Tire Diameter Question?
01-24-2011, 03:01 PM
#1
Drifting
Thread Starter
Tire Diameter Question?
In tweaking my software when switching between street tires and race rubber with HP Tuners I was looking closer at listed tire diameters vs. revolutions per mile. They don't match.
For instance, a F1 Supercar EMT 285/35ZR19 at Tirerack lists a diameter of 26.9" and 775 rev/mile.
If my math isn't too hosed 775 rpm should equal a 26.02 diameter. Which diameter would you enter into the program and why?
TIA
--Dan
For instance, a F1 Supercar EMT 285/35ZR19 at Tirerack lists a diameter of 26.9" and 775 rev/mile.
If my math isn't too hosed 775 rpm should equal a 26.02 diameter. Which diameter would you enter into the program and why?
TIA
--Dan
01-24-2011, 04:04 PM
#2
Drifting
Member Since: Dec 2005
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The measured circumference will be different than the mounted with weight on it rolling circumference.
That could be an explanation for the difference.
Regardless, it seems the rolling circunference would be the one you should use.
That could be an explanation for the difference.
Regardless, it seems the rolling circunference would be the one you should use.
01-24-2011, 08:55 PM
#3
Tech Contributor
Hi Dan,
Gregg has it right. We always measure [ NOT diameter] circumfrence with the tire correctly inflated, mounted, and the corner weight on it. Also, be aware that small variations in inflation pressure easily show up in this measurement. And, to state the obvious, tire wear will change that measurement as time goes on.
Hope this helps,
Ed LoPresti
Gregg has it right. We always measure [ NOT diameter] circumfrence with the tire correctly inflated, mounted, and the corner weight on it. Also, be aware that small variations in inflation pressure easily show up in this measurement. And, to state the obvious, tire wear will change that measurement as time goes on.
Hope this helps,
Ed LoPresti
Last edited by RacePro Engineering; 01-25-2011 at 01:11 AM. Reason: Correcting CIRCUMFRENCE
01-24-2011, 09:31 PM
#4
Drifting
Thread Starter
Thank you, both. 
Then, it will be more accurate to calculate the diameter using the revolutions per mile (when known) and use that figure. That will certainly change some of the figures I have used as most tires seem to be around an inch shorter than listed sizes indicate.
I'll give it a test this weekend on the track. I'll also be interested to see what effect there is on AH/TC on the street with the more accurate rear wheel diameter plugged in.
Then, it will be more accurate to calculate the diameter using the revolutions per mile (when known) and use that figure. That will certainly change some of the figures I have used as most tires seem to be around an inch shorter than listed sizes indicate.
I'll give it a test this weekend on the track. I'll also be interested to see what effect there is on AH/TC on the street with the more accurate rear wheel diameter plugged in.
01-25-2011, 01:13 AM
#5
Tech Contributor
I am sorry, Dan,
I inadventantly used the wrong term (diameter) in my post, when I really meant CIRCUMFERENCE. Even using the circumference, this is, at best, an approximate measurement. Obviously, when the tire is "at speed" on the track, friction heat and centrifugal force combine to expand the measurement.
We take these readings simply to ensure that we are not chasing some handling difficulty brought on by mis-matched pairs of slicks. The assumption is that each pair on a given axle will more-or-less expand the same percentage on the track.
I apologise for any confusion using the incorrect term.
Ed
I inadventantly used the wrong term (diameter) in my post, when I really meant CIRCUMFERENCE. Even using the circumference, this is, at best, an approximate measurement. Obviously, when the tire is "at speed" on the track, friction heat and centrifugal force combine to expand the measurement.
We take these readings simply to ensure that we are not chasing some handling difficulty brought on by mis-matched pairs of slicks. The assumption is that each pair on a given axle will more-or-less expand the same percentage on the track.
I apologise for any confusion using the incorrect term.
Ed
Last edited by RacePro Engineering; 01-25-2011 at 01:26 AM. Reason: Spelling
01-25-2011, 04:42 AM
#6
Drifting
Thread Starter
I am sorry, Dan,
I inadventantly used the wrong term (diameter) in my post, when I really meant CIRCUMFERENCE. Even using the circumference, this is, at best, an approximate measurement. Obviously, when the tire is "at speed" on the track, friction heat and centrifugal force combine to expand the measurement.
We take these readings simply to ensure that we are not chasing some handling difficulty brought on by mis-matched pairs of slicks. The assumption is that each pair on a given axle will more-or-less expand the same percentage on the track.
I apologise for any confusion using the incorrect term.
Ed
I inadventantly used the wrong term (diameter) in my post, when I really meant CIRCUMFERENCE. Even using the circumference, this is, at best, an approximate measurement. Obviously, when the tire is "at speed" on the track, friction heat and centrifugal force combine to expand the measurement.
We take these readings simply to ensure that we are not chasing some handling difficulty brought on by mis-matched pairs of slicks. The assumption is that each pair on a given axle will more-or-less expand the same percentage on the track.
I apologise for any confusion using the incorrect term.
Ed
I am just looking for a more accurate measurement to enter into the program to control shift speeds and speedometer, etc. Calculated (by tire size) or listed diameter wasn't working.
Thanks, Ed.