How much assistance do pullup bands provide?

Marie asked this question below, and since it's a very good question I did some research. It turns out the answer's not a simple one, which sort of makes sense given the variable nature of rubber resistance/assistance.

Basically, the idea here is that the more stretched-out the band is, the more help you're getting. Say once the band is looped over your pullup bar it hangs 30 inches below the bar (as is the case with mine). That means once I hoist myself up to where I'm only 30" below the bar, I'm no longer getting help from the band. But that's okay, because once my muscles are in that almost-fully contracted position I no longer need the help.

Now, suppose I put a couple 25-lb dumbbells in the loop of the band, and one of them falls out and lands on my toe. That means I have to hop around and yell and curse while holding my injured foot. It also means I have to replace the dumbbell in the loop of the band so I can find out how much the band needs to be stretched out to give me 50 lbs of assistance. As it happens, the two weights together pull the band down another 30 inches or so, meaning that that when I'm 5 feet below the bar I'm getting about 50 lbs of assistance. Since that in fact is about my start position, at least when I do my assisted pullups with one knee in the band as opposed to two feet in the band, my work is done.

Here's a link to a thread on the Crossfit board that may explain it better than I did. (Note that they leave out the part about the dumbbell falling out of the band. That's because they're guys and afraid of looking stupid. But I'm pretty sure it happened to them, just as it did to me.)

http://board.crossfit.com/showthread.php?t=28095