Crank Arm Length: 170 through 175 mm... which one???



AmpedCycle

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Dec 29, 2004
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So I've decided o go with the Shimano Compact Double (R700), but I'm a little unsure about the specific crank arm length I should get. I know that shop guys will tell me anything, and that there should be a more reliable, mathematical way to calculate it. Does anyone know the magic formula? My inseam is 89.5-90.0 cm, which translates to 35.24 inches. So I figure that all things aside (frame size, seat height, etc), my inseam should tell me how long my leg is so that I can pinpoint which crank arm is the right length for me. There are three lengths available: 170, 172.5, and 175 mm.

Anyone know about this?
 
Can't tell fer sure, but if your inseam in 35 inches your legs at long enough to push 175s easily. Mine is about 31 inches and I ride mostly 172.5 with one bike at 175.
 
AmpedCycle said:
So I've decided o go with the Shimano Compact Double (R700), but I'm a little unsure about the specific crank arm length I should get. I know that shop guys will tell me anything, and that there should be a more reliable, mathematical way to calculate it. Does anyone know the magic formula? My inseam is 89.5-90.0 cm, which translates to 35.24 inches. So I figure that all things aside (frame size, seat height, etc), my inseam should tell me how long my leg is so that I can pinpoint which crank arm is the right length for me. There are three lengths available: 170, 172.5, and 175 mm.

Anyone know about this?
Correct crank arm length choice is affected by anatomy and riding style. By any formula used, the range of crank sizes that people of varying leg lengths need is wider than the conventional 3 sizes offered. As such most of us in the middle sizes probably ride cranks close to what we need, whereas shorter riders often ride cranks a little too long and tall riders ride on cranks that are too short. Like any other aspect of bicycle fit, there is plenty of dogma and not near as much hard data.

In general, longer cranks should give more leverage, but may not be as easy to spin fast because of the larger turning radius, but this isn't that much of an issue if long cranks are used by riders with long legs.

Lennard Zinn recommends at crank arm length of 0.21-0.216 X inseam length (in mm), which will suggest a crank arm longer than you can buy stock. He specializes in custom bikes for tall riders and does make custom cranks up to 210mm+.

http://www.velonews.com/tech/report/articles/5257.0.html

Bottom line is that there is no definitive answer, but for your leg length, I would go with 175 minimum. I have an 85cm inseam and ride 175s comfortably.
 
My inseam is about the same as yours.

I've been obsessing about crank length for a couple of years, and for the last 12 months I persisted with 180s, but I now think they're too long for me. The big problem with long cranks (which isn't emphasized enough on these "proportional crank length" sites) is that the seat has to be lower, so the rider ends up in a low/weaker position, which probably counters much of the potential leverage advantages.

I'm now experimenting with my old 175s, 172.5s, and 170s. The first night I put the 175s back, I was flying!!!! :p Some would probably say that I gained strength from the 180s, but I don't think so.

My opinion is that the Zinn formula is way too long -- the thought of me riding 193mm cranks is ridiculous!!

Anyway, what was the question? :) ...You're legs are definitely long enough for 175s, but I'm sure you'd also feel comfortable on 172.5s. So, either the 175s, or the 172.5s. That's not much help, but you won't really be sure until you try them out.
 
I can't really notice the difference between the 170 I have on one bike and 172.5 I have on another and I've never seen any reliable scientific justification for any crank length formula. I suspect whatever you are use too (have trained your legs to use) will work best.

Another consideration is the ground clearance. Hitting the ground when cornering under power would be a problem with extra long cranks.
 
gclark8 said:
I'm not having a crack at ya, I'm having a crack at the long crank guys...... :)

Are the Greenspeed recommendations for recumbents? If not, I don't know where guys get their 'science' from.
NO medium/tall pros use cranks that are relatively that long.

As many of us know, Indurain is 6ft2, and used 180s for most races and 190s for time-trials (I'd like to know his inseam). Ulrich is 6ft, and uses 177.5, which, if he has the same leg length as me (I'm also 6ft), is 19.8%. Sosenka used 190s for his hour record, but he's 6ft7!!. Booned is (I think) 6ft3, and uses 177.5mm. That's it for the pros -- as as I know there are nop pros,even the very tall, racing on longer than 180. If, for eg, Axel Merckx thought he would win more races with 200mm cranks, I'm sure he'd use them. Some would say that he's restrcited to using sponsored products, but the sponsors like nothing more than victories.

Andrew Bradley, Lennard Zinn and Kirby palm (proportional crank length advocates) are all tall guys who, I reckon, have generalized from their own experinces, which was the frustration of being retrsicted to cranks no longer than 175mm when they were racing. Just because a bunch of short guys rode 170s (Palm's "logic") doesn't mean that 6ft6 guys should be on 220s! Andrew Bradley's site is by far the most balanced, offering stacks of impartial info, but his preferred formula is for a longer than normal crank fro medium/tall riders.

This article, which Kirdy Palm laughingly 'bags' on his site, counters the long crank stuff. http://www.bsn.com/Cycling/cranks.html


These are Chris Carmichael's 'general' recommendations for road racing:

inseam less than 29", 165mm cranks
inseam 29" to 32", 170mm
inseam 32" to 34", 172.5mm
inseam over 34", 175mm


Here's another more reasonable chart
http://www.myra-simon.com/bike/cranks.html