A quick tyre question

[ChemicalChaos]

Does the stated size of a tyre (ie width(mm)/profile(%) ) relate to the size of the carcass, or the size over the tyre overall including tread blocks? I ask because I have long wondered in the case of, for instance, knobbly off road tyres - do they start out massively oversize and wear down to stated carcass size, or start off at stated size and wear down to massively undersize?

[Hooli]

I think it's the carcass as when I had a 4x4 on knobbly tyres they were massively oversize.

[Dog Star]

Anything that needs the aircon regassing.

[ChemicalChaos]

Dog Star said:

Anything that needs the aircon regassing.

Come again?



Thanks Hooli

[daveenty]

Dog Star said:

Anything that needs the aircon regassing.

Why do I think that this was meant to be in the "deal breakers" thread

[Captain Muppet]

ChemicalChaos said:

Does the stated size of a tyre (ie width(mm)/profile(%) ) relate to the size of the carcass, or the size over the tyre overall including tread blocks? I ask because I have long wondered in the case of, for instance, knobbly off road tyres - do they start out massively oversize and wear down to stated carcass size, or start off at stated size and wear down to massively undersize?

It's a nominal size, so the actual tyre size can be larger than nominal when new, and wear until it's smaller than nominal.

From ISO4000-1:2001...

Maximum width in service = design section width x coefficient “a” OR design section width + 8mm whichever is greater.

Maximum diameter in service = Rim diameter + (2 x Section height x coefficient “b”)

Minimum width in service = design section width x coefficient “c”.

Minimum diameter in service = Rim diameter + (2 x Section height x coefficient “d”)

All rounded to the nearest mm.

For radial tyres the coefficients are: b=1.04, c=0.96, d=0.97. A depends on aspect ratio but is 1.04 for any tyre of 70 ratio or lower.

So for a 195/50R15 tyre:
Rim diameter = 381
Section height = 97.5
Max width = 203mm
Min width = 187mm
Max diameter in service = 584mm
Min diameter in service = 569mm

Or thereabouts, depending on whether I hit the wrong button on my calculator or not. I should have done it in Excel really.

ETA: I definitely should have done that in Excel as the section width of a 195 tyre is 201 and the section height of a 195/50 is 98, both according to the standard and fitted to the correct rim. So those numbers are all a bit small. If you really care then do your own maths.

Edited by Captain Muppet on Thursday 13th March 14:51

[Captain Muppet]

You're welcome.

[ManFromDelmonte]

Captain Muppet said:

You're welcome.

Bet you're glad you didn't bother with Excel now eh?

[ChemicalChaos]

Only just seen this. Thanks for the very detailed explanation, I appreciate the effort

[Captain Muppet]

ChemicalChaos said:

Only just seen this. Thanks for the very detailed explanation, I appreciate the effort

Great, now I feel like a graceless idiot.

Glad to have been useful