I was an early adopter of the concept of singlespeed mountain bikes, starting with my old Hercules, and later, my Bianchi Osprey.
While I liked both of these bikes a lot, neither of them was entirely satisfactory.
The Hercules, dispite very pleasant road riding characteristics was a bit tall for me for off road use, and it's rather low bottom bracket was a bit of problem.
The Osprey was a bit small for me, also a bit heavy, and the vertical dropouts were a somewhat limiting factor.
When Bianchi came out with their B.u.S.S. singlespeed in 1998, I loved it, except for the orange color which would have made the ghosts of all of my mother's ancestors back in County Mayo spin in their graves. Despite the color, I lusted for one of these because they were so light and lively.
In 1999, the B.u.S.S. was surplanted by the B.a.S.S., basically the same only in bright green. This was even more tempting, but I couldn't bring myself to buy a whole bike that expensive, since I already had two singlesped mtbs!
Bianchi made a major marketing goof, however! The orignal B.u.S.S. came in two sizes, 17.5 and 19.5. It was well enough recieved that the B.a.S.S. was offered in 3 sizes, with a 21.5 size being added...but they forgot to tell the catalogue folks, so the catalogue didn't list the 21.5 size!
Fast-forward to fall of 2001, Bianchi still had a bunch of 1999 21.5 B.a.S.S. framesets left over, so the offered them to dealers at a very good closeout price. I bought 10 of them for the shop, and I was the first customer!
I cannibalized most of the parts that had been on the Osprey. I just love this bike!
Lately, however, I've been experimenting with a two-speed fixed-gear setup, using a Shimano cassette hub
with two BMX sprockets and a stack of spacers.
I removed the Freehub body and brazed it solid from the back.
This is a somewhat experimental setup, don't know how durable it will turn out to be in the long term, but I like it a lot as it is. Having both fixed sprockets on the same side of the hub avoids having to remove the rear wheel to change gears.
Removing the wheel from this frame is a particlar hassle because it has those stupid old-fashioned rear-opening forkends, instead of proper horizontal dropouts.
Having two chainrings, with the same 4 tooth difference as the rear sprockets lets me keep the axle in the same place, and provides excellent chain line for both gears.
At first, I was stingy with the brazing of the Freehub body, just tacked it in 3 places. Also, I was too lazy to take it apart and properly degrease it...I just removed the Freehub body from the hub shell and took the torch to its backside. I'm really not an expert brazer, it was definitely a mono-buttocked job.
It worked OK for several hundred miles, then one evening toward the end or a ride, it suddenly started coasting down a steep hill. The brazing had let go.
It's a real tribute to the quality and design of the Shimano Freehub that even though I had cooked it pretty well while brazing it up, the Freehub ratchet mechanism still worked perfectly! I made it home with no problems. Coasting was a little stiff due to the residual brazing material in the Freehub body, but driving was completely reliable.
After this I re-did the brazing job, running a bead of brass all the way around the backside of the Freehub body. This has held up fine, and I don't anticipate any further problems in this area.