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Old 06-18-03, 09:10 PM   #1
SamDaBikinMan
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Crank arm length

My old Cannondale is now in dire need of a new crankset and bottom bracket. It held up today with increased clicking and by the end of the ride noticable additional play.

I was considering going to a 175mm rather than the current 172.5. I also ride mountain bikes and all my mountain bikes are 175mm.

Has anyone done this and did you notice any adverse affect on your spin?

I know there will be a slight leverage advantage with a 175 but not much. Maybe not noticeable. I cannot see where 2.5mm will make a lot of difference, this is the bike I pull my kiddie cart with so the extra leverage may be a godsend.
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Old 06-19-03, 02:58 AM   #2
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The extra leverage gives you extra torque so you can turn a larger gear more slowly. It does not give you more power. Pulling your trailler will still take ther same amount of work, whether you spin in a lower gear or turn slowly with long cranks in a larger gear.
2.5mm is not a great change, but take care with toe-clip overlap ate the front wheel, and cornering cleararnce; compare your bottom bracket height to your MTBs
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Old 06-19-03, 05:39 AM   #3
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It would be about like taking 3/4 of a tooth from your chainring. If you notice it, you are far away from a good fit on the crank.
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Old 06-19-03, 10:14 AM   #4
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When I went from 170s to 172.5s, I noticed it big-time. Not the leverage, the fit. It's about 1/4" difference from top of a rev to the bottom. Not much, but I sure noticed it, and it took me a while to adapt. Now when I ride my commuter w/170s, they feel tiny. It's weird.

Since you're used to the larger cranks on your mtb, it probably won't be a big deal.
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Old 06-20-03, 12:46 AM   #5
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Hi Sam,

What you gain in leverage you lose in cadence. I think that perhaps the limiting factor is foot speed.

Check out:

http://www.cranklength.info/

and

http://www.nettally.com/palmk/crankset.html

for more than you ever wanted to know.

I asked the same question a few weeks ago and ended up realising that it doesn't matter a great deal in terms of the maths involved regarding power, pedal speed and torque. A 5mm change in crank length seems to alter your gearing by half a sprocket tooth or so. I created some models to try to understand what goes on.

I twisted my brain in knots and now suspect that the important thing is to go with what feels right and that different lengths suit different types of musculature (as in people with a predominance of fast or slow twitch fibres)rather than different leg lengths.

I think that my own slow twitch muscles prefer the longer effort and recovery durations provided by longer cranks but I haven't experimented yet and could be completely wrong... I think that what Roadbuzz says about fit rather than leverage is probably spot on.

cheers,
Ants
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Old 06-20-03, 04:16 AM   #6
MichaelW
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Ants, You are partly right. In physics modelling, we like to fix one variable, and vary another to see what happend. If you fix leg length, then you shpould use a crank length based on your musculature(fast/slow ie cadence rate). If you are a spinner, go for a shorter one, mashers prefer longer ones.
If you fix the musculature type, and vary leg length, then longer legs need longer cranks.
Of course longer legged people often have slower moving legs.
For any leg length there is a medium crank, you can then vary about this medium according to your preferance for long/slow or short/fast.

If you are very short, your medium crank may be 160mm, if you are very tall it may be 180mm.
Crank length (eg http://www.coloradocyclist.com/bikefit/) advice tends to advise variation with leg size with the medium leg lengths (ie from 165 to 175mm), but no variation above and below the "normal" size range. I detect a flaw here.
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Old 06-20-03, 05:14 AM   #7
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i am 1.70m tall, and i find 170mm crank arm length fits me well.
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