Simulation
Try a design before you cut anything. Set your car's sizes, run a test, and see how far it rolls and where the spring's energy goes. Every setting is explained on the Guidance page.
Point at a setting to see that part light up on the car. Your settings stay saved in this browser.
Speed over time
Where the spring's energy went
Drivetrain numbers
Using it on your project
- Start from the Starter car. Then type in your own numbers: the six design settings from your sketches, and the measurements once you have a trap and parts to weigh.
- Pin a run, then change one setting. The pinned run stays on the chart in gray, so you can see whether the change helped.
- Read the build checks under the results. A red or amber check is a problem a real car would have too. Each one says what fixes it.
- Look at the energy bar. The biggest slice is where to work next. The Guidance page explains each one.
- Calibrate after your first real run. Type in how far your car actually went. From then on, the simulator is predicting your car.
- Predict, then test. Before each real change, write the simulator's prediction in your iteration log. Write the real result next to it.
It's a model, not a promise. The first prediction is usually too far, because a real car wobbles, steers off course, and slips its string. That's what calibrating fixes. Your score comes from the real test track.
How the model works and what it assumes
- Lever angle. 180° is fully wound, lying back toward the rear axle. 0° is at rest, lying forward on the trap. The spring's torque drops in a straight line from your measured value at 180° to 40% of it at 0°.
- Winding. Winding the axle shortens the string and pulls the lever tip toward the axle. Winding stops at 180°, or earlier if the lever runs into the axle or its tip hits the floor.
- Pulling force. String tension = spring torque ÷ the string's moment arm (the perpendicular distance from the hinge to the string). The force at the floor = tension × axle radius ÷ rear wheel radius.
- End of the pull. Near the end of the swing the string lines up with the lever and stops pulling. The string is assumed to slip off a hook on the axle there, so the car coasts freely while the lever drops onto the trap.
- Axle friction. Friction torque = friction factor × load on the axle × axle radius, for both axles. String tension adds to the rear axle's load. The front axle is a 3/16 in (4.8 mm) rod.
- Other losses. Floor rolling resistance = factor × car weight. Air drag = ½ × air density × 1.0 × frontal area × speed². If the drive force is more than the rear wheels' grip allows, the wheels spin and that energy is lost.
- Mass. Wheels are solid discs, about as heavy for their size as a CD, and the energy it takes to spin them up is counted. The trap (about 30 g) sits at the trap position; the rest of the body mass sits at the middle of the chassis. That sets how much weight the drive wheels carry.
- Hardware. The trap base is 11.4 cm (4.5 in) long. The spring hinge is 1.6 cm above the rear axle's center line.
- Left out. Lever weight and bending, string stretch, string piling up on the axle, wheel wobble and misalignment, and the car steering off course. Real cars usually go a shorter distance than the first prediction. That is what calibration is for.