FR - I think Ive seen every airhead variant used as a basis for a bike in this thread. Most of those wouldve come with an 18 rear, stock. It only gets tricky when you start with a paralever if you want an 18 wheel. It would seem that a twin shock model might give you a greater range of options for longer suspension in the rear, but its do-able regardless. Just a matter of what youve got to start with and what parts you can get your hands on with the budget you have. Find a bike and start from there. Also bear in mind that not everything you see here will function as well as it looks. That said, airheads have fairly long travel suspension as stock bikes, and people have been scrambling them since well before someone decided to call it that.