Bicycle Crank Arm Length

Ahhh, there go our engine ears again!!! I understand 10% of it. But I love it anyway. Keep those equations coming!!!

 

So what (I think) I'm hearing is the shorter cranks can be a good thing taken the overall picture: the knee angle at the top of the stroke is the important thing. Crank length will depend on leg length, at least to some degree. Makes sense ergonomically. And the comparison to a load bearing column is an interesting one. To me the engineering/machine is a good approach as the body is, for all intents and purposes, a machine (albeit a very complex one).

On a Diamond Frame we're told that seat height should be at the point that the leg is just slightly bent away from straight at the bottom of the pedal stroke. The top of the pedal stroke is not mentioned and I can see where this would be much more critical on a 'bent. In fact it could be argued that the top of the pedal stroke and the leg in relation to it is even more important than the bottom of the pedal stroke.

I have some minor (!) knee problems with my right knee and it is important to me to get this right from the start so I don't screw up my knee worse. The desire to ride in spite of the knee is one of the reason I've gone supine to begin with. So I need to get it right on order to keep riding.

Thanks for the advice and it is starting to tick in my brain. And it only took two pages!

 

Actually it is also quite important that the bottom of the stroke is set right. If your extended leg is such that your knee straightens all the way, you will hyperextend your knee and really give yourself problems. Art Hagood recommends setting your extended leg length such that the top of the pedal is one inch from the bottom of your bike shoe when your leg is fully extended. That way, when you are clipped in, you can slightly extend your calve muscle (extend your toes) to get all of the 'toggle' force possible, but your knee can stay slightly bent as you do it.

 

There is such a thing as too short which isn't workable either. How short is too short? That depends only trial and error will tell you what is best for you. For instance, I find 145 to 150mm to be as short as I want to go. Any shorter and I have too hard a time climbing. Laurie, on the other hand, loves her 110mm cranks.

Mike

 

Certainly the bottom of your stroke needs to be set right too. Clearly that's the measure for appropriate leg extension, and the shorter crank affects the seat distance. Everything changes when the cranks shorten up. The necessity of moving the seat back for the shorter crank on the bottom stroke also helps open up the angle at the top stroke. All the geometry changes. Mike's right, it takes some experimentation. If you're happy where you are though with no pysiological problems, it's hard to make yourself chase after rainbows. That's my situation, I guess I may be just dumb and happy (but not fat, dumb and happy....).

Don

 

I read all the posts about Pedal Extenders and shorter cranks and toe clips and faster cadence.

I bought pedal extenders. I bought shorter cranks (160 mm). I added toe clips. I really started paying attention to my cadence.

I just got back from a ride and my knees are happier and I was able to maintain a higher mph. IMHO it was worth all the changes.

 

Don, don't think that way. Whereas all these comments are fine and dandy IF you have correct proportion in both shin/calves and thigh length. Unlike most people I have short thighs. So even though I have had three (3) knee surgeries on my right knee, (unrelated to biking, pre-bike life). I have had no problems with my knees unless I have my seat mal adjusted. Now I have had some pain in my tendons due to riding my bike more than 75 miles. 65 miles is a good distance to ride. More than that and my tendons ache, But I rub in a good dollup of Myoplex and I am right as rain the next morning.

 

Short Cranks

I may be looking at this all wrong but.... it seems to me that no matter what length the cranks are, one complete turn of the peddle creates a movement of the bike regardless of the length of the crank. So being interested in speed, I'm interested in cadence, turning the peddles more times and for me each shorter length of crank increases the cadence more turns with same energy and more turns with more energy. A consistent cadence of 100 is pretty good for me but I can't do it on 170's or 165's but I can on 155's. Knees were never a problem for me as long as the the boom was adjusted correctly for my leg length. I love to power the peddles from a red light with other bikes etc. and the shorter arms take me to my top cruise speed quicker. I use 52, 38 large chain rings and I think 11-26 cassette down from the 34 cassette. This is on my 20" Z-bone but it has become my standard set up. But this is just my ride! And I bought Origin 8 155 cranks since they were cheap to try --- I'm still liking them a lot. And Amazon comes through once again.
joe

 

Excellent series of articles!

I like my 175 cranks but now I'm curious about longer crank arms.

155s were way too short for my bike, for me.

I imagine that recumbents in general are pretty different from the upright bikes in the article and all recumbents differ from each other.

Even so, there's really not enough range in crank lengths available is there!

 

crank arms

The difference from 170 to 155 in force applied to turn the crank is not measurable to my legs. Actually the increased speed of turning the cranks (cadence) makes it seem much easier to turn the cranks faster with much less effort. And my computer says that over the same route I travel faster.

 

No, not at all. 'Bout ruined my worn out knee trusting
Art. No substitute for a long fulcrum and I can get up
on the front of the seat and spin a 175 real fast. Plenty
fast enough. BTW, that's the way Fast Freddy rode the
Gold Rush that's hanging in the Smithsonian.

 

I have always believed that shorter cranks should be for shorter folks. I mean, if you look at childrens bikes, they have shorter cranks that the adult sized bikes. But here lately I come to realize what Serenity said. "If the seat is in the correct position, there should not be a knee problem." I changed out my 170mm cranks for 155mm's and could not tell the difference in leverage except on extremely steep hills. Ones where I was down in the granny gear and turning over at about 3 turns per minute. I never noticed an automatic increase in my cadence. And I'd bet that anyone can turn a 170mm crank just as fast as a 155mm crank. If there is a difference I doubt that it would be more than 2-3 turns per minute. Increasing the cadence by 20-25 turns per minute is more in the mind than in the length of the crank. Now I'm talking about folks that are in good health, not someone with a limb length discrepetance. (Buddy) There just isn't enough difference in the crank length to make that kind of an increase.

Just my opinion folks,
Safe Rides,
Yoda