Gyrocar

Trace around the plastic cap on the plywood sheet. Use a saw or router to cut out a disk that size, then file and sand it down until it fits flush just inside the cap.

• TIP: If you use a hole saw, back the plywood with another piece of wood. This ensures that the saw won't rip the grain on the exit side, so the disk will have clean edges. A hole saw also drills a hole in the center, which is fine.

Measure the diameter of the motor, then scribe, cut, and sand 2 plywood rings whose inner diameters are just smaller than the motor and whose outer diameters are about twice the motor's diameter. File or sand the holes until the motor fits snugly inside. You can also cut a matching hole in the larger disk, which will give the gyroscope wheel more vertical space to fit inside the cap.

The battery pack attaches flat on top of the large disk, so you need to make a way for the wires to run through the disk to the motor underneath. If you cut a motor-sized hole, use a small file or hobby knife to make a small channel deep enough to accommodate the wires under the battery pack. Otherwise, drill small holes that the wires can pass through, as shown here.

Drill 3 roughly equidistant pilot holes into the circumference of the large disk, and temporarily insert 3 small wood screws.

Glue the 2 small rings together on the bottom of the large disk, with all their centers aligned so that the motor fits in vertically.

Test-fit the motor into the disk assembly and route the leads out the top. Use the remaining 2 small wood screws to attach the battery holder centered to the top of the large disk. I added a small switch (optional).

Remove the gyroscope wheel from its wire housing and try tapping the axle out with a hammer.

If it comes free, you can fit in a plastic rod, which will make things easier. If the rod is a bit too wide to press-fit through the hole, chuck it in an electric drill and turn its diameter down by running it against a file.

If you can't extract the axle, saw it off and file or grind it down on both sides of the wheel.

Before drilling the gyro wheel, you must find its exact center point. If you inserted a plastic rod, push a pin into it where you think the center should be, then hold the pin pointing upward and gently spin the wheel like a top to see if it wobbles. Keep adjusting the pinpoint location and spinning the wheel, using trial and error until you find a good balancing point.

If you filed down the original metal axle, you can scribe crosshairs on the wheel using a try square set to 45°, mark the point with a hammer and center punch, and test-spin the wheel with the pin in the mark.

• NOTE: If you have access to a metal lathe, you can chuck the wheel on its outer diameter and drill a center hole on the lathe. You might even try making your own gyroscope wheel.

Use a caliper or micrometer to measure the diameter of your motor shaft. Find a drill bit that's the same size, or a few thousandths of an inch smaller if you've fit in a plastic rod. I used a #55 drill bit. The center hole you'll drill in the wheel must grip the shaft tightly, so that it doesn't detach at high speeds.

At the exact center point of the gyro wheel, drill the hole for the motor shaft, as perpendicular to the wheel as possible.

Press or tap the wheel onto the motor's shaft, as close to the motor as possible without interfering with it turning freely.

This connection needs to be tight and strong. You can reinforce it with hot glue.

Download the track wheel mounting bracket template from above, under Files, and print it at full size.

Cut and trace the outline onto sheet metal, then cut it out with tinsnips and bend it with pliers, following the notes on the template.

Drill the holes in the mounting bracket, as noted on the template.