Need definition of COG and COM.

[Xanderrock]

My 5 yr old grandson and I are pinewood newbies. I know COG is center of gravity. What is COM? Do they refer to the same thing in the pinewood design context? I've seen several posts discussing how to determine COG but nothing explaining why it's important. Once I determine the COG for the car, then what? Is the COG location relevant? Is there an optimum location and if so how do I change my existing COG?

Puzzled

[whodathunkit]

Xanderrock,

COG center of gravity.. usually refers to the distance from the back axle to the front and back balance point.

COM center of mass.. usually refers to the exact center of weight front and back & left to right.

[Vitamin K]

Xanderrock,

COG center of gravity.. usually refers to the distance from the back axle to the front and back balance point.

COM center of mass.. usually refers to the exact center of weight front and back & left to right.

Are you sure you're not reversing those definitions, Whoda?

[whodathunkit]

COG center of gravity the location at which an object balances in all 3 dimensions.


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[Speedster]

Xanderrock, Welcome to Derbytalk.
Yes, the COG location is very relevant. I recommend you buy the book, "Build a Winning PINEWOOD DERBY CAR" by Troy Thorne. It can be purchased from Fox Chapel Publishing Company for $9.99. The Michaels Stores usually have a few copies also. It will give you a good start in building a fast car. If you find it difficult in acquiring the book, email me and I'll sell you one.
Enjoy the time with your scout.
Best
Speedster

[john4840]

Most people use the terms COG and COM interchangeably. COG or COM is the distance from the balance point of your PWD car to the rear axle. Why is it important, most PWD derby tracks are either Ramp-flat or S-shaped. You will want to have more aggressive COG 1 to 3/4 of an inch. This will get the weight of the car higher up and more potential energy. The car will have a longer fall distance and will achieve a faster speed at the bottom of the hill. On a Continuous-slope track, which is less common these days, COG of the car has only a minor effect on car performance, but it is best to locate the COG near the middle of the car body. You can check this out from a news letter from Randy of Maximum Velocity here. It gives a good explanation on the subject and how to set up the COG or COM on a PWD Car for the different types of tracks (Continuous-slope, Ramp-flat, and S-shaped).



John

[Stan Pope]

You will also see occasional use of CM, CMx, CMy, CMz, CMloc, etc. In a PWD context, the reference (0 in the coordinate system) is usually the center of the rear axle, with the x-axis extending through the front axle and y-axis across the car in the axle plane. Some folks worry about the z-axis location (CMz) as well, since it also affects the available energy. CM is important because of its effect on available potential energy based on track profile AND because of its effect on the weight carried by the various wheels. The latter is important to limit the friction losses from one or more wheels touching the center rail. Friction increases as the force between the surfaces increases, so cars which guide by intentionally turning a front wheel into the rail lose less energy if they carry less weight! CM is most accurately quantified by weighing the car, one wheel per scale, with the car in various attitudes.

Rail guided cars (AKA "rail riders") work because they give up a small friction loss in order to gain a straight run in which the more heavily loaded rear wheels do not lose energy by touching the rail. With this in mind, one might find that lower CMx and rail guiding work well on "straight" (constant slope) tracks.