Pinewood Derby-Awana Race Car Secrets?

My son once won the Pinewood Derby and took second in his next year’s appearance.

Make the car minimal weight. All objects fall under the force of gravity regardless of weight. A heavier car will only increase friction at the axles.

The three main issues you have to deal with are air resistance, wheel (contact with the surface of the track), and axle friction. Our car was a simple wedge shape, nothing fancy. Use the Prius as an example. He had to add a little weight in the middle to make the car of legal weight. For the axles we ensured we used a generous amount of graphite powder on the axles (which are really nothing more than chrome plated nails). I don't know how you can get the car to go on two wheels, but my son's ran unintentionally on three wheels, which probably helped. I did not notice that until after weigh in and thought we were doomed from the start.

The person who is in charge of the race should have either a proper computer program or logic sheet designed to ensure all cars are able to utilize both lanes of the track and eliminate each other properly.

In the first year when my son took first, the organizer had a laptop computer that figured out the proper race orders and lanes for each race. In the second year when he took second and the scoring was done by hand, I noted he did not race the first place car and won over a car that won over the first place car in one of the heats. I did not say anything as so not to create a problem. After all, it's for the kids, they all had fun regardless.

Good luck,

Rick

 

Wish I could help, but all I can see is my older brother going out of control and landing in a gigantic rose bush with his Big Toe still connected to the pinewood racer. It was a practice run, no one else around and in our neighborhood, late in the afternoon...about 50 years ago.....

 

Wow, this brings back some memories.

The wedge shape is important... cut down as much wind resistance as possible, When I built one back in the early 1970's, I drilled holes in the back near the rear axle and filled them with lead. This allows you to get right up to the maximum weight, and to balance the car well. Filling the holes with lead allowed you to just drill out some of the lead if you went overweight. My lead was primarily just behind the rear axle, but there was a bit in front of the front axle too. This has the effect of pushing the car, but also serves to keep all four wheels firmly on the ground

Graphite on the wheels helps too. Any resistance at the wheel level will slow the car down drastically. Oh, and make sure the wheels are absolutely straight... one little wobble will lose you the race.

Best of luck. I placed second overall, and the devastating loss haunts me to this very day! Actually, the second place finish was still exciting... most of all, I cherish spending the time with my Dad building the car... your son probably will too.

Dan

 

Actually, in this situation, mass is important. The equation of motion (simplified to exclude the wheels) here is:

a=(g)sinT-(fg)cosT-(1/m)cv^2

where

a = acceleration down (along) the ramp (the main goal)

g = gravitation constant (can't do anything about that)

T = angle of the ramp (don't get to change that either)

f = coefficient of friction (this is where the wheels get figured in as there mass is relativly small)

m = mass of car

c = a coefficient of drag

v = velocity along the ramp

So, there are three things you get to work on: friction, mass, and drag. In (fg)cosT, friction is a multiplier, so it obviously should be as small as possible. Your main source of friction will be the axles. In (1/m)cv^2, drag is also a multiplier, and be made small. Mass, though is a devisor. The larger mass is, the smaller the term (1/m)cv^2 is. For drag at the speeds these cars move at, you will want to keep a few things in mind. The fineness ratio, which is basically how fat it is over how long it is, should be as small as possible. Long and skinny is good. Surface area should be at a minimum. The surface should also be as smooth as possible. A flat surface on the rear will cause considerable seperation of flow and extra drag. A rear tapering to an edge will help. Basically, the more it looks like an airfoil, the lower the drag will be. Make sure there is no downward lift, though, as that will increase the friction force.

Also, if the rules allow, see if you can mount the wheels underneath the body, with most of the wheel up in the wood. Wheels are very bad for drag. The less of it that is exposed, the better.