18 replies to this topic

[Don't Panic]

so a 175/50/13 wheel would have a diameter of 19.88976 (managed it myself :tongue: ) not a huge amount of difference between all three in diameter then.

oh and i didn't know the middle figure was a percentage of the tyre width in mm, cheers for correcting me.

[Steve@RetroDash]

i'm a busy lad no time for sums :grin:

[Dan]

There is a significant difference between some Mini tyres as the diameter when calculated like that is the unloaded diameter of the tyre. When loaded up the tyres all compress by different ammounts depending on how low the profile is, how high the tyre pressure is, how heavy the car is, temperature, camber, tread wear, etc etc. The effective size of the tyre you need to know when working out things like road speed from gearbox ratios is the rolling diameter which I believe there is a table of in the FAQ section.
To check the difference for yourself, calculate the diameter of your wheel based on it's size as above and then go out and measure it from the center to where it sits on the road and double it. That's the rolling diameter.
Incidentally pretty much every tyre for a Mini is low profile, normal profile tyres are known as super baloon tyres and have a sidewall height the same as the width so if there's a number in the size after the width as in 165/70 then it's low profile.

Edited by Dan, 26 January 2006 - 09:56 PM.

[Sprocket]

Standard 10 tyre is 145 x10. A tyre without profile designation is normaly, rule of thumb 80 profile or above, but profile designation do not start until 70 as this is a fairly modern term. Most tyres these days will have at least a 70 profile with the rubber bands being in the region of 40 and 35 profiles. These are the LOW profile tyres and IMHO anything 50 and below is a low profile tyre. The thing to watch when the profile gets smaller is the width will normaly increase. I wish they did a tyre in 175/45x14 as 14s would then be no problem in fitting