11 replies to this topic

[Cooperman]

There has always been a belief that using a 10" wheel will make a car rev much higher for a given final drive ratio and speed.

 

I just did some calculations on the rolling radius of 10", 12" and 13" wheels with all the usual tyre sizes:

 

145/80 x 10                 Nominal Rolling Radius 9.567"

165/70 x 10                                                        9.547"

145/70 x 12                                                        9.990"

165/60 x 12                                                        9.890"

175/50 x 13                                                        9.940"

 

So it can be seen that the difference between the smallest rolling radius and the largest is a mere 4.64% and that is with new tyres. So in reality it makes virtually no difference. The highest profile is a 12" wheel with a 145/70 tyre fitted. Some tyres, such as a 145/70 x 10" winter tyre will have a higher rolling radius due to the greater tread depth.

[Swift_General]

Interesting. I'd have guessed the difference would have been more. Does this take into account the 'squash' of the tyre. I guess even if it doesn't the a comparison will still yeild similar relative results?

Edited by Swift_General, 13 July 2015 - 05:14 PM.

[ChopperHarris]

thanks for that, I was thinking about this as I was driving around this morning

[nicklouse]

Cooperman what do you mean by rolling radius?

[Cooperman]

The 'Nominal Rolling Radius' is the distance measured vertically from the centre of the wheel to the ground with no allowance for deflection of the tyre-wall due to vehicle weight. Thus it determines the circumference of the tyre and from that the number of revolutions per mile travelled.

In practice the deflection due to vehicle mass can be ignored as it is almost the same for each tyre at the same pressures. If a more accurate figure is required, for example, for calibration of trip meter or speedo, then a physical measurement of that dimension can be made which will factor out tyre deflection, tread depth remaining and tyre pressure.

For our use here the nominal rolling radius is adequate to illustrate the comparison between wheel/tyre sizes and their effect on overall gearing.

 

I hope this helps.

[nicklouse]

Right nothing special.

Just like when running cross ply slicks they give the diameter.

And for Minis we tend to look at 19.5" or 20" diameter.

[Cooperman]

Right nothing special.

Just like when running cross ply slicks they give the diameter.

And for Minis we tend to look at 19.5" or 20" diameter.

Yes, with racing tyres the overall diameter inflated is normally given, but with road tyres it's the tyre width and wall profile together with the wheel diameter which is given and the rolling radius or circumference has to be calculated, or measured if you already have the tyres and wheels.

The reason I posted the figures is because owners often state on here that they have a certain gearing and feel it would not be suitable for a smaller road-wheel diameter, e.g.: a 10" wheel, whereas in practice it makes no actual difference.

[nicklouse]

Exactly.

Again most wheel and tyre sites will give this info.

The reason for my first question I was thinking you were adding some magic formula but it could not see it.

[Cooperman]

No I was just clarifying the situation regarding the change in effective gearing with different size wheels/tyres.

[Boycie]

It doesn't look like much but the difference in revs (and nvh) between a 165/10 and a 145/12 is certainly noticeable in my opinion.  So much so, that I did used to put wheels with 145/80r12 tyres on the front of my '75 mk3 temporarily if I had a long motorway journey to do...

[Cooperman]

It doesn't look like much but the difference in revs (and nvh) between a 165/10 and a 145/12 is certainly noticeable in my opinion.  So much so, that I did used to put wheels with 145/80r12 tyres on the front of my '75 mk3 temporarily if I had a long motorway journey to do...

Yes, it makes about 4% difference, so if you have the smaller 165/70 x 10 you would be pulling 160 rpm more at 4000 rpm than with the 145/70 x 12". Not that much difference really. That's not much difference from the additional revs when the tyres are down to about 1.8 mm tread depth from the as-new tread depth.

[Guess-Works.com]

or...

 

http://www.guess-wor.../Tech/ratio.htm

 

a 5% difference in rolling radius is equivalent to a drop or gain in a diff ratio... so running a 2.9 instead of a 3.1, or a 3.6 instead or a 3.4

 

and you will hear people banging on about how a diff ratio can affect the attributes of a car or engine.

Edited by Guess-Works.com, 14 July 2015 - 08:01 AM.