Shaft Drive

[The Kalmyk]

I thought this was pretty off beat... This drive system wouldn't stand a chance in the woods...The animation show the guts of the design

http://www.dynamicbicycles.com/?gclid=CIPO6rTphp4CFYJx5Qod3mgToA

 

[pinkshirtphotos]

It's popular in Asia as a town bike. I think ridemonkey had a pretty good thead on this.

 

[mtn_goat]

There is a company making 2 wheel drive bikes using a shaft with a ring and pinion gear setup similar, and it is very durable, I know someone that has one and it is pretty good. Check out this site:
http://www.christinibicycles.com/
They also do 2 wheel drive motorcycles, so the system is pretty tough actually....