I am trying to construct a pinewood derby racer for my son’s pinewood derby race this weekend. I am trimming it down to make it more aerodynamic, but I have a question about what would be the best weight distribution. You may have a maximum weight of 5 oz., but is it better to have a car that maxes out the limit? It seems like you would want to have a lighter car that can overcome its inertia faster, but not so light as to not be able to overcome its air resistance.
Thanks for your help,
Rob
You want to max out the weight and put as much weight at the back of the car as possible. That’s because the track starts out on a downhill slope, but then turns level. The back of the car will travel further down the ramp than the front, so the weight in the rear will provide extra driving force.
This was tested and proven with my kid winning the derby for his pack.
Its been a while since I took physics so other may correct my mistakes, but here’s my take.
I would think that you would prefer a high weight. Both the inertia and the forward force due to gravity are going to be proportional to weight, and so in an ideal world weight is irrelevant. Other forces that might come into play are air resistance and axle friction. Air resistance is going to depend on cross sectional area. Axel friction will probably be proportional to mass.
So putting it all together, all factors but the air resistance are going to be proportional to the mass. So to reduce drag you want the greatest ratio of (mass/cross-section), and then also do what you can to reduce axle friction
One of my childhood memories was at the cub scout pinewood derby where my car beat all others in all age classes because it was exactly at the maximum weight limit. We even had to chip away some wood at the weigh-in to get it exact.
Since the track in an incline slope, greater weight will simply go faster. Aerodynamics are negligible compared to potential energy and friction. Distributing the weight as high as possible with respect to the other cars may provide a slight edge.
The other key tip is to make the axles smooth and straight and lubricate with graphite.
As stated, you want the car to be as close to the weight limit as possible, and you want the weight as far back on the car as possible.
Also, if possible, smooth the nails that you’re using as wheel axles. Standard nails have a rough surface which causes wheel drag. Get a power drill and put the nail in the chuck. Spin it and hold sandpaper around the nail shaft where the wheel will ride to make the surface as smooth as possible.
IIRC, the graphite is illegal. You might want to check the rules before applying anything to your car. I agree with others- the most important thing is to get weight high and limit friction, though.
If you want to split hairs, the weight should be near the bottom of the car because that falls farther than the rest of the car. At the top of the hill, it’s lower than the highest point by some fraction (actually, cos(incline)) of X, where X is the height of the car.
Make sure you keep the wheels straight and don’t allow them to wobble. That’ll just make the car crooked and waste the energy.
Good to see the tradition of fathers designing and building the cars rather than the kids is still going strong.
It’s been well over 20 years, but I think the rule is no liquid lubricants. Powdered graphite is okay.
The weight at the bottom of the car has little to do with how much the weight will fall. You want the weight low because that is more stable. A top-heavy car has a greater chance of jumping the track.
You also want the weight at the rear for the reasons stated above (greater potential energy at the start of the race).
In theory, the car should be as long and short as possible (remove as much wood as you can). I’m not sure if the max length is equal to the original block of wood or not. If the max length is longer than the wood block, then you will want to extend the length of the car. Then make it as heavy as allowed by putting weights around the rear axle. You will probably want the center of mass to be at the rear axle and not behind it so the car doesn’t ‘pop a wheelie.’ The result will be an ugly car, but it should be fast.
In theory anyway.
My Dad dominated our Pinewood Derby race when I was in 4th grade… and it actually looked like a car.
He followed the smooth the axles philosophy that AndrewL prescribes. In addition to sanding and smoothing the nails. My Dad put a disc in his power drill then used that to spin the plastic wheels on the axle so that everything was smooth.
I was hoping someone other than I noticed this…
My understanding is that there is a fine line between the wheel/nail axle assembly being a smidge too tight and a smidge too loose.
Too tight is a real killer wasted energy wise. But woobling while too loose is too. Another thing is how well the wheels are aligned. Given that its just nails into wood, its probably fairly easy to not have the “axles” particularly parallel, which cant be good.
Bahh, just hide a magnet inside the front of the car and be done with it.
My son won first or second in 6 or 7 derbies (he did a couple as a Boy Scout), and our method was to max out the weight, polish the axles, and use graphite, as others have said.
After cutting and shaping the block of wood, we would gouge out a hole behind the back axle and melt some lead solder in there. (We’d drive a screw into one side of the hole first so the lump of solder wouldn’t fall out when it cooled.) Then, to get it the rest of the way to 5 oz., we’d add some of those screw-on lead weights that you can buy at a craft store or your council store (scroll to middle of page):
These we would mount in a shallow hole gouged out just forward of the rear axle. The advantage of using these is that you can snap off a little piece at the derby in case the car weighs in a little too heavy (as opposed to lopping chunks of wood off the car at the derby). If you wind up a little light at the weigh-in, just scotch tape a dime or two to the top of the car.
While you’re working on the car and checking its weight, don’t forget to include the wheels and axles.
Good luck, and remember to have your son do as much of the work as possible!
The temptation is to make it areodynamic like a race car that is propelled by the wheels. But this thing is propelled by gravity, not the wheels.
Perhaps it is better to make it more like a lifting body that doesnt quite fly. All the weight pushing down on the wheels is just creating more drag.
First Dad to break out his old grad student fluid dynamics code wins!
Quoth Anthony N:
If aerodynamics really were negligible, then weight would be irrelevant, since aerodynamic drag is the only force in the problem that isn’t proportional to mass. What you probably meant is that shape has a relatively small relevance to aerodynamics. Making it a nice smooth curve will help, of course, but it’s much more important to get the cross-sectional area as small as possible.
And to the OP, you’ve done your part. Now show your son this thread and let him take it from here.
He can’t use sharp tools yet. I let him design and paint it and I do the carving.
Also, how the hell do you get the axle nails to go in straight? My idea is to put the thing on it’s side in the drill press, screw the chuck all the way down and use the tip of the chuck to press the nail in. Then get a flat piece of wood to support the car so that the wheels aren’t touching the table and do the same thing to the other side.
Thanks,
Rob
I agree with the near max weight and at the back.
Just remember use the axle slot nearer the edge as the rear axle. Last year my son’s car sadly popped wheelies in 3 of the races but when it stayed down won each of those races easily.
I made the cut from top of back to a very narrow cut at the bottom, maybe ¼". I then had removed enough wood this way to put all the weights in the rear. These are the little drum weights, two groups of three. I drilled out the holes and had my son pack them in with just a little glue.
This made for a car with wheels, paint and decals that weighed 4.95 oz. Also very fast.
Oh, those axle nails, it is probably worth it to take a fine drill bit an open the axle slot just a little. But be careful. You still want the nail axles to be tight and you want to ensure the nails don’t slip in to far to where the wheel rubs the wood.
Sure, year 1 my sons car won (unanimously), car most clearly made by Scout. He came in second to last and was heart broken. Meanwhile most of the cars were clearly made by the fathers.
So year 2 on, I went out to the scout site with the recommendations for fasted cars. Showed my son why I was making the cut for him the way I was. Explained the gravity to him. Explained about how having him start with a course grit and working down to a 600 grit would make the car really smooth. Taught him how to paint the car with long strokes with the grain and why a sealer and multiple coats were needed.
Who actually expects a Cub Scout to design the car? No one expects them to cut it or use a drill. But these kids do hopefully start to learn tool use and painting and design. My son knows how to sand now. I’ve been teaching him knife safety for other scout projects. He has a healthy respect for the band saw and other power tools.
So what exactly was your point?
Before Cub Scouts, my sons were in the YMCA Adventure Guides program, which had car races, but the cars were up to 11 inches long and weighed up to 24 ounces (!). The kits they gave us had wood blocks that were 10 inches long and about eight ounces.
My technique was to first extend the length to 11 inches, to get that weight higher up the ramp, and to put it as far back as I could. However, one year I got it so far back that the front tires bore hardly any of the car’s weight, and this was a problem because the front end lifted off at the end of the sloped ramp, and the car would sometimes spin out (there was no track, just an open area walled off from the car you’re racing against).
I finally figured out the right ratio of about 1.5 ounces of weight on the front axle and the remainder 22.5 ounces on the rear axle, and my son took first place.
Oh, and about the questions of dads or boys doing the work - these were little kids who couldn’t work with power tools. I would involve my sons in every step and let him do as much as he could, especially in the aesthetics department, but there is no way a kid that age could build the car himself.
We don’t? When I made my Pinewood Derby cars, I used the power tools myself. Mom was watching over my shoulder, of course, but that’s all she was doing: Watching. We are talking about Cub Scouts here, right, not Tiger Cubs?
Well, you’re only a Tiger Cub in first grade. Cubs range from second graders up through fifth graders. You don’t magically become trustworthy with power tools upon graduating from first grade. I would expect cubs to gradually take over more and more of the job of building the car as they get older. And each set of parents will have their own ideas about what is safe for their children to do. I’m not about to second guess them.