Friday, October 04, 2013

Cover Up Those Wheels With Fenders
By Randy Davis


Looking at photos of pinewood derby cars in races across the country,
especially the highly competitive league races, virtually every car
has some kind of fenders. The intent of the fenders is to improve air
flow over the wheels, squeezing out a few milliseconds. Given the
slow speeds of pinewood derby cars, a few milliseconds is all you can
hope for, but of course a few milliseconds is all it takes to win or
lose a race.

But do fenders really help? Well, there is only one way to find out.
So let's do an experiment.

FACTORS TO CONTROL
In order to make sure that the presence or absence of fenders is
actually being measured, we must construct a car on which:

1. Fenders can be added and removed without affecting the wheels,
axles, lubrication, and alignment.

2. Ballast weight can be added and removed to compensate for the
weight of the fenders, again without affecting any other factor.

3. The ballast weight is added/removed at the center of gravity
of the fenders.(1)

THE TEST CAR
My solution to meet these criteria was to use full fenders and mount a
1/4 inch dowel pin at the center of gravity of the fender. A hole
was then drilled through the car at the center of gravity of the
fenders. This hole would accept the fender pin or the ballast weight.


Figure 1 - Test Car With Fenders


Figure 2 - Test Car Without Fenders
(Ballast Weight Shown as Inset)


Figure 3 - Underside of Car

The fenders were shaped by tapering the leading edge, and by beveling
the portion that hangs below the car (this must be done to prevent
scraping on the track guide rail - see Figure 3). The COG of the
fender was located and a 1/4 inch hole was drilled at that location.
Short pieces of 1/4 inch dowel pin were then glued into the fender
hole.

The ballast weight was a bit of a challenge as I only had a 1-3/4 inch
long x 1/4 inch diameter hole in which to put the weight. I ended up
acquiring a 1/4 inch rod of solid tungsten and then cutting it to
length (not a practice I recommend). I then super-glued two tungsten
disks and two short pieces of dowel pin to the tungsten rod, resulting
in a solid unit that was 1-3/4 inches long and just slightly less than
the weight of the fenders. To adjust the weight, I sanded off a
portion of the dowel pin attached to the fenders, resulting in an
exact weight match.

The car body was made with a 1/4 inch hole drilled from side to side
at the location of the fender COG. Weight pockets were added, and the
car was brought up to five ounces (when the fenders are attached or
the ballast weight is inserted). Packing tape was applied over the
underbody pockets so that they did not affect air flow. The car was
equipped with a raised front left wheel.

Other static car factors include:

- Pro-Stock Speed Wheels
- BSA Speed Axles (polished)
- Krytox 100 lubricant
- Down-canted front right wheel for rail-rider alignment

THE TEST
The car was assembled with the fenders; two adhesive dots were applied
on the inside of each fender to keep it in the proper position. The
alignment was set to drift left five inches over eight feet, which
works well on my track. After a few trial runs, the testing
commenced.

First, four runs were made with the fenders in place. Then the fenders
were removed, the ballast weight added, and the alignment verified (no
change needed). Eight runs were then made without fenders. Finally,
the ballast weight was removed, the fenders re-installed, and the
alignment re-checked (again, no change needed). An additional four
runs were then made.

The high and low results were then removed for the fender heat times
and the non-fender heat times. The remaining data was averaged and the
standard deviation calculated.

THE RESULTS
To my surprise the results were clear(2):

Fender Heat Average - 2.531
Standard Deviation - .00146

No-Fender Heat Average - 2.536
Standard Deviation - .00163

So on average, fenders improved the speed of the car by .005 seconds,
and slightly reduced the variance in the heats.

THE TRADEOFF
One thing that needs to be pointed out is that fenders add weight to
the car, and not at the desired location (the fenders in this test
weighed 0.66 ounces). So, the question must be asked: Would the same
improvement be seen if the weight was put at the back of the car and
the fenders eliminated? Unfortunately, I did not set up the car to
specifically test this, but I did tape the ballast weight on top of
the car just in front of the rear axles. The heat times did improve
to 2.533. But this test is not conclusive as the weight was not
necessarily optimally placed and it was not inside the car.

But the point is that for best results the effect of the fender weight
must be minimized. There are several ways to do this:

1. Use half fenders instead of full fenders.

2. Use balsa fenders instead of basswood or pine fenders.(3)

3. Minimize the weight of the main car body as much as possible.

CONCLUSION
In a tight race, if implemented properly, fenders can give your car a
slight edge. So, on your next project, consider covering up those
wheels.

(1) For the purist, the ballast weight should really be spread all
along the side of the car, but that would be extremely difficult to
do.

(2) Actually to my relief. I was concerned that the data would be
inconclusive and I would need to try again with half fenders. Half
fenders would be more of a challenge to get the correct weight when
the fenders were removed.

(3) I am not a big fan of balsa as it is more difficult to use,
but it is very popular for fenders. Both pine and balsa half fenders
are available from Maximum Velocity

From Pinewood Derby Times Volume 13, Issue 1

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