I wrote this post some weeks ago in a bout of enthusiasm at a unicycle
idea I had. Since then I've been wondering if I've actually got the
correct idea of what splines are because some recent posts seem to be
saying that splined cranks aren't particularly easy to take on and
off.
However, here's the post in its original form, it's about an idea for
adjustable cranks, all comments welcome.
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The most obvious way to make adjustable cranks is to simply have extra
holes for the pedal to screw into, this has the benefit of simplicity
but also several drawbacks: -
1. when in 'long mode' the inner hole is a weak area for the crank
2. changing length necessitates unscrewing the pedal and rescrewing it
into a different hole i.e. inconvenient if changing a lot and also, I
suspect that there is a limit to how often they can be changed before
the threads start taking damage
The other method, and one used in commercial models, is a sleeve
arangement; it works and seems to be intended for tandems, but involves
compromises between strength, ease of adjustability etc.
The few tests of available adjustable cranks by unicyclists seem to have
not been successful, with the cranks breaking; though we should
differentiate between cranks used for commuting/transport and those used
for muni/trials (with big drops etc).
So here's my idea, in theory it looks like it should work well, but,
lacking the tools/knowledge necessary to make a prototype, I'm not able
to make and test it out. (see attached image)
The idea is, rather than adjusting the actual length of the crank, the
distance betweeen the crank and pedal is altered by a mechanism that
effectivly creates a bend in the crank.
So, if it is set to '125 mode', the pedal axis is 125mm from the wheel
axle, creating the same leverage effect as if a 125mm crank is used.
The benefits of this system is: -
1. Use of present unicycling technology; splined axle/cranks are
becoming commonplace and the same tools could be used to create the
elements of the adjustable crank.
2. Strength. I'm no engineer, but, given that splined axles/cranks are
used in top of the range muni/trials unis, and, as far as I can see
there is no reason why the stress on pedal/crank splines should be
greater than the present axle/crank splines; then this setup should be
strong enough for all uses of uni.
3. Ease of changing length. As all the stress of riding should be mainly
in the direction of the circumference of the spline hole, rather than at
right angles, it shouldn't require a hefty mechanism to hold the pedal
in the crank hole. This means that something as simple as a hole drilled
through the protruding spline part with a retaining pin should sufice,
enabling quick and easy changes.
Like I said, I'm no engineer so maybe there's some flaws I've missed and
it'd be great if someone with experience in making uni parts/engineering
gave some feedback.
It'd be nice if it did work because one of the things I love about
unicycles is their mechanical simplicity. As someone whose experienced
the pros and cons of different crank lengths for a given wheel size, i
know that a workable variable crank would be a great boon to
unicycling.
Of course, it looks like geared unicycles will soon be on the market,
but my feeling is that they will have issues of cost and lack
simplicity.
While gears may have the edge in terms of a large range, I would be
quite happy with a crank that adjusted from 150-125mm, and I see no
reason why a range of 100-175mm should not be feasible with a splined
angle crank mechanism.
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--
onewheeldave - Semi Skilled Unicyclist
"He's also been known to indulge in a spot of flame juggling - but it's
the Muni that really fires him up."
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