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Bicycle Frame Building Components

    

Bottom Bracket Standards

There are now a wide variety of bottom bracket shells available in the bicycle industry, making "industry standards" a thing of the past.  The following article will help clarify which bottom bracket shells we manufacture so that you can choose the right one for your bottom bracket assembly.

A Brief History

The bottom bracket originated around 100 years ago in Europe during a time when taxes and the high cost of transport limited European trade.  Italian frame builders sold most of their goods in Italy, the French concentrated on French customers, and the British made bikes for their home market.  From the beginning the result was a hodgepodge of design specifications, which continues today.

Overview of a Bottom Bracket Assembly

The bottom bracket assembly on a bicycle connects the crankset to the bicycle and allows the crankset to rotate freely.  It contains a spindle that the crankset attaches to and the bearings that allow the spindle and cranks to rotate. The chain rings and pedals attach to the cranks.  The bottom bracket assembly fits inside the bottom bracket shell which connects to the seat tube, down tube, and chain stays.

 

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Bottom Bracket Shells

Threaded Bottom Bracket Shells

Threaded bottom brackets have the advantage of being compatible with the largest range of cranks.  Traditionally they have two bearing cups that screw into the frame, a right hand (fixed) cup, and a left side cup (adjustable).  There are three common types of threading for bottom bracket shells:  English, Italian and French.  All of our threaded bottom bracket shells are English style, meaning they are 1.370" in diameter, have 24 threads per inch with a thread length of 0.50”, and are right and left threaded (also known as British Standard, ISO/English or BSA); making them compatible with most modern threaded bottom bracket spindles.  Our threaded BBs are available in titanium, steel, and stainless steel in varying widths, and are 0.5 mm oversize to allow for finishing after welding.

Threadless Bottom Bracket Shells

BB30:  Cannondale released its BB30 system as an open standard in 2006.  BB30 bearing cartridges press directly into a precision-fit shell, and stop at removable snap rings that mate with two snap ring grooves in the shell.  The key benefits to this system are that it is lighter than a threaded assembly because of the elimination of separate bearing cups, and stiffer due to its 30 mm diameter aluminum spindle.  Additional benefits include more room in the bottom bracket area to attach larger down tubes, seat tubes, and chain stays; and because the shell and spindle increase in diameter and not width, the cranks can be made to provide more heel clearance.  Our BB30 compatible shells are available in titanium, steel, stainless steel, and aluminum in 68 (road) and 73 (mountain) mm widths.  The bearing bore is supplied undersized, 1.640"/1.643" (~41.7 mm), and must be finished to size by the frame builder after welding; the bearings have a corresponding OD of 42 mm.

PressFit 30:  The downside of the BB30 system is that it requires more stringent dimensional tolerances to work well, resulting in higher manufacturing costs.  SRAM addressed these issues with the introduction of the PressFit 30 (PF30) bottom bracket standard in 2009.   Both the PF30 and BB30 use the same 30 mm ID bearings and 30 mm spindle cranksets; the difference is that PF30 bearings are contained inside cups and the cups are pressed into the frame.  The shell has a smooth bore due to the lack of snap ring grooves; the outer lip of the cup contacting the frame is what stops the press action.  According to Chris Hilton, drive train product manager for SRAM:

"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 a stand-alone bearing.   PF30 bottom brackets also have an integrated sealing system, which helps prolong bearing life."

The large ID of the PF30 bottom bracket shell gives it the further advantage of being adaptable to a wide variety of bottom brackets assemblies, including eccentrics.  We make our PF30 shells in titanium, steel, stainless steel, and aluminum in a variety of lengths and widths; the ID is supplied .5 mm undersized, to 45.5 mm (1.791"), and must be finished to diameter and width after welding by the frame builder.

41 mm Press Fit/Shimano Press Fit (BB86/BB92):  These bottom bracket assemblies are identical to the PF30 except that the OD of the bearing is 41 mm instead of 42 mm.  We have recently begun manufacturing these in titanium and steel in will offer them in 88, 91, and 106 mm lengths which allows for facing after welding.

BBRight:  Up until 2010, bottom brackets incorporated bearings that were symmetrically positioned around the frame's centerline. Cervélo changed that in 2010 with the introduction of the asymmetric BBright bottom bracket system.  BBright and PF30 share the same 30 mm diameter aluminum spindle, cartridge bearings, drive side bearing location, and bottom bracket cups.  However, the non-driveside bearing for the BBRight is pushed outward by 11mm.   This allows the designer to widen the frame tubes that connect to the bottom bracket shell, making it stronger and stiffer.  Our titanium BB0030 measures 2” OD x 80 mm wide, providing 1 mm for facing, and isa compatible with the BBRight system.

Why all the alternatives?

The demand for alternatives to threaded bottom bracket shells grew out of a desire to provide a greater variety of frame sytles that are lighter, stiffer, and cost less to manufacture.  With today’s emphasis on cutting edge technology and innovation, new standards are the norm rather than the exception.  Our decision to manufacture a given standard will be based on its anticipated applications and longevity in the market.

References

To the Point:  Understanding Bottom Brackets

Bike Radar:  Complete Guide to Bottom Brackets

Problem Solvers:  Bottom Bracket Standards