Crankarms

Decisions on appropriate crank length are often steeped in mystique and considered part of the "art" of coaching for the competitive cyclists. Knee pain, especially chondromalacia, is a common cycling complaint that can occasionally be an indicator that your crank length is incorrect for you.

A few links that might be of interest:

• Making a Bicycle Fit has a link to a crank length calculator.

• Bill Boston Cycles also has a reference to a page dedicated specifically to crank length.
• I did stumble across a nice review on the subject of picking your crank length.

1. It has been recommended that competitive cyclists use shorter cranks than the normally recommended to reduce the dead spot at the top of the cycle, from 9 to 12 o'clock (viewed from the rider facing to the right) and to allow a shorter stroke through the strongest leg movement. This would avoid having the knee bed less than 90 deg.

2. There is data showing that the shorter the crank, the higher the cadence that is possible (ultimately up to the rider). Shorter crankarms allow for faster cadences and improve cornering clearance on velodromes and in criteriums. They also avoid the effect of long crank arms to close the hip angle and reduce power - a benefit for velodromes and time trials.

3. An increase in crank length leads to an increase in the lever arm and the ability to more force. Longer crankarms have long been touted as superior for hard steady efforts such as hill climbing and mountain biking. But studies have shown that while they change torque, power is unchanged and they require the rider to pedal a larger circle.

4. Competitive cyclists ride comparatively smaller gears on the track in sprinting events (as opposed to the road) to allow optimal cadence in the shortest time. This is especially the case on banked tracks where the sprinter needs to changed direction suddenly at times. This facilitates reaching optimal cadence in the shortest time ( accelerating out of the bends). The same effect occurs with shorter cranks. The use of longer cranks allows for more leverage and so more power but can lead to a dead spot at the top of the stroke for those with a less than perfect pedal action & strength . In endurance events where the requirement for high cadence is not as great, the extra leverage is of benefit.

If you change your crankarm length on your current set up, don't forget to adjust your seat height as changing crankarm length will change the distance from your seat to the lowest point of the crank cycle.

Here's a starting point for suggested crank lengths based on traditional wisdom - you can start here and then tailor them (up or down) to your own style and preferences:

• inseam < 29 inches - 165 mm crank
• inseam 29 - 32 inches - 170 mm crank
• inseam 32 - 34 inches - 172.5 mm crank
• inseam > 34 inches - 175 mm crank

And for those of you wanting to delve further into the subject, here is a link to a beta test version of a Crank Calc Program being developed by Nigel Jones of Machinehead Software.

Here is a question posed to "Coach Fred" at www.roadbikerider.com entitled "Do Long Crankarms Help Time Trialing?"

Question: I'm a 36 and a competitive triathlete, usually placing top-5 in my age group. I use 170-mm crankarms but have heard that for time trialing, longer is better. Is that true? -- Harold F.

Coach Fred Matheny Replies: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual. So, longer crankarms aren't a panacea for time trialing. In fact, there are dangers associated with them. The added length makes your knees bend more at the top of pedal strokes and extend more at the bottom -- both of which can lead to biomechanical injuries if you jump from 170 mm to, say, 180 mm. Also, longer cranks reduce cadence -- and a brisk cadence is the key to good time trialing. All this said, many time trialists use crankarms 2.5 mm longer than those on their normal road bike. Because 2.5 mm (one-tenth of an inch) isn't much, it rarely causes an injury. But the jury is still out on whether that bit of extra length actually improves performance.

In Summary - Crankarm length, it's about saving your knees!

These two posts on roadbikerider.com (one and two supplement the above links/references and were the source of the following CPTIPS facebook post in 2024:

With a longer crankarm, the excursion of the foot between the top and the bottom of the pedal stroke is greater, and for each pedal stroke, rpms being held constant, more energy is delivered to the drive chain per revolution of the crank. This is akin to climbing a flight of stairs with 10 inch versus riser of 10 inch versus 6 inch risers. You expend more energy moving your body up a single 10 inch step versus a 6 inch step, and it take more total energy to climb a flight of a dozen 10 inch stairs in the same time as a flight of a dozen 6 inch stairs.

I am going to quote directly as I highlight 3 key points:

1. >At a constant power level (energy from the quads delivered per time interval to the pedal) and a constant drive chain gearing ratio, a longer crank means a slower cadence.

   "Many production road bikes today come with 172.5 mm cranks. If you reduce this to 165 mm, you are shortening the vertical stroke by 15 mm, a little over 1/2 inch. This means your knee is going to move a little less than 1/4 inch lower at the top and higher at the bottom of the stroke. This is the change in vertical motion of the knee. Work done to move the pedal through one revolution is changed by (172.5-165)/165 which is about 4%. Shifting between a 16 tooth and a 17 tooth gear gives you a change of approximately 6%. Hopefully this demonstrates the relationship between crankarm length and gearing."

2. The increase in the force delivered per pedal stroke with the slower cadence (above example) means more stress on the knee. It is is much healthier on the knee joints to use shorter cranks and spin them faster.

3. The increased excursion of the knee with a longer crankarm(top to bottom of a crank revolution) increases the biomechanical stress on the knee as well.

   "The human body was built to walk and run, not ride a bicycle. Walk around the block and/or run around the block and measure knee flexion. Now get on your 175-220 mm cranks and measure knee flexion. There's much more on the bicycle....you would tremble in sheer horror knowing how long crankarms are destroying one's knees. You mention the older riders talking about the benefits of longer cranks, then why did most world tour pros retire in their mid 20's with blown out knees?....it is much safer and healthier on the knee joints to use shorter cranks and spin them faster. In closing, I am 6' tall and use 170mm cranks on all my bicycles except my track bike with 165mm. No more knee pain!"