Cranks are a vital contact point with your bike, and judging by the specificity with which you can select their length, it would seem there should be a clean cut way of choosing the right one. How else could you differentiate between 172.5 and 175 millimetre cranks? Currently this topic is about as clear as a rainy day, but the good news is that you don’t need to worry too much about it.
In almost all circumstances, your choice of crank length will not put you at a mechanical advantage or disadvantage.
Crank arms can vary between 162.5 and 177.5 millimetres long, but generally come standard on bikes as 170 to 175mm, in 2.5mm increments. A common myth among cyclists is that if you run longer cranks, you’ve essentially got a longer lever, and therefore you can push more power out at the same cadence. Whilst this does hold true for single speeds, the advent of gears renders this point moot, and therefore your crank length does not equate to your power output. In fact, in almost all circumstances, your choice of crank length will not put you at a mechanical advantage or disadvantage.
Choosing a crank you like the look of can be just as important as its length.
Where crank length is important is in comfort, in particular for shorter riders. The most common issue with crank lengths is caused when a shorter rider runs cranks that are too long. This is due to an error whilst setting your saddle height. The most common method of setting saddle height is by sitting on your saddle and putting your leg at the six o’clock position. Your saddle is at the right height if your leg is at a slightly sub-maximal extension. The issue with this method is that you inadvertently compensate for cranks that are too long by lowering your saddle height accordingly; only when your leg is raised to the 12 o’clock position will you notice any issues. Initially, the position may feel fine, but over time you may develop back problems, or if you use a power meter, you will notice a dead spot in your pedal stroke, as you are unable to exert maximal power in an overly contracted position.
Crank length is generally only an issue on road bikes, with very few mountain bikers even considering the issue.
So how do you know what crank length is right for you, and if you should change the cranks you have? If you’re five foot tall running 177.5 mm cranks with no issues, then stick with them; most riders wouldn’t even notice a change in their crank length even if they weren’t initially using an ideal ratio. But for those having back or knee issues that can’t be rectified by a shorter stem or a better fitting saddle, you might want to try changing your cranks. There are a few rules for getting the right crank length for your height. The first and simplest method is that your cranks should be 9.5% of your body height, or, if you’re wanting a more specific fit, measure your inseam in centimetres, multiply that number by 1.25, and add 65. The result is your recommended crank length in mm. These two methods do give different results, especially for shorter heights, but the second method, known also as the Machine Head method, is generally seen as more accurate.
Method 1: Crank length (in cm) = Rider height in cm x 0.095.
Method 2: Crank length (in mm) = Inseam in cm x 1.25 + 65
By rounding this to the closest standard size, you will get an idea as to what length you should be running.
There are also indications for changing your cranks based on your riding style or your physiology. If you’re a track cyclist, or looking to get into the discipline, you should run shorter cranks than you do on the road. This will allow you to corner more aggressively on the velodrome, with the extra clearance awarded by a shorter crank. This benefit will outweigh any extra power you gain from a longer ‘lever.’ If you’re looking for the ultimate marginal gain, there is a theory which states that if you have a greater composition of fast twitch muscle fibres, i.e. if you’re a naturally good sprinter, you should run a longer crank, as you have to move your leg further, and therefore faster, to keep up the same cadence, whereas a natural climber should run a slightly shorter crank length. However, this theory yet to be proven, so you probably shouldn’t buy a new crank set off that postulate.
If you’re five foot tall running 177.5 mm cranks with no issues, then stick with them; most riders wouldn’t even notice a change in their crank length.
If you’re having back or knee issues, or you just don’t think your crank length is right, be sure to get a proper bike fit – it’s the best value-for-money upgrade there is.