Adjustable crank idea
 

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: -

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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Adjustable crank idea
 

This is the set up for 125mm mode: -


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Adjustable crank idea
 

And 150mm mode: -


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| Attachment filename: 150mode.gif |
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Adjustable crank idea
 

That is a really, really cool idea, and one that sounds as though it
would work well. Why not take a couple of sets of cranks down to a
local machine shop, show them your drawings, and see what they say?


--
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Adjustable crank idea
 

That's a great idea! I have never thought about doing it that way. One
part i can't understand or figure out is how the two parts of the cranks
connect. I would assume that the pedal side would be on the outside of
the axle side. right? Also, would there be something like a splined axel
on the crank part that goes into a splined hole on the hub part?

Over all I think the idea is worth seriously looking into.

Just imagine how many crank lengths you could get out of a 48 spline
setup. :D

Keep the ideas coming.


--
daino149 - How's it going, Texas?

there ain't enough body armor in the country for me to try that. -- Ken
on the mtbr forum in reference to MUni
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Adjustable crank idea
 

daino149 wrote:
*That's a great idea! I have never thought about doing it that way.
One part i can't understand or figure out is how the two parts of the
cranks connect. I would assume that the pedal side would be on the
outside of the axle side. right?
*

Initially I was thinking that the short arm (with the pedal attached)
would connect on the outside of the long arm (the one that connects with
the axle).

This would mean that the pedal would be slightly further outwards from
the axle than on a normal uni.

However, the short arm could connect on the inside of the long one, this
would leave the pedal no further out from the axle than normal; it would
slightly restrict the shortest effective crank length a little as, in
the short mode the pedal axle would rest on top of the long arm.

Then again, as some people prefer the pedal further out, as it stops
their heel accidently catching the axle, the first method may be ok
after all.
daino149 wrote:
Also, would there be something like a splined axel on the crank part
that goes into a splined hole on the hub part?

[/b]

That would be optional, but, as splining would be used on the long
arm/short arm connection, it would probably be used to connect the whole
crank to the axle as well.


--
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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Adjustable crank idea
 

That's a really cool idea. The limitations are minor. One could be
surprised by the piece of crank coming up and bumping into the bottom of
their shoe when in short mode. If that piece were significantly long it
could sneak into a pant leg also. It probably wouldn't be able to take
big drops but a device like this would most likely be used for distance
riding anyway.

Make it. Good luck.


--
harper - Old dog, no tricks

-Greg Harper

B L U E S H I F T

"In the unlikely event of someones unicycling demise, one should say 'It
was an amazing display of unicycling skill and daring, with a rather
unfortunate outcome'. Or something to that effect. - Scott Wallis

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Adjustable crank idea
 

I am impressed with the idea. Now, who's going to make the first
prototype? --chirokid--


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Adjustable crank idea
 

chirokid wrote:
*I am impressed with the idea. Now, who's going to make the first
prototype? --chirokid-- *


Sounds like a job for Steve!


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there ain't enough body armor in the country for me to try that. -- Ken
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Adjustable crank idea
 

Great idea! I might try making one for a machine shop project, but
probably not for a while.

I'm also not an engineer, but I can say that one limit would be the
range you could build with this. With a range of 125-150mm, the part
attached to the pedal could be very short (pedal axis would be 12.5mm
from the center of the spline axis), which would be a plus, because that
would put less stress on the hinge. But, as you make the range larger,
say 100-175mm, you encounter some serious issues with the strength of
the splined hinge. To have a thin splined area 75mm from the pedal would
be extremely weak. The strength of splined setups partially comes from
the length (thereby contact area) of the splined area (actually, now
that I think about it, the whole point of a splined steup is to maximize
the contact area between the crank and axle with a minimal increase in
axle diameter. That's why profile is so strong. Massive contact area
between crank and axle. Square taper cranks suck because they have a
very spmall contact area). Onza and Profile cranks have 1" contact
areas, I think. For contrast, square taper joints use about 1/2-3/4'' of
contact, which isn't as good. So, the point is that in order to put one
of these splined attachments on the middle of a crank would mean it'd
need to be rather thin, which would weaken it. Also, it would be
97.5-175mm range. YOu could work a way around this, though. You could
have a strengthening pin in it, so when you want to do extreme stuff,
you just set the crank in it's shortes setting, and put a clamp on each
piece so the joint is reinforced closer to the pedal.

With that in mind, I think a modification of this design would be to
include a crossbar that would be sleeve type of bar, which would only
get direct straight forces. You have the hinge be a freemoving hinge
(instead of splined), and have the adjustment be the length of the bar.
You could use a bolt type of tightening system where you tighten the
bolt on the outside of the bar, which would clamp down on the inside
one. It'd work like a unicycle seatpost or a router collet. That might
be a bit thinner and stronger.

My $0.02
Bevan


--
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