Originally Posted by
whitecat
...How can you be confident that the problem lies with the rim, and not the tire? If every other tire worked fine on that rim of yours, and only this one blew off, please explain me how did you get to the conclusion that the rim is defective? I had that blow off problem only with this Michelin tire, not with any other of more then a few dozen tires I used on that same rim - to me, that points in the direction of the tire as being responsible for the fault, not the rim. I will probably be exchanging them for some cheap CST tires that I used to run before - minus the puncture protection and a bit less grip, but no thinking about will it ever blow off again on it's own accord.
My conclusion is purely speculative and not in any way based on scientific data. I came to this conclusion based on the fact that the bike in question is from a Big Box Store and has cheap aluminum rims. Ironically enough, when I went to grab the bike from the garage this morning I discovered that the tire had been blown off the rear wheel! This got me thinking that maybe whitecat was on to something and the tires are indeed defective. Maybe, maybe not, but I have to take a couple of other variables into consideration:
First, my rims: After removing the rear wheel today I carefully inspected the rim. Although I could see no obvious defects I also noticed that there were no markings on the rim at all: no brand name, no sizes, no manufacturer's stamp, nothing. I double checked with a magnet to make sure they were at least aluminum and yes, they are. Based on the images I have seen of double-walled rims mine seem to be single-walled. There is a few mm of material which seems to double over upon itself creating a bead around the circumference of the rim. This strip of material also appears to have been knurled, which I assume is meant to grab the tire bead more securely.
Second, my new pump: I recently purchased
this pump because my new bike has Presta valves and my old pump only handles Schrader (BTW the bike in question has Schrader valves). The pump is well-reviewed by MEC members and "feels" quality in my hands. As I mentioned earlier, I double-checked the pressure against a digital gauge that I have and it was spot-on. This leads me to believe that maybe my previous pump (also cheap, and also purchased at said BBS) was displaying too high a pressure, and therefore I was unknowingly running my tires at lower pressures than I assumed. Perhaps when I later topped them up with my new pump the resulting pressure overcame the seal between the tire bead and the rim.
In any case, I am going to try to find a happy medium; a lower pressure that is high enough to prevent pinch flats, but low enough to avoid blowouts. Other than that I will be riding on faith (and at slower speeds - just in case). Wish me luck!
p.s. I'm just waiting on a rack for my new bike so I can use it as my primary commuter, and then the "blowout bike" can wait for better rims (and maybe tires).