Quantitive Axle Failure Anaylsis

The weakest part of most high end unicycles (if there is such a thing) is
usually the axle. There simply is not a quality axle commercially available.
Anyone who has done a significant amount of high impact riding or freestyle has
broken an axle or two. MUning is especially hard on axles because it combines
high impact riding, long distance and high torque. It is necessary to be able to
predict axle failure to prevent getting stranded on the trail. It is impossible
to ride with a broken axle, so if it breaks on the trail, you are walking home
(which can be a long walk for all the long distance riders). I have been doing a
lot of 30 + mile rides where a broken axle could put me in a very perilous
position – being up to 15 miles away from my destination. So I have developed
the following technique for predicting axle failures.

Characteristic axle failure precursors:
1) “Twisting” of the axle – Most evident by taking off the cranks and
looking at the squared end that the cranks attach to. The squared
edges on both sides of the axle should be in a plane. If they are
not, the axle has twisted and is nearing failure
2) Bending or bowing of the axle – When viewed from the front, the axle
is an arc instead of a straight line

Case (2) is easy to identify, just eyeballing the axle can show that it is
bent. Case (1) is not so easy to identify. Taking off the cranks is a pain, and
it can be bad to remove cranks since taking them on and off can round off the
contact points on the crank. Sometimes, you can see that the cranks do not line
up when viewing them from the top, but this is very inaccurate since it is
difficult to determine if the problem is in the axle or the crank, or if the
cranks are not lined up perfectly to the axle due to rounding of the crank. A
more accurate, and non intrusive method of identifying twisting of the axle is
to use spoke tension. In the past couple weeks I have broken a couple spokes.
Interestingly, broken spokes were on opposite sides of the axle and 180 degrees
apart from each other

-- > | |
     |     | <--

(A bad diagram of the axle from the front with arrows pointing to the location
of broken spokes) It is important to note that the spokes were on opposite
sides of the flange, not of the rim. I theorized that the position of the
spokes indicated that the axle was twisting. I determined that axle was most
likely to twist in the direction of my most dominant pecking (or hopping)
position. I peck with the left peddle back, so the left side of the axle would
twist clockwise looking at it from the left side (or twist toward my dominant
foot). The spokes that broke were basically parallel to the cranks on the same
side as the crank, which meant that under a twisting condition they would be
compressed and support more load than the spokes around them, and therefore
break sooner. A spoke’s tension can be very precisely determined by its
resonant frequency. Similar to a guitar string, tighter spokes will make a high
pitch sound when hit with a metallic object, and loose spokes make a lower
sound. I wanted to substantiate my conclusion of a twisted axle, so I used this
technique to test the tension of the spokes. Sure enough, the spokes that would
be compressed by axle twisting (like the spokes that failed) resonated at a
lower frequency, and the spokes that should be stretched rang at a higher
frequency. (Make sure you’re not tone deaf before you do this – I am and so I
had my musically talented roommate help me)

So what’s the bottom line? If you have spokes that are opposite to each other on
the flange and would be compressed by stressing the axle with your dominant
riding position, the axle is twisting and could be nearing failure. If you want
to verify this and rule out spoke failure due to normal fatigue, compare the
frequency that spokes pointing opposite directions to each other make.

Does anyone else have any suggestions on predicting axle failure?

    Brett Bymaster "Altitude Addict" bymaster@purdue.edu

Re: Quantitive Axle Failure Anaylsis

<< Does anyone else have any suggestions on predicting axle failure?

I once asked Sem why he and Theresa have their spokes radial, straight out from
the axle, instead of criss-crossed. I thought maybe it was just because it looks
neat. But he said that it puts extra stress on the spokes. They will break
before the axle breaks. They can still complete a show O.K., but then they stop
using that unicycle. If you’ve seen them perform, with Theresa held high in the
air on his outstretched arm, you can see why safety is so important to them. A
broken axle could be a disaster.

There’s a thorough discussion of broken axles on George Peck’s mountain
unicycling video. He’s broken over a dozen of them. Finally he had one made of
titanium. It never broke.