Friday, 5 November 2010

Choosing the correct crank length

An often overlooked consideration on road bikes is the length of your crank arms, relative to your leg length.

As a first time rider, you don't even realise there are differing lengths, nor their importance. It's just the thing that turns the chain round and you screw your pedals in to, isn't it?

Most shop bikes come with a 170mm or 175mm crank arm as standard, which if you're over six foot (which I am), 175mm is generally the right length due to your inside leg relative to your height. However, if your legs are a bit shorter, it may well suit you to have a shorter crank arm.

Here's why. When you're new to road cycling, most training manuals point you to a cadence (pedal rotations per minute) of around 85pm, in order to help you build aerobic capacity and reduce muscle fatigue (the Lance Armstrong method pretty much). If you have shorter legs and a long bike crank arm, it can mean your pedalling isn't as efficient as it could be as the overall rotation has to go through a bigger arc, relative to your leg, meaning you may not be getting the most from your muscles.

Now, on your first bike, this won't be a big issue as you're starting out, getting used to things, doing all the learning and things like crank length won't have any noticeable impact on your new hobby.

However, if you start to get addicted to cycling (as I did) and you hanker after your next bike, you want to get faster, look at the science a bit more and get more into things like geometry and fit, you should make sure you ask the question at your local bike shop. Selecting the right crank length for your new bike should be one of your considerations.

I had a body scan, which established the right frame size, optimum crank length etc, however there are plenty of articles on the web about it, like this one if you want to read up about it. I also found this article which offers a crank length formula if you really want to go to town.

Final thought is if you do change your crank arm length, you'll also need to adjust your seat height to compensate. If you're in the North West, come see me and I'll get you all fitted up on your bike.

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