Originally Posted by Bianchigirll

what kind of derailluer is it? is this a nonindexing derailluer? there is no spring under the 'nut'? without knowing a bit more about the derailluer I am going to say tou need it. that nut screws on to the pivot (mounting) bolt and some sets or stops in a certain spot. then when you attach the derailluer to a hanger it acts as the stop to allow the derailluer to pivot fore and aft when changing gears. the derailluer may mount on the frame without it but it may not pivot and if it does it may come loose.