you cant do anything about it except get a good new bike. this used to happen to me when I had a crappy bike, just because it had oittle ground clearence. if youre running a department store bike right now I suggest you go to the bike shop and get at least a 400 dollar bike, It is unsafe to ride those things on trails. I have never had this problem with my 800 dollar diamond back response comp, even my dads 300 dollar giant boulder does not do this.
Well... peter griffan is a top contributer so i guess he knows what hes talking about but I stand by my anwser, i have never had this problem with my bike, and i have never heard of crank stretch, but dont google it.