How to Build a LEGO Mousetrap Car

Mousetrap cars are simple toys that are easy to make. The mousetrap car runs on a very basic principle; a string is tightly wound around one of the axles and tied to a spring-loaded mousetrap atop the vehicle 1. When the trap is allowed to spring, the arm pulls the string, causing the axle to rotate as well. Building a mousetrap car is not a precise science, but allows a designer to try new ideas and experiment with the principles involved 1.

How to Design and Build a LEGO Mousetrap Car

Create the car's body. This should be based around a LEGO platform piece that is large enough to support the mousetrap. You may choose to add styling, or you may decide upon a minimalist design for less weight and more speed.

Mount the mousetrap on top of the car's body using hot glue, model cement or some other strong adhesive. Make sure that the "jaw" of the mousetrap is facing the rear end of the car when the trap is in the loaded position. The mousetrap will provide the power for this toy car.

Mount the dowel as a rear axle on the car's body. You can use washers, eyehooks or even special LEGO pieces with holes in them as axle mounts. It is best if the axle mount's hole is not much larger than the diameter of the dowel, ensuring a tight fit. Be sure that the axle is located at the rear of the vehicle.

Mount the wheels. The rear wheels should fit on the dowel axle. You can design a similar axle for the front wheels, or use the pre-made LEGO pieces intended to hold wheels.

Using hot glue (or a similar method), attach one end of the string to the center of the rear axle. Be sure that the string is well attached, as it will undergo a considerable amount of tension — one possibility is to drill a hole through the axle and tie the string through the hole. Run the string behind the rear of the car and tie the other end to the "jaw" of the mousetrap.

Add any finishing touches. Apply paint if you wish, or add any extra LEGO pieces that you wish to add visual appeal or functionality. Perhaps you'd like to include a LEGO driver in your car!

To operate the car, simply leave the trap in the unloaded position, then carefully pull the vehicle back while the wheels are held firmly on the ground. This action should cause the axle to wind the string, pulling back the arm of the trap. Then, simply release, and the arm will pull forward, providing power to the rear wheels.


Experiment and find out what works best for your vehicle. For example, try using different lengths of string to discover what works best. Perhaps you'd like to add a pulley to the rear of the vehicle, so that the string winds and unwinds more smoothly. Try experimenting with different size LEGO wheels, or wheels with differing amounts of traction. By trying new things, you can find out what makes your car run best.


Always leave the mousetrap in the unloaded position when your car is not in use. Leaving the trap loaded causes the spring to lose tension, as well as the added danger of getting fingers pinched by the trap.