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 .

Cheers
Col

I use something like this to calculate the circumferences, any use?

https://tiresize.com/tyre-size-calculator/

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.

The best is either the distance from the centre of the wheel to the top of the tyre (unloaded radius), or one of the tyre comparison sites like this:

Hope that helps

Guy 2006 RRS S/C CONSTANTLY under modification...
1994 RRC 4.2l not LSE sadly gone but not forgotten
LR Defender 300Tdi - affectionately know as "The Black Beast"

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.


Cheers
Col

Sorry if I'm coming across as a bit thick, a very long time since I was at school but how can the circumference of the tyre change, I would have thought that it would have been a constant

That's correct but I don't think a tyre would 'stretch' along it's circumference, just change shape. The circumference should stay constant.

EG take a 30cm piece of paper and make a tube out of it. The circumference will be 30cm. If you then flatten one spot the circumference will still be 30cm... Very over simplified as paper doesn't stretch and rubber does subject to its design parameters. That's why tyres are belted.

Further if tyres did change circumference significantly depending on loading and air pressure then speedos would constantly vary in their accuracy. This would further be amplified by centrifugal force as they spin more quickly at higher speeds.


Guy 2006 RRS S/C CONSTANTLY under modification...
1994 RRC 4.2l not LSE sadly gone but not forgotten
LR Defender 300Tdi - affectionately know as "The Black Beast"

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.

Cheers
Col

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.

You can calculate the circumference from the radius, or the diameter, using simple maths. However, the rolling circumference is sometimes given by the manufacturer. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

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.




The circumference may change at high speeds as you see on dragsters, this is why regular tyres are belted and radial to prevent ballooning otherwise sports cars couldn't run the tyres so close to the fenders etc without risk of shredding them at speed...

Guy 2006 RRS S/C CONSTANTLY under modification...
1994 RRC 4.2l not LSE sadly gone but not forgotten
LR Defender 300Tdi - affectionately know as "The Black Beast"

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?

The point is the radius between the point of contact on the road and centre of wheel will always be the same lower figure, thus in principle lowering the circumference figure surely. As the rolling circumference is not actualy the circumference of the the wheel but the distance covered in on revolution.

Cheers
Col

Last edited by Col on Wed Sep 02 2020 10:29am. Edited 1 time in total

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.

If the contact patch angle is 45 degrees, the contact patch would be 283mm long as opposed to the arc length (assuming the contact patch was part of the continuous circle) 290mm. So the change in total circumference is only going to be 7mm, assuming a 370mm radius. That's probably a long contact patch so the real situation is probably less than that.

I think.  2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

I think this could go on and on what I will do tomorrow is mark the tyre on the car and corresponding mark on the floor and move the car to turn the wheel one revolution and see how far it travels, and do the same with a loose wheel just to see if there is any real difference and leave it at that as my brains starting to hurt.

ROLLING CIRCUMFERENCE = The linear distance travelled by a tyre in one revolution.

Cheers
Col

Last edited by Col on Wed Sep 02 2020 10:46am. Edited 1 time in total

To prove it, why don't you
1. drive over a length of string then wrap it around the rest of the tyre & measure it
2. By maths measure the diameter, then apply the formula 3.142 x Dia, which gives Circumference

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.

Dave 2010 3.6 TDV8 HSE (Gone)
2011 Discovery 4 Commercial SDV6 (Gone)
1980 OBLIC 4.0ltr Range Rover (went a long time ago)

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. as if both size wheels do the same amount of revolutions if my calculation are some where near correct the smaller 20" tyre/wheel having traveling 1 mile (1609 Mtrs.) the larger 21" would have travelled approx. 1735 Mtrs. and I really ought to have better things to do.

Cheers
Col