19 Apr 2016


They come as standard sizes on most bikes but different limb and torso lengths mean you might not be riding the most efficient crank for you.

Your crank is possibly the most important contact point with your bike. It’s where your fitness, energy and enthusiasm are channelled in the search of ever-greater speed. It’s also where art eclipses science because finding the perfect crank length for you is a very personal thing. And that’s why we begin by dispelling a myth: that an increase in crank length leads to more power. It doesn’t. Studies have shown that while they change torque, power is unchanged and they simply require the rider to pedal a larger circle. More important is comfort and, with that in mind, there are broad principles that you should adhere to.

Inhibit power
Most modern bicycles come with cranks measuring between 170mm and 175mm depending on frame size. That might be okay, especially when solely gauging the foot in the six o’clock position. However, when it comes to crank length, it’s more important to note what’s happening with the foot at 12 o’clock.

If the crank is too long, for instance, it forces your knees to bend more. This can lead to a kinetic chain of events through to the hip and the lower back that, either instantly or over time, results in pain. It will also impact efficiency because you’ll find it’s difficult to sustain a good power output. It inhibits aerodynamics, too, because you can’t maintain a streamlined position because you’re uncomfortably ‘tucked up’ (see our feature here about finding an ideal aero TT position).

It’s a slightly different and shorter story when it comes to cycling with too short a crank length, as it shouldn’t cause injury and comfort should be okay, but you might find the naturally increased cadence becomes tiring or simply affects enjoyment.

Calculate perfection
That’s why it’s beneficial to calculate the ideal crank length for your size, and why Vision offer the Metron TT BB386Evo //product link// in five sizes from 170mm to 180mm, while the Trimax Compact TT MegaExo come in six sizes from 155m through to 175mm

In stealthy gray Vision’s top-of-the-line Metron crankset has superior aerodynamics and stiffness, and is available in five different crank lengths

If you have the money, you can visit a credible professional bike-fitter who’ll utilise cutting-edge measuring tools, computers and sensors to determine your optimum crank length. That’s great but another financial outlay. Thankfully, there are alternative methods that require the minimal cost of a tape measure and pencil.

Scottish cyclist Graeme Obree who once held the hour record devised one method. Innovator Obree suggests that 9.5% of your height provides your perfect crank length. So if you’re Bora-Argon uber-sprinter Sam Bennett, who uses Vision products and measure 1.78m tall, by Obree’s method Bennett should use 170mm cranks (169.1mm).

Another popular method is to measure the inseam from the top of your thigh (crotch) to the floor. You take this figure in millimetres, divide it by 10, multiply by 1.26 and add 65. So if Bennett’s inseam is a not-unreasonable 830mm and you put it through this equation, his ideal crank length again comes in at 170mm (rounded up from 168.75mm).

So both methods have merits though Obree’s method diverges at the extremes – if you’re very tall or short. As Obree’s method also doesn’t take into account riders who might have long legs and small torso, and vice-versa, we’d recommend the inseam method.

Then again, many professionals choose crank lengths that don’t fit either of these equations but are chosen on feel and experience. It doesn’t affect their speed, stamina or comfort but just remember that many bikes are designed to fit cranks between 170mm and 175mm, so too far either way could negatively impact comfort and handling.
Right, time to get your tape measure out!