Swap your set-up to shorter crank arms and you could reap benefits on both the bike and run, says Garth Fox
With wheels, handlebar, tri-bars and saddles all fighting for attention when it comes to upgrading your bike spec, you could be forgiven for overlooking your crank, a complicated and costly upgrade.
Yet it’s the one kit swap that could save you minutes on both your bike and run splits.
Traditionally crank-arm length has been determined by the size of your bike frame. A large frame would have 175mm cranks, medium 172.5mm and small frame 170mm. Hardly scientific.
Yet at the elite end of triathlon the trend in recent years has been to shift to 165 or 170mm crank arms – even for the tallest athletes. There are several performance enhancing reasons why they do this.
If you drop your crank length from 175mm to 170mm that means that at the bottom of the pedal stroke your foot is now 5mm closer to the seat. This means you can now raise your seat by 5mm to give the same leg extension as before.
This in turn means that your knees are further away from hitting your chest. So you can now lower your handlebars and still have ample clearance between your knees and chest. Bingo. Your drag coefficient just got lower and you will likely go faster for the same power output.
The other beneficial effect is that an open hip angle when cycling (ie knees not hitting your chest) can help you to run better afterwards. A few years ago I conducted a study on the effect of both a narrow and a wide hip angle on average cycling power production as well as running off the bike.
The conclusion was that a more open hip angle resulted in higher power and better running economy. A shorter crank will help you achieve this.
Hip angles aside, does changing your crank length effect your power output and cycling efficiency? Yes, but only to a very small degree and even then, positively.
Research shows that the oxygen cost of producing a given power output (efficiency) is only modestly affected by using crank lengths of between 145mm-190mm. So moving from 175mm to 170mm is not going to damage your power output but it will give you more room to optimise your aerodynamics.
So if shorter cranks can yield a better riding position without any discernible loss of power or efficiency, should everyone look at dropping a size or two and are there any drawbacks?
Changing your cranks means buying new ones and this may also necessitate a change in your cassette due to the slight loss of leverage in any gear ratio. Another consideration is that many triathletes own more than one bike, so it might require two or three new cranksets. Not cheap!
Despite the drawbacks, changing your cranks may still be worthwhile though. Your performances will likely be improved because of the potential for improved aerodynamics as well as a slight gain in efficiency. Add these two things together and you might notice a difference in your bike splits. The icing on the cake is that you may also run faster off the bike.
MAKE IT WORK FOR YOU
If you are serious about reducing crank length in order to improve aerodynamics then it’s worth visiting a specialist fitter. They will measure the angle between your shoulder, pelvis and knee when the foot is at the top of the stroke, using different crank lengths. This angle is important to triathletes. Shorter crank lengths should allow you to maintain or increase it, even when rotating your torso forward and down in a more aero position.
Garth Fox is a sports scientist and coach (garthfox.com). He works with world-class and age-group athletes, transferring the latest techniques across endurance sport disciplines.