Shorter cranks for recumbent

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Kerosene-Tin
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Shorter cranks for recumbent

Postby Kerosene-Tin » Thu Oct 25, 2018 7:43 am

Im looking to start experimenting with crank size on my long recumbent due to the 175 stock standard ones interupting my pedal rotation each time when my right leg reaches maximum compression

Adjusting the seat fixes it one way but then has me over extending losing power and hitting a dead spot so repeating the cycle

Shorter cranks seem to be the go
Thing is I dont know what size would be the sweet spot.. Shorter cranks seem horribly expensive just for experimentation

It has to be a common recumbent problem..
Anyone got a good solution?

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MattyK
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Re: Shorter cranks for recumbent

Postby MattyK » Thu Oct 25, 2018 11:29 am

Short cranks on recumbents seem to be the thing. Also in the triathlon world, though fancy options get expensive.

150-160 would be the typical range for "short", though maybe even shorter if you are also short.

Assuming you can tolerate a square taper BB:
Trisled has some cheap 155mm cranks. (probably worth talking to the man there too)
Otherwise Origin8 cranks on ebay come in various lengths.

skyblot
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Joined: Tue Sep 22, 2009 8:52 pm

Re: Shorter cranks for recumbent

Postby skyblot » Sun Oct 28, 2018 10:30 am

Some cranks can be shortened, others not. It depends on the thickness of the crank where the new hole will be. Off the shelf short cranks are rare, some manufacturers do 165mm but you would be unlikely to find them at a local bike shop or distributor. As mentioned, Trisled have some, and also Greenspeed.

I run 150 mm on one bike, and 140 mm on another.

I'm reading that your knee flexion is the limiting factor? See if you can work out what is comfortable for you before buying cranks. A 350mm step height is equivalent to 175mm cranks, maybe see what height step is your limit for your knee? (Use a pile of books for instance....)

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MattyK
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Re: Shorter cranks for recumbent

Postby MattyK » Sun Oct 28, 2018 11:11 am

Reviewing your other posts, you have a front triple chainring. Assuming you want to keep that, it will make finding a short crank extra hard (the ones I linked are doubles only)

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Duck!
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Re: Shorter cranks for recumbent

Postby Duck! » Mon Oct 29, 2018 11:51 pm

Short cranks definitely do make a difference on a 'bent; I ran 150mm on my race trike for several years because 170+ just felt wrong due to the posture, but I have no problems with that length on an upright bike.

Short cranks enable you to spin a higher cadence, which helps because you can't stand on the pedals and mash your way up a climb; you need to spin to stay on top of the gear, and the shorter crank radius means that for a given linear leg movement you're scribing a greater portion of the crank's rotation.

The trade off is that you have less torque than with a longer crank, especially at lower cadence with higher load, so you really need to ensure it's adequately geared to allow you to keep the cadence up.

If you're going to re-drill a crank to shorten its effective length, you need one at least 25mm longer than what you want to shorten to so there's enough material left around the new pedal hole; Eg. if you get hold of a 175mm crank, the longest you can redrill to is 150mm, and that will leave about 10.5mm of material around the hole; too little you can risk cracking the end of the crank and blowing the pedal out.
I had a thought, but it got run over as it crossed my mind.

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MattyK
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Re: Shorter cranks for recumbent

Postby MattyK » Tue Oct 30, 2018 12:08 am


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