Mountain bike crank length isn’t usually an issue riders worry about, but it can be a major source of extra power and control if you get it right. Stock cranks, usually coming at 175 or 170mm work just fine for most people, but there are benefits to longer or shorter crank lengths depending on a riders physiology and riding style. This article will examine the major benefits and costs of different crank lengths on a mountain bike, so you can decide whether making the change is right for you.
Mountain bike crank length is spec’d based on the bike’s intended use, generally coming in either 175mm and 170mm length. Due to the lower bottom brackets on long-travel bikes like the Norco Range and Aurum, these bikes come with shorter crank arms to lower the risk of crank arm strikes when pedalling through rocky terrain. Shorter travel bikes like the Norco Optic and Revolver come with longer crank arms to increase the riders mechanical advantage and power output. The common range of mountain bike cranks across the industry is 165mm to 180mm (in 5mm increments) giving very tall or very short riders room to play around with their setup if needed.
So, why would you change your crank length? If shorter cranks provide better ground clearance, why would anyone run longer crank arms? Mechanical advantage is what longer crank arms provide, and mechanical advantage is speed. Longer crank arms mean more leverage, and thus more power when pedalling. In testing, it has been shown that this power benefit from longer crank arms drops off sharply after the rider passes 60rpm, but under that threshold it provides an important advantage. While road cyclists spend the majority of their time pedalling above above 60rpm, grunting up steep inclines on a mountain bike rarely sees cadence exceed this number. At these low cadences, the extra mechanical advantage of long crank arms can be capitalized on to produce more power, and thus increase control.
If you do end up changing your mountain bike crank length, there are some other considerations that you’ll want to make, and changes you may need to make to your bike to ensure that it fits the same. Shorter cranks mean that you effectively reach a shorter distance to the bottom of the pedal stroke; your saddle should be raised to make up for this change, and your bars may have to be raised to make up for the change in saddle height. Longer crank arms have the opposite effect; you will likely have to lower your saddle to maintain the same leg extension.
Your local Live to Play Sports dealer can help you choose the ideal cranks for your bike- a balance of strength, weight and cost, in the length that is ideal for your riding style and bike setup. Here are some options to consider.