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Pinewood Derby Stories and Photos from Maximum Velocity
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Eliminating Pinewood Derby Race Variation
By Randy Davis
Have you ever looked at heat times for pinewood derby cars? If your
event uses an electronic timer, then the heat times are available and
can provide important insight on race performance.
Quite often the heat times vary considerably, while in some cases the
heat times are quite close. This difference in heat times is sometimes
called "race variation". Today we will look at what causes race
variation, identify what can and what cannot be controlled, and then
suggest ways to minimize race variation.
UNCONTROLLABLE FACTORS
Some factors that cause heat variation cannot be directly controlled
by the car builder, however, in some cases they can be minimized by
adjusting the car design. The main factors in this category are: Lane
Variation, Staging Variation, and Finish Line Variation.
Lane Variation
Under racing conditions, race variation will be introduced due to lane
differences. On many tracks there are fast lanes and slow lanes. This
lane variation can be due to defects in the track surface and lane
guides, and/or warping of the track due to poor setup, improper
storage, or exposure to heat or moisture. Although the race
participant cannot directly control these factors, lane variation can
be minimized by:
1. Setting the balance point of the car - If the track is known to be
rough or warped, then setting the balance point to be less aggressive
(more stable), will help the car to stay on a more true line as it
runs down the track.
2. Setting the alignment - As long as the track guides are reasonably
smooth, setting the car to rail-ride will minimize variation because
the car will hold a straight line. Also, when rail-riding, the balance
point of the car can generally be set more aggressive to improve
performance.
3. Running on three wheels - keeping one wheel off the ground reduces
track contact, which can help minimize variation.
Staging Variation
Variation in heat times is also introduced by differences in the way
the car is staged from heat to heat. If the car owner places their
own car on the starting line, then variation can be minimized by
staging the car consistently. But if a race official stages the car,
then little can be done (other than to make sure the official knows
which end of the car is the front end!)
Finish Line Variation
The finish line can also introduce variation. If the front of the car
is quite narrow, then the point at which the finish line sensor is
tripped can vary depending on whether the car is centered, or is
shifted left or right as it passes the sensor (see Figure 1). To
minimize this source of variation, make sure the front of the car is a
minimum of 3/4 inch in width.(1)
Figure 1 - Finish Line Variation
CONTROLLABLE FACTORS
There are many factors that can cause race variation which can be
controlled by careful car design and construction. Some of these
which have already been mentioned are weight position, alignment, and
3 versus 4 wheels on the ground. Additional factors include:
precision components, better fitting components, aerodynamics, and
lubricant choice.
If there are no limits on components and if the track is very smooth,
then heat variation can virtually be eliminated. In an experiment
using needle axle outlaw wheels, and Krytox 100 lube, the heat
deviation was measured at .0016 seconds (standard deviation).(2) But
even with more stock components, the heat deviation can be kept to
.0030 seconds (standard deviation).(3) So, proper design and
construction can make a huge difference in heat variation.
Precision Components
Regardless of the kit type, stock wheels and axles are not perfect.
Wheels can be out of round, have a left-to-right wobble, and/or out of
round bores. Axles also have defects and variation. To minimize heat
variation, the wheels are axles should be as accurate as possible.
Some possible remedies include:
Wheels
1. Truing wheels on a lathe or with a Pro-Wheel Shaver XT.
2. Measuring wheels and using the most accurate ones.
3. Polishing the bore of the wheel.
4. Purchasing trued wheels.
Axles
1. Polishing axles to a high shine.
2. Beveling the axle head.
3. Grooving the shaft.
4. Purchasing accurate replacement axles.
Better Fitting Components
When wheels and axles have a sloppy fit, then the wheels have an
opportunity to move in undesirable ways in response to track defects.
So, to minimize heat variation, the wheels and axles should be sized
to fit. This may not be possible due to rule restrictions, but if
allowed, size the axle to be no more than 5 thousandths of an inch
smaller than the bore. After-market axles with larger diameters are
available for BSA and other wheels with a sloppy wheel/axle fit.
Aerodynamics
Low-profile cars and narrow wheels both reduce aerodynamic drag and
turbulence. Although these are minor factors, some improvement in
heat variation can be achieved by reducing the cross-section of the
car.
Lubricant choice
Another possibility for reducing heat variation is the lubricant
choice. Although I do not have firm results, it appears that Krytox
100 produces slightly less heat variation than graphite.
CONCLUSION
So, if you want to improve heat variation - and performance as well -
consider implementing some of the options discussed above. You will
find that your car is more consistent, and consistently faster than
most (if not all) of the entrants in your race.
