Crank length - is shorter better?

Recently, a lot of my clients have been asking the same question, namely, “should I go to a shorter crank?”

Now, as with most things mankind is involved in, the pendulum of fashion constantly swings from left to right, new discoveries are made, and subject matter is better understood the more time goes by. In cycling, one of the newest “fashions” is to go to shorter cranks. Perhaps I'm being unfair by calling it a fashion – the truth is that a lot of riders could benefit from a shorter crank – however, it's the nature of how people jump on these ideas which makes the theory become a fashion.

When faced with the question “should I move to a shorter crank” my response is always “shorter than what?”? Now that might sound facetious but really it's an important question, as “shorter” is a relative term so by definition can't automatically be defined as better. Should you go to a crank;

- shorter than the crank you have?

- shorter than the “standard” crank length for your size bike?

- shorter than the length historically considered as “ideal”?

- shorter than the crank length as calculated using x2.16 of inseam?

- the length your friend suggested…?

Humans like to talk in absolutes, and when reading about things such as the benefits of shorter cranks they will always assume the principle applies to everyone. It doesn't. Thus, when clients ask me whether they should go to a shorter crank I say “possibly”.

The reality is that the crank lengths available to the public, in relation to the shortest and longest inseams, do not offer riders a consistent leg/crank proportion across the size range. Leg lengths differ in size over more than 25cm whilst standard crank lengths have a size range, longest to shortest, of just 15mm.

So, to understand where we stand, lets do a random, non-scientific calculation based on the assumptions of history, and extrapolate that out to the smallest and largest riders to see what it comes out with.

The most common male leg length is approx 86cm so we'll use that as a starting point. The crank length prescribed to males of that height on the matching size 56 bike, is 172.5mm.

So, our calculation is 86 x 2.006 = 172.5

Now let's take a fairly common inseam for the shortest riders, 74cm, keeping in mind that this would not be the absolutely shortest leg out there. Using the calculation above this rider would require a crank length of 146.4mm. Commonly, cranks offered to this size rider are 170mm, or sometimes 165mm, both patently longer than what the calculation prescribes.

Similarly, going up to a fairly common leg length for very tall riders, 95cm, this rider would require a crank length of 190.5mm as per the calculation. Typically, crank length offered on the largest bikes to these tall riders is 175mm.

It's easy to see where the discrepancies lie though it doesn't solve our problem, should one go to a shorter crank? Hopefully you now understand why I ask “shorter than what?”. Each of the riders above lives with a different reality when it comes to crank length, and thus any proposed benefit to going shorter might apply to them but also might not.

Taller riders are often in a situation where the crank length they have is proportionally short. For them, they would already be in the “benefit zone” if one accepts that shorter cranks are better. Then, as riders get smaller the crank leg increases proportionally to the point where the smallest riders are about as far from the “benefit zone” as one could be. And for them, going to a shorter crank is only going to take them closer to a normal zone of leg/crank proportion rather than to any area of benefit.

A word of caution. I'm not here to argue the benefits of shorter cranks over long ones – there is plenty of material out there doing that - but rather to point out that for some the situation is already good whilst for others going to a shorter crank is absolutely necessary, just to get closer to what many would consider normal. With this in mind, the logical answer to the question of whether to go to a shorter crank is;

A shorter crank is beneficial to those riders who are negatively impacted by the length of the crank they have.

This has to be decided on a case by case basis. The bike fitter and rider have to decide whether a crank is having a negative impact on the pedalling action. Is the rider turning too large a circle? Is there hip flexor impingement at top-dead-centre? And most importantly, is there too much flexion in the knee at top-dead-centre, causing a massive dead spot and less than fluid pedalling, or even excessive shearing force on the knee socket?

Once a rider has settled on a crank length which has no negative effect, this is the point where considering going to an even shorter crank is possible, if that rider is interested in going down the rabbit hole of theories relating to leverage, cadence and how various crank lengths positively and negatively affect the body.

As with a lot of fit related theories, the answer is rarely absolute, and - notwithstanding all the studies which have been done on shorter vs longer cranks – the decision to go to a shorter crank should be considered in your own circumstance and not simply because it's a fashion.

Ronan Descy

 

Ronan DescyComment