Racing tire question about pressure

[chrisreale]

I so far only AX. I run Victoracers at 30-32 psi hot. When I get back from a run that lasts a minute, the pressures have gone up a few pounds. How the hell do you manage your tire pressures when you are racing? Seems to me that they get hotter (duh!) and yo are out there longer than one minute (duh#2!). Do tires get to a certain temp and the pressure stops going up?

[Jack Olsen]

Yes. On the track you'll see a 5-10 pound jump, depending on the tire and the track, but it does level off.

[jakermc]

Let me add my own pressure question. I am about to run for the first time on R-compound and am not sure what to target for hot temperatures or where I should start cold. The tires are Toyo RA-1 and they will be for the track, not Auto-x. Anyone else running these and can share some experience?

[APKhaos]

Jake - start your RA1s at 26f,28r. Monitor carefully when hot. I aim for 36f, 38-40r hot pressures. NEVER let them get above 40 - if the car starts to feel sketchy under brakes its well worth checking the rear pressure to be sure its not north of 40.

Alternatively, use nitrogen - far lower pressure increases from cold to hot, which is why serious racers say no to air.

[jluetjen]

chrisreale;
My experience is that it is virtually impossible for a 3rd party (short of the tire manufacturer) to make a specific recommendation for tire pressures. Even when I've used the tire manufacturer's specific recommendation, If found that nothing surpasses the value of good record keeping. As a rule I have always kept the following information from every run/session:

- The driver
- The Track, weather, ambiant temperature and track temperature (from a pyrometer).
- Suspension settings
- cold pressures
- Pyrometer readings after the run at the inside/center/outside of each tire.
- hot pressures

I've genenerally found that with that information I can fairly quickly zero in on the cold pressure settings that will equate to the desired hot pressures. Generally once they are set, I don't need to touch them again unless the tire is loosing air.

Keep in mind that it seems to be a somewhat unstable relationship, so an intitial pressure that is too low or too high will result in excess heat being generated either by the tire flexing if the pressure is too low, or the tire scrubbing across the pavement from the lack of traction if the pressure is too high. Either can cause a bit of a runaway situation as pressures will increase further due to the heat, which can cause the situation to worsen further.

The correct cold pressure will result in the hot pressures being such that the temperatures are even across the tread with a slight increase (~10 degrees) towards the inside of the tire.

I know -- you asked what time it was and I told you how to build a watch.

[J P Stein]

While we're all gathered here. Is there anything to be learned from these tires? Their last work was driving home after 150 minutes of track time.

[ttweed]

Originally posted by chrisreale
How the hell do you manage your tire pressures when you are racing?

Besides using a nitrogen fill which will yield more stable pressures, some racers use pressure relief bleed valves (or "popoffs") which are set at their ideal pressure, and when it gets higher, the valve will bleed off the excess. See this link.

TT

[addictionMS]

I run Toyo RA1s and start at 25psi all round, 205/55/16 F and 145/45/16 R.

In the Toyo Cup Series, the starting pressures are all over the map, some guys like 29 F/R others like me prefer 25 F/R other I am sure are running mixed pressures (different on the front and rear) so there is only rules of thumb, and even then there is a big range, so the best advice is play around with the pressure and see what you like, but you shuold pretty much start within that range.

As for what can be learned from the pictures of those tires, if they are RA1s they just got good....


Jim

[J P Stein]

Other side.....outside tire for right hand turns which dominate at PIR. Hoosier R3SO3. .......

[jluetjen]

Is there anything to be learned from these tires?

I'm gussing that you could use a little more negative camber, the insides of the tires don't look particularly used, especially in the front. They also may have been slightly over inflated, it does't look like the car was using the full tread surface, just a band around the center.

In both cases it is pretty hard to tell for sure without a pyrometer.

[J P Stein]

John:

I read with interest your bit about the correct tire temps being 10 deg hotter on the inside than outside.

Last time I checked temps was at an AX (with these tires).
the fronts were even all across. The rears were 10-15 deg hotter on the inside. I am running -2 deg camber at both ends. At the track day, I ran 35 psi cold raising to 39 psi hot....but had no pyrometer with me. I normaly drop the prressure 3-4 psi for AX. These tires have around 120 AX passes on em'.

I am fresh out of negative camber at the front....nothing more to be had. The offset mounts are hard up against the body work. As it is, I had to remove the dust shields from the front struts. I have also reached the point of diminishing returns from lowering. Naturally, there is more available at the rear

[jluetjen]

JPIII; here's some musings (and no more)

I ran 35 psi cold raising to 39 psi hot

* That sounds kind of high to me considering how light the 911 or 914's are. Have you ever tried running lower pressures? It's a free test. I'm guessing that you'd want to wind up with hot pressures in the 30-32 psi range. If you are unsure, I'd read through the Hoosier technical recommendations which recommend a minimum cold pressure of about 24 PSI assuming that your car weighs about 2400 lbs.

Last time I checked temps was at an AX

* Could you post the actual readings? Hoosier seems to recommend ideal temperatures in the 180-200 degree range. The front/back balance is also meaningful.

I am fresh out of negative camber at the front....nothing more to be had. ... I have also reached the point of diminishing returns from lowering.

* Keep in mind that lowering a McPherson strut equiped car works to cross purposes with the suspension geometry. Once the lower arm of the suspension is level when at rest, any further compression (as would occur on the outside corner when cornering) will cause a camber loss, especially after you figure in the body lean. What are the rules of the particular class you are driving in? Some options to resolve the problem are...
- Raise the car a little bit and tune the shocks to manage the roll.
- Use RSR style front strut assemblies which lower the knuckle on the strut and thus return the geometry to a favorable zone.
- Try using a camber bar to pinch in the tops of the front strut mounts.
- Get some camber plates
- Increase the front caster which will do a couple of things, some desireable - some not. The good news is that it would increase the negative camber on the outside wheel when the steering wheels are turned, especially when they are turned a lot which often helps when autocrossing. It will also make the steering heavier which may or may not be an issue.

[J P Stein]

AXs don't warm tires worth spit....even with two drivers.

Last I checked, the fronts were 120 deg all across. The rears were
135 inside-120 outside.

I checked the full range of motion( for camber) at the front before I set the ride height. I can (and will) do a bit of grinding to get another deg or so of caster....it's presently at 5 deg. I can live with the ride height. Raising the spindles crossed my mind, but it's a PITA and $$$$$....better spent elsewhere....like tires.

My competitors are running slicks. I did spring for some rear swingarm needle bearings to match the fronts. It'll be nice to get rid of the squeaky SOBs presently in there. Also wanna get the shox revalved....that oughta wipe out my car fund for the winter...spring & summer, too.

My SCCA AX class is FP so there is a fair amount of wiggle room. I'm under powered, under tired, overweight, *under driven*.....that about covers it.

Tire pressure is a black art that I haven't figured out.....but I'm working on it.

[ttweed]

Originally posted by J P Stein
AXs don't warm tires worth spit....even with two drivers.

JP-

That's why those fast guys are running Hoosier R25A or Goodyear 160 compound slicks. Of course it would help if the dang sun would come out in the PNW every once in awhile.

TT

[campbellcj]

LOL, JP neglected to point out that they regularly autocross in the RAIN up there! Sheesh, down here we're battling 120+° cockpit and asphalt temps a significant portion of the year. I figure in another month or so it just might rain, once or twice...

[jluetjen]

Raising the spindles crossed my mind, but it's a PITA and $$$$$....better spent elsewhere....like tires.

JP;
Actually, I don't think it is that much if you can take apart your struts yourself. Then its just a case of taking them over to your favorite welding shop to have the spindles removed and raised about 18mm. If you're running 16 inch wheels you might be able to get more so you should check your wheel clearence before you take everthing apart.

Raising the front spindles will do a few things for you, all good if your car is lowered:

1) Raise the front roll center so that it is closer to the front's CG. This will make it seem like the car has a stiffer front sway bar.

2) Return the lower A-arm to a better part of it's geometry. The result will be that you won't lose as much negative camber under compression, and may even gain a small amont.

3) Increase the front shock travel so you won't bottom the suspension.

Ideally you'd also raise the rear roll center by a similar amount to return the car to it's original geometry relationship front to rear. Smart Racing sells some pieces that you use to slot the inner mounts of the rear trailing arms. I believe that Jack has them on his car. I don't know if they work for 914's.