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.
==============================================
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: -
3. 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.
4. 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.
5. 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.
+-------------------------------------------------------
---------+
| Attachment filename: graphic1.gif |
|Download attachment:
unicyclist.com186959Open ↗| +-------------------------------------------------------
---------+
--
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."
------------------------------------------------------------------------
onewheeldave's Profile: unicyclist.com874Open ↗
View this thread: unicyclist.com30079Open ↗