Odd recommendation for tire pressure worked well
#1
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Thread Starter
Odd recommendation for tire pressure worked well
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?
#4
Rennlist Member
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
Yet, very surprised to see 40-plus pounds as optimal hot pressure on a street tire - let alone a track tire.
Best,
Matt
#5
Drifting
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:
This is a pretty good visual of what's happening:
Last edited by sjfehr; 03-08-2013 at 08:40 PM.
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#8
Rennlist Member
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?
#9
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#10
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?
#11
2nd Gear
Join Date: Feb 2013
Location: Santa Clara California
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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.