Street tire autocross guy now on race tire question

[ponyracer]

Hello all,

I have autocrossed for a few years now on street tires. I have recently aquired a set of Kumho Esta 710's from a FWD racer and am wondering where I should start with the pressures. I was thinking in the 35 psi range but thought I'd ask and see if I could get a better idea.:dw:

 

[David Hester]

I've autocrossed since '84. Beat a guy named Strelnieks 20-21 years ago. I finished 5th, Eric was 6th at a SEDIV event. I think he got better, though. Randy Pobst and Jeff Altenburg were autocrossing with us in the SouthEast back then, too. They were already good. Autocrossing you want to be at optimal pressure from the start, so I used 38-40 starting out. I found Kumhos like 40-42 best with a Mustang. Maybe 36 or so rears. If you get understeer, try another couple of lbs in the REARS. For some reason people want to pump their fronts way up to avoid roll over. Best to set fronts and then adjust rears to what works.
from tire rack
Inflation Pressures

The ECSTA V710 will work well on most cars with inflation pressures between 35 and 40 psi ("cold" inflation pressures for autocross use and "hot" inflation pressures for racing use). KUMHO Tires has found that most stock suspension wheel rates require hot inflation pressures in the 35-40 psi range. With aggressive suspension rates and good roll control wheel rates, you can reduce your hot inflation pressures to around 28-32 psi. Cars that are light, well balanced and relatively easy on tires will find slightly lower pressures may work for them, while heavy, high horsepower cars that are generally harder on tires may find that they need slightly higher inflation pressures.

Typically, adding air pressure in small increments (2 psi) to the tires on the opposite end of the car that loses traction first works best. For example, if a car is understeering, add pressure to the rear tires. If the car is oversteering, add pressure to the front tires.

 

[ponyracer]

Thanks for the reply Dave!! It was so cold(32 degrees) when we started that I had to lower them even more than I had been told. I started with the fronts at 38 and the rears at 34 and ended up with the fronts at 34 and the rears at 32 just to get hooked up. It was like driving a skid car. Of course, the afternoon group got a clear dry sunny track and faster times. The even is so big for our club that they dont allow NFT runs or some one to race at a higher class no matter what you bribe them with, believe me I tried.

There was a bad turn out for the ESP class with only two of us there but I took first and unfortunately my camera crew(wife and daughter) were late so no pics this time. The other ESP driver and I were actually watching the CP guys and would have placed in the top part of their class if we would have chose to run that class.

Well, up next is another month wait for the points events to start and see how I can do with race tires. If we dont get a good turn out in ESP I will jump into CP for competion.

 

[nightstalker865]

I ran my 710's on my Cobra at 32 front and 30 rear. It did not roll the sidewall and gave great grip and balance. That was on an 03 so It weights a bit more than yours.

 

[David Hester]

As important as what you start out with is what you end up with.
If 35-40 is where the engineers designed the tire to work, you want to be in that range most of your run. If you find you are 45-48 at the end of a run, lower starting psi a couple of lbs before next run.
If you start at 34 and end up with 40 or so at the end, you are good.
If you start at 38 and end up with 45, you are still good.
If you start with 32 and never get to 35-40, you are missing out on performance you paid for.
I start with 35-36 front 34 rear roadracing and after 35-45 min race, I'm around 40-42 hot. That works for me.
Remember for autocross, you want to start near where you want optimal to be, as you don't have but 30-60 seconds to get to the "good" area.

 

[ponyracer]

then dont you have to take in concideration the temperature(32-35)? I am also usig nitrogen instead of O2.

 

[David Hester]

Yes. Cold track/ ambient air takes more to heat tires. More reason to start with 35-40 optimal autocrossing. Nitrogen is dry, so you don't have moisture which swells. Issue in humid summer, maybe not so much in winter with less moisture in the air. Relative humidity may still be 100% in the winter, but since cold air holds less moisture total, not to worry, and you can start a little closer to 35-40 knowing there is no water vapor to swell.
Think pressure cooker with water. Heats up and steam comes out from, well pressure.
Same pressure cooker with just air, and it takes forever to move the thingy, if at all. Pressure is there, but not as much and at much higher temperature.
Hoosier, Kumho and BFG way back when they were big into autocross and roadracing (I think they are working back now, since the drifting thing didn't sell as many tires as they thought), have engineers at many events. They all tell me 35-40 psi and around 200 degrees is where their R compound tires are meant to be run. Any adjustments after that are for altering handling due to style, conditions, or other variables.