carlfogel@comcast.net wrote:
> On Mon, 28 Mar 2005 19:51:36 GMT, NiteRider
> <stingray@bowlander.bike> wrote:
>
>
>>In article <c6ig41d8kkngphf4omilfs81vttr9bjkf9@4ax.com>,
>><carlfogel@comcast.net> wrote:
>>
>>
>>>On Mon, 28 Mar 2005 16:57:47 GMT, NiteRider
>>><stingray@bowlander.bike> wrote:
>>>
>>>
>>>>In article <114f5q4kuu4caa3@corp.supernews.com>, A Muzi
>>>><am@yellowjersey.org> wrote:
>>>>
>>>>
>>>>>I do not believe that you can damage a rim by inflation
>>>>>pressure of 80psi no matter how poorly mounted.
>>>>
>>>>You can "believe" whatever you want. I know what happened.
>>>
>>>Dear NiteRider,
>>>
>>>Andrew's point is simply that any non-defective bicycle
>>>wheel rim can easily restrain a tire inflated to 80 psi. No
>>>matter how the tire is mounted, the maximum pressure remains
>>>80 psi. A normal rim doesn't fail with excessive
>>>inflation--the tire just creeps over the edge, the inner
>>>tube goes bang! when exposed.
>>>
>>>He probably bases his opinion on common sense and over
>>>thirty years of inflating bicycle tires on every imaginable
>>>kind of rim at a high-volume bicycle shop.
>>>
>>>If your rim failed when the tire was inflated to that
>>>pressure, then it was a defective rim, either from the
>>>factory or else due to road damage or even brake pad wear.
>>
>>That's not what he wrote. He didn't differentiate between defective and
>>non-defective rims. He simply wrote that it's not possible. Period.
>>It's so easy to be a critic and a skeptic when you don't have something
>>happen to you personally. I accept that such a failure is possible. The
>>skeptics simply wave their hands and say it's impossible. Who is the
>>one with a closed mind here? People like this laughed at the Wright
>>brothers, because their "years of experience" told them
>>heavier-than-air flight was impossible. Not that I'm comparing myself
>>with the Wrights, but it's the same kind of mentality they encountered.
>>
>>The fact is that it happened at home. That's no lie. The fact is that
>>there are two such dents roughly 12 inches apart. No road damage could
>>account for two dents. The fact is that that wheel has a hub brake and
>>is not subject to brake wear. Instead of accepting these facts and
>>trying to figure out what really happened, all I seem to hear around
>>here is "not possible. You're a troll, a liar. You don't have any idea
>>what you're talking about." Sure, there's the occasional reasonable
>>person who says the rim may have been defective, like yourself. But
>>nobody likes being called a liar.
>
>
> Dear NiteRider,
>
> I agree that some posts have treated you to the usual
> obnoxious responses common on this newsgroup. But Andrew
> really wasn't behaving that way--you've just had to put up
> with such rudeness that you're easilly upset.
>
> For example, I should have re-read the whole thread and seen
> that you've mentioned that the hub brake eliminates pad wear
> as a culprit. So I was rude, but not intentionally.
>
> But we're all prone to such slips. I don't think that you're
> trying to be rude when you say that no road damage could
> account for two dents--but I do think that you should
> reconsider that notion. Why can't a wheel hit two things if
> it hits one?
>
> Most riders would have trouble telling if they hit one bump
> or two bumps a foot apart at higher speeds. At 20 mph, the
> wheel is rolling about 30 feet per second, so banging over a
> pair of nasty inch-high rocks a foot apart would take only a
> thirtieth of a second--and we can't tell that camera film is
> showing still pictures 1/24th of a second apart.
>
> And the odds are fair that a rider might hit the same damned
> bump coming and going--which in turn gives good odds that a
> different section of the wheel would hit the bump coming
> home.
>
> Consider also that the failure is on the rear wheel, which
> is the more heavily loaded wheel and far more prone to road
> damage. The front wheel is not only less heavily loaded, but
> also enjoys a much more flexible frame arrangement than the
> rear diamond.
>
> (You've indicated that you don't think that there was any
> impact, much less two, but anyone like Andrew Muzi is
> painfully aware that customers bring bikes in with obvious
> road damage and sincerely believe it couldn't have happened.
> They're not lying--they just can't believe that the dents
> and cracks came from impacts that they didn't notice or
> don't remember. I learned long ago that my clients aren't
> lying when they insist that they didn't do such and so on
> their computers--if they'd realized that they were doing it,
> they wouldn't have done it in the first place.)
>
> While no one would mistake me for an engineer, it does
> strike me that two short sections of a rim bulging out
> doesn't seem like excessive tire pressure on a non-defective
> rim. If you get a tire strong enough to withstand pressures
> that will damage a non-defective rim and pump it up higher
> and higher, a much larger section of the rim should bend
> outward. A pair of small, localized failures suggests either
> some irregular defect in the manufacturing process, two
> places where road damaged occurred, or a combination.
>
> If a mis-mounted tire somehow crept and applied significant
> extra pressure on one small section, there would be only one
> local failure--the shape of the tire and a non-defective rim
> would prevent the tire from creeping out elsewhere to cause
> a second failure.
>
> If pumping up the tire to 80 psi causes dents and bulges to
> form in the rim, then the rim is extraordinarily defective.
> A much more likely explanation is that there were already
> dents and bulges, pumping the tire up added pressure to the
> already defective rim and bent things even more, and then
> the tire slipped off, with the usual inner-tube explosion.
>
> As Andrew pointed out, 80 psi doesn't bend normal bicycle
> rims. He could have added all the qualifiers and exceptions
> that I'm adding, but he has more common sense than I do and
> less time for beating the obvious to death.
>
> I hope your new rim lasts longer.
>
> Carl Fogel
for a good quality rim in good condition, it is indeed most unlikely
that a tire can be pumped sufficiently to bend the rim before blow-off,
but that doesn't mean it can't happen - there has to be some pressure at
which it occurs. once that principle is established, it's then a matter
of quantification.
now, on the pics you're hosting,
http://home.comcast.net/~carlfogel/download/rim_nite.jpg
the third frame shows clearly that the rim has cracked. this results in
material compromise to the integrity of the rim, and as mentioned
previously regarding fatigue & stress orientation perpendicular to the
extrusion axis, excess pressure is definitely going to play a
significant role in crack formation & propagation. if cracking is
present with our subject, then it absolutely /is/ possible for
niterider's rim to bulge where cracked, at the pressures mentioned, and
there's no reason why it can't happen in two places at once. fatigue
/does/ occur below yield and /does/ result in failures at stresses below
what might be considered "normal service".
