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Wednesday, 27 February 2013

Crank Length: The Long And The Short Of It.

Crank length is a heavily debated subject in bike fitting as there hasn't been any definitive studies showing that either longer or shorter is better. In fact, a number of studies have shown that there isn't much difference in maximum power output between a big range of crank lengths:

Martin and Spirduso (2001) tested cranks from 120mm to 220mm and didn’t see a significant difference in maximum power between 145mm and 170mm.

Barratt, Korff, Elmer and Martin (2011) also tested cranks ranging from 150mm to 190mm and didn’t find a significant difference in maximum power output.

I think that this can be explained by the fact that both studies found that the length of the crank affected the cadence that the riders used. The shorter the crank, the faster the cadence. So why is this?

Muscles fibres have a speed that they are most efficient to contract at. This leads to the joints (and therefore the pedal) to move at a certain speed. As the picture above shows, when using the same cadence, a pedal on a longer crank has to travel further than a short crank in the same time and therefore moves faster (requiring a faster muscle contraction). Therefore, as we lengthen the crank, the cadence slows so that the muscle fibres contract at their most efficient rate. So why isn't power significantly affected?

Power is a result of torque (the twisting force on the crank) times speed (the cadence of the pedals). A longer crank can produce more torque (due to the longer lever) but uses a lower cadence. The short cranks make up for the lower torque by using a higher cadence to get near enough the same power output.

So should you be worrying about your crank length?

On the face of it, if there aren't any power gains to be had then the short answer should be no. But I believe that if there isn't much to be lost power wise when dropping a crank length then why shouldn't we consider it for certain situations. I believe there are 3 exceptions to consider:
1) Riders that have severe restrictions in their hips,
2) Shorter riders,
3) Time trial and triathlon

Hip Restrictions

If a rider is very tight around their hips (tight rotator muscles, hamstring, hip flexors, joint capsule etc) then the higher the knee needs to travel up to get over the top of the pedal stroke, the less efficient they will be. This often causes the knee to move away from a straight up and down as the body finds a way around the restriction (either around the outside or rocking the hips to the side). This causes knee traces like the circular pattern above. A shorter crank means that the knee doesn’t have to travel as high to get over the top of the stroke and therefore the rider can get on the power quick.

Shorter Riders

Shorter riders have proportionally shorter legs and therefore will have to further into hip flexion to get over the top of the pedal stroke compared to taller riders on the same length crank. Again, a short crank may be more efficient for a shorter rider and put less strain on the knees as they won’t have to bend them as much. I have had great success putting riders that have saddle heights of 71cm and less on 165mm cranks. Here are a couple of testimonials:


“Up until June 2012 I had previously been riding between 8 and 12 hours a week on 170mm cranks. During this time I had problems with my back and knees and spent painstaking time and money at sports doctors and changing different equipment on the bike to try and rectify the problems. I changed  shoes and had custom footbeds and numerous seats! At the end of 2011 I entered the Haute route Alps Geneva to Nice 780 k and 21,000 metres of climbing 7 timed stages. There would be no hiding from injury and bad fit during this event! I went to see Ben Hallam at Bespoke cycling who used the Retül system and adjusted my cleats and the recommendation of going to 165 mm cranks. I am not the tallest of men and was a little sceptical about a mere 5mm difference in crank length. Well size does matter after fitting 165mm Rotor cranks with standard round rings, the difference was remarkable my average cadence on my usual loops round Kent were up between 7 and 10 rpm and my pedal stroke was noticeably smoother and my knee problems eradicated. It is so noticeable how much easier it is to spin the pedals and push earlier in the stroke.”


“I just wanted to let you know that your advice to change from the 170mm down to the 165mm cranks has really worked for me. I made the change in August and have noticed how I pedal much quicker and more efficiently now. I used to like pushing a harder gear, but have since realised that just focusing on keeping my legs spinning fast to maintain speed saves my legs getting tired.

Last year I did my first 25 mile Cycletta event in 1 hour 53 mins and I did two 25 mile Cyclettas, one in September and the other in October, both in just over 1hr30 mins. I attribute those faster times to being able to maintain my speed easier and climb easier with the 165cranks.”

Time Trial and Triathlon

During my career, I time trialled on everything from 175mm to 180mm cranks but last year started experimenting with shorter cranks. I'm currently using 170mm (I’m 6 feet 1) and I’m considering trying going shorter than that. The theory behind using shorter cranks is as follows: Using a shorter crank means that you don’t have to come as high with your knees (into hip flexion) to get over the top of the pedal stroke. If you hip flexion angle isn't as acute, you can more comfortably lower the front end of the bike and keep the same hip angle over the top of the stroke. A lower front end and resultant flatter back should reduce your drag coefficient and if your power doesn’t significantly drop with the shorter cranks, the end result is that you go faster. The picture above is of Courtney Ogden. While this is taking things to the extreme (115mm cranks) is a good example of how the reduced crank length allows a more aerodynamic position. The time that short cranks may be a disadvantage is accelerating out of corners due to the reduced torque. Therefore may not work as well for short distance, tight and technical sprints or prologues.

Are shorter cranks for you? 

The cheapest way to find out is to book a follow up fit at Bespoke. We will recreate your set up on our Retül Müve jig and we can then experiment with everything from 155mm to 185mm. Give us a call to book a session in.


Nick W said...

Chalk another success up Ben, moving me to 165mm cranks has had a significant impact on performance over long rides, with knee pain no longer evident and I believe a smoother pedal stroke. A definite convert here!


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