Spoke Breakage

Sorry if this has been already discussed.

But I am somewhat interested in why spokes are not hollow? Is it the bending
moment that breaks the spokes most often (J bend region) and if so a larger
diameter hollow spoke would increased polar moment of inertia considerably
without any increased weight. However if it is fatigue related what type of
implications would a hollow spoke have with the same cross sectional area,
but with a hollow center and a larger diameter? Would this greatly reduce
aerodynamics of the wheel?

Just wondering and Jobst... yeah yeah I'm a moron- I'm not a mechanical
engineer, just a lowly biomedical engineering student.

Thanks for your time-

David

"david" wrote:

Just wondering and Jobst... yeah yeah I'm a moron- I'm not a mechanical
engineer, just a lowly biomedical engineering student.

Beautiful attempt at a preemptive strike. I'll be interested to see
how effective it is!

david wrote:
Sorry if this has been already discussed.

But I am somewhat interested in why spokes are not hollow? Is it the
bending moment that breaks the spokes most often (J bend region) and
if so a larger diameter hollow spoke would increased polar moment of
inertia considerably without any increased weight. However if it is
fatigue related what type of implications would a hollow spoke have
with the same cross sectional area, but with a hollow center and a
larger diameter? Would this greatly reduce aerodynamics of the wheel?

Just wondering and Jobst... yeah yeah I'm a moron- I'm not a
mechanical engineer, just a lowly biomedical engineering student.

Hey, if someone crashes badly you could remove a spoke and use it as a stent
for a stint!

(Or a /catheter/...)

Shudder Bill

good work. That's beautiful.

david

On Tue, 1 Mar 2005 13:46:33 -0500, "david"
wrote:

Sorry if this has been already discussed.

But I am somewhat interested in why spokes are not hollow? Is it the bending
moment that breaks the spokes most often (J bend region) and if so a larger
diameter hollow spoke would increased polar moment of inertia considerably
without any increased weight. However if it is fatigue related what type of
implications would a hollow spoke have with the same cross sectional area,
but with a hollow center and a larger diameter? Would this greatly reduce
aerodynamics of the wheel?

Just wondering and Jobst... yeah yeah I'm a moron- I'm not a mechanical
engineer, just a lowly biomedical engineering student.

Thanks for your time-

David

Dear David,

A) That idea is for the birds, bonehead.

B) Or maybe featherbrain?

C) Sorry, but those two puns were irresistible, even if they
shaft an innocent inquiry.

D) The previous answer sounded hollow. The next answer is
forced.

E) Spokes are cheaply and easily made by drawing wire
through smaller and smaller holes. The sides of hollow wire
would squish together, defeating the purpose.

F) Running stainless steel tubes through the process above
would deplete our precious stocks of frame tubing.

G) No, that was no joke, it Raleigh would! The 531 tubing is
now available only by special order:

https://vault2.secured-url.com/reynolds/steel531.html

H) The greatest stress is on the outside of the shaft as it
bends into a J, so removing the interior wouldn't address
that concern. (The inner curve compresses, the outer curve
stretches, and the center changes least of all. Look at your
forefinger from the side and curve it--the big skin fold on
the inside allows compression, the wrinkly knuckle allows
stretching, and the skin is almost smooth in the middle of
the side of the joint, where little happens.)

I) Fatter spokes would increase drag a bit, but the
interesting question would be whether to replace hollow
nipples with tiny bolts.

J) Here's a brief account of how actual seamed and seamless
tubing is made:

https://vault2.secured-url.com/reyno...tedtubing.html

This would be rather more expensive than drawn wire spokes,
most of which actually work just fine and last a long time
outside the hypercritical atmosphere of rec.bicycles.tech

Carl Fogel

david wrote:

Sorry if this has been already discussed.

But I am somewhat interested in why spokes are not hollow?

To maintain strength and make manufacture possible you'd need a pretty
big spoke diameter. For many riders aerodynamics are more important
than mass when it comes to wheels, and big fat spokes would be really
bad in this context. You could make long chord aero versions but then
you'd need a fancy way of attaching them to the hub and the rim.

Anyway, your average trispoke or pentaspoke carbon aero wheel does have
hollow (foam-filled) spokes; it's just that they work like cartwheels
rather than tension-spoked wheels and they are not very efficient in
terms of weight. Good for time trialling or pursuiting though.

Tubing in tension has no more strength than solid rod. The best way to
eliminate the bending stresses at the hub end of the spoke would be to make
them "straight pull" by redesigning the hub. I think it has been done on
wheelchairs, but why isn't it done on bikes?

Leo Lichtman wrote:

Tubing in tension has no more strength than solid rod.

Less strength, surely? Tubing is more efficient when you have bending
or torsional stresses; there should be no significant torsional stresses
in a spoke once the wheel is built, so that just leaves bending.

The best way to
eliminate the bending stresses at the hub end of the spoke would be to make
them "straight pull" by redesigning the hub. I think it has been done on
wheelchairs, but why isn't it done on bikes?

And therefore this is the real point. Most spokes break at the elbow.
However, straight-pull spokes (it *has* been done for bikes) have a few
problems:

1) they create a new standard for spokes, and new standards are resisted
by the industry

2) the hub will probably be heavier; the parts of the flange where the
spoke attached had better be strong, otherwise you have the radial wheel
issue where spokes can rip straight out. With a conventional flange and
tangential spoking, the spoke has around half an inch of metal between
it and freedom.

3) straight-pull spokes rotate when you're trying to tighten them. Not
helpful.

On Tue, 01 Mar 2005 21:23:58 GMT, "Leo Lichtman"
wrote:

Tubing in tension has no more strength than solid rod. The best way to
eliminate the bending stresses at the hub end of the spoke would be to make
them "straight pull" by redesigning the hub. I think it has been done on
wheelchairs, but why isn't it done on bikes?

Dear Leo,

Some bicycles do use straight-pull spokes, and so do many
motorcycles.

In all likelihood, the manufacturers see no reason to change
the J-bend spoke design that's cheap, simple, easy to work
with, and incredibly reliable, despite our fuss here on
rec.bicycles.tech.

For example, it took years for the old motorycle frame
design with twin exposed rear suspension units to be
replaced by the modern monoshock tucked inside the frame.
Bicycles came to rear suspension late, so they pretty much
started out with modern rear monoshocks.

Carl Fogel

On Tue, 1 Mar 2005 13:46:33 -0500, "david"
may have said:

Sorry if this has been already discussed.

But I am somewhat interested in why spokes are not hollow?

A tubular spoke with the same tensile strength as the ordinary solid
ones would have a much larger diameter, and thereby higher wind
resistance. This would be viewed as a Bad Thing.

Is it the bending
moment that breaks the spokes most often (J bend region)

That's just one place where they break, and the problem is commonly
addressed by appropriate stress-relief procedures during wheel prep.

and if so a larger
diameter hollow spoke would increased polar moment of inertia considerably
without any increased weight. However if it is fatigue related what type of
implications would a hollow spoke have with the same cross sectional area,
but with a hollow center and a larger diameter?

It would need to have a solid end for the J; a tube would simply
deform at the bend unless some fairly fragile manufacturing techniques
were used, and even then, the larger-OD spoke would mandate a larger
flange diameter...no, this would not be a good thing.

Would this greatly reduce
aerodynamics of the wheel?

The perception would be that it would impact wheel aerodynamics,
whether that was a significant factor or not.

Tubes are good for resisting twist and bend forces, but a spoke's load
is almost purely tensile; there is no advantage in using a tube.

Just wondering and Jobst... yeah yeah I'm a moron- I'm not a mechanical
engineer, just a lowly biomedical engineering student.

I'm not an engineer either, but this one's not rocket science. (Or
brain surgery, if you prefer.)

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