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Forum LockedShort Cranks

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AlanGoodman View Drop Down
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    Posted: 24 January 2012 at 12:05pm
There is now an article on short cranks by Mike Burrows in the Magazine Extra section of the web site...
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Bill B-J View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bill B-J Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2012 at 2:38pm
As Mike says, bodies  are complex.

In case it is a useful consideration, some generalisations:-

Muscle produces most power in mid range. (Inner range being close to as short as it gets, outer being, close to as long as it gets.)

Maxiamal compression between knee cap, (patella,) and thigh bone, ,(femural condyles,) mechanically will occur when the quads are contracting through 90 degrees of knee bend (flexion.) Which is why patello-femoral problems manifest, walking/running up and down hills/stairs; grinding on the pedals with longer cranks, and presumably why shorter cranks help those with such a problem.

If there is a theoretical down side to shorter cranks; perhaps one draws on a larger collection of muscle fibers , when contracting through the full range of a muscle, and fewer when the using a shorter range. Superior blood supply to great number of fibers, perhaps.

It would be a surprise, if like many other things in life, there was not a balance to be found!



Edited by Bill B-J - 29 January 2012 at 2:41pm
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Bill B-J View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bill B-J Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2012 at 6:13pm
Questions-

Does one reduce the gearing, in relative to the percentage shortening of the crank?

And, off the shelf shorter cranks seem to be rare. I understand hollow cranks are no good for cutting down. Are there favourite cranks for shortening?
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Post Options Post Options   Thanks (1) Thanks(1)   Quote graydog Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2012 at 6:36pm
I personally find old school cranks have more meat on (so to speak) for allowing shortening.
There are ways to do hollow cranks, by moulding in inserts.

For standard cranks I generally seek blackwidows, BMX. I think they go down to 140mm but on 110mm BCD spiders. So you may need to make carries for your 130 shams, or 135 cantis.

Not sure the are generally available new either, as heard a rumor the where not being sold anymore. But i have seen them for sale since.

Cost around 60quid.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wyndrake Quote  Post ReplyReply Direct Link To This Post Posted: 02 February 2012 at 9:37am
Billie-J - Have a look at www.unicycle.com based in Stockton. They stock a wide variety of crank lengths starting at 90mm, priced at around £30.

Regards,

Alan
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wyndrake Quote  Post ReplyReply Direct Link To This Post Posted: 02 February 2012 at 9:42am
Bill B-J - Apologies for mis-naming you.Embarrassed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul Lowing Quote  Post ReplyReply Direct Link To This Post Posted: 02 February 2012 at 1:10pm
But surely unicycle cranks don't have any means of attaching a chainring...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote graydog Quote  Post ReplyReply Direct Link To This Post Posted: 02 February 2012 at 1:19pm
Some unicycles use chains or very very long legs ;)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wyndrake Quote  Post ReplyReply Direct Link To This Post Posted: 02 February 2012 at 10:52pm
Unicycles stock a variety of splined ISIS and system cranks such as Schlumpf (which I use on velomobile and used on the Windcheetah Hypersport.

Middleburn also supply interchangeable splined cranks compatible with Shimano XTR rings. However, I believe they only go down to 160mm as a stock item.

Regards,

Alan
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Adrian Setter Quote  Post ReplyReply Direct Link To This Post Posted: 03 February 2012 at 12:31pm
Originally posted by Bill B-J Bill B-J wrote:

Does one reduce the gearing, in relative to the percentage shortening of the crank?
 
You certainly should expect to need to reduce your gearing.  Simplistic mechanical thinking would indeed suggest that it should be by the same proportion as the reduction in length of the crank, so keeping the overall gearing (in terms of distance moved by the pedal to distance moved on the road) the same.
 
This is pretty much what I've done to my road recumbent, going from 170mm to 145mm cranks and from a 50T chainring to a 42T.  My limited experience so far is that I'm spinning out at a speed about 10% lower than I did do, so either I shouldn't have gone for quite such a small chainring, or I just haven't learned to spin those short cranks properly yet.
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