probe. That will tell you what you really need to know. Someone will probably loan you one if you ask nicely. That is, before you blow past them on the straight. I usually change pressures 2 PSI at a time, but that's with DOT race tires; YMMV. Street tires generally do not have as stiff a carcass as a DOT race tire and will probably need a bigger change, maybe 4 PSI. I'm going to be learning bias ply race tires next; wish me luck.

Also keep in mind a couple of things: camber settings will move temps around on the tires. Negative camber = hot inside temp, positive camber = hot outside temps. Also, the last turn before you check your temps is going to skew the readings. For instance, if the last turn is a long right hander, the outside of both left tires will still be a good bit warmer than the inside which at first glance says 'camber problem' but it's actually the turn loading the outside of the tires.
June 16, 2009 10:54 p.m. billy3esq Dork

Jensenman wrote:

Chalk, shoe polish etc is not a real good indicator of tire usage. It's better than nothing but it gives very limited information, ie whether the tire is rolling over too far. The problem: if you jack the pressures up to stop the rollover, there's a good chance you will have overinflation which is every bit as bad for handling as underinflation.

I'm going to have to disagree. I know everyone will find that shocking.

Specifically, I disagree with the implication that chalk can only indicate under-inflation. The chalk will also tell you if the tire isn't wearing at all at the shoulder (especially on a round-shouldered tire), which is indicative of either not pushing it hard enough or being over-inflated.

Moreover, starting at the street pressure (which is an integral part of my recommendation) is unlikely to result in over-inflation. In my experience, one usually has to add a couple of psi above the street pressures to keep the tire from rolling over too far. That puts you pretty close to the sweet spot. At that point I care more about how it feels and what the stopwatch says than what the pyrometer says.

I certainly agree that having a pyrometer (esp. a probe) is more accurate/better, but there's nothing inadequate about the chalk method (applied as I described), especially for non-competitive applications.

Furthermore, your insinuation that a probe-type memory pyrometer is superior to chalk means that you're not really "grassroots." (The board hasn't had one of those "I'm more grassroots than you" threads in a while, so I figured I'd stir the pot.)
June 17, 2009 9:12 p.m. Dr. Hess PowerDork

Well, we're back. We had a good time. The FC and FD were really fast. The Panoz was really, really fast. I started at the <120MPH cold settings of 21/26. After the first session, my back tires were quite uniform across, but the fronts were cooler in the middle. Hot temps were 31PSI. I upped that to 34 PSI and after that, the fronts were evenly distributed across, low-middle-high. I got about 7 MPG on the track. Lots of boost and POOOSH (HK$ SSQBOV).

I got a half gig + of pics to look through and pick a few out. I'll post some then. There were two Lancer Evos. They both crested the same hill, got light in the rear, spun and ran into the tire wall within 5 ft of each other (different sessions), both tearing the rear bumper off. I call the hill "Evo Hill." Lessee, besides my Esprit, there was Jim's 86 Esprit, a 74 Pantera, a "rousch" or something P car, a Cobra kit car, another couple P cars, a Miata, a group of sport bikes and some others. That guy with the FC was like a half second slower than the best times on the sport bikes
June 18, 2009 1:15 p.m. Dr. Hess PowerDork

If you look carefully at the middle of this picture of a 944 Turbo motor's block, you'll see pavement.
June 21, 2009 12:17 p.m. Grtechguy UltraDork

I love the engine bay of the Pantera..... absolutely beautiful
June 21, 2009 3:24 p.m. fifty New Reader

What times were you turning at Hallett? A group of us rented the track back in March - nice track, good people.

Were Chris and Kevin Edens there yesterday (cover of June 2008 GRM)?
June 21, 2009 3:39 p.m. Dr. Hess PowerDork

My best time was 1:46. We ran anti-clockwise. My car is pretty much stock, and I ran as I drive it on the street with 5 year old hard street tires, etc. We drove to the track and drove home. I was sort of hopeing to drive home on cords, but I still have a bunch of tread left.

No Edens' there. We rented the track for the day. Brad Nelson, owner of the Pantera pictured above, organized it.
July 2, 2009 8:12 p.m. wbjones New Reader

Xceler8x wrote:

Where's you track day Hess? I'll be at VIR on Aug 27th and 28th if you're interested in coming down, or up.

I'll be there... with what will probably be the slowest car in the intermediate class (91 CRX) so I'll get lots of practice watching my mirrors

what group/car are you?
July 14, 2009 12:04 a.m. grinch77 Reader

I love that track...

You'll need to log in to post.