Spoke busted in the middle?

I'm curious about your mechanical engineering background, and about
whether or not you've read _The Bicycle Wheel_ by Brandt.

- Frank Krygowski

Though I'm an electronics engineer and embedded systems specialist I did do some extensive design in robotic systems in medical instruments and managed an engineering department where I had to have more than a passing familiarity with mechanical designs.

Please recall that the Bicycle Wheel was last updated in 1993 and aero rims were just starting to appear and I can't remember Jobst referring to them. At that time a wheel had a Mavic Reflex rim and 36 spokes.

On Friday, September 4, 2015 at 9:18:55 AM UTC-7, wrote:

Most of the common knowledge about wheel construction is mythology and it is only recently that the mythology is being challenged.
What you need to remember about aluminum - and ALL aluminum alloys
is ALL flexing causes stress, and the stress is cumulative.
Unlike steel, which can flex indefinitely as long as it never flexes
past the fatigue limit, aluminum has no fatigue limit. If it moves AT
ALL fatigue is building up in the material and eventually it WILL
fail..

That's why you generally don't see aluminum springs. (yes, there are
SOME uses for aluminum springs, some light aircraft use bent aluminum
gear, for example - but they do have a limited lifespan)

That is sort of an old-wive's tale. Firstly it depends on the alloy and secondly it is strongly dependent on the application. In case you missed it aircraft are built of aluminum and do not fail even with that spectacularly hard aircraft alloy. When you watch P51's and F82's performing at Air Shows remember that fatigue build up.

How often do you see rims failing? I have some of the original Campagnolo tubulars that have probably 50,000 miles on them. There is not the slightest sign of failure and I've never seen a fatigue failure of these sorts of wheels after riding for 35 years.

Frank and I are discussing stress failures. If you have to use Statistical Mechanics to discover a fatigue failure you can consider it an unusual case..

On 9/6/2015 10:18 PM, wrote:
I'm curious about your mechanical engineering background, and about
whether or not you've read _The Bicycle Wheel_ by Brandt.

- Frank Krygowski

Though I'm an electronics engineer and embedded systems specialist I did do some extensive design in robotic systems in medical instruments and managed an engineering department where I had to have more than a passing familiarity with mechanical designs.

Please recall that the Bicycle Wheel was last updated in 1993 and aero rims were just starting to appear and I can't remember Jobst referring to them. At that time a wheel had a Mavic Reflex rim and 36 spokes.

IIRC, Jobst referred to aero rims (and paired spoke wheels, etc.) quite
a few times in this forum.

Have you read the book? If you haven't, I highly recommend it. The
mechanics of a spoked wheel are much less obvious than most people -
including most engineers - think. The FEA results in the book are often
surprising, but logical once the knowledge sets in, so to speak.

I understand that there will be quantitative differences with deep
section rims and unconventional spoking patterns. But I see no reason
that the fundamental physics will change.

--
- Frank Krygowski

On 9/6/2015 10:31 PM, wrote:
On Friday, September 4, 2015 at 9:18:55 AM UTC-7, wrote:

Most of the common knowledge about wheel construction is mythology and it is only recently that the mythology is being challenged.
What you need to remember about aluminum - and ALL aluminum alloys
is ALL flexing causes stress, and the stress is cumulative.
Unlike steel, which can flex indefinitely as long as it never flexes
past the fatigue limit, aluminum has no fatigue limit. If it moves AT
ALL fatigue is building up in the material and eventually it WILL
fail..

That's why you generally don't see aluminum springs. (yes, there are
SOME uses for aluminum springs, some light aircraft use bent aluminum
gear, for example - but they do have a limited lifespan)

That is sort of an old-wive's tale. Firstly it depends on the alloy and
secondly it is strongly dependent on the application. In case you missed it
aircraft are built of aluminum and do not fail even with that spectacularly
hard aircraft alloy. When you watch P51's and F82's performing at Air Shows
remember that fatigue build up.

I don't think it's an old wive's tale. If there are aluminum alloys
that have a true endurance limit, I haven't yet heard of them. IIRC,
the hardest aluminum alloy typically used in aircraft construction is
7075-T6, and while it's got good fatigue strength, it has no true
endurance limit. In plain English, that means it is expected to fail
after enough applications of a repeated or reversing load, no matter how
low the stress. Planes are very carefully designed so their varying
stresses are low enough for long life. But there certainly have been
failures. Here's a classic one:
https://en.wikipedia.org/wiki/De_Havilland_Comet

How often do you see rims failing?

Rims do fail, and these days the typical failure seems to be fatigue
cracks in the spoke bed. That area is subjected to variable tensile
stresses as a result of the lower spokes losing tension due to rim
deformation under load, as explained in the book.

Do a Google image search for "bicycle wheel rim finite element analysis"
and take a quick look at some of the graphic results.

--
- Frank Krygowski

On 07/09/15 12:36, Frank Krygowski wrote:
On 9/6/2015 10:18 PM, wrote:
I'm curious about your mechanical engineering background, and about
whether or not you've read _The Bicycle Wheel_ by Brandt.

- Frank Krygowski

Though I'm an electronics engineer and embedded systems specialist I
did do some extensive design in robotic systems in medical instruments
and managed an engineering department where I had to have more than a
passing familiarity with mechanical designs.

Please recall that the Bicycle Wheel was last updated in 1993 and aero
rims were just starting to appear and I can't remember Jobst referring
to them. At that time a wheel had a Mavic Reflex rim and 36 spokes.

IIRC, Jobst referred to aero rims (and paired spoke wheels, etc.) quite
a few times in this forum.

Have you read the book? If you haven't, I highly recommend it. The
mechanics of a spoked wheel are much less obvious than most people -
including most engineers - think. The FEA results in the book are often
surprising, but logical once the knowledge sets in, so to speak.

I understand that there will be quantitative differences with deep
section rims and unconventional spoking patterns. But I see no reason
that the fundamental physics will change.


On the quantitative differences, here's a table that shows the
difference between a 30mm aero rim and a couple of more box-like rims.

http://www.slowtwitch.com/articles/i..._Stiffness.png

Almost doubling the rim height gave approx. a 4 times increase in rim
stiffness.

I assume that distributes the strain on spokes more than one would
expect from rims that Jobst wrote about in his book. It doesn't mean
the rim remains round and pulls the top spoke through the rim though.

30mm rims are not considered particularly deep. There are plenty that
are well in excess of 40mm. It would also be interesting to know the
stiffness of some of the aero CFRP rims, like Zipp 404s, for example.

There's a lot more from the web page where that table is linked to..
http://www.slowtwitch.com/Tech/Debun...ness_3449.html

I think this FEA analysis also confirms what Jobst found. Would be nice
to compare with a similar analysis of much deeper rims.

http://www.ewp.rpi.edu/hartford/~ern...inalReport.pdf

--
JS

https://www.youtube.com/watch?v=F-1zD6_Yjig

On Sun, 6 Sep 2015 19:31:02 -0700 (PDT), wrote:

On Friday, September 4, 2015 at 9:18:55 AM UTC-7, wrote:

Most of the common knowledge about wheel construction is mythology and it is only recently that the mythology is being challenged.
What you need to remember about aluminum - and ALL aluminum alloys
is ALL flexing causes stress, and the stress is cumulative.
Unlike steel, which can flex indefinitely as long as it never flexes
past the fatigue limit, aluminum has no fatigue limit. If it moves AT
ALL fatigue is building up in the material and eventually it WILL
fail..

That's why you generally don't see aluminum springs. (yes, there are
SOME uses for aluminum springs, some light aircraft use bent aluminum
gear, for example - but they do have a limited lifespan)

That is sort of an old-wive's tale. Firstly it depends on the alloy and secondly it is strongly dependent on the application. In case you missed it aircraft are built of aluminum and do not fail even with that spectacularly hard aircraft alloy. When you watch P51's and F82's performing at Air Shows remember that fatigue build up.

But in fact airplanes do come to pieces due to accumulated fatigue.
You might want to read up on the British Comet.

"The de Havilland DH 106 Comet was the first production commercial
jetliner and a year after entering commercial service the Comets began
suffering problems, with three of them breaking up during mid-flight
in well-publicised accidents. This was later found to be due to
catastrophic metal fatigue in the airframes,

How often do you see rims failing? I have some of the original Campagnolo tubulars that have probably 50,000 miles on them. There is not the slightest sign of failure and I've never seen a fatigue failure of these sorts of wheels after riding for 35 years.

Frank and I are discussing stress failures. If you have to use Statistical Mechanics to discover a fatigue failure you can consider it an unusual case.
--
cheers,

John B.

question is mooooooooot

deep rims are AFAIK unrepeairable

so the point is ?

tack ?

On 9/6/2015 11:18 PM, James wrote:
On 07/09/15 12:36, Frank Krygowski wrote:
On 9/6/2015 10:18 PM, wrote:
I'm curious about your mechanical engineering background, and about
whether or not you've read _The Bicycle Wheel_ by Brandt.

- Frank Krygowski

Though I'm an electronics engineer and embedded systems specialist I
did do some extensive design in robotic systems in medical instruments
and managed an engineering department where I had to have more than a
passing familiarity with mechanical designs.

Please recall that the Bicycle Wheel was last updated in 1993 and aero
rims were just starting to appear and I can't remember Jobst referring
to them. At that time a wheel had a Mavic Reflex rim and 36 spokes.

IIRC, Jobst referred to aero rims (and paired spoke wheels, etc.) quite
a few times in this forum.

Have you read the book? If you haven't, I highly recommend it. The
mechanics of a spoked wheel are much less obvious than most people -
including most engineers - think. The FEA results in the book are often
surprising, but logical once the knowledge sets in, so to speak.

I understand that there will be quantitative differences with deep
section rims and unconventional spoking patterns. But I see no reason
that the fundamental physics will change.


On the quantitative differences, here's a table that shows the
difference between a 30mm aero rim and a couple of more box-like rims.

http://www.slowtwitch.com/articles/i..._Stiffness.png


Almost doubling the rim height gave approx. a 4 times increase in rim
stiffness.

I assume that distributes the strain on spokes more than one would
expect from rims that Jobst wrote about in his book. It doesn't mean
the rim remains round and pulls the top spoke through the rim though.

30mm rims are not considered particularly deep. There are plenty that
are well in excess of 40mm. It would also be interesting to know the
stiffness of some of the aero CFRP rims, like Zipp 404s, for example.

There's a lot more from the web page where that table is linked to..
http://www.slowtwitch.com/Tech/Debun...ness_3449.html

I think this FEA analysis also confirms what Jobst found. Would be nice
to compare with a similar analysis of much deeper rims.

http://www.ewp.rpi.edu/hartford/~ern...inalReport.pdf

Looks like interesting information. I won't be able to read them for a
day or two, but I'll get to them.

FWIW, I recall about the same time Jobst was coming out with his book,
John Forester got his first dial gage and was playing around with it.
He rigged it up to measure spoke deflections on a bike wheel, and
independently discovered that there's no significant increase in spoke
tension as the wheel revolves; there is instead a decrease in spoke
tension near the load point on the rim. He wrote this into a short
informal article.

I came across the article Forester wrote before I came across Jobst's
book. It immediately made sense to me. Jobst's book is, of course,
much more technical and thorough, covering many different load regimes, etc.


--
- Frank Krygowski

On Sunday, September 6, 2015 at 7:36:45 PM UTC-7, Frank Krygowski wrote:
On 9/6/2015 10:18 PM, wrote:
I'm curious about your mechanical engineering background, and about
whether or not you've read _The Bicycle Wheel_ by Brandt.

- Frank Krygowski

Though I'm an electronics engineer and embedded systems specialist I did do some extensive design in robotic systems in medical instruments and managed an engineering department where I had to have more than a passing familiarity with mechanical designs.

Please recall that the Bicycle Wheel was last updated in 1993 and aero rims were just starting to appear and I can't remember Jobst referring to them. At that time a wheel had a Mavic Reflex rim and 36 spokes.

IIRC, Jobst referred to aero rims (and paired spoke wheels, etc.) quite
a few times in this forum.

Have you read the book? If you haven't, I highly recommend it. The
mechanics of a spoked wheel are much less obvious than most people -
including most engineers - think. The FEA results in the book are often
surprising, but logical once the knowledge sets in, so to speak.

I understand that there will be quantitative differences with deep
section rims and unconventional spoking patterns. But I see no reason
that the fundamental physics will change.

--
- Frank Krygowski

I read his book but it was one of the many things I threw out when I was recovering from my concussion and was apparently getting rid of anything that had to do with bicycles.

I broke a rim and had a spare front wheel so I removed the rim and installed it on the back hub, tensioned all of the spokes and straightened it completely in less than 30 minutes so apparently I remember something about wheels.

I rode that wheel for about six months with no problems with it.

After putting it together I remembered that you tension spokes NOT by feel or by using a tensiometer but by the sound. This also means that the wheel has to be 99% built.