Werehatrack said:Quoted message said:Quoted message said:
[email hidden] wrote:
> On 7 Jun 2006 08:08:23 -0700, [email hidden] wrote:
>
> >How many of you would put an identical wheel/tire/pressure in as a
> >replacement and continue riding?
>
> Dear D.,
>
> I wouldn't put a 37mm tire on a rim recommended for no more than 28mm
> and 117 psi and then blow it up to an estimated 80-85 psi,
> particularly if I weighed 275 lbs.
>
> I've heard of a case where the wheel failed catastrophically within
> 500 miles on smooth pavement when this was done.
>
> Cheers,
>
> Carl Fogel
How about a 28mm tire?
Dear D.,
I'd happily put a 28mm tire on a rim rated for that width with no more
than 117 psi--and inflate it to about 100 psi or less. A recommended
maximum is a warning, not an invitation to see what happens.
In any case, the destroyed rim was wearing a 37mm tire inflated to an
estimated 80-85 psi when it abruptly tore itself in half after only
500 miles.
That 37mm is 32% beyond the CXP33's maximum recommended width.
What could be a more impressive and dramatic illustration that Mavic
was right about those rim limits? Or that wider tires exert more
leverage on rims?
If your logic is valid, then there are (and have long been) a lot of
riders out there on time bombs, and we should have been seeing a lot
more reports of such failures. I really think this was just what it
appears to be; a random defect failure. The fact that it failed in
500 miles while the other three identical wheels remain sound is
strong evidence that it's not the choice of tire and inflation which
was seminal, though the rider's weight may have a bearing. And in
point of fact, I've seen lots of oversize tires used on lightweight
rims at moderate overpressure (for the tire, the rim, or both) for
longer periods without failures. There has to be a margin of safety
in the design of such things because the maker know that there will be
instances when the limits will be exceeded for good reason, bad
reason, or lack of reason. While exceeding the published specs is not
a good idea, it's common, and seldom has such spectacular results in
such a short time. Perhaps that rim might have gone farther under a
lighter rider; I suspect, however, that it was just a unit that was
going to fail, and the rider's weight may have sped it along somewhat.
Mavic makes *lots* of rims. It's hardly surprising that failures
occur from time to time, and if you're making lots of rims, your name
will come up in such failures quite often. On the other hand, they're
far from the only large rim producer, but yet their name seems to pop
up more often than any other when rim failures are under discussion.
Given that some shortcomings in their products have been identified in
the past, I think there may be reason to be wary of using their rims
in high-stress applications. I'm beginning to think that they're just
engineering their products too close to the limits in general.
Dear Werehatrack,
A rider puts a 37mm tire on a rim whose manufacturer recommends it for
no more than a 28mm tire, about 32% beyond the manufacturer's chart
for that rim.
An independent site about tandems and heavily laden touring bikes
warns that the tire is a couple of sizes out into the daner zone for a
rim with such narrow internal dimensions.
The rider inflates it to a "thumb-test" 80-85 psi, which looks to be
about 30%-40% beyond the maximum that the manufacturer would recommend
for such a tire on that rim if the manufacturer recommended it, which
the manufacturer doesn't. (If the rider's thumb was off 10%, the tire
could have been at 95 psi.)
The rider weighs around 270 pounds.
After only 500 miles, the rim fails catastrophically.
Gosh, who could have predicted this?
Lots of other heavy riders put wider-than-recommended tires on their
rims and pump them up--well, maybe not quite that heavy, that wide, or
that high.
I have a vision of a lot of people standing at the edge of a frozen
pond.
They're staring at a hole beyond a "Danger Thin Ice" sign.
Some of them are saying that the fellow on the pogo-stick who just
vanished is entitled to sue someone.
Others are explaining that all the other people skating a little past
the sign prove that it should have been safe to pogo-stick much
further out there.
Cheers,
Carl Fogel
