Exzellenter Artikel aus Bike Radar über die verschiedenen, heute gebräuchlichen Typen von Innenlagern bei Rennrädern. Auch bei diesem mechanisch relativ simplen Teil, zeigt sich die Tendenz weg von Industriestandards und weg von der Möglichkeit ohne viel Aufwand einfach selber damit zu basteln. Einen ähnlichen Beitrag würde ich mir auch über Steuersätze wünschen.
Here’s a rundown of each of the major BB systems currently available, along with advantages, disadvantages, notes on compatibility, and some input from their respective proponents on why they exist:
- Conventional threaded
- BB386 EVO
- Changing between bottom bracket setups
- Why are there so many standards?
Modern threaded bottom brackets aren’t exactly a new invention. In fact, their roots date back more than two decades to Craig Edwards‘ old Sweet Wings cranks and Alex Pong’s groundbreaking Magic Motorcycle.
The idea is simple: by moving the bearings outboard of the shell, you can then use a much larger spindle. Currently, 24mm is the accepted norm compared to square taper’s relatively miniscule 17mm diameter.
Shimano popularized the concept as Hollowtech II, with the introduction of the XTR M950 group in 2003 – even now it’s still, arguably, the gold standard. In fact, companies such as Santa Cruz continue to stand by it despite the plethora of other options.
Canyon stuck with a threaded shell on its Ultimate CF SLX road frame until last year
„Threaded bottom brackets are compatible with the largest range of cranks [and] some press-in bottom brackets have a reputation for creaking,“ Santa Cruz global marketing manager Will Ockelton told BikeRadar.
„Press-in bottom brackets require special tools, which most consumers don’t have [and] they can only be installed a few times before it’s recommended that you replace the bearings [or cups – ed.]. Most of our bikes come with ISCG tabs, but for the few that don’t a thread-in bottom bracket still allows for chain guide mounting.“
„A threaded bottom bracket is great because it isolates the bearing from the frameset,“ said Matt Harvey, president of bearing, seal, and bottom bracket maker Enduro.
„Because you have aluminum threads and surfaces, you can face the shell with a cutter and make it parallel if it is not. The bearing bores are usually round already, with aluminum cups because it is a machined surface, and if the bearing bores are too tight you can sand them out and get the fit just right for the bearings before you thread them into the frame. You cannot use a facing or cutting tool that I know of in a carbon shell.“
- Pros: Easy at-home service; interface surfaces can often be machined; huge selection of parts and accessories
- Cons: Heavier than press-fit systems; frame design limited by relatively small-diameter and narrow shell
- Crank designs that will fit: Standard 24mm external type (Shimano Hollowtech II, SRAM/Truvativ GXP, Campagnolo Ultra-Torque, FSA MegaExo, Race Face X-Type, etc), square taper, ISIS, BB386 EVO
Trek introduced BB90 in 2007, on the revamped Madone, and soon followed with the mountain bike-specific BB95. Instead of having cartridge bearings that were pressed into aluminum cups that were then threaded into the frame, Trek eliminated the middle man and pressed the bare bearings directly into the frame.
Bearing seats were molded into the carbon fiber frame structure, to save weight, and bottom bracket shell width ballooned to 90mm on road bikes and 95mm on mountain bikes, creating more real estate for frame tubes and suspension pivots.
BB90 uses the same bearings and bearing locations as a conventional threaded system
„The most significant [advantage] is a stiffer frame,“ said Trek mountain bike product manager John Riley. „Secondarily, BB90/95 offers a lighter overall solution by replacing traditional alloy bearing cups with integrated carbon bearing bores. It also provides a simplified installation process.“
Aside from omitting the cups, the BB90/95 setup is functionally identical to conventional threaded bottom brackets, in that it uses the same bearings and the same bearing locations as before. Crankset compatibility is thus unchanged, although you do lose the ability to run chain guides that sandwich between the driveside cup and frame.
- Pros: Lighter than conventional setups but retains wide range of compatibility; creates a wider BB shell without affecting crank width
- Cons: Can’t use cup-mounted chain guides; shell is wider but still small in diameter; requires a separate bearing puller and press for service
- Crank designs that will fit: Standard 24mm external-type only
Very closely related to Trek’s BB90/95 system is the PF86/92 standard used by Scott, Giant, Pivot, and many others. Once again, the bearings and their locations in space are identical to those of conventional threaded bottom brackets, only they’re mounted in small composite cups before being pressed into the frame.
The advantages are similar to those for Trek’s BB90/95: a wider bottom bracket shell that doesn’t otherwise affect crank width, plus lighter weight compared to threaded aluminum cups.
From a manufacturing standpoint, PF86/92’s dimensional tolerances aren’t as stringent as BB90/95’s, and it’s easier to adapt the system into aluminum frames. As long as you have a concentric bore of the right diameter, parallel faces, and a proper shell width, you’re good to go.
PF86’s wider shell allows the chain stays to be pushed further apart
„Because of this technology we’re able to improve pedaling stiffness some 10 percent on some of our frames, over traditional threaded bottom brackets,“ said Giant global marketing director Andrew Juskaitis. „[PF86/92 is] less subject to tolerances, too, and is able to take up +/- 0.5mm of [shell width] discrepancy.“
Scott US marketing and PR director Adrian Montgomery touts similar reasons for his company choosing BB86/92 over other standards:
„The most significant advantage is that the frame manufacturer can widen the bottom bracket shell shell without having to adhere to the old threaded bottom bracket standard. It also costs less to do a press-fit bottom bracket in regards to manufacturing. Disadvantages? None that I can see.“
- Pros: Same as BB90/95
- Cons: Same as BB90/95
- Crank designs that will fit: Standard 24mm external-type, BB386 EVO
Several companies have dabbled in alternative bottom bracket standards over the years (Gary Fisher, Klein, and Merlin, to name just three) but it wasn’t until Cannondale released its then-proprietary BB30 system as an open standard in 2006 that something actually stuck.
BB30’s bearing cartridges press directly into a precision-fit shell, as with Trek’s BB90/95 system – albeit with the additional aid of two pairs of snaprings. But here it’s the shell and spindle diameters that increase in size, not the shell width.
The key benefits here are lighter weights owing to the larger, 30mm-diameter aluminum spindle and the elimination of separate bearing cups. Plus, because the shell is still just 68mm wide, the cranks can also be made to provide more heel clearance – or, depending on the crank design, even a narrower stance width (commonly referred to as ‚Q factor‘).
Proper BB30 setups require extensive and careful machining inside the shell
„The idea was that, since Cannondale had control of both the frame and crank manufacturing, we could use system integration to design a superior solution, even if that meant not relying on a traditional industry standard,“ said Cannondale vice president of R&D, Chris Peck. „[Advantages are] a lighter and stiffer crank with the 30mm alloy spindle; more room in the bottom bracket area to attach larger down tubes, seat tubes, and chain stays; and very low bottom bracket weight.“
„[Our] Hollowgram SiSL2 crank with SpideRing weighs 484g. By comparison, new Dura-Ace 9000 weighs around 678g [we recorded 637g in our review – ed]. In addition to weight, another BB30 advantage is stiffness. By increasing the spindle diameter, you increase its resistance to twisting, which is where the bulk of the deflection in a crankset comes from. Roughly 70 percent of total deflection measured at the pedal comes from axle twist.“
BB30 shells, however, do require more stringent dimensional tolerances to work well – meaning it’s more expensive to manufacture. Moreover, with the notable exception of Felt, who uses a carbon tube, BB30 also usually employs an aluminum shell insert that has to be bonded or co-molded into an otherwise carbon fiber frame.
- Pros: Lighter weight; more resistant to axle twist; more heel clearance; larger-diameter shell and spindle; potential for narrower pedal stance width
- Cons: Shell still just 68mm wide; metal-on-metal interfaces can be prone to creaking; requires a separate bearing puller and press for service; requires high manufacturing tolerances; bearing seats can’t easily be faced post-manufacturing
- Crank designs that will fit: BB30, BB386 EVO, standard 24mm external-type, Specialized OSBB, BBright
SRAM addressed BB30’s stringent dimensional tolerances and their associated manufacturing costs with the advent of PressFit 30 in 2009. Just as with PF86/92, PF30 is analogous to BB30 in that the bearings and bearing locations are identical, but with the addition of intermediary composite cups that allow for looser frame tolerances. Not surprisingly, then, PF30 has gained widespread traction in the industry, particularly with smaller manufacturers who can’t afford high reject rates.
„I think it’s more about the simplicity of the manufacturing process,“ said Drew Guldalian of Pennsylvania-based custom builder Engin Cycles. „Boring out a [standard] BB30 is not impossible, but in my opinion it’s an unrealistic task from a manufacturer’s standpoint that needs to do something to make money.
„With the PressFit 30, they’re using the same technology with the larger bearings and the same technology with the bearings living inside the frame, but they’re utilizing the nylon to their advantage where you have a more realistic tolerance.“
„PressFit 30 fundamentally provides all the most significant advantages of BB30 but requires much less precise machining of the bottom bracket shell area, as the bearings come inserted into plastic cups that are easier to fit into the frame and less delicate than standalone bearing,“ added SRAM drivetrain product manager Chris Hilton. „PF30 bottom brackets also have an integrated sealing system, which helps prolong bearing life.“
Cannondale’s Chris Peck provided BikeRadar with specific details. While BB30’s bearing bore tolerances are 41.96 +0.025/-0mm (meaning the bore can be slightly oversized but not undersized), PF30’s requirements are 46.00 +0/-0.05mm – exactly half as rigorous.
Specialized’s OSBB system is nearly identical to BB/PF30 but with a few key differences, according to the company’s director of advanced research, Chris D’Aluisio.
The Specialized OSBB system
„The 30mm bearing is a good choice when using thinwall aluminum spindles,“ he said, „and the narrow bearing stance (relative to outboard bearings) works perfectly with our carbon arm design needing to make the turn into the spindle. The OSBB is basically the same as BB30 but with a little tighter tolerances and a 0.5mm difference in the clip-to-clip dimension. Cranks of both systems are compatible with each other.“
D’Aluisio even argues that OSBB offers alignment advantages with threaded shells:
„The tolerances are not too different but the way they’re measured is. They both have the same needs for concentricity and placement relative to the frame. The threads are at a disadvantage in that they need to be machined from both sides and have a hard time sharing a centerline, whereas our OSBB is machined from one side all at once – and round holes are easy to measure.“
- Pros: Same as BB30 but with lower costs, easier assembly, and the potential for additional bearing seals integrated into the cup/bearing assembly
- Cons: Standard-width shell; requires a separate cup remover and press for service
- Crank designs that will fit: BB30, BB386 EVO, standard 24mm external-type, Specialized OSBB, BBright
Up until now, all these bottom bracket fitments incorporate bearings that are symmetrically positioned about the frame’s centerline. Cervélo tipped that apple cart in 2010, however, with the introduction of BBright.
BBright shares its 30mm-diameter aluminum spindle, cartridge bearings, and driveside bearing location with PF30 – in fact, the bottom bracket cups are identical to PF30.
However, the non-driveside bearing is pushed outward by 11mm – the same as conventional threaded, BB90, and PF86 bottom brackets. This makes it something of a hybrid between the two widely accepted systems.
Cervélo introduced BBright on the R5ca
„BBright uses the ~11mm space occupied by an external bottom bracket’s left bearing cup to let the designer widen the frame tubes that connect to the bottom bracket shell: down tube, left chain stay and seat tube,“ said Cervélo race engineer Damon Rinard. „All these can be 11mm wider now. In the case of the chain stay, that’s nearly double the width at an important and highly loaded point – the junction with the bottom bracket – which has a several-fold beneficial effect on stiffness.“
„The resulting increase in moment of inertia has gained us so much stiffness and strength that the left chain stay weighs less than the right, despite being stiffer and stronger,“ he continued. „Because stiffening either chain stay stiffens the frame, the net result is a substantial increase in the frame’s bottom bracket stiffness, together with a small reduction in frame weight.“
Rinard insists this revised bearing spacing comes with no functional downsides, either:
„This performance boost comes with no other changes to basic dimensions: the left bearing stays in the same spot; the aluminum cup is simply replaced with carbon frame now. And since the bearing’s in the same spot, the lateral position of the left crank isn’t changed at all, so Q-factor is also unchanged. Your feet are in exactly the same place as before.“
From the outset, Cervélo introduced BBright as an open standard that anyone could jump onto, and while it seems to make good engineering sense, other bike companies have been slow to come on board. Currently, only Open – the mountain bike company started by Cervélo co-founder Gerard Vroomen – uses the system in addition to Cervélo.
- Pros: Stiffer than BB/PF30, in theory, without impacting other attributes; good crank compatibility, cheaper to manufacture than threaded or BB30
- Cons: Questionable longevity; requires a separate cup remover and press for service
- Crank designs that will fit: BBright, BB386 EVO, standard 24mm external-type
BB386 EVO is the newest bottom bracket of all those covered here, and it takes the oversized principle to extremes. The bearings have the same nominal spacing, width-wise, as conventional threaded, BB90/95, and PF86/92 formats, but it uses BB/PF30’s bigger 30mm diameter spindle.
As stiffness is directly tied to cross-sectional size it’s no surprise that, theoretically, BB386 EVO has the most to offer, with both the added shell width and diameter that many frame-makers yearn for.
In addition, it uses the same bearings and cups as PF30 and BBright and, thus, the same manufacturing tolerances and required tools.
BB386 EVO uses the same cups as PressFit 30
„[BB386 EVO] allows us to go wider, which in turn opens up big opportunities in terms of stiffness and strength in the bottom bracket/chain stay area,“ said BH president and CEO Chris Cocalis (Cocalis holds the same position with Pivot Cycles, whose frames are built around the more mountain bike-friendly PF92 format). „With bigger cross-sections driving stiffness, we also have more flexibility in being able to tune ride quality with the carbon layup.“
„For us, there are no disadvantages versus a threaded design or a standard BB30. We are able to have a full carbon shell with no metal bonded into the frame, which makes for a stronger, lighter frame structure.
„With our manufacturing methods, we also have no issues holding tolerances. [BB386 EVO] wasn’t designed as a cost reduction, but not having to post bond an aluminum shell into the frame and make sure the facing and threads are correct is slightly less expensive and much more reliable.“
Crank compatibility is another BB386 EVO strongpoint. Despite the bulbous dimensions, the shell will accept most modern crankset fitments save for dedicated BB30/PF30 and BBright designs.
„The BB386 EVO cranks are compatible with almost every BB shell standard (BSA, BB30, PF30, BB90),“ said Jason Miles of FSA. „We currently make bottom brackets and adaptors for BSA, BB30, and PF30 frames and are working on BB90 bottom brackets. Frames with BB386 EVO shells should be able to run almost any crank. We currently make adaptors to fit Shimano, Campagnolo, and SRAM 24mm spindle cranks.“
While the surrounding frames should be stiffer than those built around narrower BB/PF30 or threaded shells, Miles admits that the BB386 EVO crankset actually takes a slight step back in that regard, but only when viewed independently of the entire system. The straighter arms also have less ankle and heel clearance than those narrower formats.
„Because the crank arms are straighter, we can make them lighter then BB30 ones,“ he said. „But because the BB386 EVO spindles are longer than BB30 ones, they aren’t as stiff. Total stiffness is not a measure of crank stiffness alone, though. When you factor in the stiffness gains from a wider shell, the overall stiffness will be highest when BB386 EVO frames meet BB386 EVO cranks.“
- Pros: Theoretically the best in terms of frame stiffness; reduced manufacturing costs; generous crankset versatility
- Cons: Heel clearance isn’t as good as BB30; requires a separate cup remover and press for service
- Crank designs that will fit: BB386 EVO, standard 24mm external-type
Am I stuck with what I’ve got forever?
As you’ve likely noticed in the footnotes above, most of the bottom bracket standards offer a fairly wide range of options in terms of crankset choice. And, not surprisingly, several companies are at the ready to help you make those changes.
Numerous bottom bracket adaptors are available
Changing between setups typically requires some type of adapter. Or, in ideal cases, bottom bracket bearing assemblies that directly convert from one system to another, such as Praxis Works‘ outstanding BB/PF30-to-24mm assembly.
In general, though, the guideline is the same: try to minimize the number of parts involved. More parts added on top of each other results in greater tolerance stack-ups, plus more interfaces that could potentially creak.
Why are there so many standards?
There’s nothing wrong with threaded bottom brackets, per se, and it’s worth noting that none of the proponents of alternative systems cast threaded systems as being fatally flawed. They do, however, note the system’s inherent limitations in terms of frame design, which is why more companies – particularly ones striving to be on the cutting edge – continue to embrace non-threaded formats.
How companies decide to go about that – and precisely what design features they want – is the real issue, because everyone seems to have their own ideas on how to achieve the same goals (less weight, more stiffness, and lower manufacturing costs).
„The explosion of bottom bracket standards reflects the various design objectives and manufacturing competencies of individual frame and component brands,“ said Eric Doyne, US PR representative for Shimano. „Shimano sees and values the importance for consumers to be able to easily source a frequent consumption article like the bottom bracket.“
„The explosion of standards is not good for consumers or dealers,“ Doyne continued. „The bottom line is that bike companies are driving the standards. Threaded systems have very few limitations – they are easily cleaned and repaired at the shop level, and they do not affect the bearing fit.“
In concept, at least, all of the standards we’ve covered are functionally sound. But the realities of relaxing tolerances can create other issues. For example, save for Shimano, Rotor, Specialized, and some FSA models that use adjustable effective crank arm-to-crank arm distance, most of these alternative systems require wave washers or compressible spacers to account for inherent shell width variability (Magic Motorcycle used precision micro-shims back in the day, and some third parties offer similar systems for modern cranksets).
The BB standard boom has required the introduction of new tools to go with them
Things work alright if the installation is done correctly. But, if not, you could still end up with lateral play under load (possibly resulting in dropped chains) or bearings that are too heavily preloaded (decreasing bearing lifespan).
Weather-sealing can be a problem, too. Integrated cup and bearing systems at least have the potential for good barriers against dirt and moisture, depending on the design, but bottom bracket standards that rely solely on bearings pressed directly into the frame sometimes have to make do with the typically inadequate seals on the bearing cartridges themselves.
Some designs even offer discipline-specific advantages. For example, BB30 offers the narrowest potential Q factor, while wider shells allow for greater chain stay spacing, for more tire clearance on mountain bike hardtails and cyclocross frames.
Finally, there are aesthetic considerations. Road companies often gravitate towards BB30 for the sleek, narrow form factor, while smaller builders who deal primarily in steel might prefer the more proportional look of a threaded shell. Carbon companies looking to maximize their stiffness (or stiffness-to-weight) figures generally just want to go bigger in some way.
As to which standard is ‚best‘, the answer is undeniably straightforward: it depends.
Zum Schluss noch ein Gedanke zum Thema Torsion: Alte Stahlachsen waren aus 17 mm Durchmesser Vollstahl, neue Achsen sind auf 30mm Aluminiumprofilen. Wie dünn kann so ein Aluprofil sein, wenn es die gleiche Torsion hat? Quick and dirty:
Der Verdrehungswinkel einer Torsion läßt sich bestimmen als (T x L) / (G x I), wobei T das Torsionsmoment ist (bei beiden Achsen gleich), L die Länge der Achse angibt (gleich angenommen) und G das Schubmodul (Stahl 79,3 GPa und Aluminium 25,5 GPa) und das polare Flächenträgheitsmoment eines Kreisquerschnitts ist: Pi x (R**4-r**4) /4.
Also, wenn Verdrehungswinkel, Torsionsmoment und Stablänge gleich sind, dann gilt:
79,3 x 17**4 = 25,5 x (30**4 – r**4), oder nach r aufgelöst
((25,5 x 30**4 – 79,3 x 17**4)/25,5)**0,25 = r
r wäre dann 27,29 mm, oder anders gesagt, die Dicke der Aluminimumwandung der Achse müßte etwa 2,71 mm sein. Nicht schlecht, theoretisch.