You notice a distinct clicking sound about four hours into your ride. By the time you stop for a cookie at Milt’s, you’ve already ruled out all the normal culprits and narrowed the noise to the bottom portion of your bike, sort of in the middle, but no, it’s not the pedals or the crank. “Uh-oh, I think it’s your bottom bracket. You better get her into the shop,” your cycling buddy says.

What is it about the bottom bracket that freaks us all out? Sends us running to the shop without even bending down to investigate? The main reason seems to be that there is no one bottom bracket standard, despite attempts by several brands in recent years. Besides that, well frankly, the bottom bracket is a bit complicated – one of those rarely explored “internal” sections of your bike. So let’s get down to the bottom of your bottom bracket.

The bottom bracket is a steel or ceramic cylinder, you can think of it as an axle or a spindle, that threads to your crankset, and allows your crank arms to rotate. Don’t see it? Take the crankset off, and you’ll expose the bottom bracket, nestled neatly inside your frame on the drivetrain side, in what’s known as the bottom bracket shell. Note that the bottom bracket shell is the place on your frame where the seat tube, down tube, and stays all meet. A critical place, for sure – your bottom bracket experiences more torque than any other part of the bike. All that stress is the reason bottom brackets wear out quickly, and also the reason why a smoothly running bottom bracket is essential for bike efficiency.

Bottom brackets have evolved over the years. For a while, the reigning type was the cartridge due to a greater ease in servicing and replacing than prior versions, which involved more moving parts. The cartridge-style bottom bracket is only two pieces: the cylinder that sits in the bottom bracket shell on the drivetrain side and attaches to your crankset, and a cup made of alloy or plastic that supports the whole sha-bang from the non-drive side. There’s also a seal, to close off the bottom bracket from any grime kicked up from your tires. The cartridge concept made replacing the bottom bracket as simple as removing your crankset and the bottom bracket seal, removing the cylinder from the bottom bracket shell, and popping in a new one.

The latest innovation in bottom bracket design is outboard bearings set-ups, introduced by Shimano in 2004 with their Hollowtech crank system. The Hollowtech represented a further simplification in bottom bracket design – essentially cups inside the bottom bracket shell, with bearings held outside the frame. This design resulted in significant performance gains – bottom brackets became stiffer when the bearings were moved outside the frame because it provided a wider platform. And since the bearings were outside, they could be made larger, which increased their durability. At the same time, the bottom brackets became lighter because there was no more cylinder – they were essentially hollow.

Campagnolo came on board in 2007 with their Ultra-Torque outboard bearing system. Today, SRAM's Red, Force, and Rival cranksets are all designed with outboard bottom brackets. While this is the closest thing we’ve ever had to a true standard, don’t mix and match your bottom bracket and crankset. Mechanics will always recommend running a crankset with the recommended bottom bracket due to slight variations among manufacturers.

So now you know what’s down there. Don’t let it go to your head.

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