my last few MTBs have all been 29ers... if the shop doesn't have a proper sized 29x2.1 tube a 26x2.1 tube was fine... took a bit more work getting it in (had to make very sure the tube didn't get pinched bead and rim... but once I got that sorted it was just business as usual... and after the first install it was much easier the next time as the tube would stretch a bit