Bottom Bracket Standards
Bicycle drivetrains have been fairly consistent throughout our velo-history. Our feet push on some sort of crank, rotating a ring of some sort to drive a chain (or belt), which then turns a wheel. But the details between our feet and where the rubber meets the road have been changing. This article focuses on the center of the cranks, bottom bracket shells and bearings used to support cranks.
There are now several different crank bearing systems and standards. The different standards do not allow interchangeability between bearings or cranksets. Having some knowledge of the standard you are dealing with will help the mechanic be aware of service options.
Threaded Bottom Bracket Shells
Threaded bottom bracket shells have internal threading. A spindle passes through the frame shell and is held in place by either threaded adaptors (cartridge bearings) or threaded bearing cups (adjustable cup-and-cone type systems). The outside diameter of threaded cups (adaptors) is nominally 35mm (36mm for "Italian" threading), and the inside diameter of frame shells is 34mm (35mm for "Italian"). Shell width can vary from 68mm to 100mm and even wider. Cranksets with varying spindle lengths are manufactured for these different widths.
Threaded internal bottom bracket shell
The most common bottom bracket issue is the bearing cups or bearing adaptors being poorly tightened into the frame. The solution is to remove the cranks and then tighten the bearing adaptor fully into the frame. When the cartridge bearing units wear out, they are simply removed and replaced. The adjustable bearing types can be taken apart, cleaned, greased, reassembled, and adjusted.
Common cartridge bearing for a three-piece crankset
If the threads inside the shell are deformed from manufacturing or have been damaged, some repairs may be possible with a BTS-1 Bottom Bracket Tap set.
Frame shell being tapped and faced with the BTS-1 Bottom Bracket Tap and Facing Set
The BB30 standard was created to allow the use of a crankset with a relatively large, 30mm diameter spindle. This, in turn, meant bigger bearings and bigger frame shells. BB30 frame shells have no internal threads and a nominal inside diameter of 42mm. Frame shell widths are 68mm for road bikes and 73mm for mountain bikes. A cartridge bearing presses directly into the frame and stops at a snap-ring designed into the frame shell.
BB30 frame shell with C-clip for bearing stop
With time and use, all bearings will wear out. BB30 bearings are no exception. Do not remove the bearings from the frame unless you intend to replace them. See the tools and procedures for replacement at BB30 Service.
There can be two press fit complications with the BB30. If the press fit into the frame is too loose, the bearings may move in the shell when stressed by pedaling. This results in a creaking sound. To resolve this, first check the torque on crank bolts to determine if the crank-to-spindle interface is the source of the noise. If this fails and there is still creaking, it may be the cartridge bearing moving in the shell. The bearings should be removed and new ones installed. Press the new bearings into the frame using a retaining compound such as Loctite® RC™ 609. Even a threadlocker would be better than pressing the bearings into the shell dry.
If the press fit is too tight (the shell ID being too small relative to the bearings), the ball bearing in the cartridge will be literally squeezed and ground as you pedal. If you remove the cranks from the bike, turn the bearing inner race by hand — it should feel smooth. If you feel tightness and grinding, it is likely from the press fit being too tight. Do not confuse dirty bearings for this feeling. If the fit is too tight, it can be corrected by reaming. A professional bike shop will use the Park Tool HTR-1 and the 752 BB30 Reamer Set. This will correct the inside diameter by removing material from the shell.
Reaming the BB30 frame shell to correct a tight fit
The BB30 frame shell will accept non-BB30 cranksets, such as the Shimano® Hollowtech® II or SRAM® GXP®, when used with adaptors. These can be simple reducing bushings that slip into the cartridge bearing to reduce the diameter as necessary.
Reducing bushing that permits a 24mm crank to be installed in a BB30 frame
The PF30 standard came after the introduction of the BB30 standard. The shell inside diameter is nominally 46mm. Frame shell width may measure 68mm for road bikes or 73mm for mountain bikes. Cartridge bearings are mounted into either plastic or aluminum adaptors (or assemblies). These are pressed into the frame shell. An outer lip on the adaptor stops against the shell face.
PF30 frame shell
Like the BB30 standard, the PF30 was designed for cranks with a 30mm diameter. Cranksets that are designed for a BB30 will fit the PF30. Similar to the BB30, adaptors are available to permit 24mm spindle diameter cranks to fit frames made in the PF30 standard.
Praxis® bearing system inside a PF30 frame shell
Like the BB30, creaking in the crank system can be an issue. See also PF30 Bottom Bracket Service.
It is a good idea to press the PF30 bearings into the shell with retaining compounds or at least a threadlocker. However, because most of the PF30 bearing adaptors are plastic, the retaining compounds will generally not set up and harden. Use special "Loctite Primers" that allow the compounds to harden.
If the press fit is too tight, it will squeeze the bearing as in a BB30. If the bearings seem to wear prematurely, it may be the result of uneven shell faces (opposite shell faces not square). These can be faced to improve bearing alignment, similar to how headset faces are. The process can only face the outer surfaces of the frame shell. A professional will use the HTR-1 and the 791 Reamer Set. This can improve alignment between the two bearings, resulting in greater smoothness and longer bearing life.
Reaming and facing of the PF30 frame shell with the 791 Reamer Set
BB86, BB92, or BB121
The BB86, BB92, and BB121 standards use a threadless bore of nominally 41mm in the frame shell. The nomenclature of this system lists the nominal shell width as the second number. For road bikes it is 86mm, for mountain bikes it is 92mm, and for "Fat Bikes" it is 121mm. These standards use two-piece cranks with spindle diameters of approximately 24mm. A couple examples are Shimano® Hollowtech® II and SRAM® GXP® cranksets.
The frame shell can be made out of carbon fiber, steel, aluminum, or titanium. The bottom bracket bearings are similar to those used in ZS (zero stack) headsets. Bearing adaptors are pressed into the bore, and a lip on the adaptor stops against the frame shell face.
Bearing assembly and frame shell of a BB86, 92, or BB121 system
The bearings of these systems will eventually wear out and need to be replaced. See the tools and procedures at BB86/BB92 Service. Like other pressed type bearings, they are susceptible to issues with the interference fit. If the press fit is too loose, the bearings may creak under pedaling pressure. To prevent this, use a retaining compound as with a PF30. If the press fit is too tight, it will squeeze the bearing and it may wear out prematurely.
BB90 or BB95
The BB90 and BB95 standards are Trek® Bicycle proprietary standards found on some high-end road and mountain bikes. This system is analogous to the "integrated headset." The cartridge bearings are not mounted in adaptors as with a BB86/BB92/BB121. Instead, as with an integrated headset, the frame is molded to accept the cartridge bearing as a slip fit. The system uses two-piece cranks with 24mm diameter spindles.
BB92 frame shell and cartridge bearings
The BBright® is a proprietary design by the Cervelo® company. This is basically a version of the BB30 or PF30 system. The bottom bracket shell is very wide. However, the shell is not centered to the middle of the bike. The bike features a large left side chainstay, so the shell is offset to the left to accommodate this. The frame shell for this design is 79mm, but 45mm of that sticks out to the left, while 34mm sticks out to the right.
To further complicate the BBright® system, there are two types. BBright® Direct Fit is basically a BB30 type bearing fit, with a shell inside diameter of nominally 42mm. The BBright® Press Fit is a PF30 style, with an inside diameter of nominally 46mm. As a result of the wider frame shells, the crank must be compatible with the BBright® system.
Service issues are similar to the PF30 systems.
The 386 EVO is a version of the PF30 system. The frame shell width is nominally 87mm. The inside diameter is 46mm, similar to the PF30. The pressed bearings are the same as PF30 bearings, but the crank must have a longer spindle compatible with a 386 EVO bottom bracket frame shell.
Service issues of 386 EVO are similar to the PF30 systems.