A Gravity Bike? It is what it sounds like, a bicycle that goes down hill and otherwise has not much use. These machines are a joy to ride almost soothing; just the sound of the wind and road. Speed varies depending on the hill. We regularly get up to 50mph but speeds up to and beyond 80mph can be accomplished. While the speeds may be questionably legal, these machines are legal on all public roads where bicycles are allowed.
If you ever care to race in an organized race. Conforming to the IGSA standards might be a good idea. Listed below are the three rules we build loosely around and have not found the need or desire to go out side of those bounds. The rulebook can be found at http://www.igsaworldcup.com/rulebooks/2009_igsa_rulebook_final.pdf
The basic standards are:
Wheel size - 51cm / 20” max
Bike weight - 34kg / 75lbs max
Axle to axle length - 127cm / 50” max
Many more rules can be found in the rule book. But where we race, none apply :)
Tools needed or have access to:
Metal cutting tools of some kind - Angle grinder and cut off wheels, metal band saw, hacksaw, hole saw.
Assortment of wrenches
The more the merrier!
We dig through metal scrap bins for most of our metal material and then let the material inspire.
We did buy 8ft of .75" x 1.5" x .125" mild steel for this project.
x2 20 inch wheels
Old bmx bike to pluck any tid bits from
We truly hope you enjoy this inside look!
S.I.N. (Sir Isaac Newton) Cycles
Step 1: Prototype
As you can see in the photo our prototype is super high tech utilizing a chunk of wood, rope, zipties, a fork, and some wheels. It was just together enough that we could carefully straddle it. Pretending where the handle bars, kneelers, and rear pegs might be. Card board boxes milk crates and other random items come in handy at this stage.
Be sure to take more measurements than you think you need. Also plenty of photos sure don't hurt. Documentation is a wonderful thing.
Sometimes we will draw over the photo to play with the overall aesthetics and frame layout. It is cheaper to learn things on paper than steel.