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Old 04-09-2007, 04:45 PM   #11
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Originally Posted by mrmad View Post
"Sure the diameter doesn't grow appreciably BUT the distance between axle and road rises with increased pressure, effectively increasing the "diameter"."

I guess I should have been clearer, the effective diameter (distance between axle and road) may change, but the radial belts keep the circumference from changing, effectively meaning there should be little change in the accuracy of the odometer/speedometer by changing tire pressure.
I'm going to have to disagree a little here. I can notice my car rising a little when the tire pressure is low. How can the circumference not change if the diameter is changing? Doesn't make sense to me. Not trying to start an arguement here at all. It just mathmatics since diameter is used to calculate circumference, there must be a change in circumference if diameter changes.

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Old 04-09-2007, 05:04 PM   #12
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Tire curcumference and pressure

At first I got in an argument and then lost when confronted with what actually happens. I know it is a steel belt so the change has got to be really small. However, my Impalla looks at the wheel speed indicators and can conclude if a tire is low. It has set off the warning light twice. The tire was down around 15 psi when the warning came on. To put this in perspective after the tire is fixed and the warning is reset it can take up to 100 miles to again infer the size of the tire according to the manual.

Just a side note.

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Old 04-09-2007, 05:42 PM   #13
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Yes and there is a different amount of slip when a tire is low vs high in pressure. I have noticed that both my GPS readings are off from the SGII readings at 60mph i.e. 60.9mph on Garmin GPSIII+ is reading 60 on SGII when usually it would read 61mph - wondering if I should be rounding up past .5mph or drop down to lower reading. It would affect the distance traveled on the SGII if I increased the speed by 0.5 mph to be closer to the GPS.
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Old 04-09-2007, 07:42 PM   #14
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if the tire pressure is lower than the sidewall pressure you can increase the diameter of the tire by increasing pressure. So if the tire says 44psi max and you go from 32 to 38 then the tire effectively becomes bigger. If you go from 44 to 50 then the diameter does not change enough to be measured.

I think run the highest pressure you are willing to ride on and don't worry about it. Just make sure you run at least what the tire is rated for. I run 48psi cold in mine
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Old 04-09-2007, 09:26 PM   #15
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Originally Posted by mrmad View Post
"Sure the diameter doesn't grow appreciably BUT the distance between axle and road rises with increased pressure, effectively increasing the "diameter"."

I guess I should have been clearer, the effective diameter (distance between axle and road) may change, but the radial belts keep the circumference from changing, effectively meaning there should be little change in the accuracy of the odometer/speedometer by changing tire pressure.

I'm going to have to disagree a little here. I can notice my car rising a little when the tire pressure is low. How can the circumference not change if the diameter is changing? Doesn't make sense to me. Not trying to start an arguement here at all. It just mathmatics since diameter is used to calculate circumference, there must be a change in circumference if diameter changes.


No worries about not agreeing with me. In old bias ply tires, the overall diameter and circumference would increase as you increased pressure, like the tire was a ballon. The reason the circumference doesn't change (or at least by any significant amount) in radial tires is there is radial belts running the circumference of the tread that do not allow the rubber to act like a ballon. A tire with load on it, is not a perfect circle, it is deformed at the contact patch. The change you are seeing as you increase pressure is how much deformation occurs.
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Old 04-10-2007, 01:51 AM   #16
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Well, I went from quite a low pressure, to the rated pressure - so that is probably why I saw a difference!

As a side note - if a tyre is very flat (e.g. 8psi), imagine the shearing stresses it must undergo - it is no wonder that they blow out when this happens!.

Edit - the difference was 6% in my case, so it did make a difference. This is because I went from 28PSI (car specs) to 45PSI (tyre rating). Reading something elsewhere, I'm sorry if I caused any offence in this thread? I didn't mean to if I did.
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Old 04-10-2007, 06:48 AM   #17
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Reading something elsewhere, I'm sorry if I caused any offence in this thread? I didn't mean to if I did.

This forum is far too polite (maybe that's why I like it). Other forums I'm on, someone is bashing you if they disagree.
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Old 05-10-2007, 08:43 AM   #18
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Pressure Allert !

Hey the other day I check my tire pressure now that the temps are in the 60's and found they have climbed up to 45psi and I was running them at 40-42psi and had not checked them for about a month. You all may want to double check your tire pressures.
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