'rolling diameter' of tyres: clarification please?
Submitted: Friday, Nov 11, 2005 at 09:59
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bware
Hi all,
Yes more about tyres! This issue about 15mm/ 50mm increase in rolling diameter means what? Are they talking about the diameter or the circumference and why don't they just call it so or is it something else?
Cheers,
Brian.
Reply By: Turbo Diesel - Friday, Nov 11, 2005 at 10:18
Friday, Nov 11, 2005 at 10:18
Brian
Check out the info on 'Tyre size the debate the law' it should answer your questions, however don't be suprised if you are more confused than ever before as I'm.
Cheers
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Follow Up By: bware - Friday, Nov 11, 2005 at 10:35
Friday, Nov 11, 2005 at 10:35
Hi Turbo Diesel,
Exactly! I did read it!
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Reply By: blown4by - Friday, Nov 11, 2005 at 10:24
Friday, Nov 11, 2005 at 10:24
They are talking about diamter.
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Follow Up By: bware - Friday, Nov 11, 2005 at 10:44
Friday, Nov 11, 2005 at 10:44
Thanks mate.
We know that we can always rely on bureaucrats to make things sound more complicated than it is! The fact that they couldn't just use the word 'diameter' on its own made me think it might mean something else like the diameter from top to bottom of the tyre while on the vehicle which would be less due to the compressed part of tyre touching the ground(or some other obscure definition).
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Follow Up By: Truckster (Vic) - Friday, Nov 11, 2005 at 11:12
Friday, Nov 11, 2005 at 11:12
at what pressure - 5psi or 55psi?
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Reply By: robak (QLD) - Friday, Nov 11, 2005 at 11:08
Friday, Nov 11, 2005 at 11:08
Just having a guess -
1. An overall diameter is when the tyre off the ground
2. A rolling diameter is when the tyre is on the ground (ie rolling)
R.
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Follow Up By: Member - JohnR (Vic)&Moses - Friday, Nov 11, 2005 at 11:43
Friday, Nov 11, 2005 at 11:43
my thought is 2. actually robak and that is entirely up to the tyre pressure. I haven't been able to get a real definition from my searches but it can stand to reason unitil some guy pulls you up to challenge.
The tyre pressure then comes into play if you let your tyres down to match the specification to get the 50mm rolling diameter. Means you can go to 33 inch tyres at low pressure as long as the origial tyres were at the top of the pressures as to the plated info ;-)
Well thats for my Nissan,,,,,,,,
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Reply By: Sky Pilot - Friday, Nov 11, 2005 at 12:34
Friday, Nov 11, 2005 at 12:34
Actually, the only one that really makes sense to me is the rolling diameter which would be twice the distance from the road to the centre of the axle when the tyre is on the car inflated to normal (plated pressure). This measurement is the one that makes the gearing give the kmh/1000rpms that it does and that the speedo must be calibrated for.
The pressure would vary that measurement, but it should be at the recommended pressure for that car - probably for the higher speed.
If you want to be really confused, have you ever watched what happens to the tyre diameter of a pair of slicks on a drag car when they take off. The diameter grows significantly. Somehow, I don't think that is relevant to 4WD though:-)
I would think there are so many variables here that if you were within 3 or 4% of the legal value, it would be difficult to sustain a conviction in court. A good lawyer would make mincemeat of these waffley regs.
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Reply By: Member - Jay Gee (WA) - Friday, Nov 11, 2005 at 14:40
Friday, Nov 11, 2005 at 14:40
In the end analysis - the only definition that will really matter is the definition that a judge will accept when this finally comes up as a test case in court.
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Reply By: Ozrunner - Saturday, Nov 12, 2005 at 02:42
Saturday, Nov 12, 2005 at 02:42
Skypilot has the correct interpretation.
Although just to confuse the buggers more its would be twice the loaded radius = the rolling diameter.
Loaded radius being the height of your tyres from the ground to the middle of the axle as inflated and loaded etc etc and x 2 gives the exact rolling diameter of your actual tyre.
In my case my so called 32" A/T have a loaded radius of 15" meaning they have a rolling diameter of 30" so in reality I have 30's not 32's !!!!!!. Its this measurement that you use to calculate speedo reading v RPM's etc.
I've yet to see a tyre that is close to the exact height etc of whats on the sidewall when fitted and in a loaded state, ie weight of vehicle etc.
JD
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