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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This is the set up for 125mm mode: -


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"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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And 150mm mode: -


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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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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?


--
U-Turn - There has got to be a better way...

Weep in the dojo... laugh on the battlefield.

'29er Tire Study' (http://tinyurl.com/3248f)

'Strongest Coker Wheel in the World'
(http://www.unicyclist.com/gallery/albup39)

'Unatics Website' (http://www.newyorkunicycle.com)

-- Dave Stockton
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john_childs wrote:
*
Steve Howard posted a nice 3D rendering of both designs. *

Where?

-mg


--
mgrant - Ever hoping for the best!

The sun is the same in a relative way but you're older, shorter of
breath, and one day closer to death.

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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.

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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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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Ben Plotkin-Swing wrote:
*
Someone should certainly build a prototype of this so we have
something to talk about other than theory.
*

Agree. If there are any problems they won't be obvious until there is a
working prototype. I can't think of any reason why it can't be made to
work mechanically.


--
cyberbellum - Level 1.0 rider!

Optimists think the glass is half full. Pesimists think the glass is
half empty. Engineers think the glass is too big.
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These are some great ideas! Actually, John Child's eccentric hub crank
is something I've thought of before but Dave's idea is one I'd never
considered.

I got so excited about the two ideas, I decided to do a little design
work. The attached .jpg file shows Dave's idea on the top with John's
idea below - both mated to my extra wide hub.

Dave's crank is adjustable to five positions: 170mm, 163mm, 147mm
(shown), 129mm and 120mm. When made from aluminum with a short steel
splined shaft between the two crank parts the weight of one cranks is
1.02 pounds. The spline pattern is the 8 spline KH.

John's crank is adjustable to six positions: 170mm, 165mm, 156mm,
142mm, 130mm and 125mm. With the eccentric hub and crank made from
aluminum (and the lightening holes as shown) one crank weighs 1.11
pounds. By contrast a 170mm Profile crank weighs about .64 pounds.

I think cyberbellum is right on about the splined joints being something
with no slop. A pinch type arrangement should work pretty well in both
designs.

I know what the next question is: When am I going to make a set?

Steve Howard


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showard - ------
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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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