Bottom brackets may be the most overlooked part of a bike, hidden as they are at the bottom of your frame, surrounded by flashier and more exciting cranks, derailleurs, pedals, and so on. But though upgrading your BB might not provide the flash of new carbon parts or a fast new wheelset, it can be one of the most effective ways to improve your bike's performance. After all, it's not easy being a bottom bracket—few parts of your bike see more use than your humble, hard-working BB.
Bottom brackets are the bearing assemblies that allow your cranksets to spin. Cranksets get a lot of attention for being stiff and light and able to take the abuse of translating the power of your legs into forward momentum, but it's the bottom bracket that really gets the job done, laboring away in relative obscurity in the recesses of your frame. Bottom brackets not only have to handle the same stress, but they have to do it while still spinning smoothly and trying to keep out contaminants. The smoother your bottom bracket, the faster you're going to roll, but too many seals and too much friction will rob precious power from your legs.
Bottom bracket standards can be incredibly confusing—mostly because "standard" is hardly applicable with so much rapid evolution happening in recent history. We'll try to clear some of it up in this article.
Threaded Square Taper Bottom Brackets
Once upon a time, road bike bottom brackets featured English threading or Italian threading, and that was pretty much the only question. The spindle that stuck out of the ends of the bottom bracket was square and slightly tapered (technically, different brands used slightly different taper angles, but the basic concept was the same). The BB was threaded into the bike frame's BB shell (that part of the frame where the down tube, seat tube, and chain stays meet), and the cranks that attached to it had square holes that fit nicely over the spindle. Square taper bottom brackets are far from extinct, but since 2000, manufacturers have been busily experimenting with alternatives as well.
Octalink & ISIS
Shimano was the first company to step outside of the square taper standard, with their Octalink system, featuring a splined spindle (with, you guessed it, eight splines) that paired with specially-designed cranks. These spindles were hollow, lighter than square taper spindles, and stiffer and stronger thanks to a larger diameter compared to square taper spindles.
Shimano's system was patented, so other manufacturers wasted no time in creating a similar, ten-splined "open" (non-patented) standard that they called ISIS (International Splined Interface Standard) Drive. The catch to both of these was less room for the bearings, which had to fit around larger diameter spindles. The bearings were made smaller as a result, which caused no end of premature wear problems.
Hollowtech, Mega-Exo, GXP, & Ultra-Torque
Shimano's solution to the problem of not enough room for the bearings was to move the bearings outside of the BB shell, adding about 11mm to each end of the BB. Plenty of other designs had incorporated external, oversized bearings in the past, but Shimano developed simple cups and had the market share to make the design stick. They called it Hollowtech. Other manufacturers followed Shimano's lead with their own outboard-bearing systems: FSA created Mega-Exo, SRAM/Truvativ called theirs GXP, and Campagnolo's Ultra-Torque system is based on the same idea as well. The larger bearings and spindles were a major improvement in performance, and this technology remains in wide use.
Cannondale chose to remedy the problem not by moving the bearings outside of the BB shell, but by increasing the diameter of the bottom bracket itself—and, resultingly, the diameter of the BB shell. Since they manufactured their own bike frames, BBs, and cranks, they were able to make these changes, and their open BB30 (named after the new bottom bracket spindle diameter, in millimeters) standard has since taken off; it's used to many other companies today. BB30 bottom brackets are still 68mm wide, just like BBs before them all the way back to those oldschool English-threaded square taper BBs we mentioned earlier.
PF30 / PressFit
But there's a big difference between current BB30 designs and earlier systems. Many manufacturers have eliminated threading or the retaining rings that held the bearings of BB30 designs in favor of a press-in system, sometimes referred to as "PF30" or "BB30 PressFit." Much like headset cups, PF30 bearings are held in cups that press directly into the shell. The bearing cartridges press into nylon cups, which in turn press into the frame. The nylon cups have some give in them, allowing for more tolerance in manufacturing and making the use of full carbon BB shells without alloy inserts possible, saving weight. PF30 has largely taken over from BB30 in today's market.
Trek was the next manufacturer to innovate their own solution to the BB problem in search of the highest possible performance. They chose to move past Hollowtech and similar technologies and, rather than keep the bearings outside of the BB shell, decided to just build the frame out over where the bearings would be! They were able to make bike frames with down tubes that expanded out to a bottom bracket shell 22mm wider (11mm on each side) than the 68mm standard. This makes for an incredibly strong frame in the bottom bracket area, plus Trek was able to eliminate some parts. BB90 is a press-in system, with a pair of bearings that are pressed into the frame, and that's it. "90" refers to the new width of the BB shell (68+11+11), in millimeters. The spindle diameter and bearing size is the same as with traditional threaded systems.
The PF92 standard, used by Scott, Giant, Pivot, and other companies, is very similar to BB90 except that it uses PressFit technology, in which the bearings are pressed into composite cups which are then pressed into the BB shell. This allows for slightly less stringent manufacturing tolerances, making this system popular for use with aluminum frames.
BBright is Cervelo's standard. They picked their favorite parts of BB30 and BB90, and used PressFit technology to boot. BBright has the larger-diameter spindle and BB shell of BB30, plus a wider shell width like BB90—but only on the non-drive side (so, 68+11=79mm wide). This allowed Cervelo to use a wider and stronger down tube on their frames, without any effect on their bikes' chainlines.
FSA's BB386 standard is similar to BBright in that it uses a wider 30mm spindle and a wider BB shell—but in this case, the shell is a symmetric 86.5mm wide. In theory, this allows for the stiffest and strongest frame of any other option, and as a bonus, this standard uses the same bearings and cups as PF30 and BBright.
The BB type that your bike uses will affect what kind of crank you can install, though many options are generally available for any given BB, and some manufacturers make adapters to make otherwise incompatible sets work together.
Bottom bracket overhauls and replacement should only be attempted by an experienced mechanic. Improperly installing a bottom bracket can easily ruin a frame. New frames often need to be "faced" before bottom bracket installation; this should only be done by an experienced mechanic with the proper tools. As always, if you have any questions about compatibility or anything else pertaining to your bottom bracket, please contact us at firstname.lastname@example.org or 1-800-651-4050.
One last thing... ceramic bearings
Research has found that ceramic bearings are more efficient and more durable than their steel counterparts. Ceramic bearings have 5-10 times the durability of steel bearings, are 10 times rounder, 20% stiffer and weigh 40% less than steel. Their hardness is much greater than steel's—they literally pulverize contaminants. Choosing a BB with ceramic bearings, or upgrading your current system, can make a big difference in your bike's performance. For more information, check out our article on ceramic bearings.