Pinewood Derby Stories and Photos from Maximum Velocity
One of the five keys to performance(1) is alignment. The purpose of alignment is to adjust the steering of the car such that the car rolls in the desired direction.
But what is the desired direction? The seemingly apparent answer is "straight", and that was the answer I would have given in the past. But does straight alignment really produce the best performance?
NOTHING IS PERFECT The answer is no (this will be proven experimentally later in this article). If the track was perfectly level and smooth (no deviations whatsoever), and if the car could be set perfectly straight (again no deviations), then clearly, straight alignment would be the best. But a perfect track is an impossibility (most tracks are far from level and smooth), and in fact, perfectly straight alignment is not possible. So, what happens if a car that rolls reasonably straight is raced on a track that is not perfect? It will exhibit one of the following behaviors:
- Track generally tilts left - Car will roll until the right wheels contact the center guide rail, and will generally stay there for the entire run.
- Track generally tilts right - Car will roll until the left wheels contact the center guide rail, and will generally stay there for the entire run.
- Track tilts both left and right - Car will oscillate between contacting the left and right wheels.
Now let's add one other factor: a raised wheel. The purpose of the raised front wheel is to reduce the energy required to start the wheels rolling. If the raised wheel can be prevented from spinning, an advantage will be gained. But if the raised wheel contacts the guide rail, even one time, the advantage will be lost.
So what can you do to get the best performance within the limitations of the track and the car's alignment? The answer is: create a Rail-Rider(TM) car.
RAIL-RIDING To get the best performance under non-perfect conditions, we must minimize the number of wheels that contact the guide rail, and ensure that the raised wheel does not contact the guide rail. Thus, the alignment of the front dominant wheel must be adjusted so that it steers towards the raised wheel (thus keeping the raised wheel away from the center guide rail). So, if the front-left wheel is raised, then the front-right wheel is adjusted so that the car drifts to the left. If the back wheels are aligned straight, then the front right wheel will be the only wheel that contacts the guide rail.(2) Thus, the front-right wheel, more or less, rides the rail down the track.
EXPERIMENT This discussion is all academic, so let's set about proving whether or not rail-riding really provides the intended benefit.
First, we must create a car with a front-right wheel with a toe-in/toe-out adjustment. This allows the car's drift to be adjusted without removing the axle, thus minimizing experimental variance.
The toe-in/toe-out adjustment was accomplished using set screws. Aluminum tubing was used to hold the axle and provide a pivot point (see Figure 1).
Figure 1 - Toe-In/Toe-Out Adjustment
Additional equipment includes:
- 32 Foot Aluminum Freedom Track, the track was leveled with a bubble level and shims. - Rear weighted extended wheelbase wedge weighing five ounces, front- left wheel raised - Krytox 100 Lube(3) - Pro Stock Speed Wheels from DerbyWorx(4) - Speed Axles from Maximum Velocity(5)
Six heats were run, each with six different toe-in/toe-out settings(6). The inches of deviation is the amount of drift over an 8 foot run on an alignment board.
- 5 inches left - 3 inches left - Center alignment - 3 inches right - 5 inches right
After the heats were run, the high and low times were removed and the remaining heats averaged.
The results of the experiment are shown in Figure 2.
Figure 2 - Rail-riding results
CONCLUSIONS The experiment clearly proves the benefit of rail-riding. For this experimental setup, a drift of up to 5 inches (and probably more) provides an incremental performance benefit. But, please note that the aluminum track on which this test was performed has a very smooth center guide rail. On other tracks, especially wooden tracks, the best amount of drift would need to be experimentally determined. A rougher center guide rail may require a lesser amount of drift.
(2) To further ensure that the back-right wheel does not contact the guide rail, the front right of the car can be narrowed slightly such that the front- right wheel runs slightly closer to the guide rail than the back-right wheel.
(3) During this experiment, no graphite was used. However, the same car was used for a later experiment which used graphite. The photo in Figure 1 was taken after the second experiment, so graphite can be seen on the tread surface of the wheel.
(6) I would have liked to have had finer settings, but the threads on the set screws were too coarse to readily allow a finer adjustment. I also would have liked to have tried a larger left drift, but my alignment board was not wide enough to go further.
(TM)Rail-Rider is a trademark of DerbyWorx / Warp Speed, Inc.