This a self balancing two wheel, two person vehicle. It self balances because the center of gravity is located below the center of the wheels.

Each rider controls one wheel. Pedal together and you move in a straight line forward or backward. Anything else is a turn including a spin in place by pedaling in opposite directions. I guessed a top speed of around 7mph would be a good start. I set the gear ratio accordingly. It was designed for kids around 5' tall, but it can easily handle taller. I am 5'6" and about 160lbs. I rode it with my daughter and it worked well.

This instructable details how to make a basic version. I designed everything in Pro/E. I have exported all the parts to DXF format so you can access with free software like QCAD. The drawings have basic dimensions for overall reference. They are 1:1 so you can scale off them or add additional dimensions. There are also PDFs too. I used my homemade CNC router so I added a lot of details that are not required. Need more info, detail, pictures...just ask.

The project took me about 16-20 hours over 3 weeks to build. A lot of that was design and testing, so it could take you less time. I had a lot of left over and found material that I used. If you started from scratch this could cost $150 to $200 to build. The bearings and drive train parts are the most expensive. I bought them at a surplus store. Hopefully you can find them cheaper.

The design worked pretty much as intended without any major problems. I did learn a few things along the way. I added "possible improvements" sections to each step that might make your version better and easier to build.

The working name is SteamRoller due to the steampunk look. The kids like Wheels of Doom and Portable Porch Swing. Does anyone else have any suggestions?

Step 1: Materials

Here is the basic list of parts. The dimensional lumber is estimated because I had enough scraps to cover that. I ripped the various sizes needed on my table saw from larger lumber.

Wood
(2) 4' x 8' sheets of 3/4" Plywood (I used BC Exterior grade)
(1) 4' x 8' sheet of 1/2" Plywood (I used BC Exterior grade)
(1) 2 x 4 x 6' pine board
(1) 1 x 6 x 6' pine board

Surplus Center Parts
(4) 7/8 4 Bolt Bearing Block (#1-205-14-4)
(2) 10' pieces .40 pitch chain (#1-1163-40)
(2) 11 tooth 7/8" shaft sprocket (#1-2123-11-D)
(2) 30 tooth sprocket. (#1-2123-30-4B)
(4) 7/8" dia. shaft collars (#1-2766-87)
(2) #40 Chain Connecting Link (#1-1087-40)

(4) 7/8" I.D. bearings (eBay)

6' of 3/4" heavy wall copper tubing
4' of 1" thin wall copper tubing.
(12) 3/4" 90deg copper elbows. (buy a bag/box of them...much cheaper)
(2) 1" copper end caps.
(2) 8" long 1/2" pipe (wheel Axles)
(2) 1/2" pipe flanges (mounts axles)

26' 5/8" I.D. Heater tubing
5/8" double barbed fitting.
1/8" NPT Schreader tire value

Step 2: Seat Contruction

I started with this section because the dimension for everything else scale off this.

I drew a 46" diameter circle on the garage floor in chalk and I worked with my 10 year old son to find a comfortable and practical riding position. I traced his body and measured for the seat dimensions.

Start with the seat sides. They are made from 3/4" plywood. Draw the pattern on the plywood and cut them out. Be sure you make a right and left hand version. Drill all holes as shown on the plans. I routed a groove into the sides to accept the seat parts, but this is probably not required and may be difficult to do by hand. The top bar is 1" copper pipe with end caps. I thought it would brace the side from bending in and give something to grab while getting in. I drilled flat bottom holes the diameter of the pipe caps about a 1/4" deep. I ran self tapping screws through the sides and into the cross bar to secure it.

Cut the (3) seat parts and the (2) cross members that join them together. Screw them together. Handle the seat carefully until is is bolted to the sides.

Place one chassis side on the ground with the inside facing up. Put the better side of the plywood towards the inside. Place the seat and screw strips in place. The screw strips screw to the sides and to the seat. Double check everything, then screw and glue the screw strips to the sides. Repeat this process for the other side. Make sure it will be an identical but opposite version of the first. Now screw and glue the seat assmebly to the sides.

Just in case some things have shifted a little during assmebly I suggest making a paper or cardboard test version of the center console, so you can make adjustments if needed. Be sure the pedal holes align with the sides. Cut two of these making a right hand and left hand version. Screw and glue to the seat. I put an optional cap/arm rest on this and a copper grab handle.

Install the pipe flanges and pipe pieces for the wheel axels. I used tee nuts.

Install the top cross bar. I used copper pipe, but anything strong in compression will work like a dowel rod.

Now find some way to take it for a test swing. I used (2) work tables as shown in the picture. Make sure it is strong and safe before you climb in.

Possible Improvements: The seat sits awfully low. Raising would give more clearance, but it might be tippier.

Attachements:
DXFs.zip 2D drawing of most of the parts
Drawings.pdf PDF versions of the drawings.
Steamroller.skp: Sketchup export of the 3D model
Steamroll_asm.zip. A STEP file export of the 3D model.

Step 3: Wheels

The wheels should be as big as you can cut from the plywood. I made mine 46" dia. because that is the biggest my router could cut.

The bearings I got were are aligning. This means the bearing axis will tip away from perpendicular as need to align to the shaft. Therefore the wheel will not stay straight unless you put one on each side of the wheel. This will prevent the wheel from canting on the axis. If you can find a better cheaper solution, let me know.

Mark the center of the wheel. Use a bearing block to mark where to drill holes for the bearing mounting holes. Draw the diameter and cut out. I made some spokes to lighten up the wheels, and give visibility to the sides. They also help with the steampunk look.

I used a 1/4" radius bit in my hand held router to put a small radius around the both sides of the perimeter of the wheel. This allowed the tire to fit smoothly without sharp bends.

Mount a bearing block on each side of each wheel. Use a pipe to temporarily line them up.

Tires:

I used 5/8" I.D. x 3/4" O.D. black rubber heater hose. Lowes sold it from a spool, so I could get the 12+ feet per wheel as one peice. I cut a .25"-.30" wide strip out of the hose. I used the label on the hose as a visual guide to free hand cut with a box cutter. It cut easily in a single pass. It only took about 5 minutes per hose. Be careful not to cut the other side of the hose when cutting. I used some 5/8" long staples to tack this strip around the wheel. This will puff up the center of the tire and add some extra cushioning. I then wrapped the split hose around the wheel stapling it on each side as I went. I stapled about every six inches.

Possible Improvements:

Tires: I spent a lot of time fretting over the tires. I was going to use 5/8" I.D. 3/4" O.D. heater hose in a loop joined together with a barbed fitting and a schrader valve. I tried a test version and it held pressure for the two week test period. I would need to make some cuts in the wheel to fit the valve and I would need to put some rim edges to keep the tire on. I was not sure how well it would hold up, so I went with the simpler method. I was afraid I would have to scrap the wheels if the idea failed.

Bearings. You could easily make your own bearing block out of 7/8 I.D. bearings and wooden blocks. They might actually be better because they would not be aligning.

Step 4: Pedals

The pedals shafts are made from 3/4" thick wall copper pipe and 90deg fittings. The pipe has a 7/8" O.D. that works with the pedal shaft bearings.

Cut all the pieces and liberally flux all the mating surfaces. Put all the parts together to make sure everything lines up, is square and the ends that go through the bearings are on the same axis. I suggest cutting the center vertical piece last in case it needs to be adjusted in size. Keep all the pieces together while brazing. This will make sure they stay square while brazing and will reduce the heating time. I suggest placing an extra fitting on the ends to keep that piece on the same plane. Don't accidently braze those pieces on.

The outer diameter is slightly larger than the I.D. of the 7/8 bearings. I lightly sanded them down to fit though the bearings and shaft collars.

The pedals are made from 2 pieces of 3/4" wood screwed together and drilled 1" dia. This will allow them to be replaced if they are ever broken.

Step 5: Gears and Chain

The gear sizes I liked were all available in 0.40 pitch chain. This is pretty beefy motorcycle style chain. I calculated a 3:1 ratio (three pedal revs = one wheel rev) for a practical top speed. This turned out to be perfect. I chose an 11 tooth gear for the the pedals and a 31 tooth gear for the wheel.

The chain comes in 10 foot lengths. I needed just a little over 5 feet per side, so I needed two lengths. If you want to save a few bucks you could change the design to use a few less inches of chain. It comes with one master link per piece. I would suggest buying a few more master links just in case you need them. I would also recommend getting a chain breaker.

I started out by placing the chassis on a couple of boxes to where the wheels would spin freely. This allowed everything to freewheel.

The first task is to mount the gear to the wheel. It is a bit tricky because the gear is about the same size as the wheel bearing blocks. I made an adapter plate out of 3/16" think aluminum. I used some 1-3/8" long standoffs to hold the plate away from the wheel. Be sure to allow access to the set screws on the bearing blocks after the gear is installed. I stood the gear off the plate with a nut and a washer to allow clearance for chain. Bolt this to the wheel centered on the bearing.

Install the wheel onto the wheel shaft. Slide it in until the gears are aligned. Wrap the chian around and measure the length needed. Break the chain then install it on the gears and put on the master link. Test everything and adjust as necessary.

Step 6: Finishing

After a few preliminary test drives, I striped it down for finishing. I went with a mild steampunk look to complete the family of steampunk contraptions. See the family photo.

I used Cabernet stain which is a dark redish color. I used several coats of clear gloss water based poly varnish over that. I prefer oil based spar varish, but the water based stuff dries so fast I could complete the varnishing in one day. I added some extra brass fittings and polished them up with Brasso. I painted the pedals black.

For all wood screws, I found some with a iridescent yellowish color. I think the finish is called yellow chromate. I found them at Menard's. They almost look like brass, but are much cheaper and more durable.

When rebuilding, use some removable blue Loctite on the machine screws. They tend to rattle loose.

I plan on adding a large steam style piston to the side that will actually pump in and out.

Step 7: Brakes

Nice picture huh...Yes, I wish....

I have not yet installed brakes. My concept was to get some bicycle calibre brakes and adjust the gap between the pads to work on the wheel gear adapter plate. So far it does not really need brakes. It stops easily using the pedals and a few emergency stops have been done Fred Flintstone style by putting both feet on the ground.

Step 8: Riding

I was worried about how people would mount the thing, but it is not an issue. You just kind of crawl in. It helps to have someone steady it, but it is not required. With someone's weight way forward the nose contacts the ground and this keeps things stable.

The physics work like this: The pedals allow you shift the center of gravity (CG) of the porch swing forward or backward. The wheels then rotate to return the CG to normal. Climbing hills or getting started on soft ground will require a greater shift before it will roll. This can get a little scary (but fun) as your feet start to point to the sky. New riders might need a push if they get too stuck.

Some small kids who stretch to reach the pedals or don't have much weight behind the axels tend to rid very low in the front and may tend to bounce the nose. I added a little box to the top of the back of the seat. This allowed me to add and remove a brick or two of ballast to get the nose up as needed.

Coordination to go straight is not that tricky. Either rider usually adjusts a little to correct the heading. It is fun to watch kids riding for the first time working out the coordination. Some kids tend to just stop pedaling and freeze when they don't know what to do. This of course causes the thing to immedaitely pirouette on that wheel.

It occured to me that this would be relatively easy to motorize with some wheel chair motors. A joystick with some relays could control it. I think I will keep mine old school though.