Weird spoke breakage

Rado bladteth Rzeznicki may have said:

The problem is with rear, driveside spokes. First one broke after about 1500 km, second some 500 km
later. I predict that sooner or later next spokes will break as well. Could you point the source of
the problem:

1. Hub with too small flanges (45 mm) for disc brake applications?

No.

Quoted message said:

2. Not so straight (due to several crashes) and rather soft, flexy rim which results in worse load
distribution and problems during wheel building (spoke tension is not perfectly equal across
the rim)?

Definite candidate; conventional brakes don't stress the spokes at all, but discs stress them
heavily. Uneven tension means uneven loading, and yes, that's trouble.

Quoted message said:

3. Flawed set of spokes as the bike was bought on bankrupcy sale?

Probably not; I think you hit it on 2. Others will doubtless post their observations and comments,
and you should read them all (particularly if Jobst Brandt has a comment) before taking any action,
but my opinion is that you should consider replacing that rim with one that's straight.

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Werehatrack said:

Quoted message said:

1. Hub with too small flanges (45 mm) for disc brake applications?

No.

I forgot to mention that the hub was bought second hand and material on flanges was slightly
deformed by heads and elbows of spokes from former wheel. I laced new one in the same way, but does
it matter at all? Is it possible that spokes doesn't fit the holes perfectly and therefore are
subjected to greater stress?

Quoted message said:

Quoted message said:

2. Not so straight (due to several crashes) and rather soft, flexy rim which results in worse load
distribution and problems during wheel building (spoke tension is not perfectly equal across
the rim)?

Definite candidate; conventional brakes don't stress the spokes at all, but discs stress them
heavily. Uneven tension means uneven loading, and yes, that's trouble.

I've disassembled the wheel couple of weeks ago and tried to true the rim. The result was quite
satisfactory, but after lacing and tensioning I had to choose between equal tension and true wheel.
Eventually the second option won, but differences in tension were relatively small (I plucked the
spokes to check it).

Quoted message said:

Probably not; I think you hit it on 2. Others will doubtless post their observations and comments,
and you should read them all (particularly if Jobst Brandt has a comment) before taking any
action, but my opinion is that you should consider replacing that rim with one that's straight.

What puzzles me a bit is that both wheels have exactly the same rims and are similarly dished (disc
brakes). I've got absolutely no problems with the front however.

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Rado bladteth Rzeznicki said:

I'm experiencing strange spokes' breakage. Both front and rear wheels are nearly the same - 26"
rims Sun Ringle DS1-XC, 14/15/14 g Nirosta SS spokes laced 3x. There is Formula hub in the front,
SRAM 9.0 Disc in the rear and Magura Louise disc brakes fitted on both. I've built several
wheelsets for couple of years, all of them according to common rules eg. JB's "Bicycle Wheel" book
and prefer strong spoke tension.

The problem is with rear, driveside spokes. First one broke after about 1500 km, second some 500 km
later. I predict that sooner or later next spokes will break as well. Could you point the source of
the problem:

One thing that will kill drive side spokes is when the chain gets wedged between the cassette and
spokes in an overshift. This can score the spokes, which then start failing (I lost 6 DS spokes in
one lap in a 24 hour race this way once - fun lap!).

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of the $695 ti frame

Rado bladteth Rzeznicki may have said:

Werehatrack said:

Quoted message said:

1. Hub with too small flanges (45 mm) for disc brake applications?

No.

I forgot to mention that the hub was bought second hand and material on flanges was slightly
deformed by heads and elbows of spokes from former wheel. I laced new one in the same way, but does
it matter at all? Is it possible that spokes doesn't fit the holes perfectly and therefore are
subjected to greater stress?

I can't say for sure; the only bikes I've had with disc brakes never got to the point where the hubs
had any significant wear, but those were older touring bikes with the early Shimano rear discs. The
stress levels with those were, in general, much lower than can be generated by current units. I
would think that the wear in the lacing holes is telling you something, though. Many years ago, I
re-used a rear hub from a derelict which turned out to be a spoke-eater, but the problem in that
instance was that some of the spoke holes were worn large enough that the elbows were being deformed
and the mushroomed ends were snapping off.

Quoted message said:

Quoted message said:

Quoted message said:

2. Not so straight (due to several crashes) and rather soft, flexy rim which results in worse
load distribution and problems during wheel building (spoke tension is not perfectly equal
across the rim)?

Definite candidate; conventional brakes don't stress the spokes at all, but discs stress them
heavily. Uneven tension means uneven loading, and yes, that's trouble.

I've disassembled the wheel couple of weeks ago and tried to true the rim. The result was quite
satisfactory, but after lacing and tensioning I had to choose between equal tension and true wheel.
Eventually the second option won, but differences in tension were relatively small (I plucked the
spokes to check it).

Without something to actually measure the tension, it's hard to say if it's right, and disc-brake
wheels require better control in this area than conventional-brake wheels. There is a possibility
that the tension had to go too high in order to get the unruly rim to act round. This shows up
faster with discs (as spoke breakage) than it would with conventional brakes.

Quoted message said:

What puzzles me a bit is that both wheels have exactly the same rims and are similarly dished (disc
brakes). I've got absolutely no problems with the front however.

There is rotational force transmitted to the spokes in one direction only on the front; in two
directions on the rear.

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"Mark Hickey" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

One thing that will kill drive side spokes is when the chain gets wedged between the cassette and
spokes in an overshift. This can score the spokes, which then start failing (I lost 6 DS spokes in
one lap in a 24 hour race this way once - fun lap!).

I need to remember that one next time I turn in a slow lap 🙂

Bill "note to self: pack wire cutters" S.

Sorni said:

"Mark Hickey" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

One thing that will kill drive side spokes is when the chain gets wedged between the cassette and
spokes in an overshift. This can score the spokes, which then start failing (I lost 6 DS spokes
in one lap in a 24 hour race this way once - fun lap!).

I need to remember that one next time I turn in a slow lap 🙂

Heh... yeah, I was thinking the same thing. The lap was at about 3am, it was down to almost
freezing, my light was dimming to about nothing and the broken spokes were getting jammed in the
cassette, causing it to spool out all the "spare chain" ahead of the cassette, so every time I
coasted, I'd be in a different gear when I pushed a pedal. I was shifting probably 10 times a minute
through the rocky sections.

I had to take the rear brake altgether off to keep the rear wheel turning. Naturally, the last part
of the lap had some fairly nasty, bumpy descents that really AREN'T the place you want to use JUST
your front brake in the dark (especially with a rear wheel that's trying to taco).

I don't think I ever tried so hard to ride "light". ;-)

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of the $695 ti frame

In article <[email hidden]>, Werehatrack

Quoted message said:

conventional brakes don't stress the spokes at all, but discs stress them heavily

Hmmm. Is that corrct? I seem to recall Jobst's FEA showing the opposite, and also his comments that
hard barking can cause and overly tight wheel to warp out of true (just as happens after stress
relieving). I also STR that rim strength/stiffness- which you comment on- is one of the key factors
in determining the upper limit of spoke tension. But I could be misremembering, and my copy of _The
Bicycle Wheel_ is not handy to check.

Tim McNamara said:

In article <[email hidden]>, Werehatrack

Quoted message said:

conventional brakes don't stress the spokes at all, but discs stress them heavily

Hmmm. Is that corrct? I seem to recall Jobst's FEA showing the opposite, and also his comments
that hard barking can cause and overly tight wheel to warp out of true (just as happens after
stress relieving). I also STR that rim strength/stiffness- which you comment on- is one of the key
factors in determining the upper limit of spoke tension. But I could be misremembering, and my
copy of _The Bicycle Wheel_ is not handy to check.

As Jobst states in the book, braking forces with rim brakes add a small but measurable radial load,
increasing spoke tenison.

Joe Riel

P.S. do newer editions (I have a revised edition) include an index? If not, they should.

Tim McNamara said:

Quoted message said:

conventional brakes don't stress the spokes at all, but discs stress them heavily

Quoted message said:

Hmmm. Is that corrct? I seem to recall Jobst's FEA showing the opposite, and also his comments
that hard barking can cause and overly tight wheel to warp out of true (just as happens after
stress relieving). I also STR that rim strength/stiffness- which you comment on- is one of the key
factors in determining the upper limit of spoke tension. But I could be misremembering, and my
copy of _The Bicycle Wheel_ is not handy to check.

That is as you say, not correct, and rim braking, although not causing a large change in tension,
does so in about half the spokes, which is enough to warp a rim if tension is at maximum with no
margin. Hub brakes have the same effect as pedaling torque has. It changes tension slightly above
and below average tension in pulling and pushing spokes, and because it is borne by all spokes, is
not large.

There are FEA plots as well as the numerical data in the book.

Jobst Brandt [email hidden]

may have said:

That is as you say, not correct, and rim braking, although not causing a large change in tension,
does so in about half the spokes, which is enough to warp a rim if tension is at maximum with no
margin. Hub brakes have the same effect as pedaling torque has. It changes tension slightly above
and below average tension in pulling and pushing spokes, and because it is borne by all spokes, is
not large.

After you mentioned it, I looked for some data; I really need to get a copy of your book. Yes, there
is some stress, but I get the impression that it's still lower in total than that produced by hub
brakes. In those, however, as you point out, if the tensions are even, the stress is
well-distributed. If they aren't, the picture changes. If they aren't even *and* the tight ones are
near or over max, I would expect problems. I get the feeling that the exact results would be
impacted by the rim's characteristics as well. The original poster had stated that the rim was
"soft", which I interpret as flexible; I can't really see how that would make the problem worse, as
it would seem logical that a rim with some "give" would tend to attenuate any tension hot spots.

I suspect this problem needs close inspection by a trained eye.

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