Pinewood Derby Car Design

By Stan Pope

Contents

• Make or Break
• Top 10 in district
• Design Strategy
• Implementation
• Construction steps
• Every year...
• My Book
• Examples, Pictures of "My Boys"

Design Concepts and Priorities

Make or Break:

• 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).
  

Top 10 in district:

• 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.
  

• Never let the rear wheels touch the guide rail. These wheels are heavily loaded, so if they rub the rail, their friction losses are much greater than if the front wheels touch the rail.
  

• 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.
  

• CV:
  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.
• Aerodynamics:
  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

Design Strategy:

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.

Implementation:

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 nearly as far as rules allow, without affecting rear wheel location (nearly longest allowed wheelbase) except set back from nose about 1/4 inch to protect front wheel alignment from the stopping mechanism at the end of the track.
• 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" or active front wheel mildly toed in.
• Front wheels inset slightly so as to prevent rear wheels from touching the guide rail.
• 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)

Construction steps:

• 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 (or other ballast) 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, but watch out for the fine strands that trail when the gun is pulled away)
• 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

Every year:

• Examine rule changes, especially "boundary conditions", for performance implications.

My Book:

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.

Examples, Pictures of "My Boys":

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!