I’m riding my fixed gear winter commuting bike through the slush and salt in Madison yesterday when my rear tire explodes - blowing the tire (both wire beads) completely off the rim. I went to fix it and realized that the tire is way too loose fitting on the rim. I figured I damaged the tire somehow and decided to ditch the bike and come back later - it sat in the snow while we got 7 more inches yesterday afternoon. Picked it up this morning, took it home and tried to install 3 different (although none new) tires. All of them were way too big for the rim - like a gap of 1.5 cm when I try to pull the bead directly away from the rim with it otherwise seated all the way around. I did try to put one on with a tube but the tire blew off the rim with the pressure at less than 30psi. I put one of the tires on another 700c rim - fit perfect. Now I figured there was something wrong with the wheel. But there are no cracks in the rim. Perfectly true and dished. No loose spokes. Rim tape is still smooth and flush. Can’t bend any part of the rim in my hands. This has never been a problem before and in fact I remember struggling to get a tight tire on in the past. Rims are three years old but low quality (Alex brand) and have been ridden in terrible conditions.
I can’t figure it out. It is like the whole wheel just shrunk.
Any thoughts (other than that it is impossible or that I’m stupid or lying)
Yeah, seen that with wire bead tires…plus some wire bead tires really need a big rim “hook” profile to stay on. Just a couple weeks ago I got some wire bead cross tires to put on my fixie - they would not stay on past 50psi
yep, everything is 700c. Tried old (about 3000 miles on it -saved for a trainer tire) Continental Grand Prix 3000 700cx23(folding), and newer Specialized nimbus 700cx28 (wire bead) and WTB AllTerrainasauraus 700x30c (wire bead). The Specialized is rated at 50-80 psi and blew off the rim at 20. The hood profile is a good idea. These seem to be shallower than my open pro’s. I measured the rim width at 20mm.
When you got up to 30 psi, did the tire look like it was seated or was there still the gap? Probably no gap but not truly seated. Since the tire fit another 700, my guess is the tire was not seated properly when it blew the first time (and the second time). I have wheels that are just as sensitive and you have to make sure that the tube is up in the tire and the bead is down in the rim. When you get to 20-30 psi, make sure there are no bulges around the rim and that the tire is even with the rim all the way around.
I was pretty careful about making sure it was seated right (because I only had one “big” tube) - checked it before I pumped, then again when it had about 10 psi in it - looked and felt like it was seated well.
It is possible that rather than shrink, the rim itself has “expanded” in the direction of the continual 100+psi within it. As the brake wear thins the sidewall the rim begins to splay outward effectively reducing the “hook” on the rim that holds the bead AND slightly reducing the overall diameter.
Sounds like you’ve worn out the rim. With a blowout like you had, a large section of the rim can deform in this way.
This is not definitive, but it is plausible and something I’ve witnessed on my own fixie wheels with some very old Mavic hoops.
SuperDave got it! The shrunken rim is about 3mm wider (outside to outside) than the good one. The walls of the rim must have splayed open and that’s why tires won’t stay seated.
Brilliant. Anything can be solved (or broken) on Slowtwitch!