'rolling diameter' of tyres: clarification please?

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

They are talking about diamter.

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.

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.

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.

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