Soap Box Derby Race Car Weight | Center of Mass

The Big Hill at Derby Downs, Akron, Ohio

All American Soap Box Derby expert Paul Gale on friend of the site, ZeroError Racing’s Forum, responded to the proposition that in a Soap Box Derby car aka SBD Car that: “300 pounds is 300 pounds.” The response is written in the context of Ultimate Speed Division USD cars that weigh more than the other classes of cars in the All American Soap Box Derby.

“When a SBD car is sitting on level ground, or rolling down a constant slope (inclined plane), your statement is correct. When a SBD car is rolling down a varying slope (concave slope) like the Akron hill, your statement is incorrect. A 300 pound car with low center of mass will have an advantage over a 300 pound car with a higher center of mass. This advantage is created due to the vertical distance each car’s center of mass “falls” when rolling down the hill from start line to finish line.

A small body mass (lighter weight) driver allows more added weight to be placed low in the car establishing a lower center of mass which creates potential energy. Currently in the Ultimate division, wheels are the single most important factor in determining the car’s speed. A car’s center of mass, weight distribution, aerodynamics and driving straight play a smaller role in the car’s speed.

I expect that in time, additional sources for wheels will be developed. When that happens and wheels become more equal, the other aspects of car design will become more important. Of course, the 60 pound driver with a “good” set of wheels may be able to beat a 140 pound driver with a “very good” set of wheels due to a lower center of mass. Once all of the Ultimate cars have equal wheels sets, the smallest driver (by weight) will win, unless that driver makes a driving mistake.”

Paul Gale