Guidance
Everything from class that applies to this car, in one place. Come back to this page while you're building — it's meant to be used dirty, not read once.
Try it in the Simulation. Every setting in the simulator is explained on this page, under the same name. Each section below says which setting it's about, and each setting in the simulator has a "More" link back here.
The one idea
A mousetrap spring holds a fixed amount of stored energy. That's your whole budget. You can't add to it, and every joule that goes somewhere other than moving the car forward is gone.
So the entire project is one question: where does the energy go, and how do you spend more of it on the thing you're being scored on?
W = F × d
Work is force applied over a distance, measured in joules. It's the same equation whether you're talking about the spring pulling the string, the wheel pushing the ground, or friction stealing energy from your axle.
Anatomy: the six numbers you design
Strip a mousetrap car down and you're choosing six sizes. They're the six settings under Your design in the simulator, and the same six your sketches and Onshape model have to pin down.
| Simulator setting | What it is | Explained in |
|---|---|---|
| Rear wheel diameter | The drive wheels, on the axle the string winds around. | Wheel and axle |
| Front wheel diameter | The wheels that just roll along. | Axle friction |
| Chassis length | Rear axle to front axle. The brief caps the whole car at 24 in, not counting the lever arm. | Brief |
| Lever arm length | Spring hinge to where the string ties on. | The lever arm, The lever's swing |
| Mousetrap position | Rear axle to the spring hinge. | The lever's swing |
| Rear axle diameter | Where the string winds. Tape or a bushing makes it thicker. | Wheel and axle |
The spring: your whole budget
The lever swings half a turn, from 180° (wound), lying back toward the rear axle, to 0° (at rest), lying forward on the trap. The spring pushes hardest when it's wound and weakens as it unwinds, to roughly 40% by the end. The simulator uses exactly that.
The one number you need from your trap is how hard it twists when it's wound. That's the simulator's Spring torque when wound. Measure it, don't guess it.
τ = scale reading (N) × 0.10 m
If your scale reads in grams, multiply by 0.0098 to get newtons first. A typical trap is around 0.4 N·m.
E ≈ 2.2 × τ
The spring averages about 70% of its wound torque over half a turn (π radians), so the stored energy in joules is about 2.2 times the torque in N·m. A 0.4 N·m trap holds about 0.88 J. That's the "spring holds" number above the simulator's energy bar.
The lever arm: force or distance, pick one
A simple machine never gives you free energy. It lets you trade force for distance. The lever arm on your trap is the clearest example you'll build all year. In the simulator it's the Lever arm length setting.
τ = F × r × sin θ
Torque is rotational force. It depends on how hard you push, how far from the pivot you push, and the angle. Torque is largest when the force is perpendicular to the arm — which is why the pull on your string changes strength as the arm rotates through its swing.
T = τ ÷ string moment arm
Flip the torque equation around and you get the pull in the string. The string's moment arm is the straight-line distance from the hinge to the string, measured at a right angle to the string. A longer lever means a longer moment arm, so less tension. The simulator shows it as String tension at the start under Drivetrain numbers.
IMA = din ÷ dout
The ratio of the effort arm to the resistance arm. For a wheel and axle it's the ratio of the radii instead, because a wheel and axle is just a lever that keeps going around.
The lever's swing: how long, and where the trap goes
Lever arm length and Mousetrap position work as a pair. When you wind the car, the string pulls the lever tip back toward the rear axle. When it's released, the tip swings up and over to the front. The string it drags off the axle is the difference between those two.
pull ≈ 2 × (lever arm length or mousetrap position, whichever is shorter)
A lever longer than the mousetrap position doesn't pull more string. Its tip just swings past the axle, and the extra length only weakens the pull. The simulator shows the real number as String pulled off the axle.
Three simulator checks come from the swing. Here's what each one means on a real car:
- Winding. The lever can't wind all the way back to 180°, because it runs into the rear axle or its tip hits the floor behind the car. Every degree it can't wind is spring energy you never use. Shorten the lever or move the trap forward.
- Lever reach. The lever is longer than the distance from the hinge to the rear axle. Shorten it to match, or move the trap back.
- Floor hit. As it unwinds, the lever tip hits the floor in front before it lies flat, and then drags. Small front wheels with big rear wheels tip the nose down and cause this. Bigger front wheels, smaller rear wheels, or a shorter lever fix it.
The wheel and axle: your distance multiplier
This is where Rear wheel diameter and Rear axle diameter come in. The string wraps around the rear axle. Every bit of string the lever pulls off turns the axle, and the wheels on that axle roll the car along the floor.
distance ≈ string pulled × (rear wheel diameter ÷ rear axle diameter)
Same as R ÷ r. The simulator shows this ratio as Rear wheel ÷ axle diameter. It's the distance while the spring is pulling. After the string comes off the axle the car coasts, and in the simulator the coast is often nearly as far again. Comparing your prediction to your measured result is exactly the kind of analysis the rubric is looking for.
Ffloor = T × (rear axle diameter ÷ rear wheel diameter)
The same ratio, flipped. A 25 : 1 wheel-to-axle ratio rolls 25 cm per cm of string, but pushes the floor with only 1/25 of the string tension. If that push is smaller than the friction holding the car back, the simulator says Won't start (it never moves) or Stalls (it stops partway through the pull).
What each change costs you
Nothing here is free. Every change that buys you distance takes it out of somewhere else — usually the force available to get the car moving in the first place. The last column is the simulator check that warns you when you've gone too far.
| Change | Effect on distance | What it costs | Watch for |
|---|---|---|---|
| Bigger rear wheels | More, up to a point | Less push at the floor. Too big and the car stalls. | Won't start, Stalls |
| Thinner rear axle | More, up to a point | Less push at the floor, and more turns to wind. Too thin and it stalls. | Won't start, Stalls |
| Longer lever arm, with the trap moved back to match | More | A gentler pull, so it may not start. Longer than the mousetrap position adds nothing. | Lever reach, Winding |
| Bigger front wheels | More | Taller, a little heavier, and they need room on the chassis. | Build (wheels overlap) |
| Lighter car | More | Less weight on the drive wheels, so less grip. | Wheel spin |
| More weight over the drive wheels | Usually less | Energy to move it and more axle friction — but it buys grip. | Wheel spin (it helps fix it) |
| Better axle bearings | Much more | Build time and parts. Usually the best trade on the list. | The energy bar |
Read the third column before you change anything. Teams that only read the second column build a car with a beautiful long lever arm that cannot move itself.
Where the energy goes
Once the spring lets go, every joule ends up as one of the losses below. The simulator's Where the spring's energy went bar uses the same names and colors, so you can see which one is eating your car.
The energy bar also shows unused spring energy: energy that never reached the car at all. It shows up when the lever can't wind the full 180°, when the floor stops the lever early, when the car stalls with the spring still wound, or when the lever snaps down past the end of the pull. Those are fixed with the lever's swing, not with friction.
Friction
Ff = μ Fn
Friction depends on the materials in contact (μ) and how hard they're pressed together (Fn). Not on surface area — that's the counterintuitive one from class.
You need friction in exactly one place: between the drive wheel and the floor. Without it the wheel spins and the car sits still. Everywhere else — axles, bushings, the string rubbing the chassis — friction is stealing from your budget.
So the goal is not "less friction." It's high friction at the tires, low friction everywhere else. Teams that write that sentence in their presentation tend to have already solved the problem.
Axle friction · losses 2 and 3
The simulator's Axle friction factor is μ for your axles: how easily the axle turns in whatever holds it. In most cars it's the biggest loss on the bar.
friction torque = μ × load on the axle × axle radius
That friction works at the axle's surface, but the car moves at the wheel's rim. So the bigger the wheel around the same axle, the less you lose per meter. That's why Front wheel diameter matters: on the starter car, going from 3 cm to 12 cm front wheels takes it from about 13 m to about 21 m.
| Axle setup | Axle friction factor |
|---|---|
| Ball bearings | about 0.05 |
| Axle turning in a plastic straw | about 0.15 |
| Metal rod in a drilled wood hole | about 0.3 |
| Rubbing, tight, or wheels touching the chassis | 0.5 or more |
These are rough guides. Your real number comes from calibrating with a real run.
Floor rolling resistance · loss 4
Froll = Crr × car weight
Wheels squash a little where they touch the floor, and that takes energy every meter. Hard, true wheels on a smooth floor lose the least. The simulator's Floor rolling resistance is Crr: about 0.004 for hard wheels on smooth tile, 0.01 for a hallway floor, and 0.03 on carpet — enough to stall most cars. Run your trials on the same kind of floor as the test track.
Air drag · loss 1
Fair = ½ × air density × Cd × frontal area × speed²
The simulator's Frontal area is roughly the height × width of the car seen from the front. At mousetrap speeds drag is small — around a tenth of the energy on the starter car — but it grows with speed squared, so it matters most for cars that launch fast. The simulator uses Cd = 1.0 and air density 1.2 kg/m³.
Rear wheel grip · loss 5
max push before spinning = grip × weight on the drive wheels
The simulator's Rear wheel grip is μ between your tires and the floor: about 0.2 for a bare CD on tile, 0.5 for a balloon or rubber band tire, 0.9 for a rubber O-ring. If the string pushes harder than this, the wheels spin and the energy is wasted. Spin only happens with a strong pull — a short lever, a thick axle, small wheels. Try the Fast launch preset to see it.
Mass and rotational inertia
The simulator's Car mass without wheels is everything except the wheels: chassis, trap, lever, axles. Weigh it. The simulator adds the wheels' mass from their size, as if they were about as heavy as a CD for their size.
Every gram costs distance, because rolling resistance and axle friction both grow with weight. On the starter car, going from 110 g to 200 g drops the distance by about a third. Adding mass over the drive wheels does buy grip, though. Which one wins depends on your car, which is why you test.
Two wheels can weigh the same and still be very different to spin. Mass out at the rim is much harder to accelerate than the same mass near the hub. That's rotational inertia. The energy it takes to spin up your wheels isn't lost — it comes back as they keep the car rolling. What heavy wheels really cost you is their weight.
What to research
Curate, don't collect. Every source you save should have a note saying what you're going to use it for. A page of links with no notes scores as poorly as no research at all.
- Common mousetrap car building techniques
- How to attach an axle to a chassis so it spins freely
- How to wrap and attach the string on the drive axle
- How to calculate the total distance a car will travel
- Reducing friction in a mousetrap car — bearings, bushings, lubricants
- Mechanical advantage as it applies to a mousetrap car
- Inertia and rotational inertia in wheels
- Air resistance at mousetrap car speeds — and whether it's actually worth designing for
The six questions your sketches must answer
Sketching without a question to answer is doodling. Every sketch you turn in should be trying to settle one of these. The simulator setting each one decides is in brackets — try your answers there before you commit.
- How long will the car be? [Chassis length]
- How big will the wheels be — and are the front and rear the same? [Rear wheel diameter, Front wheel diameter]
- How long will the lever arm be, and where does the string attach on it? [Lever arm length]
- Where does the mousetrap sit on the chassis? [Mousetrap position]
- How are the axles attached to the chassis, and what lets them spin freely? [Rear axle diameter, Axle friction factor]
- What material is each part made of — axle, wheels, chassis, lever arm? [Car mass, Rear wheel grip]
Running a real trial
A trial you can't repeat isn't data, it's an anecdote. The rubric asks for trials conducted scientifically, and that means something specific.
- Change one thing between trials. If you change three, you've learned nothing about any of them.
- Run at least three times per configuration and record every run, including the bad ones. Throwing out a run because you didn't like it is how you fool yourself.
- Record the same measurements every time — distance travelled, plus anything you noticed about the run.
- Note the setup: which floor, string length, where the string was tied, wheel size.
- Write down what you expected before the run. Being wrong on the record is worth more than being vaguely right afterward. The simulator's prediction is a good expectation to write down.
Calibrate the simulator with your first real run. Set the simulator to match your car, then type your measured distance into Calibrate with a real run. It changes the axle friction factor until its distance matches yours. After that, use it to predict each change before you make it, and put the prediction and the real result side by side in your iteration log.
Tuning: two problems you will have
It moves fast and easily but doesn't go far. You're spending the spring too quickly. Look at: lengthening the lever arm (and moving the trap back to match), reducing the rear axle diameter, or going to a larger drive wheel. Expect the car to accelerate more gently as it reaches farther — that's the trade, and saying so out loud in your presentation is the point.
It struggles to move at all. You don't have enough force at the wheels, or you're losing it before it gets there. In the simulator this is Won't start or Stalls. Look at: shortening the lever arm or moving the string attachment closer to the hinge, increasing the rear axle diameter, adding traction at the drive wheels, and hunting down axle friction. Expect to lose some range in exchange, so make the smallest change that gets it moving reliably.
Before you cut, print, or glue anything, check the shop procedures. Material waste is scored on this project and on your daily grading page.