Posted: 10/12/2009 11:59:57 AM EDT
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A few years ago some guys at work started an annual Pinewood Derby. A couple of them are Scout leaders and thought it would be fun. Each year there are more participants and I'm building a car for the first time.
Most guys build their cars to the maximum allowable weight (5 ounces). This got me thinking that the more mass there is at rest at the starting line, the more inertia there is to overcome. (An object at rest tends to remain at rest, and all that...) Would a lighter weight car be quicker off the line? As I recall Galileo's experiment of dropping balls of different weights, didn't they accelerate at the same rate? How much difference would weight actually make in such a short distance (I don't know the legth of the track, but it's not much)? Am I thinking too hard about this? 
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Quoted: A few years ago some guys at work started an annual Pinewood Derby. A couple of them are Scout leaders and thought it would be fun. Each year there are more participants and I'm building a car for the first time. Most guys build their cars to the maximum allowable weight (5 ounces). This got me thinking that the more mass there is at rest at the starting line, the more inertia there is to overcome. (An object at rest tends to remain at rest, and all that...) Would a lighter weight car be quicker off the line? As I recall Galileo's experiment of dropping balls of different weights, didn't they accelerate at the same rate? How much difference would weight actually make in such a short distance (I don't know the legth of the track, but it's not much)? Am I thinking too hard about this? Friction |
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In a vacuum, weight (mass) would not matter. In the real world, the car with the least aerodynamic drag and highest mass would win. Which world do you live in? IMHO, the most critical part of the Pinewood Derby is the wheel's and their relationship to the track. Some lane/car combinations ARE faster. |
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Quite a bit of difference. A 4.5 oz car is almost always behind the ones crowding the 5.0 oz mark. Not sure of the physics behind it, but I've got 12 years of two Pinewood Derbies a year (one for Scouts, one for AWANAS) as experience.
Having said that––the shape, size, and extra crap added on to the car have NOTHING to do with the speed of the car. The secret is in the wheels. If you get the wheels mounted, aligned and polished right, the car will fly if you make it shaped like a Formula 1 racer, or leave it as a brick and tape nickels to it to make weight. |
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Quoted: In a vacuum, weight (mass) would not matter. In the real world, the car with the least aerodynamic drag and highest mass would win. Which world do you live in? IMHO, the most critical part of the Pinewood Derby is the wheel's and their relationship to the track. Some lane/car combinations ARE faster. Naw, every car is going to be pretty much the same mass. The important thing is weight distribution. By placing the weight as far back as possible, the mass sits higher, thus it has more energy. Wheel friction is also important, though. |
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My experience working with pinewood derby cars with boys in our church group is (in order of importance):
1) Get the wheels spinning true and as friction free as possible Some wheels wobble and the axle nails have burs under the head. Select and keep good non-wobbly wheels, grind off burs under the axle nail head 2) Max out the weight 3) Aerodynamic design |
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Quoted:
Quite a bit of difference. A 4.5 oz car is almost always behind the ones crowding the 5.0 oz mark. Not sure of the physics behind it, but I've got 12 years of two Pinewood Derbies a year (one for Scouts, one for AWANAS) as experience. Having said that––the shape, size, and extra crap added on to the car have NOTHING to do with the speed of the car. The secret is in the wheels. If you get the wheels mounted, aligned and polished right, the car will fly if you make it shaped like a Formula 1 racer, or leave it as a brick and tape nickels to it to make weight. I have less expirience than you but my observations agree with yours. It was always the same group of fathers and sons that placed in top 10 every year. Their key, as far as they would tell me, was in the polish of the wheels /axles and the relationship between the wheels and the track. As in all racing car set-up is the key to winning. One of the problems I had... I thought having the weight as far to the rear of the car as possible would give me more potential energy to use. Unfortunately, this causes the light front wheels to "hunt" for direction. This slowed the car enough to not finish in the top 20 of 80. |
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First place winner back in the day.
Mirror polish to the wheel pins, and use graphite mixed with 1% ball-milled boric acid as lubricant. Sand the wheels so that just one edge touches the ground. Molten wheel weights to get it right to limit. Perfect balance front to back. The front of the car –– the contact with the starting gate, should be higher than the bottom plane of the car. (If you plot the path of the gate as it lowers you will see why–– basically get the car freewheeling before the others). |
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Quoted:
Quite a bit of difference. A 4.5 oz car is almost always behind the ones crowding the 5.0 oz mark. Not sure of the physics behind it, but I've got 12 years of two Pinewood Derbies a year (one for Scouts, one for AWANAS) as experience. Having said that––the shape, size, and extra crap added on to the car have NOTHING to do with the speed of the car. The secret is in the wheels. If you get the wheels mounted, aligned and polished right, the car will fly if you make it shaped like a Formula 1 racer, or leave it as a brick and tape nickels to it to make weight. Good to see that Im not the only one who did this for AWANA. |
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Quoted:
In a vacuum, weight (mass) would not matter. In the real world, the car with the least aerodynamic drag and highest mass would win. Which world do you live in? IMHO, the most critical part of the Pinewood Derby is the wheel's and their relationship to the track. Some lane/car combinations ARE faster. Did not take a class in physics to learn that. I learned that after my first pinewood derby race. TXL |
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+1 for conservation of energy! more mass higher up = more potential energy. but then dumb luck helps out too. |
I sure don't want to live in a home built by that clown |
Best wheel set up goes something like this:
- Take the block of wood to a table saw and cut off the grooves for the nails. (They're usually crooked, and not too shallow to hold the nail in without breaking). But before doing this (  ), measure how high the nails sit in the groove relative to the bottom of the block. IIRC, it's something like 1/8 of an inch.
- Take the new block of wood to a drill press. Drill holes for the nails to go into, but NOT at a 90deg angle to the block, at the above distance from the new "bottom". You want each nail to be about 2-3deg above perpendicular to the block. This cants the wheels outward at the bottom, and as the wheel rotates, it gets "pulled" up the nail and away from the block of wood. - One of the two rear wheels needs to be about 4deg above perpendicular. At any one time, only three wheels should sit on a level surface; the fourth is there more as a guide. (This limits friction and drag against the track). - Remove any extra burrs on the edge of the wheels. Take some OOOO steel wool to the wheels themselves. Helps if you chuck the wheels into your drill press, makes the job MUCH quicker. You can by a mandrell at any hobby store (Michaels, etc) to do exactly that. - Ensure the inside edge of the wheels are exactly 90 degrees. Essentially, the entire weight of the car will rest on that one point, make it as small as possible to limit drag & friction. - Polish the shaft of the nails. Bevel the head of the nail away from the shaft. I found that if you chuck the nails into a drill and hit it with a very fine jeweler's file, that make short work of the burrs and markings on the underside of the head. - When placing the nails, make sure there's enough of the shaft sticking out (so to speak ) so the wheel's top edge doesn't rub on the side of the car.
- Graphite, Graphite, Graphite. With these tips, you can make the car look like anything you want, which adds to the cool factor. All of these were at least in the final heat. (And yes, the boys did most of the shaping for this, I only did table saw work.) 
And my favorite, the purple couch: 
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part of it is the flat part at the end of the track. you want as much inertia for that part as you can
there were always people taping quarters on the top the day of the race after weighing on the official scale my dad had a pharmacy scale at home and would pre-weigh my car and drill out some of the underside wood and fill it with molten lead so that my car would be at the max weight ahead of time and not have to disrupt the aerodynamics |
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You want to get the car right at the weight limit.
All objects of any mass accelerate at 9.8 m/s in earth's gravitational field when dropped. In a vacuum you could drop a hammer and a feather and they'd hit the ground at the same time. The higher mass car has more potential energy and can therefore maintain speed better on the level section since it will have more momentum. If you assume the forces of friction and aerodynamics are equal for two cars, the car with more momentum is going to maintain its speed better on the level section. It all boils down to F=ma. The best strategy is to get a car right at the maximum weight, and minimize aerodynamic losses and wheel friction losses. |
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Wow. I had no idea that Pinewood Derby design had become a field of engineering.
I'm impressed. Some engineers at work have been asking about alternate materials. Since this isn't an officially sanctioned event, they're taling about allowing aluminum, carbon fiber, etc., next year, as long as the cars meet all of the other requirements. |
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Heavier cars win because their mass enables them to overcome the initial friction better.
The initial friction is composed of the portion of the wheel that touches the track and the efficiency of the axle. A much lesser (but still present) source of resistance/friction is the air itself against the car's body. Think of it - a car with zero mass would be unaffected by gravity. It wouldn't roll down the ramp at all when the starting gate dropped. |
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Quoted:
Wow. I had no idea that Pinewood Derby design had become a field of engineering. I'm impressed. Some engineers at work have been asking about alternate materials. Since this isn't an officially sanctioned event, they're taling about allowing aluminum, carbon fiber, etc., next year, as long as the cars meet all of the other requirements. I would limit that to adults only. If you do that with the kid's cars, then they get sidelined while the dads do all the work with the space-age materials. And, after all, it's about the boys having fun and learning in the process. Never mind, got stuck in "Scout" mode. As adults, you could have a blast with this. You know, a .50BMG round is aerodynamically shaped, and about the right weight. Just sayin'.... |
| Start with block of Balsa wood , carve into a rail dragster shape, cut out underside, as far forward as possiable to hold enough lead to spec wt. to qualify, fill remaining hole with wood filler, sand smooth , paint, add wheels , graphite. Heavy nose of car will pull it down track faster. |