Aligning Rear Axles of Pinewood Derby Cars

Instructions to Guide by Left Front Wheel

By Stan Pope

Reformat text for Guiding by Right side of Rail

Reformat text for Generic Instructions

Introduction and Summary

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.)

Weight Biased Roll Test for Alignment

Alignment Process Overview

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.

Equipment:

Rear Axle Alignment Board (see this page)
Left and Right Side Bias Weights (see this page)

Preparation

Add attachment points to the car body. I prefer a "three point hitch" using small depth holes, one centered on the rear of the car and one on each side of the car about 2" forward of the rear. These need be only 1/16" deep and sized to fit the bias weight harness wire. A #44 drill bit works well with the suggested wire.

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.)

Right Rear Wheel Adjustment:

Attach the Right Rear Wheel Bias Weight to the car.
Place the car on the alignment board with the Left Front Wheel against the rail and the Left Rear Wheel with its inner edge just on the 1/8" line. Check that the rear wheels are at the outer end of the axles.
Release the car and watch the Left Rear Wheel. Unless the Left Rear Wheel is badly misaligned, the path that it takes is most likely caused by the Right Rear Wheel.
If the Left Rear Wheel moves away from the rail the likely cause is toe-in on the Right Rear Wheel. Decrease the toe-in for the Right Rear Wheel. This means to rotate the axle so that the top of the axle rotates toward the back of the car.
If the Left Rear Wheel moves toward the rail, the likely cause is toe-out on the Right Rear Wheel. Decrease the toe-out for the Right Rear Wheel. This means to rotate the axle so that the top of the axle rotates toward the front of the car.
Repeat the Right Rear Wheel Adjustment steps until the Left Rear Wheel stays on the line.
Remove the Right Rear Wheel Bias Weight.

Left Rear Wheel Adjustment:

Attach the Left Rear Wheel Bias Weight to the car.
Place the car on the alignment board with the Left Front Wheel against the rail and the Left Rear Wheel with its inner edge just on the 1/8" line. Check that the rear wheels are at the outer end of the axles.
Release the car and watch the Left Rear Wheel. Unless the Right Rear Wheel is badly misaligned, the path that it takes is most likely caused by the Left Rear Wheel.
If the Left Rear Wheel moves away from the rail, the likely cause is toe-out on the Left Rear Wheel. Decrease the toe-out for the Left Rear Wheel. This means to rotate the axle so that the top of the axle rotates toward the front of the car.
If the Left Rear Wheel moves toward the rail, the likely cause is toe-in on the Left Rear Wheel . Decrease the toe-in for the Left Rear Wheel. This means to rotate the axle so that the top of the axle rotates toward the back of the car.
Repeat the Left Rear Wheel Adjustment steps until the Left Rear Wheel stays on the line.
Remove the Left Rear Wheel Bias Weight.

Alternate between the Right Rear Wheel Adjustment and Left Rear Wheel Adjustment until two consecutive trials require no adjustment to rear wheel toe.

Proof:

Prove the alignment by running the car on the alignment board with no bias weights and observing that the Left Rear Wheel stays on its line! If the Left Rear Wheel strays from its line, then there are several possible causes:

The rear wheels have different effective coefficients of friction. This possibility should be checked using the Differential Friction Analysis process below.
The weight of the Bias Weight altered the car enough to disrupt the car's geometry. The process has left the axles nearly perfectly aligned, but the alignment might be improved by using lighter bias weights (e.g. 1/2 ounce instead of 1 ounce) and lower bias ratios (e.g. 1.5:1 instead of 2:1 or 3:1 along with finer adjustments to toe angles.
The process was not performed correctly. (Of course, this is not very likely, but it is possible, so I had to list it.)

Example:

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'.

Weight Biased Roll Test for Unequal Rear Wheel Coefficients of Friction

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!)

Some causes of differential frictions:

difference in alignment between the rear axles (the more lightly loaded rear wheel "drags" the car offline)
difference in camber between the rear axles
different smoothness of the axles or wheel bores or wheel hub faces
different diameters of the rear wheels' bores
different diameters of the rear wheels' treads
different slopes of the axle head undersides
different shapes of the tread contact patches with the track

Concept

The fastest Pinewood Derby Cars have their weight concentrated near the rear axles. The result is that the front wheel(s) bear very little weight. The lightly loaded front wheel(s) are very susceptible to steering by influences other than their orientation. This process detects the presence of "other influences" by comparing the path followed by the car with the path it follows when the front wheel(s) carry more weight.

Equipment:

Drift Alignment Board (see this page)
Front Wheel Bias Weight (see this page)

Test:

Measure Unloaded. - On the drift test board, run the car several times to get the average drift.
Load Front - Install the "long arm" cantilever weight assembly and adjust it to load the dominant front wheel while leaving the rear wheel weights at their original value. The weight will be approximately on a line that passes through the car's original center of mass and the front wheel, but will be forward of the front wheels.
Measure Loaded - Again on the drift test board, run the car several times to get the average drift.
Analyze Losses - Compare the two drift measure averages. If they differ significantly, then one of the rear wheels rolls with a higher net coefficient of friction than the other. The deviation in "Unloaded" measure versus the deviation in the "loaded" measure tells which rear wheel is lossier: If the drift of the Unloaded runs is to the right of the drift of the Loaded runs, then the right rear wheel has a higher net coefficient of friction, and vice versa.

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.