Why is tyre profile shown as a percentage of width?

[thr32]

Evening all,

Idle curiosity only... Convention dictates that a tyre's profile is expressed as a percentage of its width, but I wonder whether this actually serves any purpose: in order to keep the rolling circumference the same when I change rim size, I would either need to convert (and double) the profile to a real number and add to the rim size, or convert the rim size to a percentage and add to the profile. Equally, if I buy a 5mm wider tyre, the profile label will change even if the profile doesn't.

What am I missing?

Thankyouplease

TH

ETA: cringe at the typo in the thread title. I'll get me coat.

Edited by thr32 on Tuesday 10th May 18:32

[Plymo]

I think it's just following the convention from the old measurements (e.g. 4.5x14 for a 4.5" wide tyre on a 14" rim) which basically had just one profile.

I suppose it's because it's easier to see a 255x45 tyre is low profile, while although a hypothetical "255x115" size would describe it's measurements accurately, it would be harder to tell if it's lower or higher profile than a "185x100" tyre - but if it was listed as 185x55 it would be obvious.

[Pica-Pica]

thr32 said:

What am I missing?

Maths?
Quite simply it is the ETRTO convention
(European Tyre and Rim Technical Organisation)

[Munter]

Pica-Pica said:

Maths?
Quite simply it is the ETRTO convention
(European Tyre and Rim Technical Organisation)

He's looking for why, not what.

[E-bmw]

thr32 said:

in order to keep the rolling circumference the same when I change rim size, I would either need to convert (and double) the profile to a real number and add to the rim size, or convert the rim size to a percentage and add to the profile.

Alternatively you could just go to "willtheyfit.com" and see what sizes equate to what you currently have fitted.

[Scrump]

Plymo said:

I suppose it's because it's easier to see a 255x45 tyre is low profile, while although a hypothetical "255x115" size would describe it's measurements accurately, it would be harder to tell if it's lower or higher profile than a "185x100" tyre - but if it was listed as 185x55 it would be obvious.

I think the opposite of this. The current way of expressing sidewall height can be misleading.
Your example of a 255x45 being low profile whilst a 185x55 is not low profile, this makes many people think the 185x55 has a taller sidewall as it is 55 series and not low profile. In reality it has a smaller sidewall than the low profile 255x45 tyre so arguably has a lower profile.

185x100 and 255x115 immediately tells you which tyre has the taller sidewall and tells you what that measurement is.

[996TT02]

The figures are not an indication of sidewall height, but aspect ratio.

• Aspect ratio*

Why should this be construed as being an indication of sidewall height? That is secondary, and relative.

The rolling diameter is not solely a function of sidewall height, but wheel (rim) diameter. So again sidewall height is only a part of it.

The aspect ratio is what most people actually care about anyway, not sidewall height. It determines the "type" of tyre, the way it performs, and certainly how it appears. A skinny tyre with high aspect ratio but low sidewall height does not look better than a wide tyre with low aspect ratio but actually greater sidewall height. Nor does it perform better, in a "sporty" sense, but is usually better for comfort.

So aspect ratio is actually what people tend to care about. It affects performance and appearance. No one cares that a sidewall is 80 or 100mm high. Because, the relevance of that is relative, and requires further information to become relevant, whereas aspect ratio instantly gives the required information.

If you want to know how aspect ratio affects physical size for a given width, just plug in the figures into one of lots of online size calculators.