Weird spoke breakage

Hi,

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:

1. Hub with too small flanges (45 mm) for disc brake applications?
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)?
3. Flawed set of spokes as the bike was bought on bankrupcy sale?

Thanks for help.

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Pozdrawiam!
Rado bladteth Rzeznicki
http://www.widzew.net/~bladteth/rower.html

MCM #252

On Sun, 07 Sep 2003 14:42:11 +1000, 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.

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.

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 wrote:

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?

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).

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.

--
Pozdrawiam!
Rado bladteth Rzeznicki
http://www.widzew.net/~bladteth/rower.html

MCM #252

On Sun, 07 Sep 2003 16:05:30 +1000, Rado bladteth Rzeznicki
may have said:

Werehatrack wrote:

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.

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.

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" wrote in message
...
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" wrote:

"Mark Hickey" wrote in message
.. .
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
http://www.habcycles.com
Home of the $695 ti frame

In article ,
Werehatrack wrote:

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 writes:

In article ,
Werehatrack wrote:

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.

On Mon, 08 Sep 2003 03:02:00 GMT, 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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