I have been having problems with my front derailler. the chain keeps rubbing and it wont change in and out of gears smoothly. After struggling with this problem for quite some time, i took it to my local bike shop, where i was told that derailler had come the end of its useful life and i should replace it. So they took off my old Diore derailler and tryed to change it with a new Diore LX. however the new diore LX derailler wouldnt fit and in the end my they put the old broken one back on my bike. They told me that the diore LX derailler was incompatible with my bike and because the bike was so old i wouldn't be able to find a new one that would fit. I'm not sure exactly how old my bike it, as i bought it second hand, but it has a biopace chainring, so i am guessing the it was made in the 80s. Does anyone know why new deraillers would be incompatible with my bike? is it because of the biopace chainring?