Optimising Crank length for road, triathlon and time trial cyclists
After some media publicity around the topic and reading all side of the debate online, the On Form Physio team decided to publish our take on the question:
What is the best crank length on a road bike?
To break this topic down into easily digestable know;edge, we will discuss the physics of power generation, the physiology of how a rider generates this power, the combination of these two in the biomechanics of how the rider and the crank interact and influence each other.
Physics
Let’s start with the basic premise of power generation on a bicycle. How much power you generate whilst riding is based upon the formula: power output = torque x angular velocity
In simple terms, how much power you’re putting out is a product of your cadence (angular velocity) and how hard you are pushing on the pedals (torque). If you’ve got a power meter on your bike, this is the very simplified algorithm of how you know how many watts you’re putting down at any given time. Based upon this formula alone, some critics reason shorter cranks mean less power, which from a purely objective standpoint is true, but only if you pedalled around at maximum power all day every day. What is quite often considered is that the engine that provides the torque to the pedals - the rider, is limited by their own physiology.
Physiology
When the cranks are on longer side, there is significant resistance to turn the pedals at a fast enough cadence, which diminishes power production. This is similar to why your car stalls when you try and set off from the traffic lights in 4th gear. The opposite end of the spectrum is also true, short cranks allow a very high cadence but lose the ability to generate high force due to a phenomenon known as the muscle fibre force-velocity relationship. The graph below is the application of our power output formula to a rider, demonstrating the balance of the high ability to force as slower contraction speeds and high muscle contraction velocity is what optimises a rider’s ability to generate power.
What tends to happen is that when a rider changes to shorter cranks, they will start to pedal slightly faster to compensate for the loss of torque. For the vast majority of riders this accommodation is no issue as other biomechanical factors such as muscle shortening velocities have an impact on how much force a muscle can generate.
This topic has been researched by groups of sports scientists who have tried to investigate the optimal crank length based upon a riders physical dimensions came to the conclusion that the happy medium between increased the pedal cadence and the increased lever arm to generate the maximal power a rider is able to was about 166mm.
Biomechanics
Here at On Form Physio, we subscribe to the policy that most bikes, in particularly road bikes, come with cranks that are too long. We quite often recommend as part of our bike fitting service that riders consider going shorter the next time they replace their cranks. From a bike fitting perspective, where we do most of our work with crank length variation here at On Form Physio, shorter cranks allow us a number of options:
1) it allows us to set your seat height relatively higher in relation to the bottom bracket because your foot is closer to the bottom bracket at the bottom of the pedal stroke, and
2) it reduces the maximum hip angle riders reach at the top of the pedal stroke, which is great as stiff hip joints, tight hamstrings and gluts are quite often the limiting factor that causes riders to rock around on their bike and lose pedal stroke efficiency.
The combination of these two reasons also gives us as bike fitters the ability produce a more aerodynamically optimised position by relatively lowering a riders front end, which for our triathlete clients is a big win to be able to stay out of the wind for longer.
Shorter cranks also change where in your leg that the force is being produced. Shorter cranks increase the amount of force production across your knee (quads muscle group) and reduce the amount of production from the gluts and hip area. The smaller range of joint angles also optimises muscle fibre length-tension relationships, creating a more efficient muscle contraction through the entire pedal stroke.