Déjà Vu, All Over Again !
We Did This Discussion 2 Years Ago (Feb/2011).
CSA
Yes, but we have a few new players now. Actually it has been a topic several time over the years, but due to cartalk server crashes, most are lost to history.
Here are my two cents. I appreciate the responses so far, these discussions are very interesting. I am responding to the question of whether greater mass really makes a difference. In a world without friction or air (like they used to create for us in engineering school exams, to keep things simple), two cars of different masses would reach the same velocity on a given track. For each car, velocity is determined by the converstion of potential energy to kinetic energy: mgh=1/2mv*^2. The mass cancels and g and h are equal for both cars so velocity will be the same.
In the real world, however, some of the energy is consumed by friction and the resistance of air. Air resistance is independent of mass (I don’t remember all factors, but shape of car and air temperature are two parameters that are involved), so a greater percentage of the less massive car’s energy is lost to the resistance of air- since it has less energy. Whether the other friction effects are directly proportional to mass is more complicated- I suspect some of the effects are proportional and some are independent of mass.
Pinewood derby cars are restricted to a maximum wieght to help make them more equal.
Some of the simplest boxy ol’ heaps win continuously and some of the sleek aero designed cars lose.
why ?
Wheels and friction.
They give us NAILS to put the wheels on and insist we use no lubricants like graphite.
Nail them too tight, friction.
Too loose they wobble…friction.
And BECAUSE they’re nails, changing or correcting alignment is near impossible.
Yet it’s that alignment that is all important.
I’ve seen too many cars scoot over to the edge and ride or bump along the lane edge all the way down.
And toe in can cause the wheels to scrub counter to each other.
With the weight being equal among competitors and aerodynamics meaning little at .001mph,
I have found the position of the weight fore/aft to matter. As others have stated
more rearward is better.
And alignment is one of the biggest varriables a kid can unknowingly get right or wrong.
So it sound like the best way to get the wheels aligned is
- Machine block of wood to be perfectly square.
- Pre drill the holes for the nails with a drill press, drilling completely through the block of wood to get the holes perfectly aligned.
- Use a dab of glue to secure the nails to correct wheel tightness.
When did they stop allowing graphite? I think that smoothing the burrs on the nail heads, not removing them completely but just rounding them over and then installing the nails with a very slight negative caster results in the lowest friction.
You could also chuck up the nail and machine down the shaft that is hidden by the hub of the wheel so that there is only drag on the innermost and outermost part of the axle.
Car weight and location does make a difference. More weight equals more momentum. As the car comes down the ramp it gains momentum to carry it out the flat. The further a mass falls the faster it goes (up to its terminal velocity). The cars start facing down the slope so if the center of gravity is closer to the back of the car it has from 1 to 3 more inches to fall and therefore gains more momentum than a car with a center of gravity at the front or center. The ramp is 48" higher than the flat so a 2" increase in the height of the center of gravity equals about 4% more momentum at the bottom of the ramp.
I’m an engineer with a BS & MS from MIT & a PhD from UCLA. I’m also good at fixing bicycles & stuff around the house, so I was sure my son’s pinewood derby car would be fantastic. The first year, our car came in dead last – far behind even a raw block of wood with the wheels simply nailed on! So much for formal education!
Two years later we made the best car not just in our pack, but in the whole region! The dog & the winner both look like a simple wedge from the outside. The dog had the lead weight as far forward as possible. The winner had the weight as far back as possible. Here is why that matters: The launch ramp is at a 45 degree angle, so weight in the trunk gives the winner more initial potential energy than the dog with the weight in the front (at a lower initial height). I have 2 sons & we built cars for several years. In our experience, hitting the max possible weight & putting the weight as far back as possible is even more important than aerodynamics. Just make sure the front wheels stay on the track!
Keith’s correction to Gashmore’s math is backwards- velocity is proportional to the square root (not square) of the height, so the diff is about 2%, not 4 or 8.
Paz’s comment about raising one wheel won’t work, since friction is proportional to weight, so removing one wheel means the other three each carry more weight, by just the right fraction to even things out.
I stand corrected, but jmmahony, you are also incorrect, the speed is 1.04% greater, but the force is 4% greater. The formula is Vf2=Vi2+2ad. This yields a velocity of 161.88 i/s after 48" and 165.2i/s after 50". I used 273i/s2 for gravity on a 45° incline (.707g). Subtract the drag from this, right now it is based on no friction or drag.
Somehow that old g=32f/s2 formula stuck in my head and I wasn’t thinking. Math class was a long time ago.
(edit) I went to all the trouble to use the html for the subscript and superscript but it didn’t seem to take. The f and i were subscript and the 2’s are superscript, except the 2’s in 165.2 and 2ad
In the case of pinewood derby cars, because the track is so short and the weights are almost the same at 5 ounces, aerodynamics and placement of the weights plays a minor part of the equation. In my experience as a winner of the pinewood derby championship for my troop back in the day, the thing that nobody notices is the friction between the nail (axle) and the wheel. This was reduced significantly by use of graphite powder before each race. Yes my fingers were dirty, but I won, and that is what matters!
If there were no friction to worry about, it wouldn’t make a difference what the car’s weighed. They’d all reach the end at the same time, a car weighing one ounce, or a car weighing one pound. But the rate of descent with friction is governed by opposing forces, friction, and the force on the car due to gravity. The friction forces are for the most part independent of mass, but the force on the car pulling it down (due to gravity) is given by mass * g. (g is the acceleration due to gravity). There’s nothing you can do about g, but a higher value of mass will create a greater downward force to oppose the frictional forces; so to make the car go fast, make it as heavy as possible, and make the friction as little as possible. (This works as long as the increase in mass isn’t offset by correspondly more friction.)
If you watch the winter olympics, notice that the downhill race, where the skiers go as fast as they can straight downhill, those skiers are quite heavy and bulky, built like hockey players, compared to the thing agile skier who do fast turned, slow moving, slalam race.
Keith and Paz have the best ideas. My son and I have build a number of winning cars and my daughter has won a number of sibling races. After two years of bitter disappointment, we learned.
Look for the ads in “Boy’s Life” for Pinewood Derby secret sources, including books, precision drilled blocks, speed wheels, speed axles, and buy several kits. This may cost you $30. Test all wheels and use only the fastest wheel and axle combinations. Build and test three cars, all of which will only be wedge shapes; do not mess with exotic shapes. Weight to the rear; use an accurate postage scale to get the exact weight, and be prepared on weigh-in night to add or delete weight with either stick on weight, or a power drill to drill out some weight. Of your three cars, naturally, enter the fastest car; siblings can race #2 and #3. You can make your own speed axles and wheels, if you wish, but buying them is cheap and easy. Add graphite to the inside of each wheel where it rubs on the body. Elevate one front wheel slightly to reduce friction by 25%, but not enough so it is noticeable, just enough so it wobbles slightly when tapped, but will ride smoothly on three wheels.
Once you have built and tested your car, DO NOT mess with it until race day. Letting scouts run them excessively will result in knocking them out of racing condition, so save your best car for THE race.
My cub scout went on to become an Eagle Scout in eighth grade, straight A’s in Engineering Physics in college, a graduate degree in Materials Science and now at age 25, is working as an engineer for the largest oil company in the world. He may have learned something from our work with Pinewood Derby cars.
The day after your race is the best time to begin for the 2014 race! Best wishes…
Wow, speed wheels, precision blocks, speed axles, what’s next factory sponsorship? Pinewood derby sure has changed since my son and I built his first car. BTW, those little cars build up quite a head of steam by the time they reach the bottom of the track, so I think aerodynamics does play a part.
With my son’s first Pinewood Derby coming up next month I sure appreciate the timeliness of this thread. I/we plan to put a lot of these tips into practice.
This topic made me finally create an account. still reading though the threads but I know you absolutely need to get as close to that 5oz mark to have a winner. Weight and friction come way before aerodynamics. It seems the weight is needed to overcome the friction of the track if anything. The cars are not being dropped in a vacuum.
We’ve had cars with weight in front and back do well, it was all about polishing the axles. We always used the stock ones (always used the stock kit), although I saw other cars cheat an use “racing axles”. We put the axle in a rotary tool and used a file to remove the “webbing” for lack of a better term that goes from the back of the head down the shaft a bit. And then used 600-1200 grit sand paper to get them glass smooth. Then a polish at a high rate of speed with felt. Then spin the tires on the axle with the graphite lube to embed it into the plastic. Make sure it runs straight as possible. And you should have a car that will compete very well.
I can’t believe you didn’t “help” with your son’s pine car derby. Here is what will help.
- design - choose an aerodynamic design.
- put the weight in the middle so it effects all wheels equally.
- get as close to the 5 ounces as possible
- put graphite on the wheels
- put the wheels on a lathe and make sure they are perfectly round and narrow them or actually make them pointed. friction is your enemy.
- take the car and a box of doughnuts to your local auto body shop and have them paint it and bake on the paint to make the car as slippery as possible.
hope this helps
bean counter
Here’s my ‘thought experiment’ as to why it’s better to put the weights at the back:
Imagine the car is the length of the ramp, in other words the front wheels hit the ‘flat’ right after the start. In that case a car with almost all the weight at the front will have little additional acceleration from that point on, while the car with the weight at the rear will continue to accelerate until the back wheels hit the ‘flat’.
Sorry Keith, but you’re still wrong. Kinetic energy is 1/2mv^2. Potential energy is mgh. So v is proportional to square root of h. (In your terms, since Vi is 0, Vf^2 =2ad (your d is my h times sqrt(2), and your a is g/sqrt(2)), so Vf is proportional to sqrt(d)). Now h is increased from 48 to 50 (about 4%), and the square root of 1.04 is about 1.02, so v is increased by about 2%.
Note your own results: going from 161.88 to 165.2 is a 2.05% increase.
And force (gravity, or gravity/sqrt(2) when at 45 degrees) is constant. How could gravity be 4% greater?
After having gone through this many years ago, I now realize the way to really “win” is to work with your son as he builds the car, and let the final product be something shaped by his ideas - ideas that you let him know you’re really proud of.
How fast the car goes down the track is not what he’ll remember nearly as much as how he felt while building it with his dad.
I wish I had internalized this more back then.