Tyre width

[fimm]

Stupid question of the day:
The Internet Says "the width of your tyre is printed on the side wall".
So my tyre has 47 - 406 (20 x 1.75). I think (if I has understood correctly) that the 47 - 406 bit is some sort of ISO standard, and the (20 x 1.75) bit means 20 inch wheel, 1.75 inches wide.

Is this correct? If not, which of the many numbers printed on the wheel and tyre should I be looking for?

[Tod28]

Yes 47-406 is the Erto? size and the 20 x 1.75 is Usasian. Imperial would be 20 x 1&3/4 but probably a different size to the USA.  Go by ERTO size to avoid confusion. 47 is the tyre width and 406 the rim (bead seat) diameter in mm.

[Cudzoziemiec]

In tyres, 1.75 is not the same as 1³/₄. The same applies to 1.50 and 1¹/₂, etc. I can't remember what the difference is or why (though I have a feeling it does involve Usanians and mtbs) but I expect a list can be found on Sheldon.

[PaulF]

For mountain bike tyres I thought the 1.75 referred to the maximum tyre width, not the bead seat diameter which is often significantly less

[Cudzoziemiec]

Saint Sheldon of the Sprocket says:

Does Point Seven Five Equal Three Quarters?
Inch-based designations sometimes express the width in a decimal (26 x 1.75) and sometimes as a common fraction (26 x 1 3/4). This is the most common cause of mismatches. Although these size designations are mathematically equal, they refer to different size tires, which are NOT interchangeable. It is dangerous to generalize when talking about tire sizing, but I would confidently state the following:

Brown's Law Of Tire Sizing:
If two tires are marked with sizes that are mathematically equal,
but one is expressed as a decimal and the other as a fraction,
these two tires will not be interchangeable. (well, there are three exceptions, noted in the tables below...)

https://www.sheldonbrown.com/tire-sizing.html

[BFC]

Headline tyre size (eg 20") was the nominal rolling diameter of a wheel and tyre combination, but is only vaguely correct for the type and width of tyre the sizing was designed for - then others come along and want narrower or fatter tyres that fit the same rim with the effective diameter reducing or increasing. Then there is tread depth.

The two metric numbers are the tyres designed width (on a specific rim width, which is yet another variable) and the diameter of the bead seat. If you have a wheel with a tyre on that fits, go by the bead seat diameter number, and consider fitting identical width or changing to suit you better (sometimes change is forced by lack of options though). If you find yourself with a bare rim or totally rotten (illegible) tyre that is a much greater challenge and involves careful measurement with a bit of best guessing.

[quixoticgeek]

It's 2023, please please please just use the ERTO size, it will make your life so so so so much easier.

In your example, the tyre is 47mm wide. On a 406mm rim. No confusion. Simple to use.

J

[Pickled Onion]

It's 2023, please please please just use the ERTO size, it will make your life so so so so much easier.

In your example, the tyre is 47mm wide. On a 406mm rim. No confusion. Simple to use.

J

This.

If you're replacing it, you can get a tyre with a different width (within reason), but it must be 406.

Any tyre marked as 20" x a number with a decimal point will fit. But that's not really relevant as any tyre that doesn't *also* have the number 406 on it will be so old as to be unusable. Any tyre marked 20" and a number without a decimal point will *not* fit, and will not have "406" on it either.

[JonBuoy]

Shouldn't that be ETRTO ?

[citoyen]

Shouldn't that be ETRTO ?

Yes - especially since the first T stands for Tyre

[Cudzoziemiec]

And it's usually pronounced "etro" though I suppose "erto" is just as good/bad. And it's an ISO standard now. Also, QG's "just do it" attitude actually makes sense in this case.

[rogerzilla]

I think ETRTO has been absorbed into an ISO standard.  The USians won't have liked the European bit.

47mm may not be the actual width; tyres are often narrower than the "marketing" width because it makes them lighter.  You can sell a really light 23mm tyre if it's really 21mm.  Such undersizing used to be rampant in the MTB world.

[T42]

My branes hurt.

[grams]

ETRTO fixed the rim diameter to be actually measurable interface, but the width remains a made up number that may or may not come to pass, but now with more pretend accuracy.

In many ways "it'll be about an inch and a half, or something like that" is more honest.

[fimm]

Thanks all.
I'm actually trying to buy mudguards, but will note the advice about tyres as well.

[Zed43]

Any tyre marked as 20" x a number with a decimal point will fit. But that's not really relevant as any tyre that doesn't *also* have the number 406 on it will be so old as to be unusable. Any tyre marked 20" and a number without a decimal point will *not* fit, and will not have "406" on it either.

This I found out the hard way last Sunday 

Maxxis 20x1 1/4 - 1 3/8 does not fit at all on a 406 rim (too long by ~ 10cm). Yet according to bike24 where I bought them (N=2) and the official product page it is ETRTO 406. Neither the packaging nor the tube itself has an ETRTO size on them btw. Time for a 1-star review I guess...

And I was lucky to also have a Schwalbe SV6 spare with me, otherwise I would have had to push the velomobile back home. For 60km.

[LittleWheelsandBig]

20 x 1 3/8” = 451

[andrew_s]

20 x 1 3/8” = 451

Not so.
451 = 20 x 1 1/8", if talking about rim sizes.

Tyres used to be sold by outside diameter, with a fat 20" tyre using a smaller rim than a narrow 20" tyre, so a non-standard tyre would list the rim size it fitted as the size of the standard tyre for tha rim, plus the actual width.
The ISO 451 rim size would originally have been for a 20" x 1 1/8" tyre (20" - 2 1/4" = 17 3/4" = 451 mm), and a 1 3/8 tyre to fit it would be listed as 20 x 1 1/8 x 1 3/8".
Such three-part sizing was a source of just as much confusion as decimal/fractional inches.

Decimal/fractional inches isn't as reliable as it used to be.
It used to be that just 26" MTB (559) was in decimal inch sizes, which made it easy to avoid the other 26" sizes, which all used fractional inches, but size listings have been changing so that most of the common sizes are now mostly decimal inches.
There's still some scope for confusion if not using the ETRTO/ISO sizes though; for example Continental currently list a 28-622 "RIDE Tour" as 28 x 1.1 inches, but a 32-622 as 28 x 1 1/4 inches and a 37-635 as 28 x 1 1/2".

[morbihan]

A detailed report will be released on GCN in May 2024

[LittleWheelsandBig]

20 x 1 3/8” = 451

Not so.
451 = 20 x 1 1/8", if talking about rim sizes.

Given the thread title is "Tyre width" and the post I am referring to had the poster wondering why their 20 x 1 3/8" tyre was too large for their 406 rim, the correct answer is 20 x 1 3/8" = 451.

[Zed43]

Now go tell this to the reseller and manufacturer of said tyre (tube) who both list it as ETRTO 406.

It appears bike24 is in no hurry to publish a review warning their customers about faulty specifications either