Almost everyday we get asked “What crank length should I run?” People tend to over think this topic when it comes to purchasing their mountain bike cranks and it’s actually quite simple. A lot of times you hear of road bikers getting “fitted” and if you do any research about crank arm length for mountain use, there isn’t much out there other than road bike fitting sites/charts. When it comes to mountain bikes, bike crank arm size isn’t nearly as crucial.
The reason behind why road bikers get fitted and crank arm length is a big deal to them is because they are in the saddle for long rides; not saying some mountain bike guys aren't, but when you are doing close to 30+ miles non stop in the saddle, if your bike isn't fitted properly then your body will definitely be telling you! Choosing your mountain cranks is very straightforward and simple. When riding XC you might be a little more concerned about fitment if you find yourself doing 20+ miles with little to no stops, but for trail/enduro riders, and downhill riders, one crank size should work for everyone. Most XC riders will run 175mm crank arms which have greater leverage, and since you are doing more climbing typically than crazy downhill descents, you can get away with longer cranks and not smash them on rocks. Trail/enduro bikes should run 170mm cranks to still keep a good leverage ratio, but also have more ground clearance since the bike will typically have a bit longer travel and might be more likely to smash cranks in certain instances. Downhill/freeride bikes should run no larger than 165mm since pedaling efficiency isn't of huge importance and you have less of a chance to losing a crank to a rock (speaking from personal experience).
Crank sizing is fairly simple and universal:
XC - Possible sizing needed depending on ride lengths, but 175’s are what is recommended for more leverage.
Trail/Enduro - 170mm crank arm length is the most commonly used size and is recommended to keep a good leverage ratio and have a bit more ground clearance due to having more travel.
DH - 165mm crank arm length so you don't lose your crank to those “jumping rocks” and since pedal efficiency isn’t at the top of you list when you are able to take a lift/shuttle or have to hike.
The only time you really need to be sized for your bike or need to be worried about crank arm size is if you are doing long rides in the saddle but since with trail/enduro riding you are constantly in and out of the saddle and getting breaks on the descents, we find 170mm length to be the perfect size.
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