Scuba tanks
Hey, you could use compressed air as your spring.Scuba tanks are a lot lighter now than when I was fooling with them; they are aluminum, and can hold maybe a most of a cubic foot at 3000 psi? I wouldn't be surprised to find out that a jacked up scuba tank-maybe 4000 psi- could propel a bike several miles(maybe by having it turn a turbine that is hooked to a chainwheel).
Luck,
Charlie