This document describes a process for aligning the rear axles of a Pinewood Derby car. The axles may be either cambered or level. It also detects the presence of differential rear wheel frictions.
The process originated as an accurate method for aligning cambered rear axles, for which published methods were problemmatic. While working with the process, it became apparent that the process was equally effective for level axles. Moreover, it was easier to use than the shim alignment process that I introduced in my book, "Learn to Build a Winner," since its sensitivity did not require that the axles be exactly level. Consequently, it allowed direction adjustment by a wider variety of techniques, including twisting slightly bent axles, a historic method preferred by many.
The process is directly applicable to three wheel (4th wheel lifted) cars that guide using the center rail. With some thought the process can be adapted to "4-wheelers" (temporarily remove the 4th wheel) and to "straight runners" (temporarily replace the axle for the dominant front wheel with a slightly bent axle.)
The following procedure causes each rear wheel's contribution to direction to be most prominent in the observed test results. It is done by shifting the load temporarily from one axle to the other by adding a small cantilevered weight. A preliminary experiment showed that a rear wheel dominates directional control if it carries substantially more weight than the other rear wheel. (A 1/2 ounce difference is usually sufficient.) The document shows alternatives for constructing a temporary cantilevered weight harness and how to use them in aligning the rear wheels.
Here is a short video showing alignment of a sample race car. Viewing it may help the reader understand the following text.
While following this procedure, you may uncover a source of "differential friction" in the rear axles. Differential friction can cause the racer to steer in directions other than that attempted by the front wheels. The process partially isolates a specific rear axle as the source. If such sources are found, correct that defect before trying to complete the alignment.
Rough align rear wheels for parallel to body. This is most reliably done by holding the car bottom side up, allowing the wheel bores to lie on the axles and checking for equal distance from the side of the car body to the edges of the tread, both fore and aft of the axle. Adjust as needed. This setting does not need to be exact, but the better it is set, the fewer issues will be encountered and the more quickly the alignment will be complete.
Set the Left Front Wheel for gentle toe in. Roll the car on the alignment board a few times and watch that the Left Front Wheel stays against the rail. If not, increase the toe-in.
Attach the Left Rear Wheel Bias Weight to the car and adjust it so that the Left Rear Wheel weighs about twice as much as the Right Rear Wheel, while keeping the Left Front Wheel weight about the same. (see this page for important adjustment information)
Attach the Right Rear Wheel Bias Weight to the car and adjust it so that the Right Rear Wheel weighs about twice as much as the Left Rear Wheel, while keeping the Left Front Wheel weight about the same. (see this page for important adjustment information)
(Note that the better the rough alignment of the rear wheel, the less bias weight is needed. If problems are found in the fine alignment, then a bias of three times may be needed.)
Alternate between the Right Rear Wheel Adjustment and Left Rear Wheel Adjustment until two consecutive trials require no adjustment to rear wheel toe.
Here is a short video, with annotation, showing an alignment. The film clip running time is 3 minutes 24 seconds. It starts with DFW toed in (normal) and rear axles badly misaligned. It ends with the rear axles aligned properly to run the dominant side rear wheel 1/16" off the rail, the other side rear wheel running the same direction, and the drift at 2" in 2'.
This section attempts to tell whether the net or composite coefficients of friction of the rear wheels differ from each other. The idea of "net" or "composite coefficient of friction" is that this coefficient reflects the result of the various causes of friction. It does not reflect the effect of different amounts of weight carried by each wheel (that cancels out of the low speed turning moments equations) nor does it include misalignment (that has already been corrected by the procedure above!)
This test does not tell the cause of the higher coefficient of friction, just that it is higher than on the other wheel. Since you did so well on the other wheel, you can probably improve this one! The good part is that "alignment" has been eliminated as a possible cause!)
If the differences are large enough to be worrisome, then correction will require disassembly of the car and alteration of some parts. And that means that after reassembly, the car needs to be aligned again.
Update: 7/14/2013 - Addition of Example videos.
Update: 2/2/2011 - Major rewrite to PHP form.
Update: 3/24/2010 - Add Unequal loss section.
Update: 3/10/2010 - Add instruction to Step 5.
Update: 3/6/2010 - Improve introduction, retitle.
Update: 3/1/2010 - Improve introduction and trailer.
Update: 2/28/2010 - Rewrite with experiments "proposed".
Update: 2/18/2010 - Rewrite with cantilevered weight and experiments.
Original page created: 3/9/2009
Copyright 2009, 2010, 2011 © by Stan Pope. All rights reserved.
Scouting organizations may print, duplicate and distribute copies of this document provided that this copyright notice remains intact and no fee, direct or indirect, is charged for the copies.