Autox Tire Pressure - Newbie Q
 

Hello,

I did my first two autox events last year, and I'll be going to my first one this year in a week down in San Diego. Now that I've got a basic understanding of what to expect, I want to start paying closer attention to my tire pressure over the course of the 10 laps around.

I have Indy 500s (17's). I plan to chalk my tires to see how they're wearing, but are there general recs for what to go into an event for tire pressure? From what I've been able to find on this forum, it seems like you should go in between 29-32psi. As you go from lap to lap, adjust based on how much it slips or grips. Am I getting this right?

Somewhat related question... Do you find it helpful to cool down your tires at certain points?

Thank you!
Chad

A. Unless you're the next senna tire pressure are probably just a distraction for your first 5-10 events, more to be gained by focusing on driving.
B. Chalk has never ever helped me determine the best air pressure, the clock rules, I'm meh on pyrometers.
C. I've had better luck in the 28-30 psi range with the tires hot, checked right after coming off course and parking in grid.
D. I would only cool if the tires are too hot to touch, they work best around 140-180 degrees, odds are they will be cooler then that before you run unless you're hot lapping.
E. The two times I autoxed Firehawks I tried 30 psi which was alright and 35psi which may have been less grip then the 30 but I was lazy and just getting seat time and didn't bother experimenting.
F. Experiment, internet advice pales in comparison to your own data. If you like running them at 40psi and spraying them with 20 gallons of ice water between runs who am I to get in the way of your happiness.

I find both chalking and temps to be useful to at least get in the ballpark and to tell me something about alignment. Spraying depends on two big factors - ambient temp and time between runs. A single driver car probably doesn't need the tires sprayed unless it's really hot (or FWD, which these aren't). I have a new set of 660s on a car that's new to me, for the first event I'll start at 32 and go from there. I like airing down at least once as the tires warm up, you'd be surprised how much the pressure can go up.

Having said that I agree with strat61caster above, fun and times are what it's all about, don't focus on tiny details so much as to get in the way of either of those.

Every tire is going to have a different pressure it works best at. That's also going to vary by location/surface, dry vs wet, etc. You can get really into tire pressures, but it's best to focus on driving.


Generally speaking, you'll want your fronts set to be wearing just above the triangles on the sidewalls. If you have coilovers with camber, you might be able to get down into the high 20's for this, but it'll likely be somewhere in the low 30's. I'd start at 35 and go down by 2psi increments from there. You can use chalk to find that sweet spot. Then tune the rears about 2-3 PSI lower.


Again, this isn't the best for every surface, every run, etc... but once you get that pressure, I'd just stick to it. My A052's like it higher at 38PSI front 36 rear because they have soft sidewalls and I only have 1.2 degrees of camber in DS. I just set it and adjust after every run back to the pressure by letting some out.


My Yoks certainly require a spray down after every run when it's 80+ degrees even with a single driver. Indy 500's probably won't and will likely get faster with more heat (to a point). Other 200TW tires generally don't need sprayed between runs even on hot days with a single driver car. Again, don't overthink it in grid and worry about spraying when you should be thinking of the course.


Tire pressures and temps are one thing, but focusing on driving early on is much more beneficial.

Before we get any futher, what suspension and alignment do you have?

e.g. if you still have 0º camber in the front, the chalk will sure tell you that you
need more camber, but won't be as useful to determine the optimal pressure.

Please provide full size specs of the tires and wheels.
245/40R17 on 17x9, 245/40R17 on 17x8 and 215/45R17 on 17x7
will probably need different pressures.

IMO, chalk and "going by the triangles" (which actually only point to the treadwear indicators and are not intended as markers for adjusting pressure!) are not a good way to go... *Maybe* for bias-ply tires of old, and only then *if* backed up with correlation to lap times.

Modern radials usually will have a *broad* range of pressures for which times are going to be very close. IMO too many get the idea that "lower is better" and try to just see how low they can go. If you're in the middle of a very broad range of pressures that work, you shouldn't see big changes in times with pressure changes of +/- 1 or even +/-2psi.
See results here:
https://grassrootsmotorsports.com/ar...oss-and-track/