Jay,

I was following the thread on the Kumhos and saw your response. I assume
"chalking the tires and driving" is related to finding the best tire
pressure.

I have often wondered how you figure out what the proper tire pressure is
when there is no official guidance to be used.

This chalking thing sounds like the answer, can you elaborate a little on
the procedure?

Thanks,

Mike Gainey



From: Jay Kempf [jkempf@tds.net]
Sent: Friday, October 13, 2000 9:51 AM
To: 928
Subject: [928] RE: Chalking tires

Hi Mike,

Basically you use any light colored marker that will make a mark on the
surface of the tire. Draw a good visible line radially from into the tread
(contact patch) and up the sidewall a couple inches. I use chalk because
those yellow crayons that the tire guys use are hard to clean up afterwards.
Then you go drive the car at the hardest cornering level that you intend to
normally encounter. The marked line should disappear to a comfortable level
where the contact patch just breaks into the sidewall. If you are rolling
the tire too much you will lose the line up into the sidewall and your
pressure is too low. If you have some of the line left into the contact
patch you are not grounding it and you are losing some available grip. When
the entire contact patch is anchored you are putting the maximum amount of
work into the road. At this point you will get the best tire life and the
best grip.

The only other variable after this is whether the pressure you end up with
is keeping the center of the tire in contact. Some tires have very stiff
sidewalls (like these new Kumhos) that when flexed will influence the say
25% of the width of the tire basically levering the middle of the tire up.
The best way to check if this is happening is with temperatures. If there is
less load on the center of the tire it will run noticeable cooler. When it
is bad you can tell with the palm of your hand. Racers need more sensitive
devices (pyrometers) to dial it in even more.

So the moral here is that you need to balance the lowest pressure you can
stand to keeping the contact patch anchored and not rolling the sidewalls.
When you get it right it is dramatic. All this assumes that your alignment
isn't affecting tire scrub or heating.

Jay Kempf