What are Crank & Bottom Brackets on Mountain Bike?

Series Summary

The sport of mountain biking has been a growing craze for adventurers since the 1970's. According to the Mountain Biking Hall of Fame, mountain biking began, among a few other places, in Marin County, CA when Gary Fischer and a few other friends updated a few old Schwinn bikes and started riding them down Mt. Tamalpais. After that, they went on to start a mountain bike business that was eventually bought by Trek. Since then, mountain biking has become an important part of outdoor recreation culture and knowing how to maintain a mountain bike is a great tool to have.

In this free video series, watch as professional mountain biker Mickey Denoncourt teaches different mountain biking maintenance and repair. Learn what the crank and bottom brackets on a mountain bike are, how to install and remove a square taper crank, how to install and remove the ISIS, how install and remove external ball bearings, the function of a bolt circle BCD, how to apply the chain guide, and how to attach the chain on a mountain bike. Thanks to the experts at Expert Village, mountain bike maintenance and repair has never been easier. So watch these videos and be prepared for anything.

Video Transcript

"Today's mountain bikes tend to use three different types of interface systems for bottom brackets and crank arms. As the level of riding increases, people are going faster, pitting larger jumps and gaps and just generally being more abusive, what we're seeing is a change away from the traditional square taper interface, which is your square spindle and your square taper that just sort of pulls on and is held on by a bolt, to progressively more heavy-duty systems. These systems, while being stronger, they also tend to be lighter and stiffer for the same application. So it's a pretty darn good little continuum that we're moving across. One of the second most common type of crank interface that we see right now is called the ISIS crank interface, it's a splined interface. Instead of just this four-sided square taper, we have a six-sided taper, it's less tapered, there's more splines, so there's more engagement and more surface area. It's a lot, lot stronger. You're less likely to break ISIS tapers than a square taper. Look, the square taper just swims around inside of ISIS. ISIS has been popular now for about 7 years. It's pretty much getting eclipsed by external bottom bracket systems, which are a two-piece setup. Your two-piece setup is your external, X-type, Octalink 2. Different manufacturers call it different things, but what it is you have one crank arm. And it's your crank arm and then your spindle, that thing. You've gone from that to that - that's a big difference. And then your bottom bracket, instead of being housed inside a shell on the frame, like it was with ISIS and like it was with square taper - as you can see, there's some threaded cups. This guy lives inside of there. So now with these new two-piece external-type systems, your bottom bracket cup screws into the frame like that, and your crank arm slides through there. So you've got wider bearing placement, you've got a larger shaft, and with proper design and engineering, you get a really strong, rigid setup that's going to last a lot longer."