When owning an older bicycle or you’re using it to its fullest, you’re getting closer and closer to the moment of upgrade, or necessary replacement of components, especially in what regards the drivetrain parts. And with the bicycle industry loving standards, this being also the reason why it has so many of them, sometimes it’s hard to figure out what exactly do you need in order to match your bicycle.
Mainly, upgrade possibilities depend on the actual contact point of the components, more exactly, where and how do they fit on the frame. So, you can expect some headaches from the rear axle, head tube, brake mounting points and bottom bracket, but today we are going to discuss the latter issue. Bottom brackets can be split in two large categories: threaded and press-in. Further on, frames compatible with threaded bottom brackets are of several types:
Most of the bicycles that currently exit the manufacturing unit’s gates use an ISO/BSA bottom bracket standard, in which is inserted either a threaded cartridge, either Hollowtech/GXP cups. Therefore, when buying a new crankset, you’ll have to make sure that the diameter, width, and cups/cartridge type matches the thread and diameter of the bottom bracket shell.
The mountain bike segment currently features several ISO cartridges (square, octalink, etc), but also several threaded cups (Hollowtech II/GXP/MegaEXO/X-Type). While cartridges can be used only with the same mounting system of the crank arms, the threaded cups are intercompatible, as long as the width and diameter of the axle are the same. This is why Hollowtech II is compatible with MegaEXO or Raceface X-Type, and all other cups with the identical features.
Frames having an ISIS bottom bracket shell can become ISO cup/cartridge-compatible using a series of adaptors, which are simply screwed in place, that literally transform ISIS into an ISO/BSA 68mm-wide bottom bracket. However, you should note that 100mm ISIS systems cannot become fully compatible with the ISO standard for the simple fact that the latter one is narrower (68mm vs. 100mm).
Of course, besides the earlier metioned thread types, there are other standards as well, but they are rare nowadays, and hardly employed. In recent years, the trend shifted to using more press-in bearings, either straight into the frame, either in cups attached to the bottom bracket shell. These standards are usually derived from BB30, and include the likes of SRAM PressFit 30, FSA PF30, 386EVO, Specialized Carbon, etc. They also feature their very own cranksets, but are in the same time compatible with other cranksets found on the market, even if it means using adaptors or spacers.