Quantitive Axle Failure Anaylsis

Brett F Bymaster writes:
> Does anyone else have any suggestions on predicting axle failure?

Both times I have enjoyed a broken axle the failure was preceeded for several
hours of riding time by a fairly loud creaking noise when I pedalled hard. Both
axles involved were cottered axles, so I thought the creaking was from the
axle-crank interface, but attempts to tighten the cranks didn’t stop the
creaking. Post failure inspection revealed that in both cases what apparently
happened was:

(i) a crack appeared in the axle, at the point where the axle is milled for
the cotter pin (presumably a stress point).

(ii) over time the crack propagated through the axle diameter. During this time
the unicycle creaked when pedalled hard.

(iii) eventually the axle was sufficiently cracked to shear off completely.

The reason for suspecting (ii) was that in both cases the crack surface has a
section which was quite oxidised, and another (larger) section with no
oxidation. So I suspect the first part of the crack had been around for some
time, and was the source of the loud creaking.

Is this the usual way axles bust? My latest (cotterless) axle is still intact,
so I don’t have experience with cotterless axles. I ride off-road to a certain
extent, but so far Pecking is not in my repertoire, so perhaps I haven’t
experienced the most troublesome axle failure mode.

I am amazed by the attitude of certain unicycle vendors when confronted with the
inadequacy of their axles. The Australian importer (Advance Traders) of the
ubiquitous Tiawanese cycles (the only brand commonly available here) told my
bike shop that I shouldn’t be riding off-road when asked for a replacement axle!
He also refused to supply 36 hole axles (his suggestion was to buy another
unicycle). I took my business elsewhere…


James <James.Gifford@anu.edu.au