Drift Translator and Toe Angle Calculator

by Stan Pope

Summary
Various writers describe a test surface specification for measuring a pinewood derby car's "drift toward the rail" when preparing a car to guide using the center rail of a pinewood derby track. Various test surface lengths and drifts are used. Eight feet and four feet are common test surface lengths, although other lengths are often more conveniently available for most of us.
This page provides a computation tool to convert drift specs to test surfaces of different lengths. For instance, it can convert a 4 inch in 8 feet spec to the comparable drift on a 3 or a 5 foot surface. IF the car's wheelbase is specified, the tool also computes the toe angle needed to produce the specified drift. With the toe angle in hand, one can use this computation tool to guide in drilling the appropriate axle holes in a car body.
Terminology

Drift - Lateral movement of a car away from the reference line on the test surface assuming that the car started exactly aligned with the reference line.

Toe - horizontal displacement of a wheel from the car's center plane. In positive toe, the rearmost part of the wheel is farther from the car's center plane than the frontmost part of the wheel. Often toe is described in terms of the angle of the axle. When toe is positive, the outer end of the axle is more forward than the inner end.

Wheelbase - distance between the front steering axle and the axle behind it.

Equipment

Test Surface - a long, flat, smooth, gently sloping surface with a reference line drawn down the center. The surface is level side to side and typically has a slope around 1/2 inch per foot (or less). The "length" of the test surface should measured as the distance that the car's reference point moves down the reference line.

Communication

Since drift can be accomplished by toe on a front wheel or by toe on the rear wheels, or by a combination, communication would be enhanced if the drift measurement used a means of aligning to the reference which was consistent across all the ways of creating drift. That is, the drift measurement should be independent of how the drift was accomplished. When a rail guided car runs, the rear wheels travel a straight line. The car should then, be staged for drift measurement so that the rear wheels start straight down the track.
However, that straight line motion is difficult to discern. Small errors in aligning their initial path leads to large errors in the drift measure! Avoiding those large errors is more important than measurement consistency between the various ways of accomplishing drift. Therefore, I recommend that some long dimension of the car be aligned to the board's reference line and that the documentation include the method of alignment and the method for accomplishing toe.
Useful long dimensions for alignment include the side of the car that has the dominant front wheel, where the axles on that side of the car enter the car body, or the inner edges of the wheels on that side of the car body.
Caveats

When staging the car for a drift measurement, the car's centerline should be carefully aligned parallel to the reference line and the wheels should be positioned on the axles as they will be after rolling a few feet. This is especially important on a shorter test surface.

Confirm drift measurements by repeating the drift test several times.

Pick out a point on the front end of the car to measure the drift. That point should be aligned to the reference line.

Consistent staging is essential for quality results. If you are trying to replicate someone else's result, it is essential that you stage in the same manner!

Drift Translator and Toe Angle Calculator
For the following computations, enter the lengths of the test surfaces (boards) less the length of the car. For an 8 foot board, enter 89 inches (or less) which is 8 feet (96 inches) less 7 inches.
Results are computed (if enough data is available) every time one of the inputs is changed. Inputs are changed when new values are keyed in and focus is changed to another input box (e.g. by pressing TAB or by clicking in another box.)

Inputs:
    Given test surface length (inches):

    Given test surface drift spec. (inches):

    Available test surface length (inches):

    Car's Wheelbase (inches):

Result:
    Computed test surface drift (inches):

    Toe angle (degrees):

Latest revisions:
2/15/2010 New!
2/26/2010 Add Staging caveat and Communication topic.

Known problems:
None
Copyright 2010 © by Stan Pope. All rights reserved.