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 Post subject: Crank lengths
PostPosted: Thu Jun 20, 2013 11:34 pm 
Old School Hero
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Joined: Tue Sep 15, 2009 1:04 pm
Posts: 216
Location: Surrey
...are typically 175mm in my experience.

Having just bought a pair of M737s I found that they are 170mm in length...

So my question is - what the advantages/disadvantages of using the shorter length?

:?


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 12:11 am 
Gold Trader
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Joined: Fri Jan 23, 2009 2:30 pm
Posts: 12424
Location: Surrey
Not sure I'm going to be much help but I usually run the standard 175mm cranks but have a set of 170mm xt cranks on my Saracen and can't say I've noticed much difference. I can kind of feel something is not as normal, but it has never been a problem though I've done limited miles on the bike so perhaps on longer runs it may become clear as to whether the 5mm difference is good or bad.


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 6:34 am 
retrobike rider
retrobike rider
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Joined: Mon Jun 13, 2011 1:32 am
Posts: 1594
Location: Worcestershire / West Midlands
BITD most of my MTB cranks were 170mm tbh. Bought an XT737 crankset when I rebuilt the Zaskar and that's 175mm. In theory longer gives you more leverage but haven't noticed much difference tbh. My knee is buggered now... but that may be more to do with riding in general. :D


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 7:08 am 
retrobike rider
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Joined: Sun Feb 19, 2012 10:08 am
Posts: 6846
Location: Nth Somerset, UK
I have ridden 175mm cranks for years. I got a pair of 170mm XT cranks because they suited the bike I was building, and I could not feel any difference, although in theory shorter cranks should encourage higher cadence.


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 7:34 am 
retrobike rider
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Joined: Tue Jul 10, 2012 11:58 pm
Posts: 2362
Location: Bournemouth
there are 2 reasons they did 170 / 175 / 180. Leg length and leverage.


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 7:43 am 
Retro Guru
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Joined: Sat Apr 02, 2011 11:16 am
Posts: 799
Location: Camden, London
I've got an old "mtb" stronglight crankset on a road bike that's 170 that I was thinking of swapping across to see effect, while all the discussions on web re leg length seem to make sense, the bit re knees seems to be bit that's really relevant. Don't forget the impact is doubled the difference in crank length, as if 5mm longer crank, then will need to drop saddle 5mm to be in same position at bottom of pedal stroke and then at top the distance will be reduced by 10mm as pedal 5mm higher while saddle still lower.

If you decide to offload the 170mm cranks, I may be in the market.............................


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 7:46 am 
Retro Guru
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Joined: Thu May 03, 2012 7:13 pm
Posts: 2574
Location: The Cock Inn, Tillett, Herts
More power with a longer crank, smoother/faster cadence with a shorter one. I fancy myself as a knowledgeable, high mileage rider and I can't tell the difference when in motion between 175 and 170, and even tried riding with one of each but couldn't detect it.


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 11:31 am 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 8209
Location: New Forest, UK
Power is about muscle output, not crank length. However, the wrong length gives sub-optimal power output as the muscle effort is not used to best advantage.
All this leverage argument is fine, but it ignores that there is already a leverage system: gears.
Match the crank length to your leg length and then pick gear ratios to suit.

Years ago I switched from 170 to 175 for a tour. I had horrible knee pain and was clearly over-extending it.

Have a look here if you want to dig into it...
http://bikedynamics.co.uk/FitGuidecranks.htm


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 12:17 pm 
MacRetro rider
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Joined: Sat May 22, 2010 7:25 pm
Posts: 4977
Location: Edinburgh
It's a much bigger issue for roadies due to the nature of the riding style, on an MTB you never spend hours in the exact same position doing a hugely repetitive movement.


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 Post subject: Re: Crank lengths
PostPosted: Fri Jun 21, 2013 12:32 pm 
Old School Grand Master
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Joined: Thu Jul 19, 2007 7:18 pm
Posts: 3798
Location: Staffordshire
I much prefer the 170 length. This preference started on the road and migrated to the whole fleet apart from one bike. This still has 175 and I'm sure I can feel it.

The recent Scrap Heap Challenge saw me with 165 cranks salvaged from a discarded BSO and again I could feel the difference.

If you are tall and have long legs 175 would be better. Like the others say though, most MTBers wouldn't feel the difference due to the dynamic nature of the ride.


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