BB3106 Bottom Bracket.
My BB3106 Bottom Bracket.

A New Bottom Bracket Standard

If there's one thing I would change about the Stooge MK5, it's the eccentric bottom bracket. With a 1x12 drivetrain, I simply don't need it. I acknowledge that there are benefits to having one. For one, the MK5 can be set up single-speed or geared. As such, it has a wider appeal, and should I ever part with it, the circle of potential buyers would be considerably larger. My experiences with the two EBB-equipped bikes I've owned, however, were more bad than good. My Niner One9, for instance, had a setscrew EBB. The setscrews quickly ate themselves into the EBB, and the BB shell ovalized over time, leaving a wide gap between the shell and the EBB. The gap filled with dirt, and the whole setup was creaking more often than not. My Air9 Carbon came with Niner's own Bio-Centric bottom bracket, a design that clamps to the outside faces of the EBB shell. In theory, this seemed great. Machine a precise cylindrical BB shell with parallel outside faces and clamp two eccentric cups against those faces. In practice, it was pretty shitty, to say the least. For one, machining tolerances were such that a Bio-Centric cup would more easily turn on one side of the shell than on the other. But much worse was that Niner designed large cutouts into the shell so that shifter cables could be routed internally. This dramatically weakened the shell to the point that the proper tightening torque of the Bio-Centric was enough to crush it. It was an extremely troublesome design that creaked a lot as well.

So, one could think that I wouldn't touch a bike with an eccentric BB with a ten-foot pole, but when you're looking for a rigid, non-suspension corrected bike, the options are pretty small. And, I love every other aspect of the MK5. Besides, it's steel and should therefore have a much stronger BB shell. The BB shells of the two Niners I owned were both aluminum. The ovalizing seen on the One9 should not happen to a steel shell, IMHO. Since I'm not running this bike single-speed, there's no need for an eccentric BB. So I thought, why not design my own CBB (concentric bottom bracket) and simplify this central part of the bike in the process? The standard setup requires the EBB insert, two setscrews, two external bottom bracket cups, and depending on your crankset axle, various amounts of spacers. Designing my own CBB, I can get rid of the BB cups by using a CBB insert that holds two bearings. Design it to the exact width of the Rotor Boost axle, and I can get rid of the axle spacers, too. To avoid munching up the CBB shell, I'm using two stainless flat-faced ball-ended setscrews, which press onto replaceable brass shims. Being concentric and manufactured to a tighter tolerance, I should also be able to tighten the setscrews with less torque. Riding the hell out of it will tell if the design is any good.

BB3016 CAD Drawing.

Since the bottom bracket is a design of my own, it doesn't follow any current bike standards. If you will, it's a new standard that I developed specifically for Rotor Kapic cranks with Boost axle mounted to a Stooge MK5 frame. It will not fit any other cranks. Every new standard needs a designation. CBB30 was a thought. Or referencing axle diameter and BB width, such as BB386 or BB392 do, it could be called BB3106. BB3106, that's right. Remember that you saw it here first!