Who’d-a-known after 30+ years of occasional fiddling with bikes? And I thought buying the right tubeless 27.5 tyres was a faff. Turns out I need 406’s. All hail our metric overlords.

I encountered this fact significantly earlier than you, perhaps as my dad had a moulton with funny sized 20″ wheels – although it might’ve been some other reason.

Indeed, I feel if the benefits of the metric bead diameter measurements had been more widely publicised, the brexit vote would’ve surely gone the other way.

There’s at least 3 different types of 26in too, your ‘normal’ MTB (559mm), 650A (590mm rims), and 650C (571mm) which all get variously described as 650 or 26, then current MTB 27.5/650B (584mm rims) and 27inch (630mm rims) which are bigger than 700c rims (622mm) despite some people calling them 28in 😆

Welcome to the world of rims and tyres!

As above, one of the few areas where metric ISO is actually a more useful system rather than trying to label by a rough approximation of outside rolling diameter of variable tyres.

one of the few areas where metric ISO is actually a more useful system

Wot? Few areas?

Aye, there’s two sizes of 16″, over 4cm different. Two sizes of 20″, almost 5cm different. 18″ is only 6mm bigger than 16″, and both are smaller than 17″.

But with big wheels, 29″ is the same as 28″ and both are smaller than 27″ 😉

Wot? Few areas?

This. As someone educated in Scotland in the 1980s, I never learned imperial measurements in school and can’t work in them.

I do remember the midwives weighting my kids when they were born, in kg, converting this to imperial before telling us the weight, and then me looking blank and asking for that in metric.

Does anyone know why we didn’t adopt km in 1967 with the rest of the metric system?

Yeah. And a 24in bike tyre isn’t the same size as a 24in wheelchair tyre….. and a 25in wheelchair wheel is what we call 26in MTB.
It’s bizarre. The more you go into it the worse it gets. 🙂

Does anyone know why we didn’t adopt km in 1967 with the rest of the metric system?

Because it’s French and it makes things sound more impressive than they are. 100km? Phhhh. 100miles? Now I’m impressed.

Sheldon Brown has a very useful guide: https://www.sheldonbrown.com/tire-sizing.html

Yes, worth checking the ISO/ETRTO size, that tells you how big it actually is.

So youneeded 406 but got 451?

What arethe tyres imight take them?

It’s a single 37-451 Kenda Small Block Eight (kids inexplicably wear out only the rear tyre – can’t think why). Approximately 20 inches in diameter 😉

I’ve currently requested to return it and if they’ll provide an alternative and I won’t get stung for a ton of postage I may send it back otherwise open to offers.

Because it’s French and it makes things sound more impressive than they are. 100km? Phhhh. 100miles? Now I’m impressed.

If it’s good enough for those nice Canadians and New Zealanders…