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.
Step 2: Get jiggy!
Our jig is made from mild steel .5" x 1.5" x .125 wall thickness and 1.5" square for the uprights because its what we had laying around. Drill holes in the jig at the distance in which you want your axle length to be. Ours is at 48 inches. We then used old 3/8" axles from bmx wheels and mounted those into the holes. These axles will be used to mount the fork and rear dropouts during the frame build and welding.
Step 3: Frame
We don't really do mechanical drawings before the build, so for us it's a lot of holding up in place and marking with sharpies. After the material is marked we use a hole saw to cope the tubing for a nice clean fit. When using hole saws make sure your materials is fixed firmly. The hole saw is aggressive and can grab the material sending it for a loop. A round hand file or drum sander is a great tool to finalize any copes that are not snugging up how you'd like. Remember it's much easier to take away more material then to add it.
The head tube angle or rake is set at 23 degrees which we have found to be a good middle ground for long straights and tight switchbacks.
Keeping your rear end off the tire is important. For this we took a piece of scrap tubing for the kneelers and welded it between the two top tubes to act as a rear fender.
This is the time to consider how you are mounting your seat. Ours has two small metal tabs that screw through into the seat.
This frame was TIG welded, but it could have been MIG welded. Just be sure and follow all safety procedures whilst welding.
Step 4: Kneeler
Minicel L200 also known as Y20 was used for the kneeler padding. This foam is resilient and has excellent shock absorption abilities. A generous two inch thick piece was used to eliminate the chance of ever feeling any hard shock through the knee or shin. Other foams that would work well would be neoprene or sleeping pad material. Minicel can be purchased at http://www.foambymail.com/Minicel.html
Step 5: Ballast
What we have done here is taken two steel hemispheres drilled a half inch hole in the center of both hemispheres. In one hemisphere we welded a piece of 1/2" all thread through the sphere. Leave a few inches on the inside and enough on the outside of the hemisphere to mount it to the frame. Then weld the two hemispheres together. Next, ground off excess weld to create a smooth looking sphere. A tube was welded to the sphere for a "wick" to be added later making a classy looking little bomb.
Now for the awesome nasty dangerous part. Filling the hollow sphere full of heavy stuff. Using a gas forge we melted lead pellets and then poured the melted lead into the sphere. A funnel is of great assistance during this process. Ours is an Ace model for about three dollars. This created a 25lb ballast. There was still room for more weight so we drilled a hole in the bottom tube of the frame and filled that full also. Do not try to cool these parts down with water. Just let them do there thing. It may take an hour or more. If you do not have a gas forge at your disposal an oxy/acetylene torch or MAPP Gas torch will also get the job done. If you go down this road please where a proper respirator and do it outside if possible.
The excess all thread that was left protruding from the sphere was enough to safely mount it in a vise for the pouring of lead. After the lead has cooled we drilled a half inch hole in the frame and mounted the ball. You can bolt the weight on using a nylock. We chose to cut the all thread down until only .125" was protruding out of the frame, and then welded it on.
Step 6: Handle Bars
First, weld mounting points onto the fork. Welded just a few inches down from the fork crown are .875" tubes .25" wall thickness 2"long. The handle bars consist of .875" tube .125" wall. Many brake levers are designed for this size tubing. Cut two six inch pieces and two seven inch pieces.
To make the bars totally adjustable we used two piece clamp on shaft collars with a .875" bore. First take the shaft collars and weld them together into sets of two. To make sure the shaft collars are alligned with each other bolt them onto .875" tube butted firmly together. After you have four sets of dual shaft collars weld them onto the ends of the six inch and seven inch tubes. You can now position the bars to whatever suits you best.
Another option is to use a bmx handle bar stem and invert some standard bmx bars.
Shaft collars can be purchased sometimes at your local hardware store. If not go to those who have it all www.mcmaster.com
Step 7: Paint
You can use fancy two part paint or rattle can, just be sure to follow the instructions. For this project we had to run some heaters to get the room up to an acceptable temp. Good practice to spray towards the end of a day forcing you to leave it over night before handling it.
Step 8: Seat
Start with a piece of half inch plywood. Lay the plywood on the frame and then trace from underneath to get the width of the seat. Then sketch out on the plywood the rest of the shape. Use a band saw or scroll saw to cut out your shape.
For seats we again like to use the Minicel L200. It's not the must squishy thing under you bum but it is tough and dampens vibrations and impacts well enough. One of the coolest things about this foam is how you can sculpt it. It cuts with a bandsaw with ease. You can use a hacksaw or hand saw also. Once you get the general shape blocked out you can use a coarse grit sandpaper to shape it further. It has a low moisture absorption rate so leaving it raw is always an option.
Once you have your desired shape you can wrap it in fabric or leather. Oregon Leather Supply has amazing scrap barrels with plenty of pieces that are big enough for a seat. We chose a piece of red leather. Ideally we like to use barge glue for this, but we where fresh out. Super 77 spray adhesive saved the day. First apply a layer to the foam and plywood and bond those two together. Then you can attack with the leather. Applying a layer over all of the foam, and then over a smaller section of the leather to get it started. Then as you start wrapping spray the inside of the leather pull and press down. Starting with your biggest flat surface and then working out from there tends to be the best option. It's a tedious procedure and in our shop we find cursing sometimes helps.
This frame has mounting holes preplanned for this seat. Although on other builds we have glued the seat straight to the steel with good results.
Other foams such as upholstery foam will work it's just not our preference. If you do happen to use upholstery foam know that electric carving knives from the kitchen work wonders.
Step 9: Wheels
Tubes are something that should be purchased fresh we recommend anything that claims to be heavy duty or extra tough. For this build we used Kenda Heavy Duty 2.25mm wall thickness. Average tubes are around .09mm thick less than half the thickness.
Tires anything that does not have dry rot cracks or threads showing will work. We recommend something with maximum rubber contact almost slicks. Animal ASM and Animal GLH are tried and true high PSI and awesome micro tread design gives great traction in many conditions. Higher PSI allows for low rolling resistance. You can always let a little air out if your running a more technical course or rain.
Rear wheel covers are getting somewhat hard to come across nowadays keep your eyes open on ebay or craigslist and you'll find a set sooner or later. These may have some aerodynamic benefit, but more importantly they keep your feet out of the rear spokes.
Alright now bolt those buggers on!
Step 10: Final Assembly
Applying the foam to the kneelers. You'll want to use a good strong water resistant glue. We recommend barge glue. It is very tenacious. Just apply a good even layer to the steel and to the foam let it sit for a few minutes and then carefully align the foam and drop it on applying a decent amount of force. When the two coated surfaces touch each other thats it no shifting it around. Like I said tenacious.
You can use zip ties to lash down any loose items such a cable housing. Be sure and not cinch down to hard on zip ties over cable housing it can pinch the cable applying undesired cable drag.
Step 11: Best Part
Feel free to ask any questions at all. We will do our best to answer them. We work out of ShopPeople a community work space feel free to visit if you're in Portland. More info on shop people can be found at http://www.shoppeople.org it's rad.
We plan on covering some of these steps in greater detail down the road because instructables are bananas and we love bananas.