Introduction: MonoWheel

The MonoWheel is exactly what it sounds like, a single wheel. It will be driven by a high speed motor and controlled remotely. This instructable focuses on constructing the mechanical structure of the MonoWheel. This project was designed, built, and tested as the final project for our Mechanical Engineering Mechatronics class at Rowan University.

The basic layout starts with a standard sized mountain bike wheel rim with the spokes removed. Sized to the inner diameter of the rim is a large ring gear. An aluminum chassis is constructed to mount the motor and other hardware and to provide 3 pivot points to keep it concentric to the rim. The motor is mounted to the chassis with the output shaft mounting a spur gear that attaches to the ring gear. Because the ring gear is substantially larger than the spur gear, there is a large reduction is motor speed and thus a high speed motor is recommended. The inner tube of the wheel is removed and replaced with foam tubing to provide a sturdy tire.

Our final test can be seen on the last step of this instructable.

Step 1: Get a Mountain Bike Wheel

The MonoWheel size is equal to that of a mountain bike wheel. Find a wheel from an old bike or buy a rim from an online retailer (Chain Reaction Cycles is a good one)

Remove the spokes, the tire, the inner tube and the inner tube liner. The Tire, the inner tube liner will be needed later so keep those. The inner tube and the spokes are no longer needed.

Step 2: Make a Ring Gear

Tools Needed:

  • CAD software
  • Tape measure
  • Water Jet Cutter
  • Milling Machine
  • Drill Bit for 8-32 tap
  • Tap and Die kit
  • Hand Drill

Materials Needed:

  • 2' L x 2' W x 1/4" 6061 aluminum plate
  • 8-32 x ½” screws (x4)
  • 1/8" x 1/4" x 4' section of adhesive foam padding
  • Browning NSS1212 Spur Gear

The MonoWheel is driven by a spur gear on a high speed motor. This will require a large diameter ring gear. The size of the ring gear will depend on the bike rim. Measure the smallest inner diameter of the wheel rim. This dimension will determine the outer diameter of the ring gear. Make the outer diameter of the ring gear slightly smaller than the inner rim dimension. This will allow a small gap in which a strip of foam padding will fill.

Next the ring gear and the spur gear must mesh correctly so ensure that the number of teeth and the pitch diameter are compatible. The ring gear can be made with a water jet cutting machine. A DXF file of the outline of the ring will be needed.

After the ring gear is cut, it needs to have holes drilled and tapped to mount to the wheel rim. There will only be 4 holes drilled so mark 4 lines on the gear at clock positions 12, 3, 6 and 9. Make sure that they line up with the spoke holes on the rim. If they do, great! If not, Move the holes accordingly. Clamp the ring gear down and drill ⅛” holes from the outer edge through gear at each mark. When the hole is drilled, tap the hole with an 8-32 tap. Do this at each of the 4 marks around the gear. See sketch at right. With the holes drilled and tapped, the next step is to locate the spoke holes on the rim that will mount the screws that screw into the ring gear. The holes should fit the 8-32 screws perfectly, but if not, make them bigger with a hand drill. Next lay the foam strip around the ring gear leaving the areas for the screws bare. position the ring gear into the wheel rim and screw into place. Replace the inner tube liner above the screws.

Step 3: Add the Tire Back On

Tools Needed:

  • Scissors or knife

Materials Needed:

  • Foam pool noodle
  • Original tire from wheel

The original inner tube cannot be used since the valve stem would protrude through the rim into the gear. To fix this, a foam pool noodle, sliced down the middle, should be stuffed inside the tire to replace the inner tube. If it is around the summer, you should be able to find this at K-mart, otherwise you can find another rigid foam to use. Bell also makes a foam inner tube for around $20.

Step 4: Make Chassis Roller Mounts (3)

Tools Required:

  • CAD Software
  • Water Jet Cutter
  • Milling Machine
  • Tap and Die Kit

Materials Required:

  • 1’ x 1’ x1/2” Aluminum Plate

The Roller mounts are used to keep the chassis centered to inside rim and to keep it from falling out. Its design is particular to the wheel rim so some angles and dimensions may differ. The angle of the mount follows a line to the center of the wheel, to better hold the side rollers in line. After Water Jetting out the basic shape, drill a 5/16” hole according to your plans for the axle, and drill and tap the 5/16”-18 holes for mounting the side rollers.

Step 5: Make Chassis Side Rollers (x6)

Tool Required:

  • Lathe

Materials Required:

  • 2” Dia. x 12” L Delrin

The Side rollers are used to center the chassis and mount to the roller mounts. They are designed around the mounts angles. Much like roller coaster wheels these rollers keep the chassis riding along the rim smoothly. The side rollers are designed with a truncated conical design to roll without slipping, the design was calculated according to the drawing shown below. The inner diameter of the rollers was a press fit onto standard skateboard bearings.

Step 6: Make Chassis Roller Gears (x3)

Tools Required:

  • CAD Software
  • Water Jet Cutter

Materials Required:

  • 5” x 5” x ⅜” Aluminum Plate

The roller gears are mounted to the roller mounts and mesh freely with the ring gear to center the chassis. They are made with varying tooth numbers to be swapped out for a better fit when the whole system is assembled. Make the gear outline in a CAD program and save the outline as a DXF. This will be used to cut the gears out of aluminum plate on the water jet cutter.

Step 7: Make Roller Gear Spacers (x6)

Tools Required:

  • Laser Cutter

Materials Required:

  • ⅜” Acrylic sheet

The roller gear spacers are used to keep the roller gear centered in the roller mount. Laser cut spacers with a 5/16” ID (OD isn’t important, as long as it will fit in the mount).

Alternatively, you can stack washers til they fit right.

Step 8: Assemble Chassis Roller Mounts

Tools Required:

  • Press
  • Screw Driver

Materials Required:

  • Chassis Roller Mounts (x3) (step 4)
  • Chassis Roller Mount Side Rollers (x6) (step 5)
  • Chassis Roller Gears (x3) (step 6)
  • Chassis Roller Gear Spacers (x6) (step 7)
  • 5/16” Dia. x 2 ½” L Dowel Pin (x3)
  • 8mm ID x 22mm OD x 17mm THK ABEC 3 Bearings (x6) (skateboard bearings)
  • 5/16” ID x ¾” OD Washer (x12)
  • 5/16-18 Button Head Screws (x6)

Step 9: Make Chassis Sides

Tools Required:

  • CAD Software
  • Water Jet Cutter
  • Drill
  • ¼-20 tap

Materials Required:

  • 2’ x 2’ x ⅜” Aluminum Plate

The chassis sides make up the main body of the chassis. They are sized to the rim and contain positions for the 3 roller mounts. It also mounts the motor and the drive shaft bearing. There are two sides each almost exactly the same. The only difference is the holes that mount the motor on one side, and the single hole that mount the bearing on the other. Drill and tap for ¼-20 screws where the roller mounts are marked to go.

Step 10: Make Drive Shaft

Tools Required:

  • Lathe

Materials Required:

  • ½” Dia. x 4” L Steel Rod

The steel drive shaft will drive the ring gear with the spur gear. It needs to run from the output shaft of the motor to the bearing in the opposite chassis side wall. The OD is ½”, except for where it meets the bearing, where it needs to be turned down to 5/16”. The side of the shaft that meets the motor must be drilled to the diameter of the motor shaft. Drill and tap a hole for a set screw to lock the drive shaft to the motor.

Step 11: Assemble Chassis and Motor

Tools Required:

  • Torx Driver

Materials Required:

  • Chassis Right Side
  • Chassis Left Side
  • Chassis Roller Mounts (x3)
  • ¼-20 x 1” Button Head Screws (for Roller Mount) (x12)
  • High Speed Motor (Gens Ace BL5160)
  • Motor Mounting screws
  • Drive Shaft
  • Browning NSS1212 Spur Gear
  • Drive Shaft Bearing

Step 12: Test!

Here we have our semi-final test of the mono wheel. Wire up a standard remote control unit for an RC car. we salvaged one from another project, but you can find them at hobby stores, or online. Strap a good battery in and gently help it get started. In the future, a servo with a pendulum can be attached to steer it, but until then, make sure you have a large, unpopulated field r lot to run it in.



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    tools needed - Laser Printer? or Laser Cutter...

    1 reply

    oops, nice catch thanks!

    That scream of straight-cut gears!

    Just need to add some controls.

    I'm thinking something like this:


    I saw bigfoot riding one of these.

    I believe this was done by a Swiss engineer around the 1920's

    4 replies

    my mis-speak. looks like the pipe & pulleys are used as a guide. The motor uses a friction drive on the rim while pivoting on its mounts attached to the platform used to mount the seat.

    Wow! that one is cool. We were inspired by some of the ones being made today by McLean Monocycle


    notice a cable is used instead of gear. much more forgiving to the engineering. less chance of breakage.

    also look how balance is maintained by keeping the CG as a constant to plane of travel. and the steering. good luck.

    not sure what you mean by cable.

    Maybe a servo that shifts some weight would be all that's necessary to have some steering capability.

    2 replies

    That was the initial plan, but we ran out of time unfortunately. we may still finish it in the next few months, in which case we'll post the results.

    How about gyroscopic steering?

    Forgot to say: Really cool!
    What will you use fr steerieng? Gyroscopes?

    1 reply

    we are thinking of using pendulums to shift the balance.

    I like it...This gives me an idea for an electromagnetic design that uses neodymium magnets along the inner rim for propulsion. I'll post a picture if I get it working.

    2 replies

    I'd love to see that too! even if it doesn't work 100%, the theory sounds awesome to find out about :D

    Ooh I'd love to see that, we went with a traction roller design, but couldn't get any grip on the bike rim which is why we went to the ring gear. If you want any better pictures/ dimensions of it, pm me and I'll see if I can get them to you.