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![]() | Home > Wheels & Tyres > Rolling Circumference |
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Col Member Since: 02 Sep 2012 Location: Hawkes Bay NZ Posts: 5041 ![]() ![]() ![]() |
It's long time since I have been to school, but if I measure the distance from the ground to the centre of the wheel on a loaded wheel/tyre i.e. fitted to the car, multiply this by x 2 then by 3.14 will this give me the correct rolling circumference as was going to play around with tyre/wheel size settings in the CCF files .
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david007 Member Since: 20 Jun 2017 Location: Forfar Posts: 166 ![]() ![]() ![]() |
I use something like this to calculate the circumferences, any use?
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Guyzer73 Member Since: 04 Jul 2009 Location: Muscat Posts: 852 ![]() ![]() ![]() |
No, as the weight of the car will reduce the radius of the tyre, reducing the rolling circumference calculation. The difference will be minimally less.
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Col Member Since: 02 Sep 2012 Location: Hawkes Bay NZ Posts: 5041 ![]() ![]() ![]() |
Unfortunately those conversion charts don't take into account the deflection in a loaded tyre sidewall which in effect reduce the radius of the wheel thus reducing the overall circumference. If you do it on a unloaded tyre surely it will not be a true reading of distance covered when on the vehicle.
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david007 Member Since: 20 Jun 2017 Location: Forfar Posts: 166 ![]() ![]() ![]() |
Sorry if I'm coming across as a bit thick, a very long time since I was at school |
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Col Member Since: 02 Sep 2012 Location: Hawkes Bay NZ Posts: 5041 ![]() ![]() ![]() |
Because you have the height of the tyre sidewall changing when you add the weight of a 2 ton vehicle to it reducing the radius between the point of contact on the ground and the center of the wheel.
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RRSTDV8 Member Since: 12 Aug 2011 Location: Northamptonshire Posts: 9022 ![]() ![]() ![]() |
The circumference will change slightly between a non-fitted item and a fitted, loaded item. Don't forget that the tyre is a flexible item and it is the flexing, stretching and compression that heats the tyre in use. This flexing gives a local change in radius. Whether the change in circumference is significant I don't know.
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Guyzer73 Member Since: 04 Jul 2009 Location: Muscat Posts: 852 ![]() ![]() ![]() |
Check this. Here they change the tyre pressure, but it's the same principle the static circumference doesn't change, just the shape of the tyre.
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Col Member Since: 02 Sep 2012 Location: Hawkes Bay NZ Posts: 5041 ![]() ![]() ![]() |
Just did a quick measure on a loose spare wheel and one fitted to the car both same wheel with brand new Pirelli Verde tyre fitted. loose wheel radius 390mm, fitted loaded wheel radius ground to centre of wheel 370mm approx. 20mm deflection in tyre, so circumference of loose wheel is 2449mm, fitted wheel 2292mm so around 155mm difference in travel per revolution, so which figure would be most accurate when setting the rolling circumference in the CCF file?
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RRSTDV8 Member Since: 12 Aug 2011 Location: Northamptonshire Posts: 9022 ![]() ![]() ![]() |
Col, the change in radius is only local to where the tyre is deflected in contact with the ground so the change in circumference is very much less than you suggest. You'd need to know the length of the contact patch (the length that the tyre is flat on the ground). From that you can work out angle of the contact patch and thus the length of the arc that is flattened.
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Col Member Since: 02 Sep 2012 Location: Hawkes Bay NZ Posts: 5041 ![]() ![]() ![]() |
I think this could go on and on |
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Lindab Member Since: 20 Nov 2017 Location: Dundee Posts: 919 ![]() ![]() |
To prove it, why don't you
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Dave B Member Since: 10 Oct 2019 Location: East/West Sussex Coast Borders Posts: 431 ![]() ![]() ![]() |
As Lindab says, or put some gaffer tape down and mark where the centre of the tyre is on it, move the car until it comes back to it, then measure, hey presto you have your figure.
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Col Member Since: 02 Sep 2012 Location: Hawkes Bay NZ Posts: 5041 ![]() ![]() ![]() |
Did actual comparison with same wheel both on and off the car, there is approx. 50mm difference in rolling circumference of the wheel, the fitted and loaded wheel on the car being the smaller circumference at 2405mm the loose wheel was 2455mm which equates to around less than two percent which I guess is neither here nor there really that is on a 275/45x21". The factory setting in the CCF for the rolling radius on a 275/40x20" is 2220mm which is quite a difference. Although the speedo is spot on I guess it will make the mileage reading wrong and the dash mpg lie-o-meter lie even more. |
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