Why weight a Pinewood Car?

Maybe I am overthinking this (and Googling websites was no help wanting to sell weights), but why is more weight good in a Pinewood race? The generic argument is more weight, more inertia, more momentum. Does that make sense?

I’d say more weight, the more inertia to overcome at the start, but maybe with a steep slope, it doesn’t matter. Next, I’d say more weight, more friction since friction has a component of the weight [mgcos(x)] manifested as friction (f). And, if the track is straight, I can’t see a need to shift the center of gravity (which may help on curves).

So, technically speaking…what does added weight do for a Pinewood car? :confused:

Aerodynamic drag doesn’t increase when you add weight. If two cars have the same exterior shape, but one has been hollowed out and filled with lead, then the latter car will win in a gravity-powered race. Axle friction might be slightly higher, but that effect typically is minor.

I don’t know the science behind it, but I do know that more weight does move cars and bobsleds down slopes a bit faster. It might depend on how much weight or where it is placed.

Scientists will be along soon.

Because the cars start on a downslope, which then levels out, the most weight placed at the rear of the car gives the greatest advantage since the rear of the car will travel down the slope further than the front of the car.

You want both more weight, and you want the weight distribution to be as far to the back of the car as possible.
Pinewood derby cars start on a slope with the fronts of the cars lined up at the starting gate, and finish on a level stretch of track.
You want weight distribution on the back of the car because that is essentially free energy. In the large scale, if you dropped a 10lb weight straight down from 1/2" or straight down from 100 yards, which will have more energy (hit the ground harder) ? The pinewood derby is the same thing in small scale.
You want more weight period because the track levels out. Given the same aerodynamics if you drop a 100lb weight and a 1lb weight straight down they both would hit the ground at the same time, but the 100lb weight would have a lot more energy. That comes into play in the pinewood derby because once they reach the level portion the cars are no longer being accelerated by gravity and thus have to use the energy they already have to reach the finish. The increase in rolling resistance is way too small to counter the advantage the heavier car has.
Without the level portion it would be a more straightforward contest of rolling resistance and aerodynamics.

Not true, unless you drop them in a vacuum (where aerodynamics play no role).

Any aerodynamic drag represents a larger percentage of the small mass’s weight, so at any non-zero airspeed the resulting acceleration is greater for the larger mass than for the smaller. Thus, the 1-lb weight always takes more time to hit the ground.
The simple answer to the OP’s question is “Because there’s friction.” Without this, the weight of a Pinewood Derby car wouldn’t matter. In the real world (where friction is unavoidable) the heavier car has an inherent advantage.

That’s an oversimplified answer. Almost all of the forces acting on a pinewood derby car, including most of the frictional forces, are proportional to the mass of the car. If they were all proportional to the mass, then the mass of the car wouldn’t matter. But they aren’t quite all proportional to mass: The aerodynamic drag is independent of mass, and so the mass does matter.


But since it isn’t possible to have a real-world situation where all friction is proportional to mass, I’d argue it’s a useful simplification.

My dad hammered a d battery into a rectangular shape and we incorporated it into the body of the car. It was the ugliest car ever entered in the pinewood derby, the race judges held it up before the races to make fun of it, then they drilled a couple dozen holes in the bottom to get it down to legal weight. It won 3rd overall. They should record the weights and mark the center of gravity on all the entries, it would be useful information, for somebody, maybe, somewhere?

With two boys that went through Cub Scouts and as a den leader with a small workshop in the garage, I have quite a bit of experience with Pinewood Derby cars. I always made my boys do their own work, but I gave them guidance. My oldest son won the pack wide race once, but we usually lost only to the obviously Dad-made cars.

Definitely get as close to the weight limit as you can. When the car is at the top of the track, it has potential energy (PE). PE is proportional to both mass and height, so you want to get as much weight as high as possible, that is why everyone tries to get the most weight at the rear of the car. The idea is to get the car’s center of mass as high up as possible. One thing to watch out for here is that you don’t put so much weight behind the back axles that the front wheels lift off the track. It might be cool to see a car doing a wheelie down the track, but it usually results in the back of the car rubbing the track, losing a lot of the PE as friction.

The Conservation of Energy says that energy cannot be created nor destroyed, but it can be converted to some other form of energy. So that means, the car with the most PE at the top of the track has the best chance to win. The idea is to convert that PE to kinetic energy (KE). But friction is your enemy. With the small size of the cars, air resistance is almost negligible. Friction will be what kills you.

Where are you going to generate the most friction? In the wheels and axles - in most cases, the only moving parts on the car. Boys Life usually has adds where you can buy polished axles and balanced wheels and other stuff meant to reduce friction between the wheels and axles. The rules my pack used said that we could only use the part provided in the official BSA kit.

I would mount each axle in the drill press and have the boys use a fine file to smooth out the shaft of each axle and reduce the head to only as large as it needed to be. Then I would have them use a strip of 800 grit sandpaper and polish the axle and the inside of the head. If you have any polishing compound, you can get those axles to a mirror finish. I preferred the graphite over the teflon for lubing the axles before races. I would also have the boys use the fine grit sandpaper to take off any molding lines from the outside of the wheel and polish them up as well. There are also contraptions you can buy that will the insides of the wheels as well.

Per our pack rules, we didn’t need to use the axle slots provided but we had to have the same distance between axles, so I would drill different holes. I would drill them on a bit of an angle so with the weight of the car, the wheels were pushed to the outside so they wouldn’t rub against the car. I did this part since Cub Scouts aren’t allowed to use power tools.

The way the pinewood derby tracks work, is it uses a thin board that runs between the wheels - under the car - all the way down the track. So you need to make sure that the belly of the car is high enough to not rub on the track. You also need to make sure that the tires are absolutely square, so the car travels straight down the track without rubbing the wheels against the raised board. To check this, once the car is finished, find a long board and lay it on the floor. Put the car on one end and slowly raise that end off the floor. The car should travel straight down the board. You will need to adjust the wheels if the car veers off of a straight line.

Also, I would have my sons raise one of the front wheels off the track just barely. It was low enough to catch the track of it started to veer for whatever reason, but since it didn’t touch the track, there was no friction generated on that wheel.

Finally, our pack allowed us to add graphite between races. Get as much graphite between each wheel and axle as possible and run the car across the floor a couple times to breakup any chunks.

Air resistance is more significant for small objects, not less, and as already stated, it’s the only reason why adding weight matters. If air resistance were truly negligible (if the race were run in a vacuum chamber, say), the weight would be irrelevant.

Yes, at the cost of increasing the friction on the other three wheels, for the same total friction.

There will probably be a small savings if three wheels are turning rather than 4. The reason is that a bearing will have at least some friction (plausibly dependent on rotational speed) even when its wheel supports no weight.

(Bolding mine)

This is a key misunderstanding. Yes, adding weights adds mass, which means more inertia to start with. However, each gram of mass is pulled on equally by gravity, so each time you add an gram of mass, you also add an equivalent amount of downward force applied by gravity. All other things begin equal, this would mean that weighting a car would be useless, as every additional gram would be exactly balanced by the added inertia. However, since other things like friction and air resistance aren’t directly proportional to weight, weighting gives you a distinct advantage.

Not much to add, other than I just helped my son with his pinewood derby car, and we used a food scale to get the car as close to 5 oz. as possible. In fact, we included brass screws in the back of the car to drive the weight a little above 5 oz., then after the weigh-in (where we were .05 oz. over) removed screws until we met weight (I calculated each screw–#6, .75" long–was .0375 oz. He placed second in his den of ~30 scouts and 5th overall in the pack (~150 scouts).

Getting the car to exactly 5 oz. is one of two essential steps IMO. The other is graphite (add it just before the weigh-in; it begins to wear out after 4-5 races).

It also reduces the potential energy lost to the rotational energy of the wheel.

I suppose that would be true, though it’d be a pretty small effect.

You’re assuming that friction is perfectly proportional to mass. That’s a pretty good estimate for most purposes, but never exactly true – for instance, as noted there’s going to be some rotational friction in the axle even if it’s supporting almost no weight. I wouldn’t be surprised if there’s enough divergence from linearity to be significant in a race, particularly as there’s both an acceleration and decceleration regime (sloped and flat portions of the track).

Another way to give your car an edge is to give it a smooth, glossy finish (and aerodynamic profile, obviously) to reduce the air friction as much as possible. The lower the car’s profile the better, although you need enough room for your 5 oz. of weight.

I would have thought this, but my observations are that minimizing wheel friction and maximizing the weight in the rear make the most difference. Cars that resemble a modern engine in the rear dragster seem to win most often, but I saw one winner with a car that had barely been carved. The scout (and his dad) got several sets of wheels and selected the ones that ran most smoothly on the axle nails.

Well then, that bodes well for my planned “ice cream truck” entry!