Understanding the Rolling Radius and the tyre Profile

Ok we all know that calculators are available to help you work out what your rolling radius should be, but I thought I would write this so that people could have an easy understanding of what the rolling radius is, and where the figures come from.

 

In this example I will use the stock rear wheel/tyre sizes of a TT Supra (255/40/17)

 

Breakdown of Tyre Info:

255 is the width of the tyre, this is given in millimetres.

40 is the profile of the tyre, this is given in percentage.

17 is the wheel size, this is normally given in inches.

 

 

Note:

The profile is a percentage of the width of the tyre. The "40" on the tyre denotes the value of the percentage.

 

With that in mind the rolling radius of such a setup is 317.9mm or 12.5”.

 

 

How this is worked out:

 

40% of 255 = 102mm (The Profile size in mm)

 

This is multiplied by 2, as the profile thickness occurs twice over the diameter, once at the top and once at the bottom (See Drawing)

 

Either convert 17” to mm or convert 204mm to inches and then simply add them together to get the overall tyre diameter.

 

Conversion:

17” = 431.8mm (17 * 25.4)

or

204mm = 8” (204 * 25.4)

 

Overall Tyre Diameter:

17” + 8” = 25”

or

204mm + 431.8mm = 635.8mm

 

To get the rolling radius the diameter is simply halved.

 

Rolling Radius:

25" / 2 = 12.5"

635.8mm / 2 = 317.9mm

 

 

With these stock figures in mind you can then work out your own tyre/wheel combinations to see what comes closest to the stock arrangement (what your speedo and stock alignment/suspension is setup to use).

 

 

Example:

 

If you had 18” wheels and you wanted wide tyres to compliment them, say 285mm, you would need to work out which profile would be best for such a setup. This is calculated as follows…

 

Convert Wheel Size to mm:

18” = 457.2mm (18 * 25.4)

 

The difference between the stock diameter (635.8mm) and the wheel size (457.2mm) is what gives you the required profile size.

 

Calculate Profile Required:

635.8mm – 457.2mm = 178.6mm

 

This is then divided by 2, again as the profile occurs twice, giving 89.3mm.

 

We now need to convert that to a percentage. As already stated the profile is a percentage of the width of the tyre. In this instance the required width of the tyre is 285mm so the calculation is as follows.

 

Profile as Percentage:

(100 / 285) * 89.3

 

The above equation works out what percentage 89.3 is of 285.

 

In order to have the exact same rolling radius as the stock wheel/tyre combo the profile would have to be 31.3333%.

 

Clearly you cannot have an exact profile so you then decide which of the closest profiles would suit you best. In this instance 30 would be the closest to stock choice. Some people may favour 35 in order to bring more comfort, this would throw out the speedometer further though.

 

Hope that wasn't too confusing

Very good. Blimey though, it must've taken you a while to write up.

 

 

 

 

In this instance 30 would be the closest to stock choice. Some people may favour 35 in order to bring more comfort, this would throw out the speedometer further though.

Actually I've found that increasing the aspect ration of the tyres by one (5%) increment makes the speedo more accurate rather less. Due to the stock speedo over-reading, you see. I mean, most speedos over-read by ~10%, so increasing the circumference of the tyre by 3 to 5 % makes the speedo more accurate.

 

 

Anyway, great work.

Very good. Blimey though, it must've taken you a while to write up.

 

 

 

 

Actually I've found that increasing the aspect ration of the tyres by one (5%) increment makes the speedo more accurate rather less. Due to the stock speedo over-reading, you see. I mean, most speedos over-read by ~10%, so increasing the circumference of the tyre by 3 to 5 % makes the speedo more accurate.

 

 

Anyway, great work.

 

 

Wasn't bad, in work a bit bored and there have been a couple of tyre queries lately so thought it would be handy.

 

Good point. I remember reading that manufacturers are encouraged to have speedos deliberately under read by between 5 and 10 percent.

 

Will leave it like that though, I don't want the blame of anyone's points. Any excuse and all that.

That's me taught then.

 

Deserves a sticky.

 

Cheers Scott.

Good thread Scott, should be in FAQ section for future reference

 

I'll stick this link in here as it also shows how different the rolling radius will effect speed.

 

http://www.ajdesigner.com/fl_tire/tire.php

Very useful, thanks

 

Next time you're bored, may I suggest you write another one about offsets, spacers and stuff? That confuses me more!

 

http://www.ajdesigner.com/fl_tire/tire.php

 

Thats a very nice calculator!

 

I actualy get quicker acceleration with my 17s over my stock 16s

Very useful, thanks

 

Next time you're bored, may I suggest you write another one about offsets, spacers and stuff? That confuses me more!

 

Agreed!

There's also a graphical representation of this in the tyre/gear calculator here: http://www.mkivsupra.net/vbb/showthread.php?225050