Bear with me …..

I have a 26″ frame designed around using a 2.4″ tyre. It’s currently running a pair of 700 X 35 cross type wheels.

This has increased the BB height. No biggie.

Now I’ve been offered some 650b wheels.

From a quick Google it suggests that a 26×2.4″ wheel will have a very similar overall diameter to a 650b X 40 with the 650b wheel being marginally shorter.

Having a bit of a brain fart moment this morning……

Do I have this right?

Obviously a lot depends on the height profile of a given tyre, but on the assumption that the carcass width leads to a consistent height (not a great assumption, but Im going with c.75% of width for tyre height) and that:

700c bead diameter = 622m,
650b BD = 584mm and
26″ BD = 559mm

your 700c would be c.622+(2 x 40mm x 75%ish) = 682mm
Your original 26″ would be c. 559mm+(2 x 2.4″ x 75% x 25.4mmish) = 650mm

Assuming fatness fit at c.670mm, a 650b at 584mm, would leave up to 86mm, or somewhere between 2.0″ and 2.25″ of 650b tyre (50+mm) I reckon.

Hmm.

Thanks Stoner

If I’m reading this correctly Linky then it shows a 26×2.35″ tyre as having a circumference of 2083mm compared with a 650b X 1.5″ as 2079.

Allowing for a variation in tyre height and coupled with the reality of running the 26″ tyre softer than the 27.5″ the two would be very similar.

I appreciate 1.5″ is only 38mm (ish) and not 40mm but what’s a few MM between friends …….

seems about right.

are there many 40c 650b tyres out there?

I may have meant 42mm … 😕