Re: Is my tire bad?

  • Thread starter Phil, Squid-in-Training
  • Start date



On Thu, 31 Mar 2005 02:14:35 GMT,
[email protected] wrote:

>Carl Fogel writes:
>
>> If the dented rim is a fraud of some kind, how was it produced?

>
>> That is, would a normal rim crack like that when bent out on one
>> side with a suitable soft-jawed tool that left no marks (doubtless
>> available under the counter from Harris Cyclery's Covert Operation
>> Catalogue), or would it just bend without the crack?

>
>This is the usual form f a rim that encountered an object, like a
>piece of RR ballast (granite) off center of the tire. I have several
>rims that were fatally dented like this on one side in my collection
>of scrap rims. The difference is that I didn't concoct a story that
>it was the result of moderate inflation pressure and then call
>everyone who doubted that a jerk.
>
>> Would the speed of the bending determine whether it cracked?

>
>No.
>
>> That is, would a real impact cushioned by a tire at high
>> speed tend to rupture, while a fake "impact" produced by a
>> comparatively slow twist with a pair of soft-jawed
>> channel-locks would leave no crack?

>
>That is completely out of the question from my experience with such
>dents. You can't make them that smooth without a tire on the rim and
>exerting a radial overload.
>
>The speed of the bicycle in conjunction with weight on the wheel,
>needs to be only great enough that the inertial load will bend the
>rim. This is not hard to do at speeds of even 15mph, although higher
>speed ensures success. Curling the edge of a rim cannot be fatigue
>related.
>
>[email protected]


Dear Jobst,

Maybe something is wrong with my crude experiment, but . . .

I just introduced an old 27-inch steel rear rim with weep
holes to my bench vise and bent five sections of the rim
sections, some with the narrow end of the vise, some with
the wider normal grip.

Even with four inches of the rim bent out at 90 degrees to
the spokes, there's no cracking, inside or out. Not even
when I bend a section with a rusty weep hole.

Maybe an aluminum rim cracks or ruptures more easily than a
steel rim? Or maybe the hollow double-wall design affects
things?

The bends are much more dramatic than the originals:

http://home.comcast.net/~carlfogel/download/rim_nite.jpg

If there's interest, I take some pictures, but it's just an
old steel rim with sections of one sidewall bent flat and no
cracks.

Carl Fogel
 
On Thu, 31 Mar 2005 02:14:35 GMT,
[email protected] wrote:

>Carl Fogel writes:
>
>> If the dented rim is a fraud of some kind, how was it produced?

>
>> That is, would a normal rim crack like that when bent out on one
>> side with a suitable soft-jawed tool that left no marks (doubtless
>> available under the counter from Harris Cyclery's Covert Operation
>> Catalogue), or would it just bend without the crack?

>
>This is the usual form f a rim that encountered an object, like a
>piece of RR ballast (granite) off center of the tire. I have several
>rims that were fatally dented like this on one side in my collection
>of scrap rims. The difference is that I didn't concoct a story that
>it was the result of moderate inflation pressure and then call
>everyone who doubted that a jerk.
>
>> Would the speed of the bending determine whether it cracked?

>
>No.
>
>> That is, would a real impact cushioned by a tire at high
>> speed tend to rupture, while a fake "impact" produced by a
>> comparatively slow twist with a pair of soft-jawed
>> channel-locks would leave no crack?

>
>That is completely out of the question from my experience with such
>dents. You can't make them that smooth without a tire on the rim and
>exerting a radial overload.
>
>The speed of the bicycle in conjunction with weight on the wheel,
>needs to be only great enough that the inertial load will bend the
>rim. This is not hard to do at speeds of even 15mph, although higher
>speed ensures success. Curling the edge of a rim cannot be fatigue
>related.
>
>[email protected]


Dear Jobst,

Maybe something is wrong with my crude experiment, but . . .

I just introduced an old 27-inch steel rear rim with weep
holes to my bench vise and bent five sections of the rim
sections, some with the narrow end of the vise, some with
the wider normal grip.

Even with four inches of the rim bent out at 90 degrees to
the spokes, there's no cracking, inside or out. Not even
when I bend a section with a rusty weep hole.

Maybe an aluminum rim cracks or ruptures more easily than a
steel rim? Or maybe the hollow double-wall design affects
things?

The bends are much more dramatic than the originals:

http://home.comcast.net/~carlfogel/download/rim_nite.jpg

If there's interest, I take some pictures, but it's just an
old steel rim with sections of one sidewall bent flat and no
cracks.

Carl Fogel
 
Peter Cole wrote:
> jim beam wrote:
>
>>[email protected] wrote:
>>
>>>Peter Cole writes:
>>>
>>>
>>>
>>>>>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.
>>>
>>>
>>>>If the 2 spots on the rim were folded down by the force of the tire
>>>>bead after the bead lifted off but before the tube blew, I would
>>>>think either the rim would have to be extremely fragile (and
>>>>ductile) and/or the pressure very high. I wonder if the tire was
>>>>being filled from a high pressure source (compressor)? It could
>>>>theoretically be possible to get very high pressures (>200 psi)
>>>>before liftoff.
>>>
>>>
>>>This is not a reasonable pursuit. The rim would not bend at the

>
> place
>
>>>where the tire first lifted off because that is a region of reduced
>>>stress, the tire bead no longer having a hold on the rim. I don't

>
> see
>
>>>why, in face of many years of experience by professional bicycle
>>>people, this scenario is given so much consideration. This is all
>>>based on a report from someone who believes this is what occurred

>
> and
>
>>>who asks, whether the scenario is probable.
>>>
>>>It isn't.
>>>
>>>[email protected]

>>
>>did you or peter ever bother to look at the cracks in the 3rd panel

>
> of
>
>>this pic?
>>
>>http://home.comcast.net/~carlfogel/download/rim_nite.jpg
>>
>>and have you two ever bothered to consider the effects of fatigue

>
> caused
>
>>by excess tire pressure, particularly with regard to the orientation

>
> of
>
>>the extrusion axis?

>
>
> Yes, I did, but I was struck by the curvature of the rim edge, I
> wouldn't have thought that it would bend so far in a simple crack
> failure.


depends on the extent of cracking. ignoring crack tip effects, you have
to agree that a rim with 0.7mm uncracked thickness remaining is not
going to support the same load that 1.0mm of uncracked thickness can.

> It was also surprising to hear that there were 2 such spots on
> the rim, that seems a little too coincidental for fatigue failure.


imo, that makes fatigue a more likely candidate, not less. if you have
a long circumferental crack all around the rim, it's going to yield
first at the points where the crack's deepest. there can easily be more
than one point of weakness in this situation. add to that the fact that
we have a rider that's exerting substantial over-pressure in a single
application, why /wouldn't/ the rim yield in multiple weak spots?
pressure is even around the rim as long as the tire hasn't burst.

> I'd
> also find it difficult to believe that an experienced rider would dent
> a rim that badly without remembering an impact (or 2), so I'm baffled.
>
 
Peter Cole wrote:
> jim beam wrote:
>
>>[email protected] wrote:
>>
>>>Peter Cole writes:
>>>
>>>
>>>
>>>>>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.
>>>
>>>
>>>>If the 2 spots on the rim were folded down by the force of the tire
>>>>bead after the bead lifted off but before the tube blew, I would
>>>>think either the rim would have to be extremely fragile (and
>>>>ductile) and/or the pressure very high. I wonder if the tire was
>>>>being filled from a high pressure source (compressor)? It could
>>>>theoretically be possible to get very high pressures (>200 psi)
>>>>before liftoff.
>>>
>>>
>>>This is not a reasonable pursuit. The rim would not bend at the

>
> place
>
>>>where the tire first lifted off because that is a region of reduced
>>>stress, the tire bead no longer having a hold on the rim. I don't

>
> see
>
>>>why, in face of many years of experience by professional bicycle
>>>people, this scenario is given so much consideration. This is all
>>>based on a report from someone who believes this is what occurred

>
> and
>
>>>who asks, whether the scenario is probable.
>>>
>>>It isn't.
>>>
>>>[email protected]

>>
>>did you or peter ever bother to look at the cracks in the 3rd panel

>
> of
>
>>this pic?
>>
>>http://home.comcast.net/~carlfogel/download/rim_nite.jpg
>>
>>and have you two ever bothered to consider the effects of fatigue

>
> caused
>
>>by excess tire pressure, particularly with regard to the orientation

>
> of
>
>>the extrusion axis?

>
>
> Yes, I did, but I was struck by the curvature of the rim edge, I
> wouldn't have thought that it would bend so far in a simple crack
> failure.


depends on the extent of cracking. ignoring crack tip effects, you have
to agree that a rim with 0.7mm uncracked thickness remaining is not
going to support the same load that 1.0mm of uncracked thickness can.

> It was also surprising to hear that there were 2 such spots on
> the rim, that seems a little too coincidental for fatigue failure.


imo, that makes fatigue a more likely candidate, not less. if you have
a long circumferental crack all around the rim, it's going to yield
first at the points where the crack's deepest. there can easily be more
than one point of weakness in this situation. add to that the fact that
we have a rider that's exerting substantial over-pressure in a single
application, why /wouldn't/ the rim yield in multiple weak spots?
pressure is even around the rim as long as the tire hasn't burst.

> I'd
> also find it difficult to believe that an experienced rider would dent
> a rim that badly without remembering an impact (or 2), so I'm baffled.
>
 
[email protected] wrote:
> Carl Fogel writes:
>
>
>>If the dented rim is a fraud of some kind, how was it produced?

>
>
>>That is, would a normal rim crack like that when bent out on one
>>side with a suitable soft-jawed tool that left no marks (doubtless
>>available under the counter from Harris Cyclery's Covert Operation
>>Catalogue), or would it just bend without the crack?

>
>
> This is the usual form f a rim that encountered an object, like a
> piece of RR ballast (granite) off center of the tire. I have several
> rims that were fatally dented like this on one side in my collection
> of scrap rims. The difference is that I didn't concoct a story that
> it was the result of moderate inflation pressure and then call
> everyone who doubted that a jerk.
>
>
>>Would the speed of the bending determine whether it cracked?

>
>
> No.
>
>
>>That is, would a real impact cushioned by a tire at high
>>speed tend to rupture, while a fake "impact" produced by a
>>comparatively slow twist with a pair of soft-jawed
>>channel-locks would leave no crack?

>
>
> That is completely out of the question from my experience with such
> dents. You can't make them that smooth without a tire on the rim and
> exerting a radial overload.
>
> The speed of the bicycle in conjunction with weight on the wheel,
> needs to be only great enough that the inertial load will bend the
> rim. This is not hard to do at speeds of even 15mph, although higher
> speed ensures success. Curling the edge of a rim cannot be fatigue
> related.


translation: "i cannot admit it's fatigue related without loss of face,
therefore i'll keep on denying it in the vain hope that everyone gets
bored & stops giving me a hard time about it".

>
> [email protected]
 
[email protected] wrote:
> Carl Fogel writes:
>
>
>>If the dented rim is a fraud of some kind, how was it produced?

>
>
>>That is, would a normal rim crack like that when bent out on one
>>side with a suitable soft-jawed tool that left no marks (doubtless
>>available under the counter from Harris Cyclery's Covert Operation
>>Catalogue), or would it just bend without the crack?

>
>
> This is the usual form f a rim that encountered an object, like a
> piece of RR ballast (granite) off center of the tire. I have several
> rims that were fatally dented like this on one side in my collection
> of scrap rims. The difference is that I didn't concoct a story that
> it was the result of moderate inflation pressure and then call
> everyone who doubted that a jerk.
>
>
>>Would the speed of the bending determine whether it cracked?

>
>
> No.
>
>
>>That is, would a real impact cushioned by a tire at high
>>speed tend to rupture, while a fake "impact" produced by a
>>comparatively slow twist with a pair of soft-jawed
>>channel-locks would leave no crack?

>
>
> That is completely out of the question from my experience with such
> dents. You can't make them that smooth without a tire on the rim and
> exerting a radial overload.
>
> The speed of the bicycle in conjunction with weight on the wheel,
> needs to be only great enough that the inertial load will bend the
> rim. This is not hard to do at speeds of even 15mph, although higher
> speed ensures success. Curling the edge of a rim cannot be fatigue
> related.


translation: "i cannot admit it's fatigue related without loss of face,
therefore i'll keep on denying it in the vain hope that everyone gets
bored & stops giving me a hard time about it".

>
> [email protected]
 
[email protected] wrote:
> On Wed, 30 Mar 2005 17:56:26 -0600, Jim Smith
> <[email protected]> wrote:
>
>
>>[email protected] writes:
>>
>>
>>>On 30 Mar 2005 07:30:21 -0800, "Peter Cole"
>>><[email protected]> wrote:
>>>
>>>
>>>>jim beam wrote:
>>>>
>>>>>[email protected] wrote:
>>>>>
>>>>>>Peter Cole writes:
>>>>>>
>>>>>>
>>>>>>
>>>>>>>>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.
>>>>>>
>>>>>>
>>>>>>>If the 2 spots on the rim were folded down by the force of the tire
>>>>>>>bead after the bead lifted off but before the tube blew, I would
>>>>>>>think either the rim would have to be extremely fragile (and
>>>>>>>ductile) and/or the pressure very high. I wonder if the tire was
>>>>>>>being filled from a high pressure source (compressor)? It could
>>>>>>>theoretically be possible to get very high pressures (>200 psi)
>>>>>>>before liftoff.
>>>>>>
>>>>>>
>>>>>>This is not a reasonable pursuit. The rim would not bend at the
>>>>
>>>>place
>>>>
>>>>>>where the tire first lifted off because that is a region of reduced
>>>>>>stress, the tire bead no longer having a hold on the rim. I don't
>>>>
>>>>see
>>>>
>>>>>>why, in face of many years of experience by professional bicycle
>>>>>>people, this scenario is given so much consideration. This is all
>>>>>>based on a report from someone who believes this is what occurred
>>>>
>>>>and
>>>>
>>>>>>who asks, whether the scenario is probable.
>>>>>>
>>>>>>It isn't.
>>>>>>
>>>>>>[email protected]
>>>>>
>>>>>did you or peter ever bother to look at the cracks in the 3rd panel
>>>>
>>>>of
>>>>
>>>>>this pic?
>>>>>
>>>>>http://home.comcast.net/~carlfogel/download/rim_nite.jpg
>>>>>
>>>>>and have you two ever bothered to consider the effects of fatigue
>>>>
>>>>caused
>>>>
>>>>>by excess tire pressure, particularly with regard to the orientation
>>>>
>>>>of
>>>>
>>>>>the extrusion axis?
>>>>
>>>>Yes, I did, but I was struck by the curvature of the rim edge, I
>>>>wouldn't have thought that it would bend so far in a simple crack
>>>>failure. It was also surprising to hear that there were 2 such spots on
>>>>the rim, that seems a little too coincidental for fatigue failure. I'd
>>>>also find it difficult to believe that an experienced rider would dent
>>>>a rim that badly without remembering an impact (or 2), so I'm baffled.
>>>
>>>Dear Peter,
>>>
>>>I'm baffled, too--why is the damage on only one side, why
>>>didn't the rider notice hitting anything, and why is the
>>>damage so impressive?
>>>
>>>A private email pointed out that rims can be bent that much
>>>a single time without cracking. (This seems plausible, but I
>>>hesitate to sacrifice a rim before hearing what others say.)
>>>
>>>Could we be looking at a combination of Jim's fatigue crack
>>>and Jobst's impact damage?
>>>
>>>If Jim's a fatigue crack developed on one side, would the
>>>rim then be weakened enough that the next healthy bump would
>>>produce Jobst's impressive curled bend on only one side?
>>>
>>>This would explain why the impressive damage is on only one
>>>side and why the rider didn't notice the kind of impressive
>>>impact that at first seems to be indicated.

>>
>>Of course you are missing the entire conspiracy theory/troll angle.
>>Notice that Mr. Hasselhoff jumped all over Jobst allmost immediately.
>>Perhaps the whole thing was a set up?

>
>
> Dear Jim,
>
> To paraphrase Bierce's comment regarding the definition of
> an inferior lexicographer, conspiracy theories tend to be
> not the last but the first resort of--
>
> Er, never mind.
>
> And never mind who actually jumped on whom first.
>
> The rim is more interesting.
>
> If the dented rim is a fraud of some kind, how was it
> produced?
>
> That is, would a normal rim crack like that when bent out on
> one side with a suitable soft-jawed tool that left no marks
> (doubtless available under the counter from Harris Cyclery's
> Covert Operation Catalogue), or would it just bend without
> the crack?


if it didn't already have cracking from fatigue, cracks would not appear
as a result of bending.

>
> Would the speed of the bending determine whether it cracked?


that would be true of some alloy systems, but not these kinds of
aluminum alloys at these strain rates & temperatures.

>
> That is, would a real impact cushioned by a tire at high
> speed tend to rupture, while a fake "impact" produced by a
> comparatively slow twist with a pair of soft-jawed
> channel-locks would leave no crack?


as above, no. some [bcc] alloys are famous for being rate sensitive
and/or temperature sensitive, but you'd have to be straining at rates
orders of magnitude outside the ballpark we see here for any effect to
be noticable.

http://www.doitpoms.ac.uk/tlplib/ductile-brittle-transition/index.php
http://www2.umist.ac.uk/material/research/intmic/features/charpy/notes.htm

>
> Carl Fogel
 
[email protected] wrote:
> On Wed, 30 Mar 2005 17:56:26 -0600, Jim Smith
> <[email protected]> wrote:
>
>
>>[email protected] writes:
>>
>>
>>>On 30 Mar 2005 07:30:21 -0800, "Peter Cole"
>>><[email protected]> wrote:
>>>
>>>
>>>>jim beam wrote:
>>>>
>>>>>[email protected] wrote:
>>>>>
>>>>>>Peter Cole writes:
>>>>>>
>>>>>>
>>>>>>
>>>>>>>>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.
>>>>>>
>>>>>>
>>>>>>>If the 2 spots on the rim were folded down by the force of the tire
>>>>>>>bead after the bead lifted off but before the tube blew, I would
>>>>>>>think either the rim would have to be extremely fragile (and
>>>>>>>ductile) and/or the pressure very high. I wonder if the tire was
>>>>>>>being filled from a high pressure source (compressor)? It could
>>>>>>>theoretically be possible to get very high pressures (>200 psi)
>>>>>>>before liftoff.
>>>>>>
>>>>>>
>>>>>>This is not a reasonable pursuit. The rim would not bend at the
>>>>
>>>>place
>>>>
>>>>>>where the tire first lifted off because that is a region of reduced
>>>>>>stress, the tire bead no longer having a hold on the rim. I don't
>>>>
>>>>see
>>>>
>>>>>>why, in face of many years of experience by professional bicycle
>>>>>>people, this scenario is given so much consideration. This is all
>>>>>>based on a report from someone who believes this is what occurred
>>>>
>>>>and
>>>>
>>>>>>who asks, whether the scenario is probable.
>>>>>>
>>>>>>It isn't.
>>>>>>
>>>>>>[email protected]
>>>>>
>>>>>did you or peter ever bother to look at the cracks in the 3rd panel
>>>>
>>>>of
>>>>
>>>>>this pic?
>>>>>
>>>>>http://home.comcast.net/~carlfogel/download/rim_nite.jpg
>>>>>
>>>>>and have you two ever bothered to consider the effects of fatigue
>>>>
>>>>caused
>>>>
>>>>>by excess tire pressure, particularly with regard to the orientation
>>>>
>>>>of
>>>>
>>>>>the extrusion axis?
>>>>
>>>>Yes, I did, but I was struck by the curvature of the rim edge, I
>>>>wouldn't have thought that it would bend so far in a simple crack
>>>>failure. It was also surprising to hear that there were 2 such spots on
>>>>the rim, that seems a little too coincidental for fatigue failure. I'd
>>>>also find it difficult to believe that an experienced rider would dent
>>>>a rim that badly without remembering an impact (or 2), so I'm baffled.
>>>
>>>Dear Peter,
>>>
>>>I'm baffled, too--why is the damage on only one side, why
>>>didn't the rider notice hitting anything, and why is the
>>>damage so impressive?
>>>
>>>A private email pointed out that rims can be bent that much
>>>a single time without cracking. (This seems plausible, but I
>>>hesitate to sacrifice a rim before hearing what others say.)
>>>
>>>Could we be looking at a combination of Jim's fatigue crack
>>>and Jobst's impact damage?
>>>
>>>If Jim's a fatigue crack developed on one side, would the
>>>rim then be weakened enough that the next healthy bump would
>>>produce Jobst's impressive curled bend on only one side?
>>>
>>>This would explain why the impressive damage is on only one
>>>side and why the rider didn't notice the kind of impressive
>>>impact that at first seems to be indicated.

>>
>>Of course you are missing the entire conspiracy theory/troll angle.
>>Notice that Mr. Hasselhoff jumped all over Jobst allmost immediately.
>>Perhaps the whole thing was a set up?

>
>
> Dear Jim,
>
> To paraphrase Bierce's comment regarding the definition of
> an inferior lexicographer, conspiracy theories tend to be
> not the last but the first resort of--
>
> Er, never mind.
>
> And never mind who actually jumped on whom first.
>
> The rim is more interesting.
>
> If the dented rim is a fraud of some kind, how was it
> produced?
>
> That is, would a normal rim crack like that when bent out on
> one side with a suitable soft-jawed tool that left no marks
> (doubtless available under the counter from Harris Cyclery's
> Covert Operation Catalogue), or would it just bend without
> the crack?


if it didn't already have cracking from fatigue, cracks would not appear
as a result of bending.

>
> Would the speed of the bending determine whether it cracked?


that would be true of some alloy systems, but not these kinds of
aluminum alloys at these strain rates & temperatures.

>
> That is, would a real impact cushioned by a tire at high
> speed tend to rupture, while a fake "impact" produced by a
> comparatively slow twist with a pair of soft-jawed
> channel-locks would leave no crack?


as above, no. some [bcc] alloys are famous for being rate sensitive
and/or temperature sensitive, but you'd have to be straining at rates
orders of magnitude outside the ballpark we see here for any effect to
be noticable.

http://www.doitpoms.ac.uk/tlplib/ductile-brittle-transition/index.php
http://www2.umist.ac.uk/material/research/intmic/features/charpy/notes.htm

>
> Carl Fogel
 
[email protected] wrote:
> On 30 Mar 2005 15:20:46 -0800, "41" <[email protected]>
> wrote:


> >No. If you look, especially at the second picture, you will notice

that
> >the curl or bend goes all the way to the very edge, far away from

the
> >crack site which is at the base of the sidewall. That means a severe
> >force was applied to that edge, i.e. by impact. The crack is far

away
> >and has no control over the amount of force required there.



> Dear 41,
>
> I don't know enough about this stuff to know who's right,
> but I'm a little skeptical of your claim that the crack has
> no control over the amount of force need ed to bend the rim
> above it.


The key is that the deformation goes all the way to the very edge or
tip of the rim, as evidenced especially in the second picture. In order
for something to deform at some location a force has to be applied at
that location. Up there at the very edge of the rim, there is no crack
and no fatigue and so the rim is at full strength and needs a
correspondingly full strength force to deform it. This force also has
to be applied all the way up and until right at that tip, something
virtually impossible to do with straight-jawed pliers or the like, and
so force starting from head on , as in an impact, is the explanation.
Whether done in a vise or on the road I cannot say.

The explanations offered by the original poster are ludicrous, for many
reasons. For example, if it really were, amazingly, done by the bead,
then the deformation would be in the direction that the bead ran,
diagonally in his scenario, the two bulges pointing towards an
intersection. In fact the one showed is straight out.

> Come to think of it, don't rims worn thin by brake pads tend
> to crack at the base of the sidewall and then start to bulge
> outward because the rim has weakened?


Such a bulge looks nothing like this.

JS and JB have the explanation exactly right and as I summarized the
matter, it relates to the fact that the OP blew at pressures as low as
his tires.
 
[email protected] wrote:
> On 30 Mar 2005 15:20:46 -0800, "41" <[email protected]>
> wrote:


> >No. If you look, especially at the second picture, you will notice

that
> >the curl or bend goes all the way to the very edge, far away from

the
> >crack site which is at the base of the sidewall. That means a severe
> >force was applied to that edge, i.e. by impact. The crack is far

away
> >and has no control over the amount of force required there.



> Dear 41,
>
> I don't know enough about this stuff to know who's right,
> but I'm a little skeptical of your claim that the crack has
> no control over the amount of force need ed to bend the rim
> above it.


The key is that the deformation goes all the way to the very edge or
tip of the rim, as evidenced especially in the second picture. In order
for something to deform at some location a force has to be applied at
that location. Up there at the very edge of the rim, there is no crack
and no fatigue and so the rim is at full strength and needs a
correspondingly full strength force to deform it. This force also has
to be applied all the way up and until right at that tip, something
virtually impossible to do with straight-jawed pliers or the like, and
so force starting from head on , as in an impact, is the explanation.
Whether done in a vise or on the road I cannot say.

The explanations offered by the original poster are ludicrous, for many
reasons. For example, if it really were, amazingly, done by the bead,
then the deformation would be in the direction that the bead ran,
diagonally in his scenario, the two bulges pointing towards an
intersection. In fact the one showed is straight out.

> Come to think of it, don't rims worn thin by brake pads tend
> to crack at the base of the sidewall and then start to bulge
> outward because the rim has weakened?


Such a bulge looks nothing like this.

JS and JB have the explanation exactly right and as I summarized the
matter, it relates to the fact that the OP blew at pressures as low as
his tires.
 
While it's probably an illusion from the lighting, the third picture,
the one exhibiting the crack, doesn't appear to correspond to the
others. Follow the hook bead on the far (bent) edge of the rim. In
that picture it appears to be bent toward the viewer, that is, bent
toward the centerline of the rim, while in the first picture it
clearly is bent away. A clearer photo would be nice.

Joe
 
On Thu, 31 Mar 2005 05:54:40 GMT, Joe Riel
<[email protected]> wrote:

>
>While it's probably an illusion from the lighting, the third picture,
>the one exhibiting the crack, doesn't appear to correspond to the
>others. Follow the hook bead on the far (bent) edge of the rim. In
>that picture it appears to be bent toward the viewer, that is, bent
>toward the centerline of the rim, while in the first picture it
>clearly is bent away. A clearer photo would be nice.
>
>Joe


Dear Joe,

I think that I see what you mean, but I agree that it's
probably an illusion from the lighting, as well as the angle
and a less than perfectly regular bend:

http://home.comcast.net/~carlfogel/download/rim_nite.jpg

Unfortunately, our regulars here on the newsgroup aren't the
only ones who are quick to take offence, so the original
poster said to hell with us--I doubt that we'll get any more
pictures.

Since he also set Google Groups not to archive his posts,
they're disappearing, which adds to the confusion.

I'm still wondering if rims dented like that usually crack
in that fashion. I tried bending the rim out in several
places on an old steel rim to 90 degrees and couldn't create
a crack, but I don't know if an aluminum rim would crack
much more easily than steel or if the differences in the rim
cross-section would lead to easier cracking.

My junk pile produced a 4-inch wide piece of some kind of
aluminum alloy plate, 0.077 inches thick, which bent to more
than 90 degrees without cracking. So it still seems worth
asking if a fatigue crack like the one that Jim suspects
weakened the rim enough to bend on impact as Jobst thinks,
but a small enough impact that the rider wouldn't have paid
any attention to it.

Several posters are trying to explain things to me, but I'm
having a good deal of trouble following their descriptions
(probably my fault, not theirs).

Carl Fogel
 
Again, because of the lighting, it is hard to tell precisely
where the crack is. It appears to be just above the "shoulder"
in the cross-section. I've crudely depicted it below.

| /
| crack-->|
+++ +++
| +------+ |
| |
+----------+

Does that agree with what you see? Or is it in the shoulder?

Joe
 
On Thu, 31 Mar 2005 16:02:38 GMT, Joe Riel
<[email protected]> wrote:

>
>Again, because of the lighting, it is hard to tell precisely
>where the crack is. It appears to be just above the "shoulder"
>in the cross-section. I've crudely depicted it below.
>
>| /
>| crack-->|
>+++ +++
>| +------+ |
>| |
>+----------+
>
>Does that agree with what you see? Or is it in the shoulder?
>
>Joe


Dear Joe,

Oops, I think that I misunderstood you. (I thought that you
meant that the very outer edge of the rim looked as if it
bent back in, not out.)

http://home.comcast.net/~carlfogel/download/rim_nite.jpg

When I look at the picture, the crack appears to be the
lower, thicker, darker, longer, jagged black line.

Above it are two thinner, smoother, black lines, roughly
where your diagram indicates on the upper flange, but I
think that they're just lighting, not cracks.

| /
| |
+++ c++ c = crack position
| +------+ |
| |
+----------+

When I tried to duplicate the crack with an old steel
27-inch wheel, it didn't matter if I clamped the rim into my
vise above the "c" or all the way below it into the deepest
part of the rim--bending and flattening to 90 degrees
produced no crack.

But steel isn't aluminum, the cross-section of the rims
could be different, and bending in a vise may apply forces
differently than hitting a rock with a tire.

The original poster vehemently denied noticing any unusual
impact, but most of us insisted that he had to have banged
hell out of the wheel, causing both the bend and the crack.

Pondering that rim, I'm wondering more and more how an
impact could bend the rim out that far and also crack
aluminum that far down into the well without pinching the
tube and producing a flat tire that would have been
impossible to ignore.

If the crack came first from some combination of excessive
pressure, a manufacturing defect, a mismounted tire, and
fatigue, then a much smaller impact, one that wouldn't
normally be noticed and wouldn't pinch the tube, might bend
things out.

I think that some posters are saying that the crack is too
deep to make the rim bend more easily (I may be
misunderstanding them), but if so, why would the bend cause
the crack?

If anyone has a rim with a crack that far down that hit
something without the tire going flat, I'd be glad to host a
picture.

Carl Fogel
 
Carl Fogel writes:

> Maybe something is wrong with my crude experiment, but...


> I just introduced an old 27-inch steel rear rim with weep holes to
> my bench vise and bent five sections of the rim sections, some with
> the narrow end of the vise, some with the wider normal grip.


> Even with four inches of the rim bent out at 90 degrees to the
> spokes, there's no cracking, inside or out. Not even when I bend a
> section with a rusty weep hole.


> Maybe an aluminum rim cracks or ruptures more easily than a steel
> rim? Or maybe the hollow double-wall design affects things?


> The bends are much more dramatic than the originals:


> http://home.comcast.net/~carlfogel/download/rim_nite.jpg


> If there's interest, I take some pictures, but it's just an old
> steel rim with sections of one sidewall bent flat and no cracks.


You can't make that comparison valid because the steel rim is far
thinner than the aluminum rim in question. That means that although
the rim may be as strong or stiff as the aluminum one, its stress is
far lower when bent than the thicker cross section. Stress goes up as
the cube of the bending thickness and aluminum is far weaker than
steel.

Besides, I don't understand the purpose of this experiment. If you
want to recreate the claimed failure, you'll have to get a similar
rim.

[email protected]
 
Joe Riel writes:

> Again, because of the lighting, it is hard to tell precisely
> where the crack is. It appears to be just above the "shoulder"
> in the cross-section. I've crudely depicted it below.


> | /
> | crack-->|
> +++ +++
> | +------+ |
> | |
> +----------+


> Does that agree with what you see? Or is it in the shoulder?


If it were bent toward the viewer, the crack would close, not open.
It's just an artifact of lighting and reflection. The crack is the
same one seen from the inside and the bend goes away from the viewer.

[email protected]
 

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