To be ready for initial prototype production, I've been finalising the last few parts of the chassis design; last week's seat, this weeks adjustable bottom bracket assembly, and the detail of the steering assembly next week. It is all coming together well, but where problems do arise, it's invariably where the velomobile parts meet the bicycle parts. All the tradition, mixed standards and mixed metric and imperial measurements is a lot to get your head around -- Sheldon Brown's pages have been invaluable though, because I do have a finished assembly…
I was quite pleased to see that bottom brackets come as sealed bearing units that can be inserted into a threaded tube of the correct internal diameter. The only complication so far is that the one side is a left-hand thread and the other is right-hand; not a problem so far, this stops the bracket coming loose during use because of precession.
Next I find that there are at least 5 different diameters of brackets (British/ISO, Italian, French, Swiss, Raleigh) that are not interchangeable; not too much of problem, because the most common is British/ISO. However, then there are 3 (or more) different spindle lengths for this bracket; 68mm seems to be the sensible choice, so I now know the length of the bottom bracket tube.
So, what should the inner diameter of the bottom bracket mount be, so it can be tapped for the internal threads? Well, the tap size is available: 1.375" (1 3/8")and 24 tpi (threads per inch), but what size drill does this thread need?
This is definitely where more experience with cycle engineering would help. The best I can figure is that it needs a 1 5/16" drill (~33.3mm) to make a hole, to take the 1-3/8-24 tap, on a 68mm long bar. If that's correct, then an outer diameter of 37mm should be sufficient for the bottom bracket holder. Is this anywhere near? Do all bike builders carry a left/right pair of these taps, or are bottom bracket holders standard parts?
In any case, here's the completed assembly. The bottom bracket has 220mm adjustment front to back, so it should take leg lengths for riders from 5' to 6'2", with the same seat position.