A physicist's question about tensioning a wheel

[Simon Brooke]

H'mmm...

I posted a message to this thread (about hubs standing on spokes) late last night which was uncalled
for and which I'd like to apologise for (particularly to Jobst). I'm posting this apology now before
I read anything else in the thread and get self-righteous or ****** off again.

Memo to self: never post to Usenet at 2 in the morning.

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/ ;; gif ye hes forget our auld
plane Scottis quhilk your mother lerit you, ;; in tymes cuming I sall wryte to you my mind in Latin,
for I am nocht ;; acquyntit with your Southeron ;; Letter frae Ninian Winyet tae John Knox datit 27t
October 1563

 

[Ryan Cousineau]

In article <[email protected]>,
jim beam <[email protected]> wrote:

> Sheldon Brown wrote: <snip>
> >
> > It probably has carbon-steel spokes, as did most bikes back then. Although carbon-steel spokes
> > are not as strong as stainless (and prone to rusting) they are less subject to fatigue than
> > stainless spokes are.
>
> the carbon alloy steel spokes were galvanized most likely. flashy [literally] ones were
> chrome plated.
>
> technically, carbon alloy steel wire is usually stronger than stainless. it's still used in steel
> rope where strength [&/or price] outweighs weather considerations.
>
> carbon steel spokes /were/ less prone to fatigue. modern stainless steels are significantly better
> than they used to be and now the two are roughly comparable. high strength carbon alloy steels do
> not typically have a fatigue endurance limit - it's just that it's been possible to make "cleaner"
> carbon alloy steels [cheaper] longer than has been possible for stainless.
>
> if ever you travel to san francisco, there are two pieces of interesting wire that you must see:
>
> one is at the golden gate bridge. there is a section of the main span rope cut open for inspection
> at the visitor center. interesting un-twisted strand design. high strength carbon alloy steel.
>
> the other is the crowd barrier rope at the cable car turnaround on powell & market. it's a piece
> of actual cable car traction rope. unlike all the [wire] rope you normally see that is twisted to
> make the exterior strands run axially, this one is twisted to make the exterior stranding as close
> to perpendicular as possible. this is to give maximum possible friction for the cable car's rope
> clutching mechanism as it goes up the hills. again, carbon alloy steel.

Don't forget to visit the cable car museum, too. When I first looked at the cable car system I
couldn't figure out what was going on underneath the road to get acceleration and braking. Any
system I could crudely guess at seemed to be destined to have a huge amount of wear.

The museum explained it: the cable grip jaws get replaced every four days on average.

http://www.cablecarmuseum.com/Anat/Anat.html

> /utterly/ useless trivia, but thrillingly geeky!

The whole cable car system is an amazing exercise in high-wear parts: the grip jaws wear quickly,
the cable gets replaced frequently, the brakes wear out fast. If it wasn't for tourism, the whole
wonderful thing would be deader than dead.

The 3-day Muni pass, BTW, is a great deal if you like riding the cable cars.

--
Ryan Cousineau, [email protected] http://www.sfu.ca/~rcousine President, Fabrizio Mazzoleni Fan Club

 

[Pete Biggs]

[email protected] wrote:
> Pete Biggs writes:
>> You're likely to get broken spokes through metal fatigue if you continue with them slack.
>
> I read this admonition now and then and have not heard an explanation of how slack spokes cause
> fatigue failures.

I thought fatigue was caused by spokes repeatedly going from slack to taut and that the slacker the
spokes, the more difference in tension there would be in use.

"A spoked wheel relies on having all of the spokes in constant tension. A highly dished rear wheel
starts with very light tension on the left side spokes. The torque of hard pedaling can cause the
left side "leading" spokes to occasionally go completely slack momentarily. Repeated cycles of
tension and slackness cause these spokes to fatigue at the bends, and ultimately break."
- www.sheldonbrown.com/wheelbuild.html

~PB

 

[Chris B .]

On Sun, 04 Jan 2004 02:05:02 GMT, Simon Brooke <[email protected]>
wrote:

>"Paul" <[email protected]> writes:
>
>> Simon Brooke wrote:
>>
>> > All the spokes are quite noticeably slack - you can take any individual spoke and rattle it.
>>
>>
>> An interesting observation that, Simon. I take it you mean when the bike was unloaded all the
>> spokes rattled.
>
>Yes.
>
>> Worth repeating.
>>
>> > All the spokes are quite noticeably slack - you can take any individual spoke and rattle it.
>>
>> How do you suppose the load on the hub was supported when the slack spokes were at the bottom?
>
>Why, it was standing on them of course. The Great Jobst has said so, how can any further comment be
>other than superfluous? If there is any difference between reality and the pronouncements of the
>Great Jobst, it is always reality which is wrong.

You seem rattled. Do you really expect people to believe that the wheel in your story could carry
that much wounded pride around?

 

[Cal]

Quite appropriate!

On Sun, 04 Jan 2004 07:04:59 GMT, "Phil Holman" <[email protected]> wrote:

>"The power of accurate observation is called cynicism by those who have not got it." - G. B. Shaw

 

[Tim McNamara]

Simon Brooke <[email protected]> writes:

> H'mmm...
>
> I posted a message to this thread (about hubs standing on spokes) late last night which was
> uncalled for and which I'd like to apologise for (particularly to Jobst). I'm posting this apology
> now before I read anything else in the thread and get self-righteous or ****** off again.

You might be able to cancel the post. Your newsreader might be able to issue the appropriate command
(another reason why one should use a proper newsreader).

> Memo to self: never post to Usenet at 2 in the morning.

A good idea for many.

 

[Jim Beam]

[email protected] wrote:
> That's a nice story but I think you are reconstructing the event from imagination. Wheels cannot
> slowly sink to a lower rolling diameter. The failure mode is lateral collapse and that jams the
> wheel in the frame so it won't turn. It makes a nice story but it didn't happen.

get a grip of yourself man. i know the difference between lateral collapse and radial failure. and
what i described was a serial progressive radial failure. try and separate blind prejudice from
rational analysis.

 

[Werehatrack]

On Sun, 04 Jan 2004 11:18:19 -0600, Tim McNamara
<[email protected]> may have said:

>You might be able to cancel the post. Your newsreader might be able to issue the appropriate
>command (another reason why one should use a proper newsreader).

Many servers do not honor cancels anymore due to the malicious cancel floods that have been seen on
numerous groups over the years. Issuing a cancel is still a good idea, though, but it should be
remembered that a cancel won't necessarily take the post off of every server that the original
article may have reached.

--
My email address is antispammed; pull WEEDS if replying via e-mail.
Yes, I have a killfile. If I don't respond to something,
it's also possible that I'm busy.
Words processed in a facility that contains nuts.

 

[Carl Fogel]

[email protected] wrote in message news:<[email protected]>...
> Jim Beam writes:
>
> >> I don't know, but I'd like to add one completely surprising observation. Among the bikes in
> >> this household is a twenty year old Raleigh Royale with 27" wheels which, until this year,
> >> hadn't been ridden for about six years (it was my sister's; she died). The rear wheel is
> >> original. All the spokes are quite noticeably slack - you can take any individual spoke and
> >> rattle it. But the wheel is reasonably true - true enough not to rub on the brakes or interfere
> >> with the mudguard. I'm so surprised at this that I've left it that way to see what will happen,
> >> and what has happened is... precisely nothing. It's been ridden for over 400 miles over the
> >> past three months in this state, and the wheel is still adequately true. Sooner or later of
> >> course I'm going to have to take it out and tension it because apart from anything else it just
> >> doesn't look right... but I'm amazed that it has remained usable.
>
> > My mother used to go shopping on a bike like that and would come home /loaded/ with groceries.
> > Dare I say it, she was no bantam weight either. This bike used to get ridden most days of the
> > week. Loose spokes, just like you say.
>
> > Then one day, just as she was pulling up in front of our house, the rear wheel started to
> > collapse. It was a bit like watching a mime comedian walking down an imaginary staircase - she
> > just sunk lower & lower as she rolled along with more & more spokes giving up the ghost. Father
> > got her another wheel but she never rode it again.
>
> That's a nice story but I think you are reconstructing the event from imagination. Wheels cannot
> slowly sink to a lower rolling diameter. The failure mode is lateral collapse and that jams the
> wheel in the frame so it won't turn. It makes a nice story but it didn't happen.
>
> > As I recall, the wheel was never superbly true, but was not bad either. The spokes would kind of
> > rattle. Considering the use it had, I think it had lasted quite well. 50's vintage?
>
> I suppose the upshot is that "we don't need no steenkin tension in our wheels". I see no other
> reason for these testimonials, phony as they sound to wheelbuilders. Loose wheels can be ridden
> but it isn't a reasonable thing to do if you know the wheel is rattlingly loose.
>
> That such a wheel is true is an old story, something on which inept wheel builders rely, because
> they don't know how to true a wheel once the spokes are tight enough to overcome the original
> trueness of a new rim. New rims are true and remain that way in the absence of spoke tension. This
> should not be amazing. What is amazing is that some people ride so timidly that their wheels
> rarely see much stress for which they would need to demonstrate strength, strength that depends
> primarily on spoke tension, if there is any.
>
> Jobst Brandt [email protected]

Dear Jobst and Jim,

Assuming that Jim's mother's wheels were indeed as strong as her thills, as Holmes assures us was
good practice in that era, perhaps the crucial defect actually lay in her whippletree?

http://cs.wwc.edu/~aabyan/Poetry/holmes.html

The interesting failure mode of the shay invites analysis.

Carl Fogel

 

[Jim Beam]

[email protected] wrote:

<snip>

> Since these cables bend around pulleys and curves, they must be helically wound or they would
> break immediately. They are made this way for the same reason control cables on bicycles are
> helically wound...

they're helically wound, but in the opposite direction to normal so that the exterior strandlets run
close to perpendicular. but you'd know that if you'd actually seen the cable up close.

> so they will have uniform stress in all strands when bending around curves. Cable car cable has a
> special layer of external friction strands that are smooth and rectangular

absolutely incorrect. that might be a bad description of a die drawn bike cable, but it's as far
from cable car rope as it's possible to be. that's why i said it was an interesting article worth
seeing - it's completely /atypical/! you've evidently never seen it up close.

 

[Frkrygow]

Pete Biggs wrote:

> [email protected] wrote:
>
>>Pete Biggs writes:
>>
>>>You're likely to get broken spokes through metal fatigue if you continue with them slack.
>>
>>I read this admonition now and then and have not heard an explanation of how slack spokes cause
>>fatigue failures.
>
>
> I thought fatigue was caused by spokes repeatedly going from slack to taut and that the slacker
> the spokes, the more difference in tension there would be in use.

That's what I think, too. The same effect occurs in bolted joints subjected to varying loads, which
is why preload on bolts is important. With sufficient preload, the variation in bolt tension is
much less than the variation in applied load. Average bolt tension is higher, but the variable
component is a much bigger contributor to failure. Either Goodman's or Soderberg's failure
criterion show this.

Spotts' text _Design of Machine Elements_ has a good treatment of this for bolted joints. I've
always assumed the exact same logic applied for pretensioned spokes. In fact, Spotts points out
the benefit of reducing the shank diameter of a "tension bolt," which makes it look just like a
butted spoke!

--
Frank Krygowski [To reply, omit what's between "at" and "cc"]

 

[Phil Holman]

"Pete Biggs" <ptangerine{remove_fruit}@biggs.tc> wrote in message
news:[email protected]...
> [email protected] wrote:
> > Pete Biggs writes:
> >> You're likely to get broken spokes through metal fatigue if you continue with them slack.
> >
> > I read this admonition now and then and have not heard an
explanation
> > of how slack spokes cause fatigue failures.
>
> I thought fatigue was caused by spokes repeatedly going from slack to
taut
> and that the slacker the spokes, the more difference in tension there would be in use.

The correctly tensioned wheel at any point in time has a couple of spokes with a large
stress reduction compared to the slack wheel which has a large number of spokes with a small
stress increase.

Phil Holman

>
> "A spoked wheel relies on having all of the spokes in constant
tension. A
> highly dished rear wheel starts with very light tension on the left
side
> spokes. The torque of hard pedaling can cause the left side "leading" spokes to occasionally go
> completely slack momentarily. Repeated
cycles of
> tension and slackness cause these spokes to fatigue at the bends, and ultimately break."
> - www.sheldonbrown.com/wheelbuild.html
>
> ~PB

 

[Carl Fogel]

Werehatrack <[email protected]> wrote in message news:<[email protected]>...
> On Sun, 04 Jan 2004 11:18:19 -0600, Tim McNamara <[email protected]> may have said:
>
> >You might be able to cancel the post. Your newsreader might be able to issue the appropriate
> >command (another reason why one should use a proper newsreader).
>
> Many servers do not honor cancels anymore due to the malicious cancel floods that have been seen
> on numerous groups over the years. Issuing a cancel is still a good idea, though, but it should be
> remembered that a cancel won't necessarily take the post off of every server that the original
> article may have reached.

Dear Tim and Werehatrack,

While cancelling an ill-natured post might be a step in the right direction, a silent erasure is no
match for a public apology--no one ever compliments you on the character shown by a cancel.

There's more to life than technology (or so I hear).

Nice post, Simon.

Carl Fogel

 

[Jobst Brandt]

Pete Biggs writes:

>>> You're likely to get broken spokes through metal fatigue if you continue with them slack.

>> I read this admonition now and then and have not heard an explanation of how slack spokes cause
>> fatigue failures.

> I thought fatigue was caused by spokes repeatedly going from slack to taut and that the slacker
> the spokes, the more difference in tension there would be in use.

It is the span of the stress change that causes fatigue. In some materials and in some shapes that
stress reversal (tension to compression and back) accelerates fatigue failure but that is not the
vase with spokes. Rattlingly loose spokes have lesser stress changes than taught ones and do not
have reversed stress.

> "A spoked wheel relies on having all of the spokes in constant tension. A highly dished rear wheel
> starts with very light tension on the left side spokes. The torque of hard pedaling can cause the
> left side "leading" spokes to occasionally go completely slack momentarily. Repeated cycles of
> tension and slackness cause these spokes to fatigue at the bends, and ultimately break."
> - www.sheldonbrown.com/wheelbuild.html

That still does not show any effect that causes more rapid fatigue.

Besides, this is inaccurate. Left side spokes sometimes appear to verify that model, but it is the
greater bend at the elbow that causes these failures, mainly because the wheel builder failed to
improve the spoke line after initial tensioning. That is, the outbound spokes need to be pressed
down manually against the flange so that they are not free to flex repeatedly with each wheel
rotation. Lower tension on the left side does not accelerate fatigue. This notion needs to be
stamped out before it gets any more support. Meanwhile left side spokes need more support.

Jobst Brandt [email protected]

 

[Jobst Brandt]

Jim Beam writes:

>> That's a nice story but I think you are reconstructing the event from imagination. Wheels cannot
>> slowly sink to a lower rolling diameter. The failure mode is lateral collapse and that jams the
>> wheel in the frame so it won't turn. It makes a nice story but it didn't happen.

> Get a grip of yourself man. I know the difference between lateral collapse and radial failure. And
> what i described was a serial progressive radial failure. Try and separate blind prejudice from
> rational analysis.

Please explain how the wheel, or better the axle of the wheel sank closer and closer to the ground
in your event of "radial wheel collapse". Did the rim become oval and how did that rotate? What were
the spokes doing at that time? Maybe you can describe this so I can correct what I have observed in
wheel failures.

Jobst Brandt [email protected]

 

[Jim Beam]

spoke elbow failure. ~50% of the spokes just gave out and the hub sunk closer & closer to the rim.
rim was still relatively ok.

[email protected] wrote:
> Jim Beam writes:
>
>
>>>That's a nice story but I think you are reconstructing the event from imagination. Wheels cannot
>>>slowly sink to a lower rolling diameter. The failure mode is lateral collapse and that jams the
>>>wheel in the frame so it won't turn. It makes a nice story but it didn't happen.
>
>
>>Get a grip of yourself man. I know the difference between lateral collapse and radial failure. And
>>what i described was a serial progressive radial failure. Try and separate blind prejudice from
>>rational analysis.
>
>
> Please explain how the wheel, or better the axle of the wheel sank closer and closer to the ground
> in your event of "radial wheel collapse". Did the rim become oval and how did that rotate? What
> were the spokes doing at that time? Maybe you can describe this so I can correct what I have
> observed in wheel failures.
>
> Jobst Brandt [email protected]