Left hand BMX freewheels are widely listed on eBay. Presumably there are hubs to match. Here's something else though:

I found a picture, but not the explanation that is (or was) out there somewhere.

The great Canadian rider and innovator in a conservative time, Jocelyn Lovell, built and rode a track bike with drive on both sides, to give a two-speed automatic for the kilo ride. As near as I can recall and work out again, this is how it worked: The right side had the lower ratio, implemented with a single freewheel snug on the free side of a fixed/free hub. The left side had the higher ratio, and started each run with a threaded cog set loosely on the hub. As the bike was pedalled, the threaded cog would overrun the hub, pushing it outward (thread is “wrong” on the left) until it hit a reverse-threaded (and maybe jambed?) lockring. Then the higher ratio would simply over-run the freewheel, and drive for the rest of the ride would be on the left. The bigger the ratio difference, the shorter the run in low gear. He had to back-pedal the bike to re-set the gears. Implementation would have been reversed and cleaner if he had a left hand threaded hub. I will gladly stand corrected on any of this if somebody knows otherwise.

In several ways, Jocelyn Lovell was Canada’s own Graham Obree.

oldschoolbike