on a serious note though, seeing as i both cycle and drive rigs that would be tempting to draft for some (tractors and trailers) i feel i have enough experience now to say that i really don't recommend it! one that put a shiver down my spine was a honda car sticking close to my rig that day consisting of a small 65hp tractor towing a 6.5 ton tipping trailer (i would estimate my total weight that day to be about 6-8 ton and the honda must have weighed no more than 2 ton so i weighed 3-4 times what it did) and i had to quickly brake sharply and stop, the honda driver woke up darn quick and when he nearly hit my big steel rear bumper on the trailer!
i can imagine that could have easily ended worse, and it might not be so comfortable if a cyclist were to be drafting and hit my trailer full of muck, or if they were to be hanging on for a lift they might have ended up head first in my load of muck!
lesson is is pisses the driver of the large vehicle off, is dangerous and could end up with you head first in a load of cow shoot!
Cheers Ed