WHAT YOU’LL NEED:
Tools:
_ Drill, with bits for both wood and metal
_ soldering iron for wiring connections
_ hand saw
_ sandpaper or an electric sander
_ a lot of patience!

Hardware:
_ chair  (more on chair later)
_ plywood board : length, width and thickness will depend upon size/weight of chair and child
_ 1 front wheel : pivoting wheel
_ 2 back wheels (we are using clear plastic skateboard wheels after having a bad experience with black nylon wheels tracking up the tile floor) more on wheels later
_ 2 electric motors (more on motorization later)
_ joystick
_ plastic and metal boxes
_ metal bar to mount joystick box
_ screws and bolts

THE CONCEPT:
I wanted to make Max’s car as simple as possible, to use a minimum number of tools and off-the-shelf hardware. For example, I used electric screwdrivers for propulsion instead of motors.  If you have ideas to make it better, go for it, and let us know of your adaptations!  I won’t give you size details because in the US you have different size standards for hardware.

The Maxou Mobile moves forward, backward, turns left and right.  It cannot move diagonally and has only one speed.

THE SEAT:
We already had a special orthotic seat for him : that’s the one we used, but if you don’t have one, you can use a car seat or some other type of seating, but the child must be belted in.  The stop and go is pretty powerful, so the seatbelt is a must.  You may also want to make the seat incline a little to the rear (using a wooden wedge or something else, once we used a dishcloth) so the stop and go isn’t too abrupt for his neck and head control.

THE CHASSIS:
I used a plywood board about half an inch thick (17mm).  I don’t want to get too much into size details: use a board that’s thick and strong enough to support the weight of the seat, the child and the hardware drilled into it, otherwise the board will bend and the motors will touch the ground. The size of the board will also depend on the seating you use.   Consider the amount of space needed at the end of the board for the child’s feet.

THE WHEELS:
The wheels are installed in a triangular shape.  For the front wheel (and only the front wheel!), I used a pivoting wheel normally used for furniture.  The wheel I used does not have a ball bearing but the support does. Make sure the pivoting wheel can rotate easily under the seat/child load.

The diameter of the back wheels you choose is important because it will determine the torque and speed: the smaller the wheel, the better. You don’t want your kid to fly!

For the 2 back wheels I first used narrow plastic wheels. I drilled the center to fit the screwdriver (motor) axle diameter. I adjusted the final diameter with a round file, then pushed hard to fit the wheel on the axle.  It worked but the wheels I chose were made of nylon (avoid those) and slid on the ground. So I switched to clear plastic skateboard wheels.

The problem I had with skateboard wheels is that they are hollow. So I bought melting plastic : pour the plastic in boiling water and when melted, pour the melted plastic in the wheel.  Once the plastic has cooled off, drill into the center.

skateboard wheels and bag of plastic beads for melting

THE MOTORS:
You can use anything to make the motors (avoid 300hp V8 though…) but to make it easy, I used 2 Black and Decker battery-operated cordless electric screwdrivers.
Axle size : the longer the better, with a minimum of one inch! Use an axle with at least the same width of the wheels you will use.
Speed : should be about 150-200 rpm
Torque : select a 2.4V or even better : a 3.6V
Total body diameter should be as narrow as possible -- about 2 inches
Shape : round is better to be attached.
- Must have screw / unscrew capability with a switch.
- if possible with a torque limiter (so when the car runs up against the wall, the screwdriver will skid but the wheel won’t turn.
 You can see on the picture numbers 2, 3 that the ring adjusts the maximum torque.

I used pipe holder to bolt on the screw driver on the board.

Screwdriver fixation:
 

Motors electric wiring:

I nailed a domino to the board for easy wiring.

I cut the screwdrivers with a metal saw between the electric motors and the batteries.  I took off the battery of each screwdriver and soldered an electric wire directly onto the electric motor.

THE JOYSTICK:
I bought a small joystick from an online electronics store but you could use an old one (Apple computer or something equivalent). Just make sure it’s not too hard to manipulate for your kid. I had to cut a spring in half inside to make it easier.

I bought an aluminum box (for electronic circuits) to make the armrest and installed the joystick.

You can also buy end caps for this box instead of using electrical tape.

The joystick is fitted from underneath and I covered the hole with electrical tape.

THE ELECTRICAL WIRING: 
The goal is to control the direction of rotation of the wheels :

Left wheel Right Wheel Car direction
Direction of rotation of wheels  Forward Forward Forward
Direction of rotation of wheels Backward Backward Backward
Direction of rotation of wheels Backward Forward Left
Direction of rotation of wheels Forward Backward Right

To do that you must control the on/off and polarity of each motor (+/- or -/+).

THE CHARGER:
I discarded the original charger base (on the left in picture) and connected a standard audio jack on the charger. It’s easy to plug in the charger and recharge the batteries whenever they run down.  We are looking into getting a longer-lasting battery, as the one we have lasts about half an hour.

I attached the armrest with a metal bar :