From: bruce@uxc.cso.uiuc.edu (Bruce Krueger)
Subject: wild rim failure -- drive side *pushing* fatigue
Summary: two types of rims fail in similar way
Organization: University of Illinois at Urbana
Date: Sun, 8 Nov 1992 20:56:26 GMT
Keywords: material fatigue, heat treating, cracks, rim sidewall


ok, I've got a story/situation/problem (take it as you will) for ya --

it seems that various types of problems occur in 'clusters' at the shop
sometimes, for reasons completely inexplicable to me (potato-chipped wheels,
crumpled frames, BB fixed cups cracked through the threads, &c.), but most
of them are repeats of probs I've seen before, but I had a couple of guys
come in with cracks in the sidewall of their rims where nearly every drive
side 'pushing' spoke was holding it under tension, and I'm frankly baffled
as to the cause, and possible preventative measures to take in the future.

the similarities between the two cases are interesting -- both were on
sport bikes that get ridden fairly frequently/enthusiasticly/hard, and
both were 'hard anodized', and the failure consisted of cracks developing
in the sidewall of the rims (on both sides) centered on the point where
the drive side 'pushing' spokes (rear wheel) held the rim under tension.
both of the hubs were shimano 105 freehubs, both were laced three cross.

so much for similarities -- 
the riders were ~150 and ~220 lbs., former had a factory-built wheel on a
'91 bianchi alfana (105 group), and an araya rim w/o ferrules, 32 spokes,
and they were butted, the latter had a hand built (yours truly) MA40 laced
with 36 DT 14ga spokes.  this rim also had a couple of points where the 
rim had started to fracture around the ferrules under the rim strip.
the former was on a wheel that is about a year and a half old, and has
had re-tensioning and corrective truing over that period, both by myself
and the owner.  the latter was only a few months old, and is somewhat more
disturbing, since I built the wheels to stand up to the beating this guy
dishes out, with the mavic rim and lotsa heavier DT's.

questions:  was the failure due to too much tension?
            would butted spokes have helped in the latter case, due to
	    their increased elasticity?
	    is this more common with heat-treated/hard anodized rims?

my surmisal is that the metal fatigue/failure is the result of the 
oscillation/cycling in the amount of tension due to the pushing drive side
spokes being 'relaxed' as torque is applied to the hub, and possibly also
during braking (though I'd expect that to also stress the pushing spokes
on the non-drive side, where no failue was observed).

other items of note:  the failure was non-catastrophic in both cases, as 
the problem was noticed before anything really 'let go', but I can imagine
(and have noghtmares over) cases where it could cause big-time problems if
not noticed in time.  the cracks were from 1-3cm long, and ranged from barely
perceptible hairline fractures (most cases) to a couple of points where the
gap was 1-2mm.

I submit the above for your comments, ideas, suggestions, amusement, and
also possible corroboration -- anybody else ever seen anything like this??

feel free to just jump right on in, jobst, don't be shy...            *ha*
				(all seriousness aside, good to have ya back)


ride bike, write back, don't crack

bruce