Sheldon Brown was an amazing bicycle mechanic who's site is a wealth of knowledge. Especially useful for strange, old and rare bikes.
Useful for straightening steel components such as forks, frames, cranks, handlebars, seatposts, etc.
Allows you to check alignment and straighten derailleur hanger. Misaligned derailleur hangers can be the cause of shifting problems.
Instructions for use:
You can check to see if a frame is bent using the Park Tool Frame Alignment Gauge (FAG-2). If Your frame or forks are steel, they can be straightened using the Park Tool Frame and Fork Straightener (FFS-2).