In an effort to make Pinewood Derby Races at Pack 101 fairer and more fun, I have written virtually everything I know about car building in this booklet. You can easily find more detail on any of these topics on the internet. This is a compilation of what I’ve picked up over the last 3 years of building cars with Benjamin and Jamison. The reason I am doing this is that I would like to see the pinewood derbies be a fun activity for dads and boys (and daughters) both while building of the cars together and the races themselves. Not everyone can win but I would like to make a level playing field. No one should be able to win a derby by use of illegal tactics, intentional or otherwise.

Do you want to win? Really?

Building a competitive car requires considerable extra effort above and beyond the shaping and finishing of a car. There are several steps that must be undertaken with considerable care. Doing them all might give you a chance to place in your rank – depending on your skill and effort. Nothing can guarantee a winning car, fortunately, or there would be no fun at all in a derby. And remember, there is nothing wrong with concentrating on the appearance. They give trophies for that too.

I advise doing at least some of the extra steps on weight and axels. That should at least place you in the middle of your rank – win a race or 2 before elimination – and, who knows, you might even do better than that.

A Competitive Car

Building a competitive car is principally based on two areas, weight and axels. Everything else is aesthetics. Many people will debate that aerodynamics plays a part. I don’t believe it does. The “brick” design from a few years ago in Pack 101 makes a strong case. Internet sites from all over the country tell of cars winning without a cut being made from the wooden block in the kit.

A little Physics first

Before getting into the specifics of weight and axels, it will probably be of benefit to understand what makes a car fast – or not.
1. What makes a car go? Gravity
2. What makes a car slow? Friction

Stored (or potential) energy – Stored energy is your motor. It goes like this; you lift your car above the floor a certain distance and let it go. It falls under the force of gravity and gains speed as it goes. Technically the stored energy is the weight of the car multiplied by the distance it falls. Of course in the derby, you put it on the track but the principle is the same. Since all cars are limited to 5.0 oz., the car with the most stored energy is the one who’s weight falls the farthest. At first glance, they all appear to fall the same, but look at Fig. 1. Since the cars start on an angle, the rear of the car is higher than the front. However when they reach the straightaway, the car is level. So the rear of the car falls farther than the front. That says to put the weight as far to the rear of the car as possible. High or low on the car itself makes no difference.

Weights

However there is a problem if the weight is too far back. Small bumps and changes of inclination in the track can cause the front to lift and become unstable. Even though theoretically the car should tilt up and come back down with no ill effects, there is always a little unbalance causing the car to come down on the guide strip in the center of the track or to bump it causing a loss of speed. Also, even though I said that aerodynamics has no effect, having the front of the car raise up in the air will probably trap air and slow the car. Most guides recommend 60-65% rear and 40-35% front. This can be approximated by balancing the car on a pencil. See Fig. 2. The balance point should be about ¾ of the distance from the front wheel to the rear.

You can get your total weight close by using a postal scale prior to the race but those scales only measure to 1 decimal place, 4.9 oz. for example. The pack’s scale measures to 2 decimals. So a car that actually weighs 5.04 oz. will show 5.0 oz. on the post offices scale and will show 5.05 oz. on the pack scale. Consequently, you should expect to adjust the weight during weighing and certification. It is best to be able to adjust your weight by a stable means. Adding or removing brass screws is one easy method. There are other ways but whatever means you choose, you don’t want the weight to be able to shift or fall off. The cars take quite a lick when stopping after the finish line. Weight can be anything from lead fishing weights to pennies or bullets. A good rule of thumb is to attach 90% of your weight solidly and make the last 10% adjustable/removable.

Friction – You push your car along the floor and it slowly stops. Why? You start 2 cars of equal weight on the track and at the bottom of the inclined section of track, one is ahead of the other. Why? In each case it is because some force opposes the car’s motion. Since, at this size and speed, air resistance is negligible, it is friction (and possibly vibration or rubbing – more on that later). Friction is defined as the resistance to motion between 2 surfaces times the area of contact. So there are 2 parts to work on, the surfaces themselves with polish and lubricant and the area of contact. The key to all these opposing forces and the vibration and rubbing is in the car’s axels and wheels.

Axels

There are 2 things to do to make sure that your axels produce a fast car. First is to polish them. Since the axels in the BSA kits are actually cheap nails, they all have crimp marks and a fairly rough finish. You must file off the crimp marks, see Fig 3, and polish the surface. You can demonstrate the effectiveness of this step by a simple experiment. Before polishing the axels, put a wheel on an axel and clamp it or press the axel in the wood block. Spin the wheel with your finger and notice how long it spins. Repeat after polishing the axel. You won’t need a stop watch to tell the difference. All you need to do is place the axel in a drill. Start with filing the crimps off. Then use 400+ grit wet or dry sandpaper followed by polishing compound (pumice and water). Change the cloth frequently and buff when finished. Polishing kits are available at Michael’s, Hobby Lobby, etc.

You can also reduce friction by making the area of contact smaller. See Fig. 4. I do this step but I am not sure it has much effect.

This axel polishing step is not one that I recommend letting your Cub do by himself. If he has pretty good motor control, you might do it together but this requires a drill motor or dremel tool and it is possible to tear-up a finger.

The second thing is to make sure the axels are true running. That means all are parallel so the car is not trying to turn. Any tendency to turn, runs your car into the guide strip and slows it down. Also the wheels could be toed in or out. See Fig 5. If the toe is equal as in the figure, the car may roll straight but the wheels are opposing each other. This is just like putting on the brakes. I prefer to drill holes for the axels. The slots are not precisely parallel and do not hold the axels as tightly as drilled holes. Drill the holes a little undersize to hold the axels firmly. I use a 5/64” drill. A little easier insertion would come from a #45 or a 2 mm. This step must be performed on a drill press – you can not hold a hand drill precisely enough.

A common practice is to tilt the axels up from the body of the car. See Fig 6. This keeps the wheel running against the hub of the axel rather than the body of the car. The hub has been polished whereas the car body is wood or paint which will drag on the wheel hub.

Also at the starting block, make sure the car is exactly centered on the track, pointed exactly straight down the track and the wheels are pulled out against the end of the axels.

Wheels

The plastic wheels in the BSA kits are injection molded. They are round and even but small imperfections do exist. See Fig. 7. The wheels may be slightly out of round, not concentric with the hub and may have a gate mark in the middle of the tread. These imperfections cause the wheels to vibrate up and down as it rolls down the track increasing friction and providing braking. It helps to true the wheels – make them perfectly round and concentric with the hub. The best way to do this is to use a lathe however you can get very good results by mounting the wheel to a mandrel and putting it in a drill. (Mandrels are also available at hobby shops.) Spin the wheel very lightly against fine sandpaper, like 400 to 600 grit. Try to touch only the high spots and stop when you have it touching all around the tread. DO NOT REMOVE THE SERATIONS AROUND THE EDGE OF THE WHEEL. You will be disqualified for lightening the wheel if you do. See Fig. 8. There are truing tools available on the internet if you want to go to that expense and effort. Also be careful sanding and polishing the plastic. It will melt easily from heat generated by friction.

Remove any flash or protrusions from the hub that may drag on the axel or car body. If you can it is good to polish the inside bore of the hub. This can be accomplished by wrapping a layer of cloth around a smaller nail than the axel and using toothpaste for polish. Do not overdo it – just get rid of any burrs or ridges.

A note on inertia. Inertia is the resistance of an object to being moved because of its mass. Once in motion it is the tendency to keep going (momentum). When the car starts down the track, gravity pulls on it and 2 things resist the motion – one is friction (the wheels turning against the axels) and the other is inertia. Inertia for the car has 2 forms. One is the weight of the car and you can’t do much about that. The other is the resistance of the wheels to rotation. It requires energy to spin the wheels and that comes from the pull of gravity. The lighter the wheels are, the easier they spin. That is why there is a rule against shaving the tread or inside of the wheels. No wheels should be shaved on the sidewall or on the inside of the tread. When truing the tread surface, the tread wall thickness must stay above 0.070”.

However there is one thing you can do to use more of the gravity stored energy on moving the car – that is to only rotate 3 wheels. The car does not need all 4 wheels to roll straight so mount 1 front axel about 1/16” higher than the other. Since the majority of the weight is in the rear, that wheel will just float.

Lubrication – Our rules permit only dry lubricants – graphite or silicone. I have no experience with silicone and have used graphite only. The main trick with graphite is keeping a good layer on the axels. Apply graphite to both the inside and outside of the hub and some on the body where the wheel hub can touch. It is a good idea to “run-in” your wheels before the race. The easiest way is to mount the wheels on the car, add graphite and spin them. Do this 4 or 5 times with each wheel to get as much of coating on each axel as possible. There are devices like swirl cages that can be used to “run in” the wheels or you can put a buffing wheel in your drill and run that against the wheel. Add graphite after your practice runs and then do not turn the wheels any more. Over the course of several races, the graphite will run out of the hubs so start with a much as possible.

The nose of the car should be smooth and polished so that it will slip easily off the starting rod. Avoid pointing the nose severely. It must trip the sensor at the finish line and if the nose is too pointed, it may get slightly off center and not trip the sensor at the earliest possible point.

No Starting Devices – A starting device is something that adds push or pull to the car as the starting rod is dropped. The most common example is a piece of double faced tape on the nose of the car. It sticks to the starting rod and as it swings down into the track, it pulls the car forward and gives it a little boost. The car must start freely from gravity only.

Enjoy this. It is for fun.

I have tried to describe all of the key points to making a car go faster. There is much more information on the internet as well as accessories – legal and illegal. Be sure whatever you buy will not violate our rules.

The most important thing is to enjoy building a car with your boy or girl. Do not set winning as the only measure of success. Try to build a faster car than last year or a cooler looking one. If you pay some attention to the keys described here, you should be competitive and, who knows? You might even win.

Best of luck

Allen Brandenburg

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