Cycling UK Forum

I often notice discrepancies between the width measurements in French and ISO/ETRTO sizes for the same tyre. It particularly seems to be the case with 37-622 700x35C which is so common that it has made it into some standard sizing tables like this one from Wiggle:

Why don't the width measurements using the two systems always match and why is it particularly the case with this tyre size?

And while you're answering that, what do the numbers represent? where would I find a measurement of 700mm (27.5") on such a wheel/tyre? Or 622mm?

Hi
Most of your questions are answered on this webpage...... and on this one
622 is the rim bead seat diameter
700 is the approx overall diameter of the tyre

As to the 35 or 37 difference, I believe that this comes from the slight differences in tyre width according to the rim that a tyre is fitted to, pressure and production tolerances (+/- 3mm) and has been adopted by the French as an approximation, i.e. 700mm outer diameter and 35mm wide

37mm ETRTO is more precise (within tolerances )

Regards
tim-b

Yeah, I understand the 622/700C bit.

What I don't understand is the width measurement which surely ought to be exactly the same measurement of the tyre in mm. Are you saying that the French system presumes a narrower rim or a different pressure?

I've heard it suggested before that the ETRTO width ought to be more accurate, but I'm not so sure. It is my experience that on average tyres come in under size, so the 35mm French designation would generally be closer to reality than the 37mm ETRTO.

I think my answer may be in the table I shared in the OP
35mm is suspiciously close to 1 3/8in. Is the French designation a metricised conversion of 28 x 1 5/8 x 1 3/8?

A thorough guide to the subject http://www.sheldonbrown.com/tire-sizing.html . It includes a section on dishonesty in sizing.

Thanks, I've read that article before. It doesn't explain why French and ETRTO widths sometimes differ. I can't put this down to dishonesty ether. I'm not disputing that dishonesty happens, but surely if you were going to be dishonest you'd be dishonest with both width measurements not just one?

Bicycler wrote:I think my answer may be in the table I shared in the OP
35mm is suspiciously close to 1 3/8in. Is the French designation a metricised conversion of 28 x 1 5/8 x 1 3/8?

Well for 28 & 32 mill tyres the width sizes in teh chart are near as dammit the same for Iso, metric and imperial. Only for 35 is it different and the metric and imperial sizes match. Two out of three agree on 35mil so is that the real size?

there quite a few similar discrepancies in this chart

http://www.harwoodcycles.com/tyre_size.htm

Personally I think that if a tyre labelled 37-622 is from a reputable manufacturer, it should be 37mm wide on a rim of the primary sort that is used with this size.

That it might also be labelled 700x35C or even be 35mm wide on some rims may be due to a number of reasons, including that tyre manufacturers probably don't want to make two different tyres so close in size, but would like to sell tyres to people who seek either fitment....

cheers

Cheers for that Brucey. I'd hope the ISO to be more accurate though experience has shown actual tyre sizes to be a bit of a lottery.

Looking at the discrepancies in that table I think there might be something to my earlier suspicions about metric conversions of imperial sizes.

1 3/8in = 34.9mm (700x35C)
1 1/2in = 38.1mm (700x38C)
1 5/8in = 41.3mm (okay, this one doesn't fit 700x40C)
1 3/4in = 44.5mm (700x45C)

and continuing onto the other popular 700C sizes:
32mm = 1.26in ≈ 1 1/4in
28mm = 1.10in ≈ 1 1/8in
25mm = 0.98in ≈ 1in

I wonder if the standard 700C sizes were initially chosen either to make for easier sales to countries used to imperial measurements, or else because 1/8in increments had already been established as an industry standard?

The latter. The UK was a major bike manufacturing country in the eary days, until USA mass production and export arose.

LWaB wrote: ...until USA mass production and export arose.

eh? US-made bikes have never been big sellers abroad. There are big-selling US brands, but with few exceptions 'their products' have been manufactured in the far east.

cheers

The markings on my Marathon Racer (Evolution Line) are 35-622 (28x1.35-700x35C). How does that fit in?
Hypers are marked 32-622 (700x32C) and the larger ones 37-622 (700x35C) 28 x1 5/8 x 1 3/8. (All these markings are those embedded in the side wall and not those easily readable.)
I haven't checked other tyres.

I'm only surmising that the 700C common sizes were at one point based upon metric equivalents of 1/8in increments and sold elsewhere as 28 x X/8in. I can understand manufacturers keeping to this if it has become standard.

MikeF wrote:The markings on my Marathon Racer (Evolution Line) are 35-622 (28x1.35-700x35C). How does that fit in?

Your Racers, on the other hand, seem to have started as metric measurements which have been converted into an decimal imperial equivalent (35mm≃1.35in). This also makes sense in a world where metric measurements (French and ETRTO) are now much more commonly used than imperial in this size of tyre. It also allows for non 1/8in increment sizes such as 30-622 700X30C which is also becoming fairly common.

So I can understand why the industry is not consistent. What doesn't make any sense is why a company like Schwalbe is not internally consistent. Why are normal Marathons 37-622 (700X35C) but Racers 35-622 (700X35C)? The 40-622 Racer is 700X38C

Bicycler wrote:
So I can understand why the industry is not consistent. What doesn't make any sense is why a company like Schwalbe is not internally consistent. Why are normal Marathons 37-622 (700X35C) but Racers 35-622 (700X35C)? The 40-622 Racer is 700X38C

Because the '700xXXC' designation is 'the fitment' and the ISO/ETRTO size is meant to be the 'actual size of the tyre'....?

This way you can make a tyre in a non-standard size (eg 40mm) to replace one that is a 'standard fitment' eg 700x38C.

BTW this appears to be the way that Vittoria Hypers are marked too...?

cheers