Pinewood derby
weight placement
Everyone will tell you to put the weight just ahead of the rear axle. Nobody shows you what it is worth. Drag the weights below, watch the centre of gravity move, then race the car you have built against a stock one and read the gap at the finish line.
Drag the grey weights along the car.
Why the back of the car wins
A pinewood derby car has no engine. Every bit of speed it will ever have comes from how far its centre of gravity falls between the starting gate and the finish line, minus whatever is lost to friction and to spinning up the wheels.
The starting ramp is the part people miss. At the gate the car is tilted nose-down, so the tail is the highest point of the whole car. If the mass sits near the tail, it begins its journey higher up than if the same mass sat in the middle of the block, so it has further to fall. The rear weight is not pushing anything. It simply starts with more energy in the bank.
That is also why the advice has a limit rather than being “as far back as possible”. Push the centre of gravity behind the rear axle and the car will lift its nose, steer badly, and rub the guide rail all the way down. The half-inch to one-inch window is where the energy gain is nearly maximised and the car still tracks straight.
What actually makes a difference
- Get to the weight limit. Five ounces exactly, checked on the official scale. A car that weighs 4.6 oz has given away speed before it has left the gate.
- Move it to the tail, within reason. Three quarters of an inch ahead of the rear axle is the number to aim for. Use the slider above and watch the finish gap change.
- Take mass off the wheels. Wheels have to be spun up as well as carried, and that energy never reaches the finish line. Turn the wheel-mass slider down and see how much a gram is worth.
Common questions
- Where exactly should the weight go?
- About three quarters of an inch ahead of the rear axle. The useful range is half an inch to one inch. Closer than that and the car gets twitchy and starts wandering into the guide rail. Further forward and you give away speed for nothing.
- Why does weight at the back make the car faster?
- Because of where the car starts, not where it finishes. On the ramp the whole car is tilted, so a centre of gravity nearer the tail sits higher up the slope at the starting gate. It therefore falls further on the way down, and a longer fall means more energy. The rear weight is not pushing anything. The car just starts with more to spend.
- How much difference does it actually make?
- Move the centre of gravity from the middle of the block to an inch ahead of the rear axle and you typically pick up a few hundredths of a second over a 32 foot track. That sounds trivial until you watch a heat: a hundredth is roughly a wheel, and three hundredths is a clear car length.
- Is it better than lighter wheels?
- They are different levers and you want both. Wheels have to be spun up as well as pushed along, and that spinning energy never reaches the finish line, so a gram off each wheel is worth having. Weight placement is free, though, which is why it is the first thing to get right.
- Does the three-wheel trick work?
- Lifting one front wheel clear of the track removes a wheel that has to be accelerated and one more source of friction, and most rulebooks allow it. It only pays if the car still runs straight. A car that keeps brushing the guide rail loses far more than the trick gains.
- Do I have to use the full five ounces?
- Yes, get as close to the limit as the rules allow. Heavier is faster here because the energy available scales with mass while the losses do not scale as quickly. Weigh it on the official scale at check-in, not the kitchen one.
How the simulation works
The car is run down a 32 foot track (a 9 foot sloped section dropping 4 feet, then a 23 foot flat run-out) by numerically integrating its motion in half-millisecond steps. Gravity pulls it down the slope, rolling resistance works against it the whole way, and the wheels are treated as solid discs, so each one adds half its own mass to the inertia the car has to overcome. The centre of gravity you set changes how far the car's mass falls, and that is where the time comes from.
It is a model, not your pack's track. Real tracks vary in length and drop, and real axles vary enormously in how well they are polished. Treat the numbers as a way to compare two of your own cars, not as a prediction of Saturday morning.
Built by Oliver because every page about this explains the rule and none of them let you try it. More things like this under work.