tyre keeps popping off

[02GF74]

I have a 26 in mtb tyre (Vuelta brand?, 10 years old) that I tried to fit on to 3 rims - each time I pump up the inner tube, it forces the tyre off the rim.

this tyre is wire bead and is dead easy to fit onto a rim. This kinda tells me that there is a fault (made in Thailand does not inspire confidence!) with the tyre (maybe the wire bead has snapped) or it is perhaps designed for some specific rim?

Anyone come across something like this before?

 

[swee'pea99]

The 'dead easy' is indeed a bit of a giveaway. I'd say if it happens on three different wheels, it's a gonner. I'm a famously tight get, but even I'd give up on it at that point.

 

[Yorkshireman]

Check what size the tyre is (usually marked on the sidewall) and what size the rim is. Bear in mind that 26X2.5 is not the same size as 26X2 1/2"
Tyre (tire) info in abundance thanks to Sheldon RIP.
http://sheldonbrown.com/26/index.html.