This guy has been selling this setup for quite a while now, since maybe 1995. He is local to me (St Louis area) and the local newspaper did a story on him way back when. At one point for a brief time some local bike shops had MTBs in that came fitted with this setup, on custom frames & forks. They used a rear wheel on the front, and the drive cord connected to a gearwheel on the front [rear] wheel's freewheel mechanism. This way the front wheel could spin faster than the rear so it would turn properly, but the front wheel would never spin slower than the rear.
I didn't buy one but did get to test-ride one around in a muddy back parking lot. With a normal bike if you rode slowly into a big area of mud (trying to hold a constant speed) the back tire would begin to spin faster and faster and your forward momentum would slow down, until you stopped in place and had to get off and walk out. With the 2WD bike, you didn't slow down. You would slow down slightly and then the front wheel would start to spin and you just kept moving forward as long as you kept pedaling.
It definitely did work, as someone who rode it I can tell you that much. But you need to be in very slippery conditions to benefit from it. I'm not sure it would help much on clean pavement or even most casual riding at all, but it could be a big help for riding in mud, or snow and ice.
I have not seen the current offering--or any offering since then, really--so can't comment on it.
Back then the design had two major problems: first was that the drive cord spun all the time, and had quite a bit of drag from that. The second was that the bicycle was basically a wal-mart quality bike worth around $200, but they had to charge around $350 or so (there were maybe three different models at different price levels). Also I recall at that time MTB suspension was just becoming common on bikes at the same price levels, and these bikes that I saw were totally un-suspended. The bolt-on kit solves at least one of those problems.
There was also general durability and parts concerns (the drive gears are exposed on both ends) but if parts are available separately then that may not be a problem now--especially if you used it for a snow bike, since snow isn't going to grind away the plastic gears nearly as fast as mud/sand would.
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