Hi, I finally purchased a road bike. A mid 80's nishiki landau. I replaced the rear shifter cable and did not adjust the derailleur properly. It got caught in the spokes and is now fubar. The bike is a 12 speed, which derailleur do I need? The long, medium or short cage? As I understand, I can use a derailleur for a cassette with more sprockets, correct? The one that was on there was either long or medium, judging by the pictures at jenson. Thanks for any help.