Andrew Webster said:Having replaced a broken spoke I was re-truing the wheel and broke
two more in doing so. Not at the nipple, but about a quarter of the
way along where the butted spokes change gauge.
Quoted message said:I am a little surprised at this as spokes usually snap at the nipple
or elbow (as the first one did that necessitated the repair).
They usually break at high stress points from fatigue, but in tensile
test they break in the straight spans at one end or the other. Spokes
that break while truing generally break from a combination of tension
and torque, the torque being the straw that breaks the spokes back, so
to speak.
I have a warning for wheel building and truing, and that is to not
look into the spoke nipple being tightened because it can eject as an
arrow from a crossbow into the face. That your spoke broke while
truing suggests you are using thin swaged spokes and that you may not
have had sufficient lubrication in the threads.
Spokes with 1.5mm slender portions cannot be adequately tightened
without such failures anyway. That's why I recommend against them. I
had experience with several brands of such spokes and found the only
way to get them adequately tight was to unload them while tightening.
This is done in a rigid truing fixture with the rim pulled to the side
of the spoke being tightened. In a truing machine this could be done
with a radial pneumatic plunger.
Quoted message said:Also this wheel dates back to the early 70s and has been used on and
off since with (until now) the original spokes.
Quoted message said:I can't imagine I am overtightening the spokes, the nipples turn
fairly freely and I doubt if the Wolber Super Champion rims would
take that much tension.
Get out your caliper and measure the spoke diameter. It is most
likely close to 1.5mm. Besides, how many spokes do these wheels have?
With 24 1.8-1.6mm spokes the unloading scheme is required anyway at
the tension such wheels require to withstand heavy loads.
Quoted message said:So (getting to my question) what do you think is the most likely cause
- I have provisionally ruled out the likeliest candidates:
Torque and tension. How much wind-up are you getting? More than 1/4
turn is a dead giveaway that torque is killing the spokes.
Quoted message said:Fatigue - would affect elbow, but not butted part? (Though I did ride
10 miles or so with one broken spoke).
Fatigue doesn't affect spokes there anyway.
Quoted message said:Poor quality spokes - but they have lasted 30 years without problems
up until now.
What brand are they, or what is the logo on the s[poke heads?
Quoted message said:Over tensioning - spokes are not over-tight
Over torqued isn't necessarily over tight.
Quoted message said:I have already decided that it is time for new rim/spokes (I want to
keep the Nuovo Tipo hub as it still runs well and I don't see any
need to change it), but I want to learn from my experience, if
indeed there is something to learn.
What's wrong with the rims that you want to chuck them?
Jobst Brandt
