Does Crank Length Matter?

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Come on….do you really expect me to answer a question like that without at least some sniggering references to “does size matter?”, “longer doesn’t mean better” or maybe “it’s what you do with it that matters”?

Well apparently crank length does matter and it’s a topic of heated and passionate debate amongst more serious cyclists. Maybe I’m just trying to become (big dramatic voice like in movie trailers) “A MORE SERIOUS CYCLIST”, and maybe you are by reading this so let’s actually try to find out why crank length matters and what we should be doing about it.

What is crank length?

Firstly let’s have a quick look at how to measure crank length. It’s essentially the distance on the crank measured from the centre of the bottom bracket spindle to the centre of the pedal spindle and is usually measured in mm. Your cranks may well have the number stamped on the side as well but they are likely to be one of the basic stock lengths of 170mm, 172.5mm or 175mm……..And this, to a certain extent is the point in that there isn’t a vast amount of noticeable difference between a 170mm crank and an 175mm. Cranks are actually available in lengths between 145mm and 195mm although the extremes are extremely rare and expensive – most manufacturers make them in the standard lengths above and you really need to go to a specialist to get anything other than this.

Crank Length and Bike fitting?

It seems that measuring and fitting crank length is sort of the Wild West of bike fitting. We are all very concerned with seat height and handlebar angle and a whole host of other variable factors and it’s now possible to buy an almost infinite number of components to make almost every other aspect of the bike fit us perfectly.

It wasn’t always like that of course and it’s only as manufacturers and technology have developed that this has happened. It seems that offering a variety of different crank arm lengths has slipped behind and it’s one of the very last things that we think about and there isn’t any real choice.

But, it stands to reason that we all have different length legs and different proportions of length between our joints, different flexibility of movements in our leg joints and differing cadence to power abilities so surely it makes sense that the crank length should be measured and fitted as carefully as seat height or reach?

I have actually read somewhere that, after seat hight crank length is the most important bike fitting measurement! Duh! Why can we only get them in such a limited number of very similar lengths then!?

What difference does crank length make?

So, if we are all different sizes then having the correct crank length should, in the first instance help with comfort. It then naturally follows that, if you are more comfortable on the bike then your power output will be higher and more efficient so your overall performance will increase.

Some of the main symptoms of incorrect crank length include pain in the knee, hip or ankle joints and it can also cause lower back pain as well. Rather inconveniently all of these symptoms can be caused by a number of other incorrect bike fit issues so crank length is probably better considered within the overall context of a professional bike fir than just on it’s own.

Crank it up! What’s the right crank length?

It sort of makes sense that, the longer your legs are the longer your cranks should be and there are various charts around like the one below that give you an idea of how long your cranks should be rather than assuming that you have average length legs and will be happy with “standard” length cranks.

6’00” – 177.5mm

5’11” – 175mm

5”10” – 172.5mm

5’9” – 170mm

5’7” – 165mm

5’5” – 160mm

…this isn’t particularly helpful as I’m 6’5” – do they make cranks long enough? At least I can blame my mediocre cycling performance on the cycling industry’s inability to provide me with reasonably priced cranks of the correct length!

Other crank length calculations include 1.25% of your inseam in cm + 65. Another way is to calculate crank length as 9.5% height and even further methods are to measure your cat’s inside leg and multiply it by the number of energy bars you need to fuel you through 20 minutes of cycling with a head wind added to your age divided by 7 – only joking but it seems that there isn’t any real definitive guide as to how to arrive at the best crank length at all.

Crank Call – should I change my cranks?

Changing your cranks could make you cranky so be warned! Obviously the implications are quite serious as changing the crank length will mean that your gearing will change and it will, inevitably, not be inexpensive.

In an ideal world we would all have perfectly matched cranks to our bodies and man or woman and machine would work in perfect harmony. However, at the moment, it’s not easy to get the exact right crank length and there are a multitude of other bike adjustments that we can do much more cheaply and easily.

If you are an incredibly high performing athlete in search of the last ounce of performance from your bike and money is no object then go ahead and look into having your cranks changed. If you are in pain and discomfort and you have discounted every other possible cause then again looking at having your crank length changed would be a good idea.

If you do decide to go for it then it has to be done within the context of a professional bike fit. To get it right you really have to go into the realms of leg angles and proportions and it will become a case of physics and mathematical precision rather than just doing a DIY job and ordering some longer cranks off Ebay and hoping for the best!

So, does crank length matter? Yes, it clearly does. It’s actually quite important but, for the moment at least, for most of us there’s not really all that much we can do about it!

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