Pinewood Derby Car Design
By Stan Pope
Design Concepts and Priorities
Follow your district's rules. If you can't pass inspection, you can't race. If you can't race, you can't win!
Assure Ground Clearance. Tracks vary in center rail thickness. (The car straddles the center rail as it runs down the track.) You will not know until race day if the center rail is 3/16" or nearly 3/8" thick. Usual thickness is around 1/4", but subject to variation. Failure results in either grinding to a stop part way down the track or dramatically flying off the track as a high spot is passed. If you can't get to the finish line, you can't win!
Assure that the car will "run" on the track. A narrow or pointed nose may not stage correctly on the starting mechanism (bad news!) and may not trip the finish line sensor correctly (might lose a race or two for you).
Make sure that the wheels are in balance, in round, moving freely, and correctly aligned. If not, it will slow itself down as it rolls down the track! Failure here could make a potential top 10 car finish in the bottom quarter.
Maximize weight vs. wind drag. As the aerodynamics of the cars shape improve, the weight of the car becomes somewhat less important. With good aerodynamics the car may lose a few places as its weight is reduced from 5 ounces. With poor aerodynamics, the loss of weight has more significant effect.
Optimize weight distribution. As the center of gravity moves farther back in the car, there is more energy available to be converted into speed. If the center of gravity is too far back, the car loses stability and will "rattle around" toward the end of the race track. Locate the center of gravity about 1/4" to 1 3/4" ahead of the rear axle. As long as the car is stable, weight distribution will affect the car's performance only a few places.
A recent race between two champions, conducted on a stock Piantedosi Oars, Inc. track, produced an average advantage for one car of approximately 1/4 inch. For the 4 heats, alternating lanes, the orange car won by 1/8 inch to 3/8 inch. Here are some specifications:
Orange car: Council Champion, eastern Illinois area.
Green car: District Webelos Champion, central Illinois area.
- Length: Both 7 inches.
- Weight: Both 4.99 ounces.
- Wheels and Axles: Both excellent.
- Wheels touching: Both "3-wheelers"
- Alignment: Both excellent.
- Rear Axle Location: 0.57 inches from rear of car.
- Front Axle Location:
Orange car: 6.00 inches from rear of car.
Green car: 6.43 inches from rear of car.
- Center of Mass Location:
Orange car: 1.7 inches from rear of car.
Green car: 2.2 inches from rear of car.
Orange car: Fair to good - Small cross section, rough surfaces.
Green car: Excellent - Small cross section, very smooth surfaces.
Pinewood Derby Design Overview:
Fundamental equation of Pinewood Derby Car Design:
Kinetic Energy (speed) = Potential Energy - Lost Energy
1. Accelerate early and hold your speed.
2. Maximize potential energy and minimize lost energy.
Key Design Concepts:
Design your car so that it runs in the steepest, smoothest, straightest and slickest lane, no matter which lane you use.
These implementation techniques are based on the current common track design, in which the starting line is sloped about 30 degrees above horizontal, with the slope reducing to 0 degrees. Some of these techniques do not apply to other track styles.
Car as long as rules allow
Rear wheels as far back as rules allow
Front wheels forward as far as rules allow, without affecting rear wheel location (longest allowed wheelbase)
Car's center of mass as far back as car stability allows, and car's center of mass as low as possible, but "as far back" is more important (maximize potential energy)
Wheel alignment "dead-on"
Weight as close to maximum allowed as possible
Car's cross section as small as possible
Wheels "in round", balanced, and all sliding contact surfaces polished (hub and inside wheel edge)
Axle contact surfaces polished
Hub contact area as close to wheel axis as possible (minimize breaking torque due to wheel-body and wheel-axle friction)
Prepare wheels and axles
Square-up the sides of the block
Draw profile of car on side of block
Mark locations of axles
Drill holes for axles (ream slot if slots must be used) #44 drill (Test drill in other wood first and check nails for snug press fit.
Cut car's profile
Draw top view, including openings for weights
Cut top view (scroll saw is good for this)
Shape and Sand
Weigh body, axles, wheels... add lead to total 5 ounces
Shape lead slugs - hammer and "anvil" works well
Glue lead pieces to car body - (5-minute epoxy works well, hot-glue is viable alternative)
Trim to weight limit; final sanding
Paint/Decorate (keep body clean where hubs will touch)
Install and align wheels
Add graphite and work into wheels
Examine rule changes, especially "boundary conditions", for performance implications.
Over the years, I have evolved a very effective Pinewood Derby Car design which a youngster can build. I call this design "The Rail," and I include it as one of several designs in my book, "Learn to Build a Winner." Important features include short steps, low-risk operations, and few (if any) steps which require fine motor control. The tools needed are, in most cases, only a phone call or two away. A donation to my BSA Council's Friends of Scouting campaign is requested for use of the Learn to Build a Winner book... honor system, of course, as is appropriate for Scouts of all ages.
The attached photos show some of "my boys" who have built their own cars. I know. I was there. Their Dad and I watched them.
Josh, his car, his District Race trophy, and his Akela's Akela.
The Car that Josh built! (fuzzy enough to protect some of his "secrets")
Brandon and his trophy at the District Race
The Car that Brandon built!
Victor and his trophy at the Council Race (This one is related to me.)
The Car that Victor built!
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Latest update: 5/26/2002
Copyright 1997, 1998, 2002 © by Stan Pope. All rights reserved.