Pinewood Derby Car Center of Gravity

Derby Monkey 7 - Pinewood Derby Alignment 3 Comments

Question: Where should I place the center of gravity on my Pinewood Derby car?

Answer: First, I will define center of gravity (COG) as it pertains to Pinewood Derby cars. What is Center of Gravity?

The center of gravity of your pinewood derby car is the distance that the balance point of the car is in front of the rear axles. This is sometimes referred to as the center of mass, COM or COG.

Many people simply tell you to just “put the weight in the back”. While that is true, there is a lot more involved in this process. Proper placement of the COG is crucial in order to have a fast car

The exact location of the COG of your car can be determined with the use of the Derby Monkey COG Quick Ref or as follows:

1. Set a ruler on its edge on a flat and level surface.
2. Carefully lay the car across the ruler.
3. Move the car back and forth until it balances on the ruler.
4. This balance point is the COG.
5. Measure the distance from the COG to the rear axles.
6. This distance is how we express the COG location on a Pinewood Derby car.

The perfect tool for locating the COG is with the aid of the Derby Monkey’s exclusive COG Quick Ref.

Where Do You Want Your COG?

There are many different opinions regarding where the exact location of the COG should be. However, every one aggress that you want it close to the rear of the car. Most publications recommend a COG of 1″ to 1½” in front of the rear axles. A more aggressive COG (shorter) can achieve better results under very good track conditions, while a more conservative COG (longer) is more appropriate for poor conditions. Most experts agree that a COG of 1″ to 1½” is too far forward. The faster cars seem to be built with a COG of a less than 1″.

The Derby Monkey Garage builds its cars with a COG of ¾” in front of the rear axles. Some builders even go with shorter COGs.

How fast your car goes on the first flat part of the track is based on the distance that the COG of the car actually travels before it reaches the curved part of the track. The weight mass of car with the COG in front will not have traveled as far when it reaches the curved section as the weight mass of a car with the COG in the rear. The farther the weight mass travels the more speed the car picks up on the first section of track.

Your COM should also be very low in the car. The lower the COM the more stable the car will be. That’s why tungsten plates attached to the bottom of the body can help create a very fast car.

Position the weights in you car so that you have the optimum COG. Tungsten is the best weighting material because of its density. This allows you much more flexibility in controlling your COG.

Comments 3

  1. Lee

    I’m in agreement regarding the COG. However, builders beware that when you begin to reduce the COG to less than 1″ in front of the rear axle, you take a chance that the car could start to wheelie (i.e. too light in front and/or front wheels lifting off the track).
    Regarding the COM, I disagree. Tungsten is dense and creates flexibility in weight placement and mass, but evidence and physics undeniably proves that higher weight placement is superior, as that weight mass has farther to fall…creating more inertia, and ultimately, speed. You are better off testing your COG with as much weight as possible to the rear and high, and then using a smaller amount of weight wherever is necessary under the car in front of the rear axles to counterbalance and get that COG to 1″.

  2. Post
    Derby Monkey

    Hi Lee… Thanks for your input. You definitely seem to know what you’re doing. Great point. I agree that you can go too far back on the weight if its more than the car can handle. Popping a wheelie is a possibility. Usually… if the COG is too short for a car… the car will start to wobble toward the end of the track. What we do at that point is steer the car harder into the rail (assuming the car is a Rail Rider). I cannot argue with your success. For us… putting the weight as low as possible in the car adds stability thus reducing wobble and side to side movement… thus increasing speed.

    There is a fine line to getting an additional 100th or 1000th second out of your car. For the average builder… there is really no way to determine what works best for their car unless they have a test track so you can test, time and tweak the car for optimum results.

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