A few months back I went out on my SL trail riding South Ca. I swapped the front two tires to knobby, but the back tire? Ouch... Just wouldn't fit. Well I spent a few days sliding around ( back road tire spins in sandy skreed ). Anyway, I sent a message to TerraTrike hoping for dropouts big enough to place a knobby on the rear tire. Turns out that such a beast hasn't been developed.
I talked to a friend of mine who owns a metal manufacturing facility. He's willing to make the dropouts for me. Its just as easy ( and cheeper ) to make a hundred of them. Let me know if any of you are interested.