Originally Posted by Retro Grouch
Their used to be a gizmo (don't remember what it was called) that allowed you to install a freewheel cog onto your crank to give you a dinky 4th chainring. I suspect that would do the same thing with fewer associated problems. How slowly can you ride on an actual trail and not have to dab? If I can't get up it with a 22/32, I'm walking it.
Mountain quad-you install a Suntour freewheel sprocket down to 17T on your crank.
http://abundantadventures.com/quads.html
Hopefully they'll make a Shimano or Campy freehub sprocket compatible version soon.