Mouse Trap Car

This mouse trap car was built from a SECME kit intended for competition.

Materials that we used which were NOT included in the kit:
One block of 0.75in x 1.75in x 6.5in pine which I had laying around for the body
Two sections of  1.5in x 7/8in dowel rod for axle ends
One 4in piece of 1/8in steel rod
Two 1/8in washers
Two 1/4in fender washers
Two 1 inch pieces of 0.060in or smaller wire
Six small wood screws
Two old CDRs for rear wheels
Wood glue or Elmers

Tools:
Drill with 1/16in, 3/32, 1/8in, 5/64in, 3/8in drill bits
Coping saw or jigsaw
Sandpaper
Hacksaw (for steel rod)
Pliers
Screw driver
Safety glasses
Vise

Start by removing the bait hook and the locking lever.  Only the bail and the spring are necessary to power the car.

Next we secured the trap base to the pine block with four wood screws.  1/8in Pilot holes were drilled into the base to prevent the wood on the trap base from splitting.  Note in the picture below that I have yet to remove the bait hook.

We drilled a 3/8in  hole centered about 1/2in from the rear of the trap base through both the base and the pine block.

This whole was used to create a notch in the back of the trap by which the fishing line connects the bail to the rear axle to propel the car.  *It is recommended that the notch be created at this time before the car is assembled. We waited until the car was assembled (as you will see in the pictures).  Once the fishing line gets used once to propel the car, it becomes very curled from being wrapped around the axle.  It is very hard to feed the fishing line into the axle hole with your fingers with just the 3/8in hole to work with.*

We used the 5/64in bit to drill the holes in the side of the block for the axles to slide through.  Both axle holes were drilled on center through the side at 1/2in from the front and rear of the block

We also cut the bottom front corner off of the pine block to provide plenty of clearance for the front end of the car because of the fact that our car angles downwards at about 30 degrees.

The 6cm 1/8in steel rod front axle which was included in the kit was then installed.  The 3.5cm black plastic wheels included with the kit are secured to the front axle by gently tapping them in place with a hammer until the axle passes about half way through the wheel hub.

The dowel rod sections serve multiple purposes: 1) As a way to attach the CDs to the axle; 2) To build a little extra angular momentum; and 3) As a grip surface with which to prevent the rear axle from spinning once the bail is set.

We drilled a 3/32in hole all the way through the center of each 1.5in length of dowel rod.  The 3/32in hole served as pilot hole for both sides.  The CDs were then attached to the dowel rod with a fender washer and a wood screw being careful to center the CD on the dowel rod as closely as possible when the screw is tightened all the way

We then drilled three 1/16in holes into the 4in rear axle.  The first hole is drilled at the center of the axle length (at 2in).  The other two holes were drilled at 1in and 3in (or 1in on either side of the center hole)
This was perhaps the most difficult part of building our mouse trap car.

The axle was then attached to one CD wheel assembly by placing a dab of glue into 3/32in hole in the dowel rod and gently nailing it about 1/2in into the hole.

One of the small washers was then fed onto the axle and then the axle through rear block axle hole.

The second small washer was fed onto the other end of the axle before securing the other CD wheel assembly to the right side of the axle.

We then secured the right CD wheel assembly using the same process as mentioned above.

Here the two washers have been pressed against the body so that you can see the three axle holes if you look carefully.

The outside two axle holes were used together with the washers and wire to create bushings for the real axle.

The wire is twisted around the axle from both directions creating a makeshift cotter pin.

The fishing line is cut to about a 5in length and tied at one end to the middle of the bail and the other end is fed into the center axle hole. The line must be trimmed to just the right length so that it pulls free of the hole once the bail has fully sprung.  If too much of the line is fed through the hole, it will not come out and instead will rewind itself in the opposite direction around the axle causing the car to brake to a halt as shown in this video:

Carefully pull the bail up while rotating the rear wheels backwards to ravel the fishing line around the axle. It is recommended that you use a block of wood to hold the bail up a little while feeding the line to free up your hands and help minimize the chance of smashing your fingers if you lose your grip on it. Once the bail is fully set, hold the axle by one of the dowel rods to keep the spring from spinning the axle. Set on a smooth, flat floor and release the axle to propel the car.

Our car only went 32 feet, but it was still fun building it and finally getting it to work.  The plastic wheels included in the SECME kit which we used for the front probably add quite a bit of extra friction.

There are several items which were included in the SECME kit which we did not use: