My housemate has an old commuter bike with some pretty old rims on it. She’s got new Schwalbe tyres for it but they’re a loose fit on the rim, quite easily pushed on by hand. Putting 70 psi in the rear results in the new tyre blowing being blown off the rim.

Are there any tricks for getting a loose tyre to seat better or is it just a case of the bead is too loose and these tyres are not going to work?

You can sometimes thicken up the inside of the rim with a few goes round of gaffa tape. I had some old BMX rims that were mega loose, ended up with a 3mm strip of cardboard held in with tape to increase the inner diameter. More or less worked.

I think all you can do is try and increase the circumference of the inner section of the rim. Think about how the tyre is seating when it pops off – the opposite side is tucking into the rim, inwards. So you want to push that section out, which in turn will pull the opposite side (that’s too high) into the rim.

So this would require lots of rim tape ! You can just use electrical tape, i.e. anything reasonably thick and low cost – you don’t want to waste money of a project like this !

Just make sure the tyre is properly seated before riding – don’t take risks !

Are you sure the old tyres were’nt 27 1,1/4 or 28 1,1/2? (if they are really old)

… some pretty old rims on it

Are they “hooked” ones?

What Rorschach said – very common if it is an old bike.

Old and new tyres are both 27 1,1/4. Can I measure the rim diameter to be sure they are correct for the tyres?

Electrical tape on the rim seems to have made all the difference. Up to 85 psi and the tyre is still on. Will leave it over night before riding it now. Thanks all!