Crank arm length? - Page 3

Well, I have to be honest. We're broadening our War of Terror, and apparently week after next, we're going start liberating Australia....and your Uranium. We'd start sooner, but we need some time to make up a connection between Osama bin Laden and you folks.

Cooool!! I look forward to having my very own pair of Freedom Jackboots

I`m 180 cm, my inseam is 93 cm, i`ve used 172.5, 175 and 180, and there`s no doubt that the 180s works best for me. I`ll be getting adjustable cranks(180-220) in a few days and will be testing them in the coming weeks and months.

Did you read the article on Sosenkas site? It says:

I have some calculations on my desk, proving that 190 is not the ultimate length for me. And that with longer cranks my performance would be as from another planet

whats wrong with a bike that fits?

Sosenka is the world record holder!

Do you use a powermeter when testing the different crank lenghts?

I was just letting the guy know that he might need a new frame if he gets 200mm cranks. If he's very tall, chances are he has big feet, so he'd probably have big issues with his heels smashing into the stays and the rear derailleur. I knew a guy with 'normal' cranks and big feet who had problems hitting his rear derailleur with his heels.


180mm is (obviously) only 19.35% of your inseam. I'm not sure what you're saying. Are you saying you'll be better off with even longer cranks, or don't you know yet? Do you think I'm wrong dismissing Zinn's "21.6% of inseam" formula?



I'm aware of that. Once again, what are you saying? I'm saying that 21% or 21.6% of inseam is way too long, especially for taller riders. As I said, I doubt very much that Sosenka is anywhere near that. While I think that 21% is too long, I do believe that taller riders should use longer cranks, but not that long.

I guess it begs the question: why didn't he use longer cranks for his record? I'm sure he did a bucket load of testing back then. Anyway, it doesn't mean anything to us unless we know his inseam.



No, I don't have a power meter. I persisted with 180s for about a year (i'm 182cm -- 89.5cm inseam, size 44 feet); I even bought 3 pairs!
I ended up with a bit of a screwed up postition, with my knees almost hitting my chest (especially when in the drops), and I was unable to 'get over' the pedals to really pound them. I know I started going better when I went back to 175s, and I sometimes even use 172.5s. The only time I missed my 180s was when 'surging' up short hills off the saddle, so I bought some 177.5s a few months ago, but I have ridden them much; I don't need them on the flats.

21.6% of inseam puts me on 193mm cranks, and 21% would have me on 188!! I can't even fathom that

I came to the conclusion that while longer cranks may provide more leverage, they're harder to push because the rider is in a relatively lower and weaker position, so it kinda cancels out. And that's without even getting to the cadence side of things.

I'll be happy to admit that I'm way wrong if pros start using super long cranks in the coming years and smash all the existing time-trial times, but until then, I'll maintain my "dubiousity".

This is a good article which outlines a couple of the conundrums of longer cranks: http://www.arniebakercycling.com/pub...m%20Length.pdf
Check the bit on "force and optimal crankarm length"

As some 'covnential' advocates has said, despite all the testing that's gone on, particularly by Zinn, no hard conclusions can be drawn that longer cranks make us faster.

The most sensible response to this question in years. Remember that the current fashion for long cranks was set by Cyril Guilmard, the DS for Renault/Elf/Gitane during the Hinault and LeMond period. Hinault was periodically sidelined with knee problems.

ah shucks -- but I basically ripped it off Zimmerman: http://www2.bsn.de/Cycling/cranks.html (the article is pretty old, so some of the links no longer work)

He says something about lots of testing coming up with not much,

I recently pulled the cranks off my bike to clean them, and to my surprise, when I measured them, they were not the length I ordered on the bike when I purchased it in 2004! Having been professionally fitted by the top fitter in town at the time, as well as double checking with several online fit calculators that came to the same conclusion, I specifically ordered 172.5 mm, but these cranks are 175 mm!

Furthering regarding crank arm lengths, however, I am having trouble getting my head around some of the performance factors associated with longer or shorter cranks. Having had some right knee problems and surgeries, I wanted to lean towards shorter cranks with the theory that they are a little easier to spin at a higher cadence and may not put as much pressure on the knee as longer cranks. I also theorized that it would be a little easier to spin up the mountains with a shorter crank than a longer crank in my 39-27 gear.

But when I do a search on this subject, I keep coming up with information that seems contradictory to the above conclusions. For example, in this discussion on crank length at Sheldon Brown, he says "More well-rounded cyclists, who are likely to deal with wider variations in chainwheel and tire size, need a more sophisticated system to realize that the 46/16 on their mountain bike, and the 52/12 on their Moulton and the 52/14 on their Bike Friday are all about the same as the 53/19 and 39/14 on their road racer. But the fact is that a mountain bike with a 46/16 has the same gear as a road bike with a 53/19 only if they have the same length cranks. If the mountain bike has 175's and the road bike 170's, the gear on the mountain bike is really about 3% lower! If you go to longer cranks without changing any of the other variables, you will have more "leverage", which is another way of saying you'll have a lower effective gear."

To me, I associate lower gear with being easier to spin and higher cadence, and I was looking for the effect of having a lower gear with shorter cranks, not that effect from having longer cranks. What am I missing here? And most importantly, am I not correct in assuming that it will be easier to spin up the mountain in my 39-27 gear with a slightly higher cadence with slightly shorter cranks than it will be for me to spin up the mountain in my 39-27 gear with a higher cadence with longer cranks? On another note, I happen to have 170mm cranks on an old mountain bike, and I seem to be relatively fast and comfortable on that bike in situations spinning up a paved mountain road, not nearly so much slower compared to my road bike times over that same course that I would expect, further making me think I would be better off going to the slightly shorter 172.5 cranks on my road bike that all the fit calculations recommended for me.

I think it is probably way too late to get the bike shop to perform and give the size crank arms that I ordered though!

The issue is pedal force is inversely proportional to pedal speed, at the same power, and pedal speed is proportional to cadence times crank arm length, so a longer length, at the same cadence, allows for less pedal force, or equivalently at the same force a longer crank allows for a lower cadence. The argument is that muscular engagement (fast twitch versus slow twitch) is dependent on force and speed of the muscle contraction, rather than on the cadence. The same muscle speed and force occurs at lower cadence with longer cranks, at a given power.

Pedal force isn't the whole issue, of course, but then neither is cadence.

Dan

A lower gear (12) is more difficult to spin than a higher gear (24).

The thing to remember about Sheldon Brown is that although he his a very smart man he tends to over think some things through.

Although he might be on to something with shorter MTB cranks. Shorter MTB cranks would allow more rock clearance, lower bottom brackets, and lower centers of gravity. Shorter cranks with lower gears would encourage MTB riders to spin more, mash less, and enjoy more torque for overcoming small obstacles.

The issue is pedal force is inversely proportional to pedal speed, at the same power, and pedal speed is proportional to cadence times crank arm length, so a longer length, at the same cadence, allows for less pedal force, or equivalently at the same force a longer crank allows for a lower cadence. The argument is that muscular engagement (fast twitch versus slow twitch) is dependent on force and speed of the muscle contraction, rather than on the cadence. The same muscle speed and force occurs at lower cadence with longer cranks, at a given power.

Pedal force isn't the whole issue, of course, but then neither is cadence.

Dan

Thanks. Then at equivalent force a shorter crank allows for a faster cadence. But Aussie's statement above makes sense to me, which notes that crank arm length is "is more about fitting the rider than leverage or cadence." I have a strong suspicion that I not only could spin a little faster and save my knees a little better with a little bit shorter crank, but that I also might just might be circling in a slightly more efficient range of motion and be able to generate a little more power and speed overall climbing up the mountains. This is somewhat opposite from what I hear many posters saying on this crank arm length question. A search on the crank arm issue finds multiple posters noting that they like a little longer crank arm on climbs to "generate leverage" and a little shorter crank arm for spinning on the flats or sprinting.

Guess I will just have to try some 172.5s out and see how it goes. I have that fitting from supposedly the best fitter in the city that also says 172.5 mm is the best size for me, and I am getting significantly faster times on road climbs with that mountain bike with 170mm cranks than I ever got before, so it seems 172.5 is worth a try. As far as the percent of inseam deal is concerned, I am about at Zinn's supposedly conservative 21% of inseam, but I am really jamming the clipboard up there hard to get my inseam measurement, using a measurement that is about a good inch and a half more than what I normally call my pants inseam.

You may, but I don't think you'd notice much leverage difference with a 2.5mm increase. Going up or down by 2.5mm is more about a 'feel' preference of pedaling the 5mm bigger circle. Going up 5mm (a 10mm bigger circle) will give you a notable difference, especially off the saddle, but then you've gotta see if the leverage isn't countered by being in a lower/weaker position..in my opinion.

Just to recap I'm 6ft and I mostly ride 172.5 (lately). The other night I used my 177.5s for the group smash-fest over some short hills. I enjoyed the leverage off the saddle up the hills, but each time I sat down, the pedals felt a little harder to pound coz my knees were coming up roughly a cm higher. It's kinda like lowering the saddle, then hoping the leverages counters the weaker position.

I accidentally put myself in a blind experiment a few years ago. I used to commute 5 days a week over a few short hills. I was using 170s because that's all I had at the time. One day I dusted off an old bike that I hadn't ridden for a a couple of years, and when I got the regular hills, I couldn't work out why I was flying up them. Several minutes later, after staring at the bike a couple of times (mainly to make sure I was using the same gears), I remembered this bike had 175s! I've obsessed about crank length, on and off, ever since.