It seems as if the number of bottom bracket standards for road bikes has expanded exponentially over the past decade. The traditional threaded design has been replaced by larger threadless diameters that proponents claim allow for both stiffer and lighter frames and cranks. CTech Editor Matt Wikstrom takes a look at the many bottom bracket standards out there now and considers what the crowded market means for consumers.
When I started working as a mechanic in the mid-90s there were essentially two types of bottom brackets for road bikes: English- or Italian-threaded. The only other thing that varied was the length and offset of the crank axle.
Two things stand out from that time: first, servicing a bottom bracket was labour-intensive; and second, nobody ever seemed to complain about clicking or creaking. Indeed, it wasn’t uncommon for owners to carry on riding their bikes until the bearings quietly carved all the way through the cups and the whole assembly collapsed.
Shimano’s cartridge bottom bracket system ushered in the era of creaking and clicking. The bearing cartridge proved to be very robust but they started to creak or click with use.
The reasons for this are many, including the mating of alloy retainer cups with steel cartridges/bearings along with the move towards massive aluminium and carbon tube diameters for the frame. As a consequence, bottom brackets generally required more attention than the previous designs.
Time passed and a few different axle designs were introduced (e.g. Octalink and Isis) but it wasn’t until Shimano debuted its HollowTech II cranks in 2003 that there was a significant shift in thinking. The new cranks dumped the traditional three-piece configuration for two pieces as the axle was incorporated into the right-hand crank. At the same time, the bearings were re-positioned outside the bottom bracket shell.
A two-piece configuration essentially underpins contemporary crank design now. Weight and stiffness may have been improved but for me the real revolution has been the serviceability of this design. Unfortunately, modern cranks are still susceptible to the dreaded click/creak, but at least they are much quicker and easier to break down, clean, and reinstall.
The first of the new bottom bracket standards arrived in 2000 when Cannondale unveiled its BB30 design at the Tour de France. The new design was distinguished by an oversized threadless bottom bracket for the frame, mated to an oversized 30mm axle for the cranks. Rather than patent and protect the design, the company offered it openly to all frame manufacturers to encourage its adoption by the industry.
There are now at least eight threadless standards for road bikes: BB30, PressFit 30, BB86, BB90, OSBB, BBright Direct, BBright PressFit, and BB386EVO. The new standards have a larger diameter than conventional threaded bottom brackets (Figure 1) but they also vary in width (Figure 2).
The following table provides an overview of the technical specifications for the new standards in comparison with traditional threaded standards:
*Adaptors are required to fit cranks with 24mm axles and/or alternative bottom bracket standards
The new standards
The most daunting aspect of the new standards is that there are so many to contend with. Along with the eight road bike standards mentioned above, there are few more that are MTB-specific (e.g. BB92 and BB95 designed for wider bottom bracket shells) along with proprietary designs (e.g. Wilier’s BB94 and Look’s oversized bottom bracket for its one-piece Zed 2 cranks, as seen in the feature image above).
Then there are the inevitable inconsistencies in nomenclature (e.g. Shimano’s BB86-compatible bottom bracket is designated BB91) which only add to the confusion.
But wait, there’s more.
There’s no consistency in the diameter or the width of the new standards (see Figures 1 and 2 above), and strictly speaking, there is very little cross-compatibility. Thus, a frame with a BB30 bottom bracket must be matched with BB30 bearings and a BB30 crankset.
Easy done, unless you want to use the cranks in your new Dura Ace groupset, which have a 24mm spindle rather than 30mm demanded by the BB30 system.
Fortunately, the industry has responded quickly and a variety of adaptors are available to solve many incompatibilities. For example, see products made by Praxis, Problem Solvers, Wheels Manufacturing, and FSA. However, some incompatibilities remain, as noted in the table above.
I’ve found that alloy adaptor cups work well while nylon cups and reducers are more likely to start creaking and clicking. Ideally, buyers should match the cranks to the frame’s bottom bracket so as to maximise the advantages of a given standard and avoid adaptors.
If in doubt, or if you’re overwhelmed by all the new standards, enlist the help of an experienced mechanic.
Is there any value in the new standards?
Delve into any discussion of the new standards and you’ll quickly understand that there was nothing wrong with the traditional threaded design. Indeed, the BSA standard remains one of the most versatile. However, it does have two limitations that are worth addressing.
First, there is a limit to the diameter of the crank axle that can be accommodated by a 34.8mm BSA bottom bracket. Cannondale created BB30 to accommodate a 30mm crank axle for its Hollowgram cranks and now more companies are embracing oversized crank axles (e.g. FSA’s BB386EVO and Campagnolo’s OverTorque both utilise 30mm axles).
I expect crank axle diameters will increase again in the years to come, aided by current (or new) threadless bottom bracket standards. Interestingly, FSA and Campagnolo both offer BSA-threaded cups for their new 30mm axles.
Second, not all frame materials are suited for a threaded bottom bracket. As carbon has emerged as the material of choice for road bike manufacture, so too has the utility of threadless bottom bracket design.
Some consider the new standards as nothing more than a cost-cutting measure, however it makes sense to mould bearing seats into the bottom bracket of a carbon frame when the material is so poorly suited to cutting threads.
The new threadless standards are not perfect though. BB30 and PressFit bottom brackets have a reputation for creaking, there are problems associated with the use of adaptors (as noted above), and some of the oversized bearings appear to suffer accelerated wear.
The dreaded creak and click of bottom brackets was already commonplace for threaded bottom brackets, but the incidence may be on the rise because manufacturers are failing to satisfy the strict tolerances for the new threadless standards. Then there is use of materials such as aluminium for the crank axle (to compensate for the extra mass of oversized axle) that are more likely to creak once exposed to the elements.
Bottom bracket standards have evolved into a new commodity for the marketplace, yet there is no clear winner amongst the current competition. BB30 offers enormous versatility while the wider standards such as BB86 may be more robust.
Further iteration will refine the current design, but perhaps the industry needs another shift in thinking (and a whole new batch of standards) before it is perfected.