Get 2 of the same DC gear-motors where it has two axles as shown above.
Here is a 3D axle adapter we designed. You will need to print 4 of the axle adapters for each cart. Look for online 3D printer services if you do not have a 3D printer. Download: https://goo.gl/SvFRWw
Push the Axle adapters on both side at the same time slowly toward the motor.
It should look like the picture above.
Wire two 5 inch lengths of wires to the each motor. Test each motor by connect it directly to a AA battery. Then glue the base of the wire to the motor. This will prevent the wires from breaking off.
Hot glue the motors to the plexiglass frame. Drill a hole somewhere on the frame to allow the wires to go through to the other side.
Glue the switch and the battery holder on the frame.
Here is the wiring diagram. When you glue it to the frame, make sure the motors are spinning in the same direction (or you'll need to reverse the wires)
Here you have a cart ready for the wheels.
To make wheels, you will need a drill with a 2 and1/8" hole cutting bit and a sheet of 1" thick insulation foam board. Lay the foam board down on a piece of wood and drill straight down.
You'll have to pry out the foam wheel from the cutting bit.
It is a good idea to have a lot of wheels ready for the students/visitors. (Dont worry, cutting the wheels goes fast)
To explore traction, you'll need to gather a wide variety of materials that can be taped on or attached to the foam wheels such as the materials above. The plastic saws are to cut or trim the wheels.
Go ahead and design and explore traction!!!
Make a ramp that simulates different terrains/put obstacles to challenge the students/visitors. This makes students search for different type of materials to put on their wheels.
Take a look at photos of the traction activity: https://goo.gl/photos/Kpf4VnwxHFyKtqwr8