What pressures would you suggest in 275/40/17 Victoracers for autox? I have run the 16's in the past, and assume that these will use slightly less pressure since they have a shorter sidewall, but wanted to know what the experts suggest.

 

Hey Chuck,

I ran the V700s for years, I'd start them a little higher in pressure initially to help combat cupping on the outside edge of the tire. From what I remember I think I started at 40/35 cold when they were new and drifted down to 38/34 or so when they were worn. That should get you a starting point in any case.

 

Thanks

 

I just started running the Kumhos in 275/40/17 this summer and I've noticed the cupping on the outside edge of my tires. How does low pressure cause that? I'm running the tires on 11" wheels for now and keep them at 32/29 cold. They grip well at that pressure, but I've never had tires cup like this.

 

Cupping is caused by carcase deformation under corning load. Granted since you are running them on 11" wheels (why aren't you using 315s as I am now) you won't need to go as high as I did for the 275s. You also didn't mention if you were using them for road racing or autoxing. For road racing you will start with 5-6 psi less pressure due to the higher heat generated.

Jason S.

 

I came across a good deal on some AFS 17x11 wheels and already have the Kumho 275/40 tires on some 9" wheels, so I figured I'd put them on the wider wheels. I'll have 315 tires next year.

For track days I run them 32/29 cold. For autoxing I start out at 35/30. Other than the cupping those pressures have worked well for me, but I don't have a pyrometer yet to be more accurate about what I should run. Another purchase this winter I guess.

 

UnbalancedEng: are you sure about the 40/35 cold pressure? that would mean about 44/39 hot! i'm still pretty new at it, but everyone at the course today thought that 40/35 hot would be more like it?

 

Yea, I'm sure. Those are the pressures that the two of us down here in ESP were running. Pryo data is what drove these pressures.

Jason S.

 

ok then......today is the second, final day of our annual American Muscle Car Team Challenge, and i'll give those a try.....

Lee

 

Thanks guys. I ran 40-41 front and 34-35 rear today and it seemed to work very well. My best time was at 34/40.

 

What is the consensus on hot pressures here for road track use? I certainly end up higher than 44 hot ( often as high as 48) but when I lower the pressures I don't seem to go any faster.

 

as a followup.....my best time today was 40 front, 30 rear (hot)

i tried 44/39 (40/35 cold as suggested above) and it was ok, but i think i was sliding more than yesterday.....

then i talked to another guy with a 4th gen camaro running the same tires and he suggested 40/30 (hot) as that was what produced wear over the shoulder, exactly to the top of the little triangle on the sidewall.....which i didn't even realize was there....so i went over to my car and checked, and sure enough, my wear was well above the triangle.....so i tried it (40/30).....and got almost a second better times....and, my wear went right to the point of the triangle, just like the other racer

now that's obviously not to say that those are the "perfect" pressures, since other considerations apply (overall tire life, cupping, etc.), but they did in fact produce wear to the triangle, and i got better times

but i'm just learning, so maybe the triangles a generalized wear mark that works for some cars and not others?

 

Hey Lee,

Many things drive tire pressure. Surface, suspension setup, tire construction (same here obviously), driving style and agression. I was just trying to get Chuck in the ball park and to help him get rid of the cupping. All in all, tire temps and some testing will tell you the best pressures for your car and surface.

Jason S.

 

Jason......thanks

i have a lingering question though.......given your last post (many factors to consider), everything else being equal, how closely does the triangle wear method of pressure determination match up with the pyrometer method?

i guess your answer might be: that--if you don't have a pyrometer--the triangles are just a starting point, and that you adjust up from there, depending on what other factors you are concerned about...

agree?

 

The triangles are there as a method of seting rollover. That doesn't always optimize the pressure across the contact patch which is what you use a pryomiter to determine. What sort of surface were you guys running on? How much autoxing have you done?

Jason S.