A mousetrap car runs on the spring energy from a standard mousetrap
A mousetrap car is a small vehicle powered by the wound spring inside a common mousetrap. When you set the trap's spring mechanism and release it, the spring unwinds and pulls a string or lever attached to the car's wheels, making the car move forward. The car travels until the spring runs out of energy — usually between 10 and 30 feet, depending on how you build it.
This is a popular project in middle and high school science classes because it teaches basic physics: potential energy (the wound spring), kinetic energy (the moving car), and mechanical advantage (how the string and wheels work together). You build the whole thing yourself from materials you can find at a hardware store or online, and it costs between $15 and $40 to complete.
Key Takeaways
- The mousetrap's spring is the only power source — everything else is designed to transfer that energy to the wheels as efficiently as possible.
- You need a chassis (the frame), wheels, an axle, a string attached to the mousetrap arm, and a way to keep the string from tangling as it unwinds.
- Lighter cars travel farther because the same spring energy has to move less weight.
- The string must pull the wheels at the right angle and speed, which you control by how you attach it and how large your wheels are.
What you need to gather before you start
Start with a standard wooden mousetrap — the kind with a spring-loaded arm that snaps down. You will also need two wheels (usually 2 to 3 inches in diameter), an axle to hold them (a wooden dowel or metal rod works), and a chassis to connect everything. Many builders use foam board, plastic sheets, or thin plywood for the chassis because they are light and straightforward to cut.
You will need string or fishing line to connect the mousetrap arm to the axle, a way to wind the string so it pulls smoothly (usually a spool or a notch carved into the axle), and something to hold the mousetrap in place on the chassis. Hot glue, wood glue, or zip ties all work. Some builders add a third wheel at the back for balance, or use CDs as wheels because they are light and roll smoothly.
Building the chassis and attaching the wheels
Cut your chassis material into a rectangle roughly 6 to 8 inches long and 3 to 4 inches wide — the exact size depends on your mousetrap and wheels. Drill or cut two holes near the back of the chassis, one on each side, so your axle can pass through and the wheels can spin freely. The axle should be slightly longer than the width of your chassis so the wheels do not rub against the frame.
Slide the axle through the holes and attach your wheels to each end. If you are using a wooden dowel, you can glue the wheels on or use a pin through a hole drilled in the dowel. If you are using a metal rod, a cotter pin or small bolt works well. Test that both wheels spin freely — if they catch or bind, the car will not roll smoothly. Some builders add a third wheel or a drag skid at the front to keep the chassis level as the string pulls.
Mounting the mousetrap and connecting the string
Attach the mousetrap to the top of your chassis, usually toward the front, using hot glue or zip ties. The mousetrap arm should point toward the back of the car, in the direction you want it to travel. Set the mousetrap (pull the arm back and latch it) to see how far the arm swings — it should swing freely without hitting the chassis or wheels.
Tie one end of your string to the mousetrap arm, as close to the pivot point as possible. Run the string along the top of the chassis toward the back, then wrap it around your axle or spool several times. The string should wind around the axle in the direction that will pull the wheels forward when the mousetrap arm swings back. Leave a small amount of slack so the string does not tangle as it unwinds. Some builders carve a groove or notch into the axle to keep the string from sliding off as it winds and unwinds.
Testing and tuning for distance
Place your car on a flat surface and set the mousetrap by pulling the arm back and latching it. Release the car and watch what happens. The mousetrap arm will swing, pull the string, and the wheels should roll forward. If the car does not move, check that the string is wound tightly around the axle and that the wheels spin freely. If the car moves only a few feet, the problem is usually friction or weight.
To make your car travel farther, reduce weight wherever you can — use lighter wheels, thinner chassis material, and remove any unnecessary parts. You can also increase the mechanical advantage by using smaller wheels (they require less force to turn) or by winding the string around the axle multiple times so each swing of the mousetrap arm pulls more string. Test each change one at a time so you know which ones actually help.
Common problems and how to fix them
If the string tangles or bunches up, the problem is usually that it is not wound neatly around the axle. Carve a groove into the axle or use a spool to guide the string. If the car pulls to one side, one wheel may be larger than the other, or the string may be pulling at an angle. Check that both wheels are the same size and that the string pulls straight back along the center of the chassis.
If the mousetrap arm hits the chassis or wheels before it finishes swinging, move the mousetrap forward or tilt it slightly. If the car barely moves, the wheels may be too large or the string may not be wound tightly enough. Experiment with wheel size — smaller wheels usually travel farther because they require less force to turn. If the car flips or tips, add a third wheel at the front or move the mousetrap lower on the chassis to lower the center of gravity.
Why wheel size and weight matter so much
The spring in a mousetrap stores a fixed amount of energy. That energy has to do two things: overcome friction in the wheels and axle, and accelerate the car's weight. A lighter car with less friction will travel much farther than a heavy car, even with the same mousetrap. This is why many winning mousetrap cars use lightweight materials like foam board and thin plastic, and why wheel choice is so important.
Smaller wheels require less force to turn, so they let the mousetrap's spring energy go further. Larger wheels cover more ground with each rotation, but they are harder to start moving. Most successful designs use wheels between 2 and 3 inches in diameter. The string also matters — thinner string creates less friction as it unwinds, and fishing line often works better than regular string because it is smoother and lighter.
Frequently Asked Questions
Can I use any mousetrap, or does it have to be a specific kind?
A standard wooden mousetrap with a spring-loaded arm works best. Avoid electronic traps or traps with different mechanisms — they do not have the same reliable spring energy. All wooden traps are similar enough that the design works the same way, though the spring strength varies slightly between brands.
What is the best material for the chassis?
Foam board is popular because it is light, straightforward to cut, and cheap. Thin plywood works if you want something more durable. Plastic sheets are also good. Avoid heavy materials like thick wood or metal — every ounce of weight reduces how far your car travels.
How do I make the car go straight instead of veering to one side?
Check that both wheels are the same size and that the axle is level. Make sure the string pulls straight back along the center line of the chassis, not at an angle. If one wheel is slightly smaller, the car will pull toward that side. You can also add a small drag skid at the front to help guide the car straight.
Does the size of the wheels really make that much difference?
Yes — wheel size is one of the biggest factors in how far your car travels. Smaller wheels (2 inches or less) usually win distance competitions because they require less force to turn. Larger wheels cover more ground per rotation but are harder to get moving. Test different sizes if you have them available.
What happens if the mousetrap spring breaks?
The spring will no longer store energy and the car will not move. You will need a new mousetrap. This is rare if you handle the trap carefully, but it can happen if you over-wind the string or if the trap is old and the metal is brittle. Buy a backup mousetrap before you start building.