Bicycle Crank Arm Length

This is probably one of those FAQ's but humor a newbie if you will. I have heard that crank arms for 'bents should be shorter than the standard 170-175 mm used on most Diamond Frame bikes. What advantage (if any) is gained by shorter crank arms on a 'bent?
Chas. Winchester

 

Shorter crank arms, whether on a 'bent or an upright, allow for a higher cadence with less stress on the knees. The trade off is a loss of some leverage, how much depends on how short you go.

That said, I found that the riding position on a 'bent dropped my cadence by about 15 rpm. By reducing the length of my crankarms by 25 mm I got the 15 rpm back. The loss of leverage was only apparent in granny gear. Switching the cassette to an 11-34 offset that.

I recommend experimenting with Crank Shorteners first. These bolt on to your crank arms and allow you shortening options of 25mm, 40mm, and 55mm.

Mike

 

I think this is very sound advice. Though I still use normal length cranks, I am going to try some crank shorteners to see what happens. Everyone in our group who has done that, swears it is the only way to go.

Nice thing about crank shorteners you bolt on your normal cranks is you can experiment with different lengths. Once you find your "ideal" length, then you can have your cranks drilled and permanent pedals installed. (Assuming you have solid and not hollow cranks.) Don't want to drill your cranks and THEN find out they are the wrong length!

 

Last year I did about 3k on 130 mm cranks. The knee has a distinctive click when palpated with cranks anything over 130 mm. An MRI shows the knee to be "completely worn out". At the end of last riding season, the pain was almost unbearable, so started the process for knee replacement. Long story short, tried natural remedy first and the knee and leg are now pain free. No knee replacement needed.


I drill, tap and shorten my cranks, but the best way to get the feel of it is to do like AD recommends and obtain crank shorteners. I also like Pedal Extenders. They keep the inside ankle joint off the crank arms or crank shorteners, and makes riding more comfortable for my riding style.


BTW, Art at one time loaned crank shorteners. He sold me a set and then bought them back at full price when I no longer needed them.

Buddy

 

I use crank shorteners as well and it has made all the difference when I ride. My legs no longer constantly cramp while riding and they don't hurt. I had gotten to where my legs and one hip bothered me everytime we rode but not any longer. The shorter cranks have really helped. Now if I could just learn to ride like Buddy I'd be completely happy!

CTG

 

Buddy,

Good to have you back on the boards. I missed you!!!! Also good information about Art Hagood and crank shorteners. I will contact him about getting me a set. There are too many people in our group that swear by them to ignore what must be a good thing. I am slow, but not TOTALLY stupid!

 

Thanks for the kind words Jon.

Another thing about the 130 mm cranks is that a mountain bike crank set works well with them. I use the 22-32-42, and like Mike said, an 11-34 gear does well. That gives the Tour Easy with the 27" wheel a gear range of 17-103. Works for me. Helps to keep Jo in sight.

Buddy

 

Folks, one thing to remember about shorter cranks is that it must be accompanied by a significant reduction in gearing or the anticipated higher spin rate will likely not be achieved. My view is, if you don't have knee issues, or you're not concerned about keeping your cadence within an aerobic range, shorter cranks will likely not provide an advantage, and may very well reduce your climbing ability at low end and lower your spin-out speed at top end. I think a lot depends on your particular body physiology, leg length, etc. Some like 'em and some don't. Trying the crank shorteners is a good experiment, although they do increase the Q-Factor, or pedal spindle length. If it means the difference between functioning knees or not, by all means pursue it. I still run 175's on my V-Rex and Vivo and have been happy with them.

Art Hagood in Chattanooga is a rabid proponent of short cranks and achiving a knee bend at the top of your stroke of 90 deg or greater. It would take a quantum reduction in crank length to get my flex to greater than 90 deg. Buddy's 130's might be close. But I rode his some and felt like I was pedalling a squirrel cage.......

To each his own, just do what it takes to get out there. And Buddy's out there...

Don

 

Thanks for all the response. I'm going to poke around and see what I can come up with. It is good having the accessability of recumbent riders on this forum. The people at the local bike shops just look at you like you're nuts when you start asking questions like this. Again thanks for the input,
Chas. W.

 

I tried 155mm and it didn't work for me, but a friend had 165 mm and I tried his and I was impressed, so I got Ultegra 165mm triple crank set. Today if my trike will fit in my friends SUV I will have my Q-rings installed. I went with a 40t middle and a 52t outer both 130 BCD. Boy am I lost with my SUV in the shop for two more weeks. I wish I could sue the guy who ran the stop sign, but alas it wouldn't do any good. I can see it now when the judge asks "Why are you suing" and I answer "Because I can transport my trike to my favorite tiding spot" yea that would go over well.

 

We're saying the same thing, AD. Art's goal, so he told me, is to keep his knee bend greater than 90 deg at the top of his stroke, i.e. 100 or 110 deg, thigh to shin. The shorter the crank arm is, the less your knee flexes and the greater the angle. I've tried to examine mine, and it would take a LOT to keep it greater than 90 deg. I'm comfortable where I am, but I rode someone's bike with 170's on it sometime, and could distinguish the difference; whether good or bad or not, I don't know. I've been told 165's might be a good compromise for my leg length, I'd like to try those sometime.

Anywayzz, it's an interesting diversion. And Chas is right, upright riders and most bike shops don't have a clue in all this. The only thing they know is, small person = small frame = slightly shorter cranks. The upright manufacturers will put smaller cranks on smaller frames. But 165 is as small as it gets in the upright world.

Don

 

Mechanical Bike Engineering 101 factoids:

• Torque = Force x Moment Arm
• Force = Moment Arm/Torque

Given a shorter crank length, it takes more force to produce the same torque. So you actually have to push harder on the pedals to maintain the same speed if you are in the same gears with a shorter crank.

Now if you gear down and spin faster with shorter cranks, you are lowering the force on your legs and thus on your knees.

When you shorten the cranks, you open the angle of your shin to your thigh because the cumulative distance between your longest leg extension and your shortest leg extension is now shorter by twice the amount you shortened your cranks.

At 90 degrees, the force vector on your knee is close to 45 degrees with your knee. The greater the angle, the more inline the vector is with your thigh and the ends of your knees. So your patella really takes a beating at 90 degrees and it gets less force pushing it away from the shin as you extend your leg.

I think the real reason shorter cranks are better is the last reason noted above. The decreased angle with the knee more approximates walking and reduces the outward force on the knee. The civil engineers amongst us would compare this to keeping a column loaded axially. The bended knee is very similar to 'buckling'.

Bill