No single part of a bicycle drive train causes as much discussion as choice of crank length. Let’s take another look at the issue.
What happens to pedalling mechanics when a rider increases or decreases crank length?
Increasing crank length increases the multiplication of the muscular force that the rider’s legs can exert on the pedal, because a crank is a simple lever that operates in an arc. While it is tempting to think that a longer crank will increase rider performance because of that increased leverage, this isn’t necessarily so because it neglects the equally important issue of being able to fluently control that extra lever length.
I’ll use an extreme example to illustrate my point. Commonly available crank lengths for road bikes are available in 165, 167.5, 170, 172.5, 175, 177.5 and 180 mm. A rider increases crank length by 10 mm from 170 mm to 180 mm. It is unlikely that increasing crank length will somehow give the rider an ability to reach an extra 10 mm with power and fluency, so the rider drops their seat post 10 mm further into their frame which restores the same seat height (as measured from pedal to top of seat) as with the 170 mm crank. This will cause the knee to rise 20 mm higher at the top of the stroke with the 180 mm crank than it will with the 170 mm crank. A 180 mm crank transcribes a circle around the bottom bracket centre of 360 mm (radius of circle = crank length) while a 170 mm crank transcribes a circle around the bottom bracket centre of 340 mm. The rider in this example has maintained the same overall seat height so all of that 20 mm increase in pedalling circle diameter is at the top of the stroke.
In turn, a number of consequences flow from this.
i. Shear forces on the knee increase. In effect, coming off the top of the pedal stroke, the femur (upper leg bone) is trying to push forward over the tibia (largest of the two lower leg bones). If shear forces are too high, the knee ligaments and musculature protests over time. Whether an individual will be injured by the increase in shear forces on the knee depends on the length and proportions of their legs, how functional they are and how good their connective tissue is.
ii. The rider won’t be able to exert force on the cranks for the same number degrees of crank arc. The knee is bent more because it rises higher at the top of the pedal stroke. In turn the crank arm must travel further forward past top dead centre before the rider can exert any given degree of mechanical advantage on the pedal early in the pedal stroke. So a large increase in crank length provides an equally large increase in leverage but this increased leverage is available for a lesser amount of degrees of the circle that the crank arm moves through. If the crank is too long, this means that the rider will be less fluent and potentially less able to maintain good technique for long periods.
iii. The ability to accelerate really hard gradually diminishes with each incremental change in crank length. Accelerating hard on a bicycle is about more than just pushing harder on the pedals. It is just as much about revving faster too. All other things being equal, a shorter crank can be revved faster under high load with good control than a longer crank.
iv. The knee may foul the rib cage at the top of the pedal stroke. This tends to be a concern for flexible riders with good ability to ride with a low torso position. It can also be a concern for the many riders with pre existing tight hip flexors which become more cramped when the knee rises higher relative to the torso.