RUF LIFE

I was recently at a race school in Austin. The instructor walked around my car and started to point out the edge of my tires, telling me I didn't have enough tire pressure to support my car(35psi). He then put chalk on the edge of the tires and asked me to go out and do a couple laps. When I came back in, he showed me how the chalk had disappeared. He said if it was proper tire pressure the chalk would still be there. So he starting putting air in the tires and continued to send me out with chalk until the chalk no longer rubbed off. Final results were 44psi in the rear and 40psi upfront. The rear had more because all the weight is over the rear. The front being lighter because it doesn't have as much weight. I was skeptical but it turns out i felt much more grip from my tires. Has anyone else ever heard or used this method before?

968gene

Chalking tires has been used by auto-xers for many years. What was the pressure after the tires cooled down?

911 Crazy

lower

Matt Lane

No question, you don't want to be running on your sidewalls.

Yet, very surprised to see 40-plus pounds as optimal hot pressure on a street tire - let alone a track tire.

Best,

Matt

sjfehr

It's not strange at all- it's a very common method of determining minimum safe tire pressures. I used it all the time, both on my car when I try new tires, and on my students' cars when I'm instructing. This method tells you when your pressures are too low and you're damaging the sidewall, but not what optimal pressures are for grip- to do it properly, you'd augment this with a skidpad where you'd continually tweak with your car instrumented to get the best spot. FYI, for most tires, it's better to err on the side of too much air than not enough. It will vary considerably from tire to tire, as sidewall stiffness is a large factor as well.

This is a pretty good visual of what's happening:

RUF LIFE

968gene, the tire pressure barely changed because he filled them with nitrogen.

RUF LIFE

Thanks for the input guys

968gene

Thanks Ruf. I was wondering about the total pressure vs the heat being generated etc. With air (as opposed to pure nitrogen) you could have had a final cold pressure that wouldn't be too unusual. What is the manufacturer's max pressure rating?

RUF LIFE

51lbs

shizzle

Are you running a 991? I found I had to run super high pressures to get better times/wear out of the tires. 40/44 is close to the 38/44 recommended by Porsche for the 991.

borellidave

I have set up track cars and autocross cars, and the biggest problem I do see, and have, is to get the drivers to even try to run higher pressures, what they do not realize is what they are asking the tires to do. They will watch the roll over rate on the outside of the tire, without looking to the inside of the tire for the best contact patch. That is what tire pressure is all about.