Ben - you've gone about this all wrong mate. I take it the fiance got a nice chunck of expensive glass on her finger after you proposed??
You should have dropped a few seeds into her mind about having your new bike on her finger. This gets the girl understanding what a big sacrifice, (commitment - sorry darling!!), you are making. Then when they're still gooey eyed and realising how amazing you are you start dropping the "wheres my engagement present?" questions and before you know it, (12 weeks!!), you're in the bike shop and she thinks its her idea!!!