VANSON Regular Tremolo Spring for Fender Stratocaster Type Tremolos, Floyd Rose etc.
Click on image for fullscreen view and other options
VANSON High Quality Stainless Steel Tremolo Spring/s.
VANSON sell Tremolo springs in 3 degrees of stiffness; STRONG: For heavy whammy bar abuse, dive-bombs etc. REGULAR: Typical strength found on most stock 'Strat' type guitars WEAK: For those looking for a more sensitive set-up.
There's no set rules regarding the above, it just gives you a few more options if you have a specific set-up in mind. If you're not sure, opt for the Regular stiffness.