Originally Posted by
irclean
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:
No matter if the rims are cheap or not, if they are keeping in one piece and are not deformed, cracked or otherwise damaged, they should be fine. Having another blow off with the same type of the tire, is that the same tire you already had a problem with or is it the other one? This is getting very very worrying, to have them blow just like that.
Originally Posted by
irclean
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.
That is probably a single wall "C" bead hook rim. The "C" denomination means that ih has a "lip" or "hook" under the side walls where tire bead sits. Such rims are specifically designed for inflation pressures of over 5 bar or over 75 PSI. Even a non branded "C" single wall rim should be good for up to at least 6.5 bar or about 97 PSI. So that leaves us the tires as the likely culprit - if you had those tires on standard, non "C" designated rims, and pumped them up to 75 PSI or over, tire might slip off due to the rim, but you had proper rims, and had tires properly inflated, and that keeps pointing towards the tire for me.
Originally Posted by
irclean
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.
As I said earlier, if the tires are rated to 87 PSI as these are, even 100 PSI should not blow them off the rim if they are good. Real blow off pressures are much higher then the rating that the tire has written on the sidewall, typically up to twice that pressure is common. I would get concerned to tire blowing off the rim (having quality rims in mind) no sooner then at least 130 PSI in them. And they probably wouldn't blow under 150 PSI if everything is ok. I'm not saying it is ok to ride like that, just giving you a scale of pressure needed to blow the tire off when everything is working as it should be.
If you were pumping them up to full rated 87 PSI, that is nowhere near enough pressure to blow them if everything is ok and if the rim/tire combo isn't defective. Even if you pump was off by as much as 20%, that would still only give 105 PSI, nowhere near enough pressure to cause this. Something is clearly defective here, and until that is found out and repaired I would not dare ride it like it is.
Originally Posted by
irclean
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).
Sorry, but at this stage, having two consecutive blow offs with those tires; it is not about luck anymore, it is about the question when will it happen again, not will it happen, but when. Take a look again at what I said about pressures, there is something SERIOUSLY wrong with either the rims or the tires if you need to run under inflated tires in hopes of it not happening again. And remember how it was when it happened at 20 kph, so think again for the sake of your safety - what would happen if the tire gives out again at a higher speed, and if you happen to go down in traffic around heavier vehicles? I don't need to answer that one, do I? Please, rethink you decision and remember that this is not a game - sometimes, we can loose sight of what is a reasonable risk to take, and what is not.
Seriously, in my opinion this had crossed the line of unreasonably dangerous defect to ride on, so please do the prudent thing, and bin those tires before you get seriously hurt. Or the rims. Whatever is causing the problem, find out, bin it and replace. A few dollars is not worth your head or your health, trust me. I really wouldn't like to read about another fellow rider getting hurt over a defective part.