Does Size Really Matter? I’m Talking About Crank Length!
Does Size Really Matter? I’m Talking About Crank Length!
rank length is a heavily debated subject in bike fitting. I’ve heard people claim that changing a crank length by 2.5mm has had a massive change to their riding. However, there haven’t been many 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 when the cadence was adjusted to maintain the same pedal speed. They did find a slight difference when the cadence was held at a fixed RPM.
This is counter intuitive for a lot of people as they assume that a longer leaver will result in a greater biomechanical advantage to produce torque and thus power. I theorise the reason the power outputs aren’t significantly different between crank lengths when the ride can select an appropriate cadence come down to the relationship between cadence and torque production. Both studies observed that the shorter the crank, the faster the cadence that the rider naturally wants to ride at. So why is this?
I theorise that this is because muscles fibres have a speed that they are most efficient to contract at. This results in the joints (and therefore the pedal) to move at a certain speed through space. 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 through space requiring a faster muscle contraction. Therefore, as the crank is lengthened, the cadence the ride selects slows so that the muscle fibres contract at their most efficient rate. So why is power not significantly affected?
Power is a result of the rotational torque (the twisting force on the crank) times by the speed (the cadence of the pedals or how many times per minute do you apply that torque). A longer crank can produce more torque per push (due to the longer lever) but has less pushes per minute. 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, we should be worried about the crank length. However, there are a few occasions where you might consider a different crank length.
The only time that a long crank is an advantage is when you are forced to ride at a low cadence. The obvious example for this is when you’re on a steep hill and you’re out of gears with no other choice but to lean on it. Therefore, I normally try and get my road riders on as long a crank as they can biomechanically get away with.
I believe that there more reasons that people may want to consider shorter cranks however and If there isn’t going to be a huge loss in power then we should seriously consider it for certain situations. I believe there are 3 reasons to consider shorter cranks:
1) Shorter riders or riders with short legs,
2) Riders that have hip restrictions,
3) Time trial and triathlon
The obvious application is shorter riders or riders who have proportionally shorter legs. Compared to taller riders on the same length crank, a rider with short legs will have to further into hip flexion to get over the top of the pedal stroke. Additionally, the knee will have to flex a lot more over the top of the stroke putting more load on the front of the knee. A shorter 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.”
If a rider is very tight around the front of their hips (hip flexors, joint capsule etc) then the higher that the foot and knee has to travel up to get over the top of the pedal stroke, the further into this restriction they will come. This can cause a number of compensations to occur.
- It often causes the knee to move away from a straight line 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. This can be a cause of knee pain.
- Another common compensation is the rider rocking their hips from side to side as they hit the hip restriction. This can lead to lower back pain.
- Finally, I commonly see riders with reduced hip flexion range roll their hips back to try and cheat extra range at the front. Again, this can cause lower back pain as the lumber is forced in to flexion. It also puts the glutes in a biomechanically poor position, reducing their influence on the pedal stroke.
A shorter crank means that the knee and foot doesn’t have to travel as high to get over the top of the stroke and therefore the rider can be more efficient as they are not hitting the restriction and can get on the power quicker.
Time Trial and Triathlon
During my racing career, I have time trialled on everything from 175mm to 180mm cranks but since I started bike fitting, I have become increasingly convinced that shorter cranks are better for positions where you are wanting to adopt a low aerodynamic position. 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 your hip flexion angle isn’t as acute, you have scope to lower the front end of the bike and keep the same hip angle over the top of the stroke. A lower front end and result in a smaller frontal surface area and should reduce your drag coefficient. 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), it is a good example of how the reduced crank length allows a more aerodynamic position.
For triathlon, this works even better as the hip flexors can work in a more open position and not be as tight when you get off and need to run. This allows better hip extension and greater stride length in the initial part of the run phase.
The time that short cranks may be a slight disadvantage is when accelerating out of corners due to the reduced torque. Therefore, it may not work as well for short distances with tight and technical corners like prologues. Hilly time trial courses may also benefit from a slightly longer crank length.
Things to consider.
There are a few things to consider when changing crank length.
- Saddle height: If you shorten or lengthen the crank length, you need to make a corresponding change to you saddle height as the pedal at the bottom of the stroke will move closer or further away from the saddle. This is why shortening the cranks makes a big difference to the mechanics over the top of the pedal stroke as you gain double the clearance. E.g. when changing from a 170mm crank to a 165mm, you would raise your saddle 5mm. Over the top of the stroke, the pedal comes up 5mm less and your hips are 5mm further away from the pedal. Therefore, you gain 10mm of clearance overall.
- Gearing: If you shorten your cranks, you need to make sure you have an appropriate range of gears to cope with the increase cadence that you’ll want to use. The difference may not be that extreme, but if you regularly find yourself running out of gears on the climbs, it would be advisable to look into some smaller gears (either a compact 50×34 crankset or 11-32 cassette).
So, should you change your crank length?
The cheapest way to find out is to book a bike fit at Primo (http://primocycles.co.uk/bike-fitting-2/). We will recreate your set up on our fitting jig and we can then experiment with everything from 155mm to 185mm. Give us a call on 01223 500502 or email firstname.lastname@example.org to book a session in.