The effects of crank length on pedalling efficiency.
The effects of crank length on pedalling efficiency.
Published in Rouleur Magazine
Chris McCann from the bike-fitting company Inspired Cycling has worked in the bike fitting trade for 25 years and has helped some of Ireland's most successful cyclists improve their bike positions and pedalling efficiency. He believes that a good bike fit is crucial when it comes to increasing a rider's pedalling efficiency score, arguing that for many, it is an untapped performance advantage.
“Biomechanically, if you take the proportionate difference between somebody's femur, their lower legs, and their tibia and fibula, and then you throw in the next lever, which is foot length, all of a sudden, that can change the dynamics of how a rider wants to pedal." says McCann.
“For example, if you take a female rider who is five foot eight, and you take a man who is five foot eight, they could have similar femur lengths, and similar lower leg length, fib and tib, but the female rider will have a smaller foot. The application of force onto the foot and ankle will have a huge effect on how the female wants to pedal and her efficiency."
McCann explains that his first port of call when a rider comes for a bike fit is to look at their pedalling stroke and ankling range, analysing how much movement there is in each rider's foot. For example, seeing they are pointing their toes predominantly or dropping the heel on the stroke. Measuring feet and adjusting cleat position is the next step in the process.
"Most riders don't have their cleats set that well. People think, is it worth moving my cleat two millimetres? But a couple of millimetres can be a ten per cent difference in their applied force and when l put it like that, they'll take it," explains McCann.
He also argues that having the correct crank length is imperative when it comes to bike fit and pedalling efficiency. "More and more bikes are now coming with 172.5 crank lengths nearly as standard, but if you someone who's five foot eight, with short femurs compared to their fib and tib a small foot, they are going to struggle with their pedalling efficiency right away with cranks that long and have huge dead spots the top or bottom of the pedal stroke.
"If you're using the longer crank, you’ve got a bigger circumference for pedalling so that's going to give you arcs at the top and bottom of your pedal stroke, which become quite big and these arc’s are extremely inefficient during the 360 degrees. We've done some measurements in the past where you can measure that arc in millimetres. If you take the arc length and you apply that over 90 revolutions per minute over a 100 kilometre ride, the distance that you lose, it’s incredible.
Sometimes I will explain to riders that if you have that dead spot, it's like, in every minute of a ride, stopping pedalling for 5 seconds and then starting pedalling again, because that can be what equates to your dead spot. Very quickly, you are losing distance and speed. I think when riders hear that, they then realise that their pedalling efficiency is hugely important to them." McCann explains that he can often see tangible results straight away in a rider's pedalling efficiency after making changes to their position by noticing flatter feet when pedalling and also an increased number of pedal revolutions per minute and torque.
Shorter cranks, maybe this should be referred to as the correct length cranks, allows the rider to lower their torso angle with no negative physiological effect. Hip flexor angles can be eased and this can have a positive effect on the rider's ability to breath easier and as the diaphragm is not as affected, a lower torso angle can be achieved. Some research has shown a decrease in the rider’s torso angle can result in lower blood pressure, and this can have a positive effect on physiology. The correct crank length can also protect riders with knee issues.
In Summary, some benefits of the correct crank length include,
• Improved RPM
• Improvde torque
• Elimination of 'dead spots'
• Better, open hip angles
• Less ankling
• Stable saddle position
• Improved aero position
• Improved force angles on the pedal