My first suggestion for a tripod would be a compact, ultralightight carbon fiber "traveler" style tripod, like the Feisol CT-3441S. Combined with a lightweight ballhead, this will easily support a lightweight 4x5 camera and give you the most versatility.
Originally Posted by Jedidiah Smith
Or, if you're serious about turning your bike into a tripod, you might consider something like this. Although it's shown mounted on the handle bars, you could mount this adapter on any tube of the proper diameter (25.4mm or 31.8mm). If you do mount it on the handle bars, you would need some way to lock the front wheel from swinging side-to-side - perhaps some kind of small clamp.
In any case, there are two possible issues to overcome - working height and keeping the bike from tipping over. Since a bike is shorter than a tripod, you'd need to bend over/crouch down to compose and focus. I'm also not sure if I'd trust my expensive 4x5 camera and lenses to my bicycle's kick stand. I'd probably want something a little sturdier to prop up the bike to keep it from tipping over while being used as a tripod - or perhaps using it upside down would solve this problem with the Feisol Bike Mount attached to the bottom frame tube.
Really Big Cameras