Learn to Build A Winner (Cont'd)


These items were identified during my review of the book. They will be integrated into the book in later revisions. For now, I would feel remiss if they were not available for you.

Angular Inertia of the Car Body.

To the extent that the angular inertia of the entire Car is important, it may be reduced by moving the center of mass toward the center of the car and focusing the mass of the car near the center of mass. The trade-off is that maximum potential energy (on a normal track) happens when the center of mass is near the rear of the Car. This may account for experimental results which recommend that the center of mass be located about 2 to 2-1/2 inches from the rear of the Car.

G-forces from Direction Change.

As your Car runs the first 12 feet of its race, its direction is being changed. The upward force of the track on the wheels as the slope changes produces a temporary increase in the wheel-axle friction. I believe that all designs and weight distributions are affected equally, with the following exception. Lighter car bodies cause less friction between the axles and wheels. (Remember, though, that the weight is needed to keep air friction losses acceptable, and that moving weight from the Car body to the wheels increases the angular inertia of the wheels. Overcoming angular inertia does not help speed.)

Hub Inside Diameter Treatment.

It may be beneficial to run a "tap" through the hub of each wheel. The purpose is to create a reservoir for the graphite so that more can be put in and less will spill out as the car races. Choose a size which does not increase the inside diameter of the hub. Be careful! If you fail to keep the hub inside diameter true and square, you will lose much more than you gain.

Hub Inside Diameter Reduction.

Following the same idea as above, you may be able to effect a reduction in the hub inside diameter by using a machine screw of the proper size. Gentle heating of the machine screw may be necessary. The idea is to squeeze small bits of the plastic interior into ridges. The redistribution of plastic results in a smaller hub inside diameter, which could, in turn, produce lower braking torque. Since this procedure could change the center, it should probably be performed prior to turning. As with the tapping procedure described above, it should also create a reservoir for graphite. The cautions above also apply.

Height of Center of Mass Above Wheelbase.

I have run on-track comparisons between identical cars in which only the height of the Center of Mass (CM) above the wheelbase varied. My test setup was not sufficiently sensitive to differentiate performance of the cases. The reason that I ran the test is as follows: As the CM is raised above the wheelbase (on a Modern Track), the distance that the CM moves from the starting line to the finish line decreases, i.e makes the track shorter. However, at the same time, the potential energy available (on a Modern Track) is reduced. So the shortening of the track is accomplished at the expense of speed at the bottom of the fall.

Engineering simulations of a Modern Track of usual dimensions (i.e. 4 foot drop and less than 32 feet length and a long flat) show that raising the center of mass helps. All else being equal, the car with a higher center of mass reaches the flat with a slight lead and a slightly lower speed. The lead is held through the finish line. On a longer track, the lead would be yielded to the faster, low center of mass car.

Raising the center of mass decreases your Car's stability. If you will race on a rough, uneven track surface, this could cause increased losses. Weigh the risks carefully.

If the track will be available for test runs ahead of time, consider constructing the lead weight as an aerodynamic disk. Use a short length of threaded rod, say 3" X 1/8", attached vertically to your Car body just ahead of the rear axle. (Check for Car height conformance.) Drill and thread a matching hole in the radial axis of the disk. Trim the disk to provide a total weight of 5 ounces and screw it onto the threaded rod. Raising and lowering the weight is easily accomplished by just turning the weight. Run the tests by raising the weight to the point of "problem" and then back it off a few turns. If the rules require, "set" the weight with a non-permanent glue.

Head Starts.

In another experiment (also conducted on a Modern Track), I compared what would happen if a car were given a "head start", i.e. staged so that a significant portion of the car were in front of the starting peg. The "head start" turned out to be a big disadvantage, because head start cost it too much potential energy (which translates to speed).

Weighted Wheels.

While it is not permitted by most rules, the subject of weighting the wheels and reducing the body weight by a comparable amount routinely arises. The advantages seem to be a reduction in wheel/axle friction, the ability to move the center of mass farther back without loss of stability, and the return of stored angular momentum to overcome wheel friction as the car runs on the flat. The disadvantages seem to be that more energy is used in the angular acceleration of the wheels, causing the car to come off the slope more slowly.

Engineering simulations of a Modern Track of usual dimensions (i.e. 4 foot drop and less than 32 feet length and a long flat) show that minimizing wheel mass (angular inertia) helps. All else being equal, the car with lower wheel mass reaches the flat with a lead and a slightly higher speed. The lead is held through the finish line. On a longer track, the lead would be yielded to the lower axle friction and the stored energy of the weighted wheels.

If the situation warrants weighted wheels, the wheel weight should have a minimum angular inertia. This is achieved by keeping the mass close to the axle. This applies to tracks of moderate length, e.g. 60 feet.

Weight Distribution.

At least 20 percent of the car's weight should be on the front wheels (if the car is racing on a "modern style" track). The reason is that the light front end allows the car to "come loose" toward the end of the track and develop a "yaw oscillation"... the front may rattle left and right.

If there is an opportunity to "test drive" the track on which the car will be raced, make use of it to get the weight distribution "right for that track." If not, be somewhat conservative in the weight distribution and increase the percentage of the car's weight that is "over the front wheels." (On the spot corrections can be made with washers and superglue. Later, you can reestablish the correct total weight while retaining the distribution that you determined experimentally.)

Measurement of weight distribution is simple: Set the scale and another support at the same height. Place the car with its front wheels on the scale and the rear wheels on the other support. Read the scale. Then repeat the measures with the car reversed. The total of the scale readings will be the same as if the entire car were weighed at one time. If not, recheck the procedure used for error.

Distributing Wheel/Axle Frictions.

After your best preparation efforts, each wheel/axle set may still have slightly different frictions. The lowest friction sets should carry the most weight, typically on the rear of the car.

Balancing Axle Frictions.

The friction from each wheel/axle is a force (moment) around the car's center of mass which tries to turn the car. So, if you arrange your wheel/axle combinations in order of increasing friction, the two with the lowest friction would go on the heaviest end of the car. The other two would go on the lightest end, in opposite order, so that the sum of the frictions on the left is about the same as the sum of the frictions on the right.

Review of Literature.

I have purchased and devoured the several advertised books on Pinewood Derby Car design. These have been generally helpful, each in its own way. This is my candid evaluation of those publications.

Learn How to Build Fast Pinewood Racers & Space Derby, Hodges Hobby House, 1421 Linden, Box 3923E, Glendale, CA 91201. Book C-1, 39 pages, $3.50. (My copy has been loaned out and I have lost track of it, so I am not absolutely sure of the title.) Hodges is a long-time advertiser in Scouting Magazine and Boy's Life. This is a worthwhile addition to your library, but does omit treatment of some important issues, such as planning for non-standard track considerations and importance of wheel alignment.

How to Win a Pinewood Derby, The Winning Edge, 4583 Wintergreen South, Saginaw, MI 48603, 24 pages, 1993. This is a generic book, not specifically tuned to the rules normally found in Cub Scout Pinewood racing. It is worth reading but be careful about taking its philosophy too much to heart. Its view of "rules" is that "if it's not specifically prohibited, then it's permitted!". This approach produces more race-day controversy than most people want in a Scouting event. Keep in mind the traditions and past practices and rules interpretations of the event.

The How-to-Book for Building & Racing, PineCar, P.O.Box 98, Linn Creek, MO 65052, 24 pages, 1991. Talks about some important issues, but provides little information on how to accomplish them. Some performance information is incorrect for the style of tracks commonly used in Cub Scout events. Unfortunately, the conditions in which the "speed tips" apply are not specified.

Cub Scout Grand Prix -- Pinewood Derby Guidebook, No. 33721, Boy Scouts of America, Irving TX, 48 pages, 1992. Includes 13 pages an Raingutter Regatta and Space Derby. Keyed to typical Cub Scout Pinewood Derby rules, but lacking in technical detail and qualifying information. Well written, and in a style that Cub Scouts can understand.

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Latest update: 4/29/2000
Copyright 1995, 1997, 2000 © by Stan Pope. All rights reserved.