On the virtue of traditional wheels when racing Paris-Roubaix

[[email protected]]

On 16 Apr 2008 19:11:14 GMT, [email protected] wrote:

>Carl Fogel wrote:

[snip quibbles]

>> If the deep carbon rim flattened only half as much as the metal box
>> rim, the suspension travel difference would be only half of 3 to 5
>> mm.
>
>That is not the issue. Damaging the bead is a fracture with
>composites and a mild ding for aluminum. The first one releases the
>tire the other usually doesn't. I have repaired enough metal rims
>while having seen failed composite rims at the LBS.

[snip]

Dear Jobst,

The post to which I replied was specifically about the shock absorbing
ability of deep carbon versus box metal rims.

If you want to address a different issue, feel free to do so.

Cheers,

Carl Fogel

 

[Ron Ruff]

On Apr 15, 6:04 pm, [email protected] wrote:
> That's not the problem but rather how malleable the rim is so that it
> doesn't fracture when a tire bottoms on a road hazard.

This is it. A shallow aluminum rim will flex first and then dent...
but you can still ride it. The deep carbon rim is vertically very
rigid, so on a sharp impact it will crack... and if it cracks badly
enough it falls to pieces.

Deep rims would be nice on a solo escape at the end, but they are
simply a poor choice for all those cobbled sections.

 

[Ron Ruff]

On Apr 16, 9:02 am, Hobbes@spnb&s.com wrote:
> So how much does a box section rim on a 32 spoke wheel give when a 170 pound guy
> with a bike hits rocks at 25per? That's a harder question. But I'll bet real
> cash the answer is not insubstantial compared to the mere 15mm we're allowing
> from the tire.

I'll take that bet.

 

[Ron Ruff]

On Apr 16, 9:52 am, [email protected] wrote:
> Back of the envelope says that a straight 14ga spoke of 280mm length
> and 110kgf tension goes slack at 5mm of rim deflection, so that's the
> practical limit of compliance.

The back of my envelope gives 1mm of length change per 1250N force for
a 290mm 1.5mm dia spoke. So I'd say the practical limit is ~1mm.

 

[[email protected]]

On Wed, 16 Apr 2008 12:34:29 -0600, [email protected] wrote:

>Dear Hobbes,
>
>Just rolling under the hub on smooth pavement, a rim deflects only a
>thousandth of an inch or so.
>
>That's obviously imperceptible.
>
>But some riders occasionally notice spokes rattling when they hit
>things hard. The rattling means that the rim deflected enough to lose
>all its tension. That's around 3 to 5 mm, depending on initial
>tension, spoke length, and spoke gauge.
>
>That's getting close to snake-bite territory, where the tire is mashed
>flat against the rim. (For an impact flat, a tire already mashed flat
>against the rim must be given a good whack to split the rubber tube
>pinched between the rim and the road.)
>
>So what you're really wondering is how much impact is needed to mash a
>rim to spoke-rattle depth for deep carbon versus metal box rim.
>
>If the deep carbon rim flattened only half as much as the metal box
>rim, the suspension travel difference would be only half of 3 to 5 mm.
>
>Maybe someone can calculate the theoretical difference for a deep
>carbon versus a metal box rim, but I suspect that difference will
>remain more theoretical than noticeable to a rider.
>
>Cheers,
>
>Carl Fogel

Ron Ruff just posted his estimate of only 1 mm of stretch for a thin
spoke (1.5 mm), 290 mm long, 1250 N tension.

That roused me to check the equations sections at the end of Jobst's
book.

Sure enough, Jobst's calculations, corrected in the 3rd edition,
worked out to 0.75 mm stretch for a bit less tension (1000 N) on a bit
thicker spoke (1.8 mm).

In other words, the 3~5 mm estimate that I carelessly accepted was
about five times too large.

So a metal box rim will lose spoke tension and twang or rattle the
spokes if it mashes a millimeter or less under impact. Even an
infinitely stiff rim would lose only a millimeter of its shock
absorbing travel on an impressively severe impact, and a carbon rim
would lose even less than a millimeter.

Less than a millimeter difference in shock travel is unlikely to be
noticeable to a rider.

Cheers,

Carl Fogel

 

[[email protected]]

On Wed, 16 Apr 2008 15:04:29 -0700 (PDT), Ron Ruff
<[email protected]> wrote:

>On Apr 16, 9:52 am, [email protected] wrote:
>> Back of the envelope says that a straight 14ga spoke of 280mm length
>> and 110kgf tension goes slack at 5mm of rim deflection, so that's the
>> practical limit of compliance.
>
>The back of my envelope gives 1mm of length change per 1250N force for
>a 290mm 1.5mm dia spoke. So I'd say the practical limit is ~1mm.

Dear Ron,

Thanks--you're right. I should have checked that figure.

Jobst's 3rd edition calculates 0.75 mm stretch for 1000 N on a 1.8 mm
spoke, so your envelope looks right.

Cheers,

Carl Fogel

 

[Ryan Cousineau]

In article <[email protected]>,
Tim McNamara <[email protected]> wrote:

> In article <[email protected]>,
> Hobbes@spnb&s.com wrote:
>
> > On Tue, 15 Apr 2008 22:13:21 -0500, Tim McNamara
> > <[email protected]> wrote:
> >
> > >In article <[email protected]>,
> > > Hobbes@spnb&s.com wrote:
> > >
> > >> On Tue, 15 Apr 2008 14:27:28 -0500, Tim McNamara
> > >> <[email protected]> wrote:
> > >>
> > >> >In article <[email protected]>,
> > >> > Hobbes@spnb&s.com wrote:

[special traditional-type wheels for P-R]

> > So they waste days of mechanic time and inventory space and many
> > hundreds of dollars on special wheels?
>
> Wheels that they probably already have, because they don't usually use
> the high-bling carbon wheels for training. They just ride their normal
> training wheels in this particular race. And the teams that normally
> ride clinchers ride tubulars for this one for the pinch flat reduction.

That's simply not correct. The stuff that gets broken out for P-R is
often not only lower-profile stuff than they generally use for training,
it's often wheel sets using parts that are no longer even available new:

<http://www.cyclingnews.com/road/2008/apr08/wevelgem08/tech.php?id=/photo
s/2008/tech/features/wevelgem_tech208/Caisse_dEpargne_Campy_rim>

<http://www.cyclingnews.com/road/2008/apr08/rvv08/tech.php?id=/photos/200
8/tech/features/flanders_tech108/Saunier_Duval-Scott_rims2>

Here's a team caught by a canny tech writer putting their sponsor's
stickers on someone else's rim:

<http://www.cyclingnews.com/road/2008/apr08/rvv08/tech.php?id=/photos/200
8/tech/features/flanders_tech108/Quick-Step_rims>

Oh, and for those wondering at the logic behind P-R wheel choices, I
present this photo:

<http://www.cyclingnews.com/road/2008/apr08/roubaix08/tech.php?id=/photos
/2008/tech/features/paris_roubaix08/Agritubel_Kuota_KOM_Berges_tied>

Note that several of those photos are from other classics leading up to
P-R. In some cases, the races are a test-bed for P-R equipment, or may
have equipment choices completely unrelated to the team's P-R choices.

--
Ryan Cousineau [email protected] http://www.wiredcola.com/
"In other newsgroups, they killfile trolls."
"In rec.bicycles.racing, we coach them."

 

[Ron Ruff]

Ryan Cousineau wrote:
> Oh, and for those wondering at the logic behind P-R wheel choices, I
> present this photo:
>
> <http://www.cyclingnews.com/road/2008/apr08/roubaix08/tech.php?id=/photos
> /2008/tech/features/paris_roubaix08/Agritubel_Kuota_KOM_Berges_tied>

Yup... it is an extreme circumstance that calls for extreme measures.
Best not to have a busted spoke flapping around.

 

[Hobbes@spnb&s.com]

On Wed, 16 Apr 2008 08:52:08 -0700 (PDT), [email protected] wrote:

>On Apr 16, 11:02 am, Hobbes@spnb&s.com wrote:
>> On Tue, 15 Apr 2008 16:55:07 -0700 (PDT), Anthony DeLorenzo
>>
>> <[email protected]> wrote:
>> >On Apr 15, 4:23 pm, Hobbes@spnb&s.com wrote:
>> >> >> The aero and semi-aero rims shed mud well and are also stiffer and
>> >> >> more rugged. I'm thinking they don't get used in P-R as a concession
>> >> >> to rider comfort.
>>
>> >> >Oy. Umm, how much more "shock absorption" do you think box section rims
>> >> >provide compared to an "aero" rim?
>>
>> >> Don't know. They are certainly much more flexible. Are you presuming that it
>> >> couldn't make a difference?
>>
>> >Probably not as much difference as those air-filled rubber shock
>> >absorbers that are wrapped around the rims, aka the tires.
>>
>> Not as much ain't the same thing as doesn't.
>>
>> How much travel is available from the tires? Maximum is going to be about 20mm -
>> 24mm sewup minus radius of the tire bed of the rim - minus thickness of rubber
>> latex and casing. That's brave low pressure for a race without a pit every half
>> mile so let's get some safety margin and not wallow around and pump it up hard
>> enough for only 15mm of compliance. Since so many of the obstacles are
>> relatively sharp edged you might want even more pressure to make sure. Still, 15
>> stinking millimeters, maximum and that's with sewups.
>>
>> So how much does a box section rim on a 32 spoke wheel give when a 170 pound guy
>> with a bike hits rocks at 25per? That's a harder question. But I'll bet real
>> cash the answer is not insubstantial compared to the mere 15mm we're allowing
>> from the tire.
>>
>> Hey, how do I submit a proposal for a steam-punkish, Fogel-project to measure
>> this?
>
>
>Back of the envelope says that a straight 14ga spoke of 280mm length
>and 110kgf tension goes slack at 5mm of rim deflection, so that's the
>practical limit of compliance.

That's the maximum compliance you'd want to design for. But there is still
plenty of compliance beyond that point.

>Funny you should mention not having a pit every half mile. The
>cobbled sectors in Paris Roubaix run up to 3.7km, are narrow, and are
>closed to team cars. I think that overriding any theoretical
>assessment of comfort is the realization that every piece of equipment
>that rolls up to that start line is going to get crashed at least
>once. Crash survival probably gets much more consideration than
>comfort for a lot of parts.

I wouldn't be surprised if the box section rim recovers better from the
overexcursions than a more rigid deep section rim. But, yep, not breaking is
priority one.

 

[Hobbes@spnb&s.com]

On 16 Apr 2008 19:11:14 GMT, [email protected] wrote:

>Carl Fogel wrote:
>
>>>>>>> The aero and semi-aero rims shed mud well and are also stiffer
>>>>>>> and more rugged. I'm thinking they don't get used in P-R as a
>>>>>>> concession to rider comfort.
>
>>>>>> Oy. Umm, how much more "shock absorption" do you think box
>>>>>> section rims provide compared to an "aero" rim?
>
>>>>> Don't know. They are certainly much more flexible. Are you
>>>>> presuming that it couldn't make a difference?
>
>>>> Probably not as much difference as those air-filled rubber shock
>>>> absorbers that are wrapped around the rims, aka the tires.
>
>>> Not as much ain't the same thing as doesn't.
>
>>> How much travel is available from the tires? Maximum is going to
>>> be about 20mm - 24mm sewup minus radius of the tire bed of the rim
>>> - minus thickness of rubber latex and casing. That's brave low
>>> pressure for a race without a pit every half mile so let's get some
>>> safety margin and not wallow around and pump it up hard enough for
>>> only 15mm of compliance. Since so many of the obstacles are
>>> relatively sharp edged you might want even more pressure to make
>>> sure. Still, 15 stinking millimeters, maximum and that's with
>>> sewups. So how much does a box section rim on a 32 spoke wheel
>>> give when a 170 pound guy with a bike hits rocks at 25per? That's
>>> a harder question. But I'll bet real cash the answer is not
>>> insubstantial compared to the mere 15mm we're allowing from the
>>> tire. Hey, how do I submit a proposal for a steam-punkish,
>>> Fogel-project to measure this?
>
>> Just rolling under the hub on smooth pavement, a rim deflects only a
>> thousandth of an inch or so.
>
>> That's obviously imperceptible.
>
>> But some riders occasionally notice spokes rattling when they hit
>> things hard. The rattling means that the rim deflected enough to
>> lose all its tension. That's around 3 to 5 mm, depending on initial
>> tension, spoke length, and spoke gauge.
>
>Unless the wheel is under tensioned, spokes do not rattle. Spokes
>that become slack under shock load, if they make any sound, make a
>sharp non-reverberating twang. This is especially so for interleaved
>and properly tensioned wheels.
>
>> That's getting close to snake-bite territory, where the tire is
>> mashed flat against the rim. (For an impact flat, a tire already
>> mashed flat against the rim must be given a good whack to split the
>> rubber tube pinched between the rim and the road.)
>
>Snake bites occur from short length obstacles in contrast to spoke
>slackening that occurs typically from road washboard, for instance.
>In that event the contact length is sufficient to prevent bottoming
>the tire while slackening spokes.
>
>> So what you're really wondering is how much impact is needed to mash
>> a rim to spoke-rattle depth for deep carbon versus metal box rim.
>
>That requires striking a rock or root less than 100mm length in the
>direction of travel.
>
>> If the deep carbon rim flattened only half as much as the metal box
>> rim, the suspension travel difference would be only half of 3 to 5
>> mm.
>
>That is not the issue. Damaging the bead is a fracture with
>composites and a mild ding for aluminum. The first one releases the
>tire the other usually doesn't. I have repaired enough metal rims
>while having seen failed composite rims at the LBS.

Here we're talking almost entirely about tubulars. God pity the man trying to
keep clinchers alive at P-R.

>> Maybe someone can calculate the theoretical difference for a deep
>> carbon versus a metal box rim, but I suspect that difference will
>> remain more theoretical than noticeable to a rider.
>
>Leave it to theory!

Just a reminder, things happen whether or not there is a theory to explain it. I
know you grasp that, but some people here don't.

 

[Hobbes@spnb&s.com]

On Wed, 16 Apr 2008 17:46:34 -0600, [email protected] wrote:

>On Wed, 16 Apr 2008 12:34:29 -0600, [email protected] wrote:
>
>>Dear Hobbes,
>>
>>Just rolling under the hub on smooth pavement, a rim deflects only a
>>thousandth of an inch or so.
>>
>>That's obviously imperceptible.
>>
>>But some riders occasionally notice spokes rattling when they hit
>>things hard. The rattling means that the rim deflected enough to lose
>>all its tension. That's around 3 to 5 mm, depending on initial
>>tension, spoke length, and spoke gauge.
>>
>>That's getting close to snake-bite territory, where the tire is mashed
>>flat against the rim. (For an impact flat, a tire already mashed flat
>>against the rim must be given a good whack to split the rubber tube
>>pinched between the rim and the road.)
>>
>>So what you're really wondering is how much impact is needed to mash a
>>rim to spoke-rattle depth for deep carbon versus metal box rim.
>>
>>If the deep carbon rim flattened only half as much as the metal box
>>rim, the suspension travel difference would be only half of 3 to 5 mm.
>>
>>Maybe someone can calculate the theoretical difference for a deep
>>carbon versus a metal box rim, but I suspect that difference will
>>remain more theoretical than noticeable to a rider.
>>
>>Cheers,
>>
>>Carl Fogel
>
>Ron Ruff just posted his estimate of only 1 mm of stretch for a thin
>spoke (1.5 mm), 290 mm long, 1250 N tension.
>
>That roused me to check the equations sections at the end of Jobst's
>book.
>
>Sure enough, Jobst's calculations, corrected in the 3rd edition,
>worked out to 0.75 mm stretch for a bit less tension (1000 N) on a bit
>thicker spoke (1.8 mm).
>
>In other words, the 3~5 mm estimate that I carelessly accepted was
>about five times too large.
>
>So a metal box rim will lose spoke tension and twang or rattle the
>spokes if it mashes a millimeter or less under impact. Even an
>infinitely stiff rim would lose only a millimeter of its shock
>absorbing travel on an impressively severe impact, and a carbon rim
>would lose even less than a millimeter.
>
>Less than a millimeter difference in shock travel is unlikely to be
>noticeable to a rider.


Losing spoke tension does not stop a rim from deflecting or consequently
absorbing road shock. In fact it becomes more compliant.

These are not rigid spokes bound to the rim, they rest in sockets in the rim.
Bend the rim enough to remove the tension and there's nothing to stop it from
moving further. In fact the release of tension at the flattened part of the rim
will reduce tension on the rest of the spokes making the assembly more
compliant.

As for this 1mm business, how much spoke thread do you use when tensioning a
wheel from the point that the nipples are seated on the rim and the wheel is up
to tension. A hell of a lot more than 1mm.

I'll agree that my premise that a box section rim is more compliant and therefor
more comfortable under extreme conditions may be wrong. But the arguments
against have been counter to simple observation.

 

[Hobbes@spnb&s.com]

On Wed, 16 Apr 2008 14:55:26 -0700 (PDT), Ron Ruff <[email protected]> wrote:

>On Apr 16, 9:02 am, Hobbes@spnb&s.com wrote:
>> So how much does a box section rim on a 32 spoke wheel give when a 170 pound guy
>> with a bike hits rocks at 25per? That's a harder question. But I'll bet real
>> cash the answer is not insubstantial compared to the mere 15mm we're allowing
>> from the tire.
>
>I'll take that bet.

Cool, I'm going to have to work out how to test. Need some suitable rims and a
fair loading test. Oh, the assertion is "not insubstantial compared" not
"greater than" or "a whole bunch." Even 5mm is a full third of the travel I'm
allowing for the tire, so that would meet my admittedly low standards. Now I'll
figure out how to test this in a most elegant experiment.

 

[Ron Ruff]

On Apr 17, 9:49 am, Hobbes@spnb&s.com wrote:
> As for this 1mm business, how much spoke thread do you use when tensioninga
> wheel from the point that the nipples are seated on the rim and the wheel is up
> to tension. A hell of a lot more than 1mm.

The 1mm is calculated from the cross section and the properties of the
spoke, at the highest tension and smallest cross-section. For 2mm
spokes it is ~0.5mm... worst case! Cut it in half for NDS. This is the
radial movement that will cause the spoke to go slack... ie, if you
reduce the distance from the hub to rim this amount, the spoke will be
slack. When you are building the wheel there are all sorts of other
things going on like bedding in and straightening the spokes and
compressing the rim.

Certainly, you might get a fair amount of rim flex after tension has
been removed... and before the rim buckles. But note that myself and
many other people who build wheels do not use any sort of
threadlock... and the spokes *never* go slack in normal use... else
the nipples would unwind.

 

[[email protected]]

On Apr 16, 6:04 pm, Ron Ruff <[email protected]> wrote:
> On Apr 16, 9:52 am, [email protected] wrote:
>
> > Back of the envelope says that a straight 14ga spoke of 280mm length
> > and 110kgf tension goes slack at 5mm of rim deflection, so that's the
> > practical limit of compliance.
>
> The back of my envelope gives 1mm of length change per 1250N force for
> a 290mm 1.5mm dia spoke. So I'd say the practical limit is ~1mm.

Thanks for the correction. I thought I might have missed a decimal
somewhere. That should have been just under 0.5mm.

 

[[email protected]]

On Apr 17, 11:49 am, Hobbes@spnb&s.com wrote:
> On Wed, 16 Apr 2008 17:46:34 -0600, [email protected] wrote:
> >On Wed, 16 Apr 2008 12:34:29 -0600, [email protected] wrote:
>
> >>Dear Hobbes,
>
> >>Just rolling under the hub on smooth pavement, a rim deflects only a
> >>thousandth of an inch or so.
>
> >>That's obviously imperceptible.
>
> >>But some riders occasionally notice spokes rattling when they hit
> >>things hard. The rattling means that the rim deflected enough to lose
> >>all its tension. That's around 3 to 5 mm, depending on initial
> >>tension, spoke length, and spoke gauge.
>
> >>That's getting close to snake-bite territory, where the tire is mashed
> >>flat against the rim. (For an impact flat, a tire already mashed flat
> >>against the rim must be given a good whack to split the rubber tube
> >>pinched between the rim and the road.)
>
> >>So what you're really wondering is how much impact is needed to mash a
> >>rim to spoke-rattle depth for deep carbon versus metal box rim.
>
> >>If the deep carbon rim flattened only half as much as the metal box
> >>rim, the suspension travel difference would be only half of 3 to 5 mm.
>
> >>Maybe someone can calculate the theoretical difference for a deep
> >>carbon versus a metal box rim, but I suspect that difference will
> >>remain more theoretical than noticeable to a rider.
>
> >>Cheers,
>
> >>Carl Fogel
>
> >Ron Ruff just posted his estimate of only 1 mm of stretch for a thin
> >spoke (1.5 mm), 290 mm long, 1250 N tension.
>
> >That roused me to check the equations sections at the end of Jobst's
> >book.
>
> >Sure enough, Jobst's calculations, corrected in the 3rd edition,
> >worked out to 0.75 mm stretch for a bit less tension (1000 N) on a bit
> >thicker spoke (1.8 mm).
>
> >In other words, the 3~5 mm estimate that I carelessly accepted was
> >about five times too large.
>
> >So a metal box rim will lose spoke tension and twang or rattle the
> >spokes if it mashes a millimeter or less under impact. Even an
> >infinitely stiff rim would lose only a millimeter of its shock
> >absorbing travel on an impressively severe impact, and a carbon rim
> >would lose even less than a millimeter.
>
> >Less than a millimeter difference in shock travel is unlikely to be
> >noticeable to a rider.
>
> Losing spoke tension does not stop a rim from deflecting or consequently
> absorbing road shock. In fact it becomes more compliant.
>
> These are not rigid spokes bound to the rim, they rest in sockets in the rim.
> Bend the rim enough to remove the tension and there's nothing to stop it from
> moving further. In fact the release of tension at the flattened part of the rim
> will reduce tension on the rest of the spokes making the assembly more
> compliant.
>
> As for this 1mm business, how much spoke thread do you use when tensioning a
> wheel from the point that the nipples are seated on the rim and the wheel is up
> to tension. A hell of a lot more than 1mm.
>
> I'll agree that my premise that a box section rim is more compliant and therefor
> more comfortable under extreme conditions may be wrong. But the arguments
> against have been counter to simple observation.

There are all kinds of messy things going on when you bring a wheel up
to tension. A better example is how much you need to unscrew the
nipple to get a spoke on a tensioned wheel to go slack. It's not very
much. The wheel may continue to deform past spoke slacking, but it's
essentially failed.

The question by the way is not really whether or not a box section rim
is more compliant. Given similar spoking, it's obvious that it will
be. There are two things going against the comfort argument. First
is the magnitude of the compliance, second is the fact that wheel
compliance is all elastic. Elastic deflection may take the edge off
of very high speed impacts, but for the most part cobblestones are
being hit at the fairly low rate of around 100Hz. Only the parts of
the bike with some viscosity, like tires and bar tape, are going to
contribute much to rider comfort.

 

[Ben C]

On 2008-04-17, [email protected] <[email protected]> wrote:
[...]
> There are all kinds of messy things going on when you bring a wheel up
> to tension. A better example is how much you need to unscrew the
> nipple to get a spoke on a tensioned wheel to go slack. It's not very
> much. The wheel may continue to deform past spoke slacking, but it's
> essentially failed.

Note however that the strength of the wheel is just the same when the
spokes are slack as when they are tight, it's just less stiff. If the
spoke tension is excessively high, it actually reduces the strength of
the wheel (in that case the rim would yield before the spokes went
slack).

The reduced stiffness when the spokes go slack means the rim will deform
more for a given load and that extra deformation _may_ cause it get out
of shape and buckle. But spokes can go slack and wheels not fail.

 

[[email protected]]

On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
> On 2008-04-17, [email protected] <[email protected]> wrote:
> [...]
>
> > There are all kinds of messy things going on when you bring a wheel up
> > to tension. A better example is how much you need to unscrew the
> > nipple to get a spoke on a tensioned wheel to go slack. It's not very
> > much. The wheel may continue to deform past spoke slacking, but it's
> > essentially failed.
>
> Note however that the strength of the wheel is just the same when the
> spokes are slack as when they are tight, it's just less stiff. If the
> spoke tension is excessively high, it actually reduces the strength of
> the wheel (in that case the rim would yield before the spokes went
> slack).
>
> The reduced stiffness when the spokes go slack means the rim will deform
> more for a given load and that extra deformation _may_ cause it get out
> of shape and buckle. But spokes can go slack and wheels not fail.

You can't really decouple strength and stiffness that way. It's
exactly because the wheel with the slack spoke is less stiff that an
incremental increase in load gets the rim closer to yield strain than
a wheel without a slack spoke would be. So the strength of the wheel
is not the same. Fatigue failure from overtensioned spokes is a
different issue all together and has been beaten to death elsewhere.

Yes, spokes can go slack without catastrophic wheel failure. You can
also knock quite a few holes in an aircraft fuselage without crashing
it. Once you're outside of design conditions you're bumping up
against the edge of, "how much farther until it collapses?", and the
real world is far too variable to consider that zone to be safe
operating conditions.

 

[Ben C]

On 2008-04-17, [email protected] <[email protected]> wrote:
> On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
>> On 2008-04-17, [email protected] <[email protected]> wrote:
>> [...]
>>
>> > There are all kinds of messy things going on when you bring a wheel up
>> > to tension. A better example is how much you need to unscrew the
>> > nipple to get a spoke on a tensioned wheel to go slack. It's not very
>> > much. The wheel may continue to deform past spoke slacking, but it's
>> > essentially failed.
>>
>> Note however that the strength of the wheel is just the same when the
>> spokes are slack as when they are tight, it's just less stiff. If the
>> spoke tension is excessively high, it actually reduces the strength of
>> the wheel (in that case the rim would yield before the spokes went
>> slack).
>>
>> The reduced stiffness when the spokes go slack means the rim will deform
>> more for a given load and that extra deformation _may_ cause it get out
>> of shape and buckle. But spokes can go slack and wheels not fail.
>
> You can't really decouple strength and stiffness that way.

But strength and stiffness _are_ different things, I'm not decoupling
them. They're already decoupled.

Strength is yield stress, stiffness is strain per unit stress.

> It's exactly because the wheel with the slack spoke is less stiff that
> an incremental increase in load gets the rim closer to yield strain
> than a wheel without a slack spoke would be.

I don't think that's right. Never mind strain, just consider yield
stress.

When the spokes are slack, the structure as a whole is less stiff. But
by definition the rim yields when the total stress on the rim reaches
its yield stress. The more stress already on it from the spokes the less
additional applied stress you need to bring it to yield.

But the other side to the story is you also have to consider the wheel
as a structure, and whether there's any way in which it "collapses"--
i.e. fails as a structure before any of the components in it actually
yield, a bit like a tent folding up in a strong wind. In that case
higher spoke tension may mean it collapses at a higher applied load.

Peter Cole explains the structure well here:

http://groups.google.co.uk/group/rec.bicycles.tech/msg/444e4c7184eef863

There are explanations elsewhere in that thread about how increasing
spoke tension "borrows" compressive strength from the rim as jim beam
puts it.

Which failure mode is significant when you get a buckle or a flat spot:
the materials yielding, or the wheel collapsing? I don't know and I
don't think it's an easy one to call.

Having collapsed in the structural sense bits of the assembly may then
yield because the geometry has changed and you may get more leverage on
parts of the structure that you wouldn't have had before. So post mortem
demonstration of yielded parts doesn't prove the failure actually
started with the components yielding rather than with the structure
collapsing.

 

[Ben C]

On 2008-04-17, Ben C <[email protected]> wrote:
[...]
> Strength is yield stress, stiffness is strain per unit stress.

I meant of course stress per unit strain...

 

[[email protected]]

On Apr 17, 6:04 pm, Ben C <[email protected]> wrote:
> On 2008-04-17, [email protected] <[email protected]> wrote:
>
>
>
> > On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
> >> On 2008-04-17, [email protected] <[email protected]> wrote:
> >> [...]
>
> >> > There are all kinds of messy things going on when you bring a wheel up
> >> > to tension. A better example is how much you need to unscrew the
> >> > nipple to get a spoke on a tensioned wheel to go slack. It's not very
> >> > much. The wheel may continue to deform past spoke slacking, but it's
> >> > essentially failed.
>
> >> Note however that the strength of the wheel is just the same when the
> >> spokes are slack as when they are tight, it's just less stiff. If the
> >> spoke tension is excessively high, it actually reduces the strength of
> >> the wheel (in that case the rim would yield before the spokes went
> >> slack).
>
> >> The reduced stiffness when the spokes go slack means the rim will deform
> >> more for a given load and that extra deformation _may_ cause it get out
> >> of shape and buckle. But spokes can go slack and wheels not fail.
>
> > You can't really decouple strength and stiffness that way.
>
> But strength and stiffness _are_ different things, I'm not decoupling
> them. They're already decoupled.
>
> Strength is yield stress, stiffness is strain per unit stress.
>
> > It's exactly because the wheel with the slack spoke is less stiff that
> > an incremental increase in load gets the rim closer to yield strain
> > than a wheel without a slack spoke would be.
>
> I don't think that's right. Never mind strain, just consider yield
> stress.
>
> When the spokes are slack, the structure as a whole is less stiff. But
> by definition the rim yields when the total stress on the rim reaches
> its yield stress. The more stress already on it from the spokes the less
> additional applied stress you need to bring it to yield.
>
> But the other side to the story is you also have to consider the wheel
> as a structure, and whether there's any way in which it "collapses"--
> i.e. fails as a structure before any of the components in it actually
> yield, a bit like a tent folding up in a strong wind. In that case
> higher spoke tension may mean it collapses at a higher applied load.
>
> Peter Cole explains the structure well here:
>
> http://groups.google.co.uk/group/rec.bicycles.tech/msg/444e4c7184eef863
>
> There are explanations elsewhere in that thread about how increasing
> spoke tension "borrows" compressive strength from the rim as jim beam
> puts it.
>
> Which failure mode is significant when you get a buckle or a flat spot:
> the materials yielding, or the wheel collapsing? I don't know and I
> don't think it's an easy one to call.
>
> Having collapsed in the structural sense bits of the assembly may then
> yield because the geometry has changed and you may get more leverage on
> parts of the structure that you wouldn't have had before. So post mortem
> demonstration of yielded parts doesn't prove the failure actually
> started with the components yielding rather than with the structure
> collapsing.

You can think about it in terms of stress if you want to. Yield
strain and yield stress are interchangeable, and are related by
modulus. If a rim is able to deflect more under a given applied load
because of a slack spoke (altered boundary conditions), then the
stress in the rim must be going up simply by virtue of the fact that
it's deflecting more. The incremental load thus brings that rim
closer to failure than a rim that is properly supported by the spokes.

The link you provided does a very good job of explaining how the wheel
works as a structure. It is meant to operate with all of its
components intact. If you take a structural support out of play (de-
tension a spoke) the structure becomes both more compliant and
weaker. Thus, strength and stiffness are in this case coupled because
the function of the spokes lends both properties to the function of
the wheel as a whole.