I have picked up a bike for a friend and checked it all out and it seems fine. It is just a cheap steel MTB style, non suspension, road bike. The problem I have found is that when changing from the middle chain ring to the big chain ring the chain drops down between the two rings. Changing from big to middle is ok and I can change from middle to small and back and from small 'thrown' all the way to big and back. It seems odd that the chain is dropping down the gap between the middle and big rings. What's wrong? Edit to add: The bike was from freecycle so no taking it back. Thank you.