Tire pressures for Kuhmo MX'x

[ronaldtrotter]

I a running Kuhmo MX's on my 911SC with pressures cold of 32 front and 35 rear. What are some of you running with the same tire.

[pdy]

Hi Ron:

I used to run these, but I don't have any AutoX pressure notes.
What I have done for years, and did when I was using the MX
on my SC is the following;

Mark the tire on the outer edge from about an inch on the tread,
to an inch on the sidewall using white shoe polish. Do this in two
or three places around the circumference of the tire, on all four.
Start with stock pressures, or maybe a couple pounds higher.
After the first lap or two, look at the marks on the tires to see
how far they have scuffed on the sidewall. You don't want it on
the sidewall, but you're looking for how far the tread is deforming
and the tire is rolling over. I found that it shouldn't go more than
about half-way over onto the sidewall - kinda like a 45° angle of
bevel. You want to run the lowest pressure that doesn't roll over
too far. Starting with a higher pressure allows you to easily let
some air out until it just is getting to that point.

Also, if you feel the car understeering (plow), you can add air to
the fronts, and if it's loose (oversteer) add it to the rears. Or, take
air out of the opposite end if it isn't close to rolling over too far.
You should see a correltaion between what you feel and the marks
on the tires. Like if it's understeering, you'll probably see that the
front tires are rolling over more onto their sidewalls than the rears.

Sorry, it's more wordy than just two numbers, but I hope this
makes some sense.

Best,

Paul.

[Jad]

Hi
Also, if you feel the car understeering (plow), you can add air to
the fronts, and if it's loose (oversteer) add it to the rears. Or, take
air out of the opposite end if it isn't close to rolling over too far.
You should see a correltaion between what you feel and the marks
on the tires. Like if it's understeering, you'll probably see that the
front tires are rolling over more onto their sidewalls than the rears.

Paul.

Paul,

I would have done the opposite? I thought you generally get more traction by letting air out. Of course, if the tire is the perfect pressure, any change reduces grip.

Ron,

Many people in the pits have pyrometers to read the temps of the tires and give you a good idea of how the tire is doing. Ask around and you can try that method as well as Pauls to dial in your tires to your car and driving style.

A ROUGH rule of thumb for street tires that I use is to try for ~39 lbs HOT all around and adjust from there.

[pdy]

Street tires behave differently, and generally want higher
pressures to keep them more on the tread rather than the
sidewall. This is especially true for cars with near-stock
camber (like Ron's). You DO want to run the lowest pressure
you can while keeping minimal roll-over. If it is rolling over
onto thesidewall more than a little bit, reducing the pressure
will make that tire roll over more and stick even less.

Pyrometers are good, but sometimes difficult to use at an
autocross, since there is not the steady heat like there is
at time trials. I use a pyrometer at time trials and DEs. For
autocross, I use hot (or tepid?) pressure as a guide with
soft tires. For street tires (like the MX), I find the marks to
be a good guideline, and that's what I did when my SC had
stock suspension and Kum-Ho MX tires.

BTW Jad, how on earth did you win KP AND BRI with that
front engine beast with turbo lag and all those tight turns,
especially that wretched swale crossing? Awesome!

Paul.

[Don Middleton]

Ron, you don't need me to tell you - Paul knows what he is talking about. I found that street tire performance is definitely about lowest pressures while minimizing rollover.

But, I found that driving style (throttle steering) and car performance (like the torque in the 3.2L in your SC) can vary hot tire pressures, especially in the rear of a 911. So, I found the shoe polish approach to be best.

The only aspect I would add to the process is to get the tires up to pressure before using the shoe polish. I would make 3 runs and then have someone add the shoe polish before my 4th run. this way, you minimize the error in reading rollover while the tires are still cold and coming up to heat/pressure.

[Jad]

Thanks.

Ironically, I think the swale hairpin is what saved me. I worked that corner and 95% of the drivers turned in and apexed the cone BEFORE the apex and fought the corner and pushed into the track out cones, then got on the gas. I turned in really late and WAY to the left of the track and was full lag, I mean throttle, well before the apex cone and came out without pushing or fighting the car. Carried enough speed that first was not even possible and I saw several people having to do that with the other line. Add that to the long straight that followed and I figure I got 1-2 seconds there

Score one for corner working. It really can be fun and educational if you let it....

[ajackson]

I have found my street tires to be quite a bit faster when pressure is in the high 30's (rather than low to mid 30's). I've done all the mark the tire type tests but finally just started playing with it. We get lots of laps at AX's -- set a base line and then for one session add or remove 3 or so pounds and see how it feels.

[Dan Chambers]

Score one for corner working. It really can be fun and educational if you let it....

I......KNOW......!

[ttweed]

Add that to the long straight that followed and I figure I got 1-2 seconds there

I wondered where you got that extra second out of that course! That swale crossing was nothing but painful for me all day.

TT