[email hidden] wrote in message news:<[email hidden]>...
Quoted message said:Carl Fogel said:Quoted message said:Your wheel would surely have broken its spoke in the next quarter mile had you continued
because it was already cracked so far through that it was like a piece of soft material
stretching the last strand still connected so it could snap.
draco.acs.uci.edu8c.1.htmlOpen ↗
Quoted message said:Quoted message said:There's more about this in "the Bicycle Wheel".
Quoted message said:Do elbow-spokes really break equally well at either end?
Quoted message said:If so, are there bad practices that might favor breaking far more spokes at the hub?
That depends more on the flange size, spoke pattern and skill of the wheel builder. The point is
that a spoke nipple not entirely in-line with the spoke line causes a bend and this bend is
concentrated in the first thread. Without a hefty stress reliving stretch, this will be a high
stress point. If the wheel is loosely built, it may not causer a break. The reason for carrying
spare spokes when touring WITH nipples threaded on is that when a spoke breaks off in the thread
the nipple is usually useless.
Quoted message said:I ask because my spokes have always broken at the hub, never at the nipple. They're almost
always the rear drive-side spokes with more tension, just as you predict, but even my rare front
spoke breaks at the hub, not the nipple.
That's usually for lack of "improving the spoke line" and insufficient stress relief.
Quoted message said:I have an embarrassing suspicion that a really badly matched derailleur sometimes rattled
against the rear drive-side spokes, closer to the hub than the rim.
These are fatigue failures unless you can see cuts in the spoke. I can't imagine an aluminum
derailleur cutting a spoke.
Quoted message said:Incidentally, where exactly would you expect spokes to break at the hub? Out in the open and
across the main shaft? At 45 degrees in the elbow? Or all the way around the bend, just short of
the flared spoke-end?
Most often right in the bend, but then I have seen them break elsewhere, usually from having been
bent by some external force. Gouged spokes from a dropped chain often last a long time even with
gouges in them if they weren't bent. This shows that it is not tension that causes failures, but
bending (forming) stresses.
Quoted message said:I have a vague notion that my spokes usually crack just before the flared end, leaving a spoke
with a jagged elbow and a button still sitting flush in the hub-hole that I have to knock out.
You should get over that syndrome. Build some good wheels with good components with 36 spokes.
Mine have lasted about 25 years with rim replacements now and then.
Jobst Brandt [email hidden]
Dear Jobst,
Prompted by some private email, I rummaged through my workbench and found half a dozen broken drive-
side spokes. (I used to save them to make crude tools.)
Under a magnifier, all six appear to have broken in the same fashion, which was not at all what I
expected. (Even spokes have little trouble outwitting me.)
The crack in each case appears to have started just below the flared end of the spoke on the outer
(not inner) curve, judging by the progression from smooth surface to increasingly rough surface on
the break. Here's a crude ASCII diagram, with the spoke standing on its threaded end and the broken
elbow surface facing you:
crack begins
from this direction?
. . . .
. .
. .
. . <--smoother upper half
. . . .
|. o . . .| <--rougher lower half
| . . o . |
| . . . . |
| |
| |
| |
| |
| |
| threads |
| far below |
Instead of pulling apart beginning on the inner curve of the elbow where it pulls directly agains
the hub, the spokes seem to have torn their flared ends off with the cracking beginning where the
outer curve reaches the flared end, as if the flare provided a weak spot and a bit of leverage under
tension against the far side of the hub-hole.
I also noticed a large dent at the very end of the outer curve of each elbow. When I looked at
replacement DT spokes, I saw the same dent, but much smaller. Possibly this is where the
manufacturing machine pushed to bend the elbow? It seems to have been the weak point on all these
broken spokes. Here's another ASCII diagram:
__________
\ / \....../ ..... <--direction of break <--threads / ( <--dent with
mushroomed metal / / ___________________________/ / /
_____________________________/
As private emails suggested, my broken spokes turn out to have gentler and longer elbow bends,
possibly to make machine lacing easier--the DT replacement spokes have noticeably sharper and
shorter elbows. (Well, noticeable when someone kindly points it out and you take a second look.)
Naturally, I like the idea that my spokes broke so often and in an odd way because they were cheap
spokes and not as good as DT, but I'm cautious about blaming them.
Still, I'm wondering whether this explains why spoke-squeezing stress-relief never seemed to help--
if the dents were too large and the spoke quality was low, I might have been doomed from the start.
Unfortunately, the broken spoke ends are too small for decent pictures with my primitive equipment,
but I'll give it another try.
Carl Fogel
