*Update* Friday, 7:38PM, as I was putting away our finished car, it fell out of the blue bin and broke in half. Thankfully we had made extra frame pieces and was able to just screw the whole thing back together. Only hitch was waiting until 9:30PM to laser cut broken front wheel.

*****

With Liza Singer.

We optimized our car for looks and modularity. We’re pretty sure the car is capable of going 10 feet at a rapid clip. But random grooves in the floor can easily throw its 1/16th” thin wheels off course.

It also has the unfortunate characteristic of having built-in brakes. We opted for a very long rubber band to turn the axle which gives us more power but at the expense of I’d say a 4 in 5 chance that the rubber band will fling itself under the front wheels before the car has gone 10 feet.

We didn’t want to have to clue or tape anything into place. With the magical powers of the laser cutter, we were able to make wheels, frame pieces, teeny bearings and washers that fit perfectly around the threaded plastic nuts and bolts we got from McMaster.

The whole thing screws and unscrews together allowing us to test different configurations easily without stressing out our materials too much with a lot of ungluing and un-taping.

Aside from “having enough power” issues, the biggest challenge of designing this device was figuring out how to keep the axle spinning freely in the frame, while the same time squeezing the wheels tight enough such that no power is lost between the axle and the wheel.

I think a couple of extra small lengths of female threaded rods would have helped our cause enormously by allowing to have bearings that were just slightly thicker (wider) than the frame, allowing the nuts to screw tight against the bearings without clamping down on the frame as well. Instead, we’re going to have to tighten/loosen the nuts on our axle “just right” for the race.

Materials

• 1 4″ length of threaded rod – 8-32 screw (back axle needs handles for winding up car)
• 1 2″ length of threaded rod – 8-32 screw
• 12 nuts 8-32 screw
• 2 1/2″ long eyelet screws
• 4 1″ washers for the back wheels
• 4 3/4″ washers for the front wheels
• 2 4″ back wheels – 1/16th” thick acrylic
• 2 2″ front wheels – 1/16th” thick acrylic
• 2 1/2″ thick rubber bands for treads for the back wheels
• 2 1″ female threaded rods – 8-32 screw
• 2 11″ strips of acrylic for the frame (with cut-out seat for mousetrap)
• 1 piece of foam core cut to fit the mousetrap with grooves so it sits on the acrylic frame
• 1 long-a$$ rubberband

(Picture below is not our final parts list. Also includes optional giant wheels for extra-slow-going mousetrap car.)

Early test runs of the car before we got better fitting bearings to help it stay straight.