tyre pressure and chalk marks
05-20-2003 | 07:59 PM
#1
Thread Starter
Drifting
Joined: May 2002
Posts: 2,777
Likes: 2
From: New Zealand
tyre pressure and chalk marks
where exactly are people placing chalk marks on their tyres whilst autocrossing, and how does this give you feedback regarding tyre pressure/rollover?
ive just replaced my rear tyres with brigestone so3 rubber, 255/40/17 and i think they need more pressure than the current 36psi - really grippy though!
im currently spending some time on motorways at east of 80mph, any opinion or thought on suggested tyre pressures? fronts are brigestone so3 as well - 225/45/17.
on a side note, can anyone suggest a reputable workshop near cornwall for a 24k service? the opc in exeter wants £470+!!!
ive just replaced my rear tyres with brigestone so3 rubber, 255/40/17 and i think they need more pressure than the current 36psi - really grippy though!
im currently spending some time on motorways at east of 80mph, any opinion or thought on suggested tyre pressures? fronts are brigestone so3 as well - 225/45/17.
on a side note, can anyone suggest a reputable workshop near cornwall for a 24k service? the opc in exeter wants £470+!!!
05-20-2003 | 08:33 PM
#2
Racer
Joined: Oct 2002
Posts: 393
Likes: 0
From: Commerce, Mi
Nick,
I can't suggest a tire pressure for you since every make of tire is different. The chalk marks are placed on the outer edge from the tread to the rim of the wheel. On most performance tires today you will find a triangle pointing towards the tread. This is the point that you want your tire roll to reach. If after a run you find that the chalk has not reached the triangle, let out 2lbs for every 1/4 inch until you reach the point. If you are beyond the triangle then add air in the same measurement.
After the auto cross, or next morning, check you tire pressure and record the results. This is what you want your cold tires to read so that when they get hot you will be on the mark.
A few other things to consider is that the type of surface that you are running on will cause the tires to run at different temperatures and affect you pressure settings. Also if you run back to back laps you will have to check the pressure after each run because the pressure will go up with the temp of the tires.
Good luck.
I can't suggest a tire pressure for you since every make of tire is different. The chalk marks are placed on the outer edge from the tread to the rim of the wheel. On most performance tires today you will find a triangle pointing towards the tread. This is the point that you want your tire roll to reach. If after a run you find that the chalk has not reached the triangle, let out 2lbs for every 1/4 inch until you reach the point. If you are beyond the triangle then add air in the same measurement.
After the auto cross, or next morning, check you tire pressure and record the results. This is what you want your cold tires to read so that when they get hot you will be on the mark.
A few other things to consider is that the type of surface that you are running on will cause the tires to run at different temperatures and affect you pressure settings. Also if you run back to back laps you will have to check the pressure after each run because the pressure will go up with the temp of the tires.
Good luck.
05-20-2003 | 08:33 PM
#3
Rennlist Member
Joined: Mar 2003
Posts: 4,328
Likes: 0
From: Nashua, NH
Make a chalk line from the bead on the inside of your tire to the bead on the outside. Make a lap at speed and inspect the tires. The optimal condition is that none of the chalk on the sidewalls is scrubbed off and all of the chalk on the contact patch is gone as well as a little on the sides of the contact patch from tire flex. Too little pressure and you'll scrub your sidewalls and leave chalk in the middle of your contact patch. Too much pressure and you won't scrub the chalk off the entire contact patch. The optimal pressure for an autocross is NOT the same as the optimal pressure for daily driving.
