Of course, when teaching someone how to whittle, make sure they are aware of the importance of their blade being as sharp as possible. It sounds counterintuitive at first, but whittling with a dull blade is more dangerous, because it requires more force to cut with a dull blade. Once the blade completes the cut, if you were using more force, you are more likely to uncontrollably thrust the blade, possibly into something (or someone).
On a related note: Never carve towards yourself.
When I was a cub in the 60s we whittled our cars (and we carried our pocket knives to school), and dads weren’t helping much in our pack. By the time my little brother was a cub in the 70s the winning cars looked like they were designed and built by NASA. I helped my son with his car. He did almost all the work, even using the bandsaw. I did the work of pouring hot lead into the car for weight just to keep his mom from freaking out. I’ll go along with the idea of father/son teamwork.
We also had a seperate race for the dad-built cars. My ‘Death-Mobile’ won that one too.
Hint: Take your car to a post office or deli to get weighed on an accurate scale.
They are much harder than balsa though still fairly soft. If the kid has already had some training in whittling I could see trying it with a nice sharp knife but it would be a terrible way to start and especially for a 6 year old when these Derbies begin. As you know, balsa can be carved with a potato peeler. Good luck trying that trick on even soft pine.
My bad. I was confusing the Pinewood derby cars and the Space Derby rockets, which are made out of balsa. I guess whiting a block of pine that size (even if it is pretty soft) would be too time consuming.
I don’t think anyone is trying to make the parents feel bad, just that it gets a bit ridiculous when the scout’s contribution involves sanding and painting his father’s car. It’s like the Dad Derby now. I’d put an Estes engine in mine. With all the paint fumes, my car might be the last one standing. What are they going to do, kick me out of cub scouts?
Is air resistance a major factor for something as small and slow-moving as this model car? And how much does the block weigh before you do anything to it? I’m wondering how much of it needs to be removed to get the weight down.
Well no proof but it does appear wedged and low profile cars consistently are faster than cars that look like real cars or rolling blocks of wood. So that would seem to be air resistance in play.
Also I saw many recommendations to try and keep down the number of wheels with weight on them and thus the thoughts of putting the weight in the back. I even saw recommendations that one front wheel be raised slightly to have a mostly 3 wheel car going down the track.
But air resistance in practice appears to have a major effect.
I was a Cub Scout over 30 years ago, and the wood for the Pinewood Derby car kit was pine. It’s still pine today. So far as I know, it’s always been pine.
From experience I will say that the density of the pine in the kits varies greatly, from harder, denser, pine to softer, less dense pine. It’s kind of the luck of the draw what you end up with. Some people buy several kits so they can pick out the best wood, the best wheels, etc.
While you could whittle the wood, I think it would be somewhat difficult to end up with a workable car, even if the person was a very good whittler.
There is a balsa wood rocket ship kit that our pack had an event for one year. This wood was so soft that the rocket could be completely shaped with just a piece of sandpaper.
As I mentioned above, Cub Scouts are not allowed to use power tools, even with supervision. I guess parents can let them do whatever they want, but it’s not officially sanctioned and is strictly prohibited at any official meeting with leaders present.
Are you kidding? They have trophies for 1st, 2nd, and 3rd place for each of the five age groups (Tiger, Wolf, Bear, Webelos I, and Webelos II). Then they have an overall 1st, 2nd, and 3rd place. No Scout received more than one speed trophy, so all in all, 18 Scouts receive speed trophies. (Our pack had between 50-60 boys, so a significant percentage went home with a trophy.)
Then they have a “Best in Show” trophy for the best looking car, voted on by the Scouts themselves in a secret ballot.
One year my son and I made a Spongebob Squarepants car that was just a block of wood! It was painted yellow, and had very good Spongebob decals from a greeting card. We were actually competing for “Best in Show” rather than speed, but interestingly enough, my son ended up with a speed award (3rd place for his age group).
In four years, my son got three trophies (one each of a 1st, 2nd, and 3rd place for his age group). The year he got a 1st place, he went on the next higher round (the District derby), but got knocked out in the first round.
In my sons’ pack there was an “unlimited” class that kids and adults could enter. Not really about speed but to see how extreme the designs could get. One dad fitted his with a R/C plane propeller and drove it backward up the track, nearly decapitating the starter. Mine was the basic block from the kit decorated with plaid (I was a Webelos den leader) topped with a wood-splitting wedge - it weighed about 32 oz compared to the scouts 5 oz max. It was a monster on the track! Good times.
I doubt it. As I mentioned way back, the major effect of weight in the back comes from the greater length of the ramp that the back of the car travels than the front because the track turns level before the end. The wedge shapes just move more weight to the back. Smooth and balanced wheel rotation is probably a bigger factor than air resistance.
Both air resistance and rolling resistance are factors. When I did it, the best cars had a very low profile… most of the wood was gone and replaced with lead. The wheels all came well above the airfoil-shaped body of the car.
IIRC the front of the car body as perhaps 1.5cm think and tapered to nothing at the back. You need to keep it thick enough so that the rear wheel’s axle has enough material to hold it.
Quoth robby:
My council, too. My biggest upset in a Pinewood Derby was one year when I was eliminated in a round by a car that was later found to be illegal (iirc, it had solid axles instead of the official nails). Yeah, I probably wouldn’t have won anyway in an honest competition, but I’ll never know. Incidentally, if anyone seems to be hyper-defensive about anyone seeing the underside of their car, mention it to an official.
As for the various forms of drag, I’m pretty sure that all of the forms of rolling resistance are also proportional to mass, with air resistance being the only force on the car, at all, that isn’t. Which means that halving the cross-sectional area of your car should give you exactly as much benefit as doubling the mass.
The weight limit for the finished car is 5.00 ounces.
The raw block of wood (plus the wheels and nails) is usually just a bit more than 5 ounces, so if any wood is removed to make the classic wedge shape, you have to add metal weights to get the weight back up to the maximum allowable. Some less dense blocks of wood might actually start off at less than 5 ounces.
When my son and I made his “just a block of wood” Spongebob Squarepants car, we sawed off a quarter-inch of wood off the back to get the weight right at 5.00 ounces (once the decals and paint were added).
Finally, I do think air resistance comes into play, but the most important factor are the wheels, IMHO. We always shaved off the burrs off of the nails, smoothed the wheels, made sure the wheels were on straight, and used lots of powdered graphite. These simple measures alone helped him get a speed trophy every year for his age group, even the square Spongebob car.
At the higher end of competition, however, the shape of the car, weight distribution, and air resistance come into play. My son’s cars were usually fast enough to land up in the top third or so, but were never in the same league as the very fastest cars.
The cars fast enough to win overall and go on to the District and Council races were sleek, highly polished, aerodynamic, long wheelbase, perfectly balanced and weighted cars. I was happy if my son could win a few races and hopefully get a trophy in his own age group.
BTW, the way they ran the races was very clever. There were three lanes on the track. Each car ran in three heats, one heat on each of the lanes (to account for any speed variation in the lanes). The times for each heat were electronically determined to the thousandth of a second, and the times for each of the three heats were added together. The car with the lowest overall time won.
For the first heat, the cars were all randomly raced together. For all of the subsequent heats, the cars were matched up so that cars with similar speeds would be raced together. This made it so that even boys with slow cars had the opportunity to maybe win a heat or two.
Unfortunately, some boys didn’t really understand this, and thought that because they won their last two heats that they were going to get a trophy, which might not necessarily be the case if they had one of the slower cars.
Oh well, that’s part of life, too. Everyone at least got a participation patch.
I keep having visions of little Billy holding his craptakular POS derby car, crying his eyes out, while the winner is being hoisted up on the shoulders of his many admirers. Meanwhile, his dad is screaming “You’ve diappointed me for the LAST time Billy! You are DEAD to me! When we get home, you are packing your bags and its Hasta La Vista Billy!”
Let’s dig into this some more, because it goes against my gut feel. Let’s say you take two completely equal cars, and push them both at a fairly slow speed across a level tabletop. Let’s say the initial speed is low enough that air resistance is completely negligible.
Wouldn’t you think that if they’re both given the same initial speed, that the heavier one would roll farther?
I thought all Pinewood races required you to use the same wheelbase, which is determined by the slots that are pre-cut into your block of wood.
I really like that system. I’m the “racemaster” of my son’s pack, and our pack’s track has six lanes, but it only indicates the finish order - first, second, third, etc. No time measurement, so you can’t compare across races.
Before I took over, the pack had a system that involved racing each car once in each lane, and with all the coordination that had to go into each race, the whole thing lasted three or four hours and the kids all lost interest. I changed it to a system based on double-elimination, where the fastest three cars advance in each race and the slowest three drop. I give each kid a ticket at the end of a race that indicates his next race number, so they’re aware of when they need to race next, and they’re ready. We can knock the whole thing out in about an hour. The downsides are that there’s no compensating for one of the lanes possibly being slower, and that some of the kids are eliminated after only two races. But at least the race is over before their attention span is, and that’s a really big factor IMHO.
I think it’s counterintuitive because in most cases you’re familiar with, the air resistance is relevant, so your intuition isn’t well-tuned to the cases where it’s not. But consider, for instance, a case where the “heavier car” is actually two identical light cars side-by-side, and connected by a thread. You would agree, would you not, that such a “heavy car” would have exactly the same stopping distance as a light one?
Of course, this isn’t a perfect analogy, since we’ve changed the number of wheels. The assertion then would be that rolling resistance increases with increasing number of wheels. But it should serve as a start.
What if the Derby car is on a treadmill?
But the number of wheels is the key part of what I would think would cause it to slow down more. It seems that the force that a wheel would encounter as it hits little bumps on the table surface would be maybe not independent of the weight, but not proportional to the weight.
I’m basing this on intuition, which I realize is not a reliable guide with questions like this. But still, there’s a huge difference with these cars between the heavy ones and the light ones, and I have a hard time believing that the difference is due to air resistance alone (or even predominantly air resistance).
Or the track is exactly nine yards long?
OK, look at it from a dimensional analysis point of view. The roughness of the track, or the deviation from symmetry of the wheels, would be characterized without any mass in them, but force does contain mass in its units. In any formula you care to come up with for a force, you’ll have to get something with mass in it somewhere in your inputs. Where would you get the mass from, if not from the mass of the car?