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.
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, and where your energy leaks
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.
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.
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 wheel and axle: your distance multiplier
distance ≈ string length × (R ÷ r)
Where R is the drive wheel radius and r is the drive axle radius. This is an ideal number — friction and slip will make your real distance shorter. Comparing your prediction to your measured result is exactly the kind of analysis the rubric is looking for.
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.
| Change | Effect on distance | What it costs |
|---|---|---|
| Longer lever arm | More | Less pulling force — may not start |
| Larger drive wheel | More | Less torque at the ground |
| Thinner drive axle | More | Harder to wind, string can slip |
| More mass over drive wheels | Usually less | Energy to move it, more axle friction — but buys traction |
| Lighter wheels | More | Less coasting momentum once rolling |
Read the third column before you change anything. Teams that only read the middle column build a car with a beautiful long lever arm that cannot move itself.
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.
Adding mass over the drive wheels increases the normal force, which increases traction. It also increases the load on your axle bearings, which increases the friction you're trying to eliminate. Both effects are real. Which one wins depends on your car, which is why you test.
Rotational inertia
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.
It cuts both ways for you. Heavy rims eat spring energy during the launch, when you can least afford it. But once the car is rolling, that same mass helps it keep coasting after the string runs out — and on a distance run, the coast is a real part of your total. Worth testing rather than assuming.
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.
- How long will the car be?
- How big will the wheels be — and are the front and rear the same?
- How long will the lever arm be, and where does the string attach on it?
- Where does the mousetrap sit on the chassis?
- How are the axles attached to the chassis, and what lets them spin freely?
- What material is each part made of — axle, wheels, chassis, lever arm?
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.
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, reducing the drive axle diameter, going to a larger drive wheel, or using a longer string. 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. Look at: shortening the lever arm or moving the string attachment closer to the pivot, increasing the drive 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.