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@gbj_tester
Sorry for the delay, but just reviewing this and I cannot let it pass. Royce BBs have been made with many things in mind, but stiffness has never been at the top of their agenda. For a start, nobody concerned with maximal stiffness would ever use a titanium BB axle, nor would they persist for long with small ID bearings mounted a long way inboard inside the BB shell. To make the stiffest possible internal cup square taper BB (taking it as read that you are so constrained), you place the bearings as far apart as possible, use steel for the axle, and use the biggest bearing you can squeeze inside the standard shell. It is possible, and I know because I have one, to make a square taper BB which fits in a normal 68mm BSC shell with only the usual thickness of cup flange, with 20mm ID bearings spaced 68mm apart centre to centre. It's a bugger to assemble, because you have to carefully shim the axle between crank and bearing to control the end float, but you do end up with a BB axle at least twice as stiff in torsion as the 17mm diameter Royce Ti one, and also much stiffer in flexion due to both the larger diameter and the support being closer to the applied load.