I volunteered to spruce up a bike that my sister has acquired from a friend, think that it would take no more than a couple of hours.

Then I saw the bike. It’s an old shopper that weighs about a metric tonne and has obviously spent the last few years sitting unloved outside. The chains is rusted solid, the cables are all screwed, the cassette may come clean with a good scrub with a wire brush etc etc.

Anyhoo, the tyres are completely perished and need to be replaced, so much so that I can’t even read the tyre size on the sidewall. The rims are bigger than MTB, but smaller than roadie. Has she unwittingly gone all 650B niche? Any ideas?

Try Sheldon

Looks like an older continental size. Measure the bead seat diameter carefully. Then try SJS cycles for esoteric tyre sizes.

sounds like 26 x 1 3/8

useful link here if you can see any of the numbers

http://www.users.globalnet.co.uk/~pattle/ddgcs/tyres.htm

It’s probably a 451 rim, commonly known as 20″ x 1 3/8″. Used on old shoppers. Tyres are usually on the cheap side and a bit limited, though some more modern versions of rims and better tyres for recumbents are around.