Bottom bracket systems

BB92 - Bottom bracket Shimano Press Fit System

BB92 – Bottom bracket Shimano Press Fit System for mountain bikes

The BB92 bottom bracket standard, also known as the “Shimano Press Fit” guarantees greater efficiency when pedalling due to more rigidity in the bottom bracket segment, e.g. when pedalling out of the saddle. It is based on the 24 mm Shimano standard crankshaft.

The BB92 involves a simple, circular press-fit connection without any complicated threaded solutions, which is suitable for hydroformed aluminium frames and above all carbon frames. In the case of carbon frames, this enables cemented aluminium inserts to be eliminated in the bottom bracket, as the bearing shells are pressed directly into the carbon bottom bracket housing. In design terms, the bottom bracket width of 92 mm permits a broader connection for the bottom tube and chainstays to the bottom bracket shell, which improves the bending and torsional strength of the frame.

New axle standard for wheels

The new axle standards of Boost 148 on the rear wheel and Boost 110 on the front significantly enhance stiffness for 29” and 27.5” wheels.

New axle standards of Boost 148

Boost 148 rear wheel

The Boost 148 design creates tyre clearance and permits chainstays of maximum shortness while using standard 27.5 tyres. At the same time, however, it is also possible to use the additional design space gained for 27.5+ tyres.  

Boost 148 designates the dimension on the rear axle with an installation width of 148 mm. Compared to the most common axle standard of 135/142 mm installation width, the flanges of the hub move outwards 3 mm on the left and right. This creates an improved, shallower spoke angle (= spoke array) and, as a consequence, greater wheel stiffness. Parallel to the flange of the hub, the brake disc mount also moves out by 3 mm, making new designs for the frame/swing arms necessary. Similarly the chain line on the right-hand side is relocated 3 mm to the outside. As a result cranks with special Boost chainrings are required or, in the case of e-MTBs adapted spider/chainrings (direct mounting).  

Example in the X1 All Mountain: The outward shift of the chain line enables us to build a short chainstay of 422.5 mm with standard tyres 27.5“ (max. 27.5x2.4); our MDI III dropouts additionally make it possible to integrate significantly broader 27.5+ (27.5+ x 2.8) tyres of the new plus-size generation, which have a larger outside diameter. On the frame, the pivot points of the main swing arm bearing can also be moved out further to increase stiffness.  

Boost 110 front wheel

On the front wheel too, a larger space between the flanges of the hub produces a distinct improvement in wheel stiffness. In order to create space for the plus-size generation of tyres, the hub flange has been moved out by 5 mm on each side, with the axle shaft diameter of 15 mm having been adopted. As is also the case with the new Boost 148 frame/rear swing arm, the suspension forks have to be completely redesigned as the position of the brake discs has also moved outwards by 5 mm.

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