Introduction: Gravity Bike

About: The word I hear most explaining me is whimsical. I'm an artist designer crafter person. By day I work for Michael Curry Design making super nifty puppets. Every other moment I spend creating things of wide arr…
Howdy, We are S.I.N. Cycles builders of gravity fueled machines. This is our most recent build, hope you enjoy! Just want to add that if you are interested in some of our other machines check out 

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  

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
Drill press
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.
Rim Brakes
Brake Levers 
Brake cable/housing
L200 foam
Wheel cover
x2 20 inch wheels
Old bmx bike to pluck any tid bits from
Shaft collars

We truly hope you enjoy this inside look!
S.I.N. (Sir Isaac Newton) Cycles

Step 1: Prototype

This is a critical step to create a comfortable ride. Everyone has there own body length and preference of how something should feel. So play around. We joke sometimes saying every bike is our prototype for the next bike we build.

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!

Great thing about gravity bike frames are the minimalistic needs. The front and rear axle must be aligned and the head tube needs to be true. Aside from those two things the rest of the frame design is up to where you want you body on the bike. 

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

The materials for the frame are all over the place. We had an old donor bike that we salvaged parts from: the head tube, rear drop outs seat stay, and fork. We salvaged the old seat stay mainly for the v-brake tabs already on the frame. The two top tubes and lower tube are mild steel .75" x 1.5" x .125wall. The down tube and seat tube are cro-moly 1.5" diameter .125 wall thickness.  The two top tubes are a straight shot from the head tube to the rear dropout. The down tube was bent on a hydraulic tubing bender. Many custom metal fab shops and railing shops will have a machine that can do this for very reasonable rates. The down tube was bent in this shape purely for aesthetic reasons. There are endless ways to get from the head tube to the rear dropout, be inventive and have fun.

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

The kneeler is an area that takes a lot if not most of the weight from the rider. Material for this was found in scrap bins. We found 2"x2" box tubing scrap and cut triangle segments off of either side to get the structure seen in the photo. Then 4" x 4" x 3/8" thick steel plate and welded them to the box tubing. We strongly recommend not using pegs or round tubing for kneelers, your shin bones will agree. The idea is to have a wide surface area to distribute the energy from bumps.

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

Step 5: Ballast

Weight, its your friend in this sport. Where the weight is located is also important. Pushing as much weight as possible below the axle height will give you a much more maneuverable bike.  So what you can do is drill a hole in the frame and fill parts of it full of lead or concrete. Some builders will bolt on lifting weights this is nice because you can easily move the weight on the bike and or increase decrease the amount.

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

This bikes handle bars are mounted directly to the front fork.

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

Step 7: Paint

At this point all metal fab that needs to be done is completed. Prep the frame by sanding the entire surface with 100 grit sandpaper. This is a good time to give the frame a full once over checking for any sharp areas, think what might be sharp wrecking at 45mph. Baby proof it, make it soft like butter. Then to take care of all your greasy paw prints with some kind of surface cleaner. We prefer acetone, remember your protection. At this point you can tape off any areas where you don't want paint. Like the head tube where the bearings go and on the kneeler plates where you will be gluing to. After everything is nice and clean give it a coat of primer and then top coat. After letting the topcoat kickoff you can tape off any areas for creating two tone effects. For this build tape off the frame around the bomb area. Then give that bomb a nice flat black coat. At this time we also installed the "wick" resin coated rope with some two part epoxy.

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

For this bike the seat is more for when your slowing to a stop or just sitting around before you bomb. The geometry of this frame puts most of your weight on the kneelers.  This allows you to throw the bike around underneath you, so we wanted a low profile 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

No need to spend a whole lot of money here. This build is using two no big name loose ball bearing hub wheel sets. We rebuild our hubs cleaning out all the old crud and replace the old grease with a good quality lube. We are kinda low tech and just use regular white lithium grease and then blast a little Remington Teflon gun oil  in there before sealing it all up. A clean proper adjusted looseball hub is a beautiful thing. Adjusting loose ball bearing hubs is no big deal if you've made it this far in the build you can do it.  If you feel you need assistance most cycle shops will walk you through it. 

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

This is where you install the front fork with fresh grease double check the nuts. Install brakes and brake cables leaving plenty of cable slack around the head tube that the cables will not pull tight when you turn the handlebars. Brake levers and grips. A neat trick to getting your grips on easily is to use hair spray. Just blast a bit down inside your grips and then quickly slide them on to the handle bar. You can also use a high pressure air nozzle ballooning the grip as you glide it on. 

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

When you build something with wheels you get to ride it. Really is an amazingly rewarding experience. BUT, lets be realistic in the haze of that final assembly something might have been over looked. Roll around the drive way make sure the brakes work and the wheels are on right side up and all of that good stuff. Now get your wrenches out and double check everything. OK now we are ready for a small test hill to see how it feels after you feel comfortable step it up. Have fun tell others what they are missing and get a group together.  Remember to ride within your limits, and maybe push them every so often...
Y'all asked for a video so here it is. Top speed on this run was 58mph.

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 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.
Spring Bike Contest

Finalist in the
Spring Bike Contest