Hambini SRAM Dub Bottom Brackets
For the past few months I have been working with NTN and one of my racing teams in developing a bottom bracket suitable for SRAM dub cranksets. SRAM's Dub is their current bottom bracket standard which uses a 29mm (28.99mm) axle with a bearing landing spacing of ~90mm. The 29mm bore diameter is non standard and no tier 1 bearing manufacturer makes a bearing with a bore of this size.
In order to accommodate the axle a thin delrin insert was required. This had the advantage of prolonging crank life because the hardened bearing did not come into contact with the relatively soft crank axle. The technical difficulty was manufacturing an insert of only 0.5mm wall thickness at a nominal diameter of 30mm. My competitors were using blow moulded nylon inserts but I found the tolerances on these to be quite poor. Instead I elected to machine the adaptors in delrin. New jigs were manufactured to hold the workpiece.
A systemic problem in machining any object that is thin is deflection. An analogy would be akin to cutting a loaf of bread. As the knife starts to cut the bread, it compresses before breaking the surface and the cutting commences. As the cutting begins, the bread springs back. The same thing happens when machining thin wall metal, it is even more pronounced on thin walled plastic. The workpiece has to be supported internally to prevent it collapsing. Tool angles must also be carefully controlled to prevent the swarf coiling
This bottom bracket has been trialled for the past few months in intercontinental racing conditions. Feedback has been good and no problems have been reported. Once the manufacturing methods have been finalised it will go on general sale, it will be avaialble in racing specification (both black and silver)