Wheel deflection

Ben C? wrote:

[...]

You are right, I do not allow for lateral movement when I am
applying a radial load. In fact I have an indicator to ensure
that the lateral deflection is kept to a minimum. Â*The reason
for this was that if you put a perfectly radial load on a wheel
there shouldn't be any lateral movement of the rim. The reason
most wheels fail the way you described is because there is
inevitably a lateral component to the force being applied that
would taco the wheel. Â*Most of my testing is non- destructive in
order to get an idea of how stiff the wheel is and at what point
the spokes loose tension.

An interesting test you could try (which unfortunately might be
destructive) is build a wheel with very high spoke tension (as
high as you can get it without the wheel taco-ing). Then apply a
radial load with the bottle jack until just before the spokes go
slack.

Then remove the wheel from the apparatus and check for permanent
deformation of the rim. If there is any, it shows excessive spoke
tension can contribute to flat-spotting.

It's something we've debated on RBT before but have only been able
to estimate whether it can happen in practice.

That sounds interesting. I will see what I can do about that.
Though, just thinking about it, it seems nothing abnormal should
occur. The only weak point I can see with lacing at super high
tensions is that the spokes are very likely going to pull through
the rim.

Yes, that's the main problem. They don't necessarily pull through
on day 1, but a few hundred miles later the rim starts to crack from
fatigue.

While the spokes are borrowing more strength from the rim as there
tension increases, they should give it right back to the rim as the
rim starts to deform from an outside force. Therefore, unless the
spokes overcome the strength of the rim in a static state I don't
see how the rim would permanently deform more easily than if the
spoke tensions were lower and a radial load was applied.

They will go a bit slacker as you apply the outside force, but the
total load on the rim still goes up, unless they go completely
slack.

Here's a simpler example: suppose you have a rod with a rubber band
wrapped tightly around it lengthwise, compressing the rod.

Now squeeze the two ends of the rod together. The rod compresses a
bit more and the rubber band loses some tension. Although the
rubber band is now applying a bit less force to the rod, the total
load on the road is still higher: it's whatever you're applying plus
the force from the rubber band.

Note that the rod is under more load than if you applied the same
squeeze without the rubber band there, and it's also under more load
than if you didn't squeeze it at all. It must be or you wouldn't be
able to compress the rod until the rubber band had gone slack.

The total load is always preload + applied load. If the effect of
applying a load is to reduce the preload a bit, then it does reduce
the total a bit. But the total load is still going up.

I don't think your example has a parallel in bicycle wheels. Total
load (what stress are you visualizing?). Compressive load in the rim,
caused by spoke tension, does not change with radial load (weight on
the axle). That is apparent from FEA evaluation. Radial loads are
supported by slackening downward spokes in "the load affected zone".

With current, un-socketed rims, high spoke tension, varying between
static tension (the highest) to loaded tension (the lowest) in the
load affected zone, causes local fatigue and cracking around spoke
holes. This was formerly not a problem with quality rims used at the
time "the Bicycle Wheel" was written. Tubular rims from Fiamme,
Mavic, and others, performed well for long use with tension as high as
the rim could bear without warping (in compression).

You may wonder why many riders who build their own wheels are yearning
for a Mavic MA-2, or Torelli socketed rim. I believe it is
dissatisfaction with rims that cannot be tensioned high enough to not
have rattling spokes under heavy loading.

Low spoke tension came to the business from machine built wheels that
could not deliver higher tension because at higher tensions, spoke
twist from thread friction was as large as the adjustments required to
try wheels. Therefore, truing robots went into infinite loops of
tightening and loosening spoke nipples with no progress. By setting
tension to levels where spoke twist was no longer a problem, various
glues such as SpokePrep (R) and linseed oil came into play to cover
for slack spokes that would otherwise unscrew in use.

As this trend spread through the wheel business, rim manufacturers
began making rims that were strong enough for loose tensioning and
away went the socketed rim. Sockets add cost and weight, and what
better advertising quality that these two parameters? So here we are
with less suitable rims and ones that make up for the loss by and
aerodynamic appearance which gives the spoke support that was lost.

So here we are, as even hubs in component groups by the major
manufacturers are vanishing along with sturdy standard rims. I am not
amused by the trend of "high tech materials and designs" considering
how durable and inexpensive wheels were in the days of yore.

Jobst Brandt

On Oct 29, 11:16*pm, steve wrote:
On Oct 29, 10:18*pm, Frank Krygowski wrote:

Second, it sounds like your framework and bottle jack will be
preventing any lateral movement of the rim. *But one of the common
forms of wheel failure is by the wheel assuming a potato chip shape,
which involves lateral movement of the rim. *It sounds like you're
reinforcing against a failure you're trying to measure. *Also, I think
such impact loads may have a lateral component, and it sounds like
your setup duplicates only an ideal radial load.

You are right, I do not allow for lateral movement when I am applying
a radial load. In fact I have an indicator to ensure that the lateral
deflection is kept to a minimum. *The reason for this was that if you
put a perfectly radial load on a wheel there shouldn't be any lateral
movement of the rim. The reason most wheels fail the way you discribed
is because there is inevidably a lateral component to the force being
applied that would taco the wheel. *Most of my testing is non-
distructive in order to get an idea of how stiff the wheel is and at
what point the spokes loose tension.

If you're trying to be realistic, you need to allow for lateral
buckling.

It sounds like what you're doing is analogous to measuring the
compressive strength of a yardstick while preventing the yardstick's
buckling. Buckling is the most likely mode of failure, so it can't be
ignored.

I could be wrong, because it may be you're after some more limited,
theoretical piece of data. But in real life, wheels buckle into
potato chips. I doubt there's a lot of value in finding how strong a
wheel would be if only it didn't buckle.

BTW, on another matter: If you're interested in the effect of an
inflated tire, it seems you should take some data with and without an
inflated tire in place, and compare. Have you done that?

- Frank Krygowski

Frank Krygowski wrote:

Second, it sounds like your framework and bottle jack will be
preventing any lateral movement of the rim. Â*But one of the common
forms of wheel failure is by the wheel assuming a potato chip
shape, which involves lateral movement of the rim. Â*It sounds like
you're reinforcing against a failure you're trying to
measure. Â*Also, I think such impact loads may have a lateral
component, and it sounds like your setup duplicates only an ideal
radial load.

You are right, I do not allow for lateral movement when I am
applying a radial load. In fact I have an indicator to ensure that
the lateral deflection is kept to a minimum. Â*The reason for this
was that if you put a perfectly radial load on a wheel there
shouldn't be any lateral movement of the rim. The reason most
wheels fail the way you described is because there is inevitably a
lateral component to the force being applied that would taco the
wheel. Â*Most of my testing is non- destructive in order to get an
idea of how stiff the wheel is and at what point the spokes loose
tension.

If you're trying to be realistic, you need to allow for lateral
buckling.

It sounds like what you're doing is analogous to measuring the
compressive strength of a yardstick while preventing the yardstick's
buckling. Buckling is the most likely mode of failure, so it can't
be ignored.

I could be wrong, because it may be you're after some more limited,
theoretical piece of data. But in real life, wheels buckle into
potato chips. I doubt there's a lot of value in finding how strong
a wheel would be if only it didn't buckle.

BTW, on another matter: If you're interested in the effect of an
inflated tire, it seems you should take some data with and without
an inflated tire in place, and compare. Have you done that?

Let's get practical. If you ride bike much, you must have flattened a
rim and also damaged rim beads from rough terrain. Both occur long
before a wheel is in buckling mode as is evident from years of riding
that caused such failures. I have a stack of cast off rims that have
flat spots (about 200mm long) from a hard landing on flat ground as
well as rims that landed on a "knob" and dinged the bead hook.

When spokes go slack the rim is on its own, trying to bridge the
unsupported gap and readily yields a flat spot. I can recall racing
down steep meadows full of gopher holes that would male spokes twang
when they regained tension. Some of those wheels didn't come away
unscathed but none of us folded a wheel as you suggest.

Jobst Brandt

On 2008-10-30, wrote:
Ben C? wrote:
[...]
While the spokes are borrowing more strength from the rim as there
tension increases, they should give it right back to the rim as the
rim starts to deform from an outside force. Therefore, unless the
spokes overcome the strength of the rim in a static state I don't
see how the rim would permanently deform more easily than if the
spoke tensions were lower and a radial load was applied.

They will go a bit slacker as you apply the outside force, but the
total load on the rim still goes up, unless they go completely
slack.

Here's a simpler example: suppose you have a rod with a rubber band
wrapped tightly around it lengthwise, compressing the rod.

Now squeeze the two ends of the rod together. The rod compresses a
bit more and the rubber band loses some tension. Although the
rubber band is now applying a bit less force to the rod, the total
load on the road is still higher: it's whatever you're applying plus
the force from the rubber band.

Note that the rod is under more load than if you applied the same
squeeze without the rubber band there, and it's also under more load
than if you didn't squeeze it at all. It must be or you wouldn't be
able to compress the rod until the rubber band had gone slack.

The total load is always preload + applied load. If the effect of
applying a load is to reduce the preload a bit, then it does reduce
the total a bit. But the total load is still going up.

I don't think your example has a parallel in bicycle wheels. Total
load (what stress are you visualizing?). Compressive load in the rim,
caused by spoke tension, does not change with radial load (weight on
the axle).

I don't see how that's possible. Sit on your bike and the rim flattens a
bit at the bottom and some spokes lose a bit of tension. The rim has
deformed therefore there must be more load on it.

That is apparent from FEA evaluation. Radial loads are supported by
slackening downward spokes in "the load affected zone".

The load is supported both by the spokes and by the rim. Both deform--
the spokes lose tension and the rim flattens.

With current, un-socketed rims, high spoke tension, varying between
static tension (the highest) to loaded tension (the lowest) in the
load affected zone, causes local fatigue and cracking around spoke
holes. This was formerly not a problem with quality rims used at the
time "the Bicycle Wheel" was written. Tubular rims from Fiamme,
Mavic, and others, performed well for long use with tension as high as
the rim could bear without warping (in compression).

You may wonder why many riders who build their own wheels are yearning
for a Mavic MA-2, or Torelli socketed rim. I believe it is
dissatisfaction with rims that cannot be tensioned high enough to not
have rattling spokes under heavy loading.

The high dish on rear wheels these days is also a big part of the
problem-- you have to make the right side very tight (and risk cracking)
or use glue on the left side.

Low spoke tension came to the business from machine built wheels that
could not deliver higher tension because at higher tensions, spoke
twist from thread friction was as large as the adjustments required to
try wheels. Therefore, truing robots went into infinite loops of
tightening and loosening spoke nipples with no progress. By setting
tension to levels where spoke twist was no longer a problem, various
glues such as SpokePrep (R) and linseed oil came into play to cover
for slack spokes that would otherwise unscrew in use.

As this trend spread through the wheel business, rim manufacturers
began making rims that were strong enough for loose tensioning and
away went the socketed rim. Sockets add cost and weight, and what
better advertising quality that these two parameters?

You can still get socketed rims-- the Mavic Open Pro for example.

Ben C? wrote:

[...] While the spokes are borrowing more strength from the rim
as there tension increases, they should give it right back to the
rim as the rim starts to deform from an outside force.
Therefore, unless the spokes overcome the strength of the rim in
a static state I don't see how the rim would permanently deform
more easily than if the spoke tensions were lower and a radial
load was applied.

They will go a bit slacker as you apply the outside force, but the
total load on the rim still goes up, unless they go completely
slack.

Here's a simpler example: suppose you have a rod with a rubber
band wrapped tightly around it lengthwise, compressing the rod.

Now squeeze the two ends of the rod together. The rod compresses
a bit more and the rubber band loses some tension. Although the
rubber band is now applying a bit less force to the rod, the total
load on the road is still higher: it's whatever you're applying
plus the force from the rubber band.

Note that the rod is under more load than if you applied the same
squeeze without the rubber band there, and it's also under more
load than if you didn't squeeze it at all. It must be or you
wouldn't be able to compress the rod until the rubber band had
gone slack.

The total load is always preload + applied load. If the effect of
applying a load is to reduce the preload a bit, then it does
reduce the total a bit. But the total load is still going up.

I don't think your example has a parallel in bicycle wheels. Total
load (what stress are you visualizing?). Compressive load in the
rim, caused by spoke tension, does not change with radial load
(weight on the axle).

I don't see how that's possible. Sit on your bike and the rim
flattens a bit at the bottom and some spokes lose a bit of
tension. The rim has deformed therefore there must be more load on
it.

You throw around the term "load" loosely. Lets talk about stress.
Thee is local bending stress from that load but that is not large
because radial stiffness of a wheel comes from spoke elasticity, not
rim elasticity. Farther on in this thread, I propose that
participants in this discussion should have the answers to all this if
they ride to the limit of their wheel strength.

That is apparent from FEA evaluation. Radial loads are supported
by slackening downward spokes in "the load affected zone".

The load is supported both by the spokes and by the rim. Both deform--
the spokes lose tension and the rim flattens.

Bot so. The load is supported by the spokes. Only after spokes have
gone significantly slack does bending of the rim become a problem.
That is why a strong wheel must have high tension, so the rim does not
go out of round to the point of plastic deformation. When staying in
the elastic range of spokes (not going slack) deflections are in one
or two tenths of millimeters, less than is required to flatten a rim
permanently.

With current, un-socketed rims, high spoke tension, varying between
static tension (the highest) to loaded tension (the lowest) in the
load affected zone, causes local fatigue and cracking around spoke
holes. This was formerly not a problem with quality rims used at
the time "the Bicycle Wheel" was written. Tubular rims from
Fiamme, Mavic, and others, performed well for long use with tension
as high as the rim could bear without warping (in compression).

You may wonder why many riders who build their own wheels are yearning
for a Mavic MA-2, or Torelli socketed rim. I believe it is
dissatisfaction with rims that cannot be tensioned high enough to not
have rattling spokes under heavy loading.

The high dish on rear wheels these days is also a big part of the
problem-- you have to make the right side very tight (and risk
cracking) or use glue on the left side.

So? Is that my fault that riders need 3x11 gearing? As you must have
seen, I managed to find some 7-speed Shimano hubs, expressly for that
reason.

Low spoke tension came to the business from machine built wheels
that could not deliver higher tension because at higher tensions,
spoke twist from thread friction was as large as the adjustments
required to try wheels. Therefore, truing robots went into
infinite loops of tightening and loosening spoke nipples with no
progress. By setting tension to levels where spoke twist was no
longer a problem, various glues such as SpokePrep (R) and linseed
oil came into play to cover for slack spokes that would otherwise
unscrew in use.

As this trend spread through the wheel business, rim manufacturers
began making rims that were strong enough for loose tensioning and
away went the socketed rim. Sockets add cost and weight, and what
better advertising quality that these two parameters?

You can still get socketed rims-- the Mavic Open Pro for example.

Just the same, it is not as sturdy as it seems and has a less
desirable cross section that earlier rims.

Jobst Brandt

On 2008-10-30, wrote:
Ben C? wrote:
[...]
The load is supported both by the spokes and by the rim. Both deform--
the spokes lose tension and the rim flattens.

Bot so. The load is supported by the spokes. Only after spokes have
gone significantly slack does bending of the rim become a problem.

The rim has to bend for the spokes to become slack.

That is why a strong wheel must have high tension, so the rim does not
go out of round to the point of plastic deformation.

When the spokes go slack, the wheel becomes much less stiff. But the
rim's strength is not affected. The load required to deform it
plastically can only be reduced by high spoke tension.

High spoke tension does not make anything stronger. It just increases
the threshold at which spokes go slack and the wheel suddenly becomes
less stiff.

But you may be right that in practice plastic deformation of the rim
does not occur until after the spokes have gone slack. The purpose of
the experiment I was proposing to Steve was to investigate this.

When staying in the elastic range of spokes (not going slack)
deflections are in one or two tenths of millimeters, less than is
required to flatten a rim permanently.

That may well be the case.

steve wrote:
By the way, does anybody have an idea of the max amount of force in
lbf or Nm that a human can produce at the center of the hub?

http://groups.google.com/group/rec.b...c8ff 1243105c

steve wrote:
The reason for this was that if you
put a perfectly radial load on a wheel there shouldn't be any lateral
movement of the rim.

Not so. Rear dished wheels will see lateral deflection... especially
if lighter spokes are used on the NDS.

steve wrote:
Unfortunatly I don't really understand how that force is transferred,
whether it is transferred to a point on the rim or to a broader area.
If it is transferred to a point on the rim then how does the tire
allow the rim to handle greater forces before permanent deformation?
Is it the energy required to compress the tire?

It spreads the load out...

steve wrote:
So what does a tire do to allow the rim to handle a greater force
before it permanantly deforms? If there is a force applied at a point
on a tire that is inflated then there should be some rim deflection
since some of the force should transfer from the tire to the rim.
Unfortunatly I don't really understand how that force is transferred,
whether it is transferred to a point on the rim or to a broader area.
If it is transferred to a point on the rim then how does the tire
allow the rim to handle greater forces before permanent deformation?
Is it the energy required to compress the tire?
Steve

Whack a bare rim with a hammer.

Taking great care to avoid the hammer's rebound, try that with an
inflated tire.

A pneumatic tire spreads the impact over area and time.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971
** Posted from http://www.teranews.com **