THSCC Forums

I've read in several threads where people have mentioned
running pressures as high as 55 to even 60 psi in autocrossing.(':shock:')

This sounds really excessive to me and I'm concerned that those pressures are high enough to potentially result in a tire failure. The maximum design pressure for many tires is around 44 psi, as stamped on the side wall. Going slightly above that for racing conditions (a few psi) may not be an issue, but going 10 to 15 lbs or more above that number makes the mechanical engineer in me say.. "waaaaait a minute....(':eek:')

I dug around on the net a little and admittely didn't find too much related to tire failure due to over inflation. Most tire problems, especially thosed used by consumers, are due to under inflation.

Since the risk could vary greatly depending upon the particular tire brand you use, I'd be interested in asking those who run those pressure to contact the manufacturer's racing or high performance tire division and ask them about the risk of running those pressures.

I would also be willing to challenge the assumption that pressures that high will result in lower grip and non-optimal temperature distributions. You can't always tell from "feel", but the use of a pyrometer to measure tread temps would tell the story.

Incidentally, for measuring tire temp distribution, you need a probe type pyrometer that can be inserted into the tread, not an IR type.

Miles

Obviously the manufacturers will tell you not to do it.

Based on testimony from many an autocrosser, I run high pressures with no worries. I've never heard of a failure due to a tire coming off of the rim when the pressure is high, and it should have happened by now.

On my Celica I've run as high as 60 in the rear and as low as 17. I was a lot more comfortable running the 60 than the 17!

I do agree with Miles that by the time you hit 55 psi you probably will not be helping grip, the better solution would be to bleed the pressure down 5 or 10 psi and slow down a bit more for the turns.

Scott

I was actually directly told by Hoosier Tom to run 60 in the front of my car and go higher/lower in the rear based on individual driving style. Good luck _ever_ getting Hoosier Tire to put that in writing though.

The WRX is a special case though as the race Hoosiers are actually undersized and need a lot of pressure to simply support the weight of the car.

--Kevin H.

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2003 WRX (again!)

I've personally never gone over 50 PSI (Azenis) but I've *heard* that a certain Subaru driver runs as much as 70 PSI in the rear to induce oversteer. You know the car understeers pretty bad if it takes 70 PSI to get it around.

_________________
Vincent Keene
'06 Ford Mustang GT (track rat)
'15 Dodge Charger R/T (yeah, it's got a HEMI!)
'07 Ford Fusion SE (205,000 miles and counting)
'98 Chevy Z-24 (retired)
'93 Acura Integra (Team SWB 24HOL Car)

Gee whiz, I don't even run 70 psi in my bicycle tires!!

Somewhere there's a tire engineer about to go nuts!! (':stick:')

I've run some astronomically high pressures in the rear of the Celica as well, and with no ill effects. Rick Cone regularly runs mid 50's in the front of his Celica with Hoosiers mounted.

Granted I've only been autocrossing for 3.5 years now, but I've competed in well over 100 autocrosses, and have yet to witness a tire blowout. I did blowout a Hoosier at our intermediate school a couple of years back, but that was due to flatspotting through all 3 layers of fiberglass cord - AB

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'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

I've run some astronomically high pressures in the rear of the Celica as well, and with no ill effects. Rick Cone regularly runs mid 50's in the front of his Celica with Hoosiers mounted.

Granted I've only been autocrossing for 3.5 years now, but I've competed in well over 100 autocrosses, and have yet to witness a tire blowout. I did blowout a Hoosier at our intermediate school a couple of years back, but that was due to flatspotting through all 3 layers of fiberglass cord - AB

_________________
'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

I ran 65-70psi in the rear hoosiers on the WRX in DS to get it to rotate all season long without any issues. 65 psi in the Kumho V700s did the trick as well. A couple of nights events ago, I was playing with pressures on the KDs and it took me 75psi in them to get them to break loose.

I'm sure it probably is a safety issue, but I'm too dumb to worry about it

I wrote to Hoosier just to see what they'd have to say. It is somewhat educational. Their response is below. I also left my original question to them at the end of this article.
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Miles,
Thanks for writing and for your concern of recommended air pressures. I believe most all of our tires, especially DOT approved tires will have the Max 44 psi stamped or molded into our sidewall. This pressure recommendation is really for tire mounters not to exceed this pressure when mounting a tire. More often than none, tire mounters will have difficulty mounting a tire and exceed this pressure to seat the bead of the tire. This pressure has nothing to do with operating pressure. Your 44 psi front and 42 psi rear sound just right for your car and for autocrossing. On some road courses, in the heat of California or Arizona sun, exceeding 100 degrees air temperature, cars may experience 46 to 48 psi hot.

There is no danger of the tire exploding from running higher air pressure on an autocross course. The main problem with running pressures at 65 psi +, is the car will not handle properly, or not stick to the course and cause the car to spin out. Another attribute of running higher air pressure is wearing the tire out quickly in the center, because of crowning of the tire.

I would strongly suggest not running that high, expecially 65 psi + for possible wheel failure. If you could pass on to your competitors to keep their air pressure below 50 psi, that would be appreciated. I cannot imagine anyone being competitive at those high air pressures. The weak point between the wheel and tire is the wheel bead flange. The tire bead will actually push out as the pressure increases inside the tire and then bends the wheel flange which causes a quick deflation and dangerous situation.

I hope I have helped.

- Jeff Speer, Hoosier Racing Tire

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----- Original Message -----
From: Miles Beam
To: info@hoosiertire.com
Sent: Monday, October 13, 2003 5:10 PM
Subject: Unsafe Tire Pressures???

Dear Hoosier Tire,

I autocross with a club in NC. Personally, I run a 91 Corvette. With Hoosier R3S03s, I've found that cold tire pressures of about 44 in the front and about 42 in the rear work well on asphalt. On a concrete surface, I have found that bumping that up 2 psi is about right.

I recently found that some of my fellow autocrossers are running much higher pressures. High enough that as a mechanical engineer, I became really concerned from a safety standpoint. Some of these guys are running 60 to 65 psi and I even heard one guy talking about 70 to 75 psi in his Hoosiers!

I know the pressure stamped on the side of the R3 and A3S03 tires is Max 44 psi, but what I want to know is at what point does exceeding the maximum pressure become a safety risk? It's hard for me to believe that autocrossing on a tire with a 70 psi pressure is safe. Is it? As a reference, what is the typical "burst pressure" for a tire? Are larger tires more likely to burst at a lower pressure than smaller tires?

If these pressures are the safety risk I think they are, I'd like for you to give me some kind of data or response that I can share with my friends to convince them that 70 psi is just plain nuts. If I'm wrong though.... please tell me that too!

Thank you,

Miles Beam, P.E.
Autocrosser in NC

Exactly the reason that we do run the odd pressures.

Not everyone is blessed with a car that has a 50-50 weight distibution or enough camber. In a perfect world we would be *fine* tuning our cars with tire pressures, but in stock class that isn't always an option.

On a nose heavy pig like a WRX, sometimes the only way to make it turn is with excessive tire pressure (to reduce grip).

Would you feel better knowing that there are people running 1/2 inch or more toe (in or out) on their cars a tthe autocrosses - sometimes even driving to the events with those toe settings?

It is interesting to hear about the wheels being a concern rather than the tires...

Scott

I, too, have gone as high as 60 psi to prevent a swing axel car from rolling the tread, or flipping over. I was surprised at what I did not see as a response from the Hoosier engineer. What I had found (years ago, so long that I've forgotten much of the finer details) was that 1.) at higher pressures there could be belt damage; not necessarily bursting of the belts, but potential delamination. And 2.) at higher pressures, any impact with stones or pavement irregularities could cause fracture of the cord/belt. That did not seem to make a lot of sense at the time because I thought that the higher inner pressure would help the tire resist the impact. Actually what hapens is that the tire, at lower pressure flexes and deforms around the impact zone in a smooth, gentle curve. At higher pressures, the tire resists deformation except very close to the impact point. This causes the belts to bend around a sharper corner and can actually over stress the fibers and lead to a belt failure.
If you resort to gross overinflation absolutely remember to lower the pressure before driving home. And, I for one, would never use at high speed a tire that I had over inflated to the point of possible belt damage.
Charlie Guthrie

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1998 BMW Z3
1987 BMW 325is
2000 BMW
1996 F250 Turbo Diesel

"Would you feel better knowing that there are people running 1/2 inch or more toe (in or out) on their cars a tthe autocrosses - sometimes even driving to the events with those toe settings? "

I ran almost 3/4" toe out in the rear COMBINED with the 65-70psi on hoosiers to get the rear of the car around

-Tom

Would running a narrower tire in the rear help your dilemma? I've known guys that ran wide tires in the front and skinny tires in the rear on other cars for this very reason.

Just curious...

With the AWD system you need to keep the tire diameter as close as possible in order not to damage the center diff. Running the 215/40/16 Hoosier, there is not a skinnier tire that keeps the same diameter available

These are reasons that I'm in STX. I don't have to do all that crap in order to make the car handle right, I can use the suspension to do that.

I guess I am going to AS next season, BUT with the STi and the nice front limited slip and bigger rear bar, we will be able to run just a *tab* of toe out and sane tire pressures and get the same handling that we want.

-Tom

Not to mention the STi handles in a straight line too.

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2005 GMC Sierra
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