Crank Length - Crank It Up!
Connecting the pedals to the chain rings (to make the bike go!), cranks are one of the most overlooked components on a bike. They are usually a similar size across most bikes, despite the huge difference in leg lengths of cyclists. A 5ft and a 6ft cyclist should not be using the same cranks!
Longer cranks give more torque on the chain, enabling more peak force.
Shorter cranks reduce the circumference of the pedal stroke, enabling more consistent power.
Crank width can vary significantly, ideally matched to hip joint flexibility and glute power.
For information on how to get the right crank length for your size and style of riding, check out this blog article by John Kelly (6S Health’s resident Biomechanical Physiotherapist) evaluating the latest biomechanical science behind selecting the best cranks for you.
To book a biomechanical bike setup assessment with John, visit 6s.com.au or contact our Mingara or Ettalong clinics.
Cranks are an essential component of any bike, connecting the pedals to the chain rings. That is, the cranks allow power to be transmitted from your legs into the chain, which drives the rear wheel, which makes you go forwards! The cranks act as a big lever to allow force to be applied, where a longer crank arm would theoretically allow you to get more torque into the chain (like using a large wrench allows easier removal of bolts compared to trying to twist them off with your fingers). However because a longer crank length forces your feet to travel in bigger circles each pedal stroke, maintaining a high cadence is more challenging.
The result? Crank length has to be a compromise between peak torque, circumference of pedal revolution, and speed of revolutions. Whilst this seems like a confusing system to balance, fortunately there have been plenty of scientific studies to evaluate what is the most efficient way of getting a bike to move forwards.
Before we get into the nitty gritty, time to clarify a few things.
-Cranks have always come in various sizes, but over the past 15 years most bikes have been fitted with 170mm cranks.
-Shorter riders are generally more efficient with slightly shorter cranks
-Taller riders are generally more efficient with slightly longer cranks
Now, the science of why these things are relevant. From preliminary studies in the second half of the 20th Century, researchers were able to find cyclists could get maximum power if the cranks were ~41% of the tibial length (distance from the base of the medial bone of your ankle, to the medial knee joint line), or ~20% of full leg length (distance from the floor, to the bone on the side of your hip). However follow-up studies found some variability in the ideal length, based on the rate of pedal revolution and maximal glute and quadriceps power.
As more and more research was done for maximal power, experimental crank sets were developed (often called eccentric cranks), where the cranks lengthen on the pushing down movement (to give more torque), and shorten on the way back up (to decrease the circumference of the spin, and counter-torque). Unfortunately despite the logical rationale for these cranks making faster cyclists of us all, the difference was measurably insignificant. There was less than 2% increase to maximal power output, and no change to metabolic efficiency or performance in normal cycling, including racing. Hence, you don’t find fancy cranks like these stock on any bikes.
So why are most stock cranks 170mm. The short answer, is research. Multiple studies have found 170mm to be the most metabolically efficient when couple with a cadence of ~100. This is slightly variable between cyclists based on height. Greater metabolic efficiency means the same speed for loss effort, which is crucial for cycling whether it is for racing or even recreational riding. The speed that a pedal moves in its rotation (related to the crank length x the cadence) has been shown to be the greatest contributor to metabolic cost. So if your bike had long cranks, and you spun them very fast (e.g. cadence >120) riding a light gear, you are wasting energy!
Studies on mountain bikes have shown slightly shorter cranks may be helpful since maximal power can be achieved more rapidly, making the rider faster in especially technical terrain. Another benefit is a small improvement in ground clearance with shorter cranks.
So what cranks are right for me? Obviously there are a number of factors influencing the selection, but most weighting should be towards your height, and style of riding. If you prefer to ride with a high cadence, slightly shorter cranks will be more metabolically efficient. If you’re a gear grinder or love hills, longer cranks will be more efficient. Whether it is mainly road riding, or mountain biking will also change what is ideal.