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PINEWOOD DERBY CAR SHOWCASE

(As an exception, I am dedicating the Car Showcase this week to a car
built by Stephen Parks).

I was browsing your newsletter archives, and saw the article on
the direct drive outlaw car that you built -- Volume 8, Issue 3

It was interesting to me because, as an engineering student, I
competed in the ASME student design competition, and designed a
"string transmission" for a transport device.

For last year's pinewood derby event at work, I wanted to make an
electric car for the Outlaw class, and initially thought of using the
same type of transmission. However, it has some issues. If the
wheels spin, then you may not have enough string to power all the way
to the end, and if the wheels don't spin, then you probably should
have used more power! Eventually I abandoned the idea of a string
transmission, and decided to just use a gear drive. A co-worker races
electric RC boats, and sourced a Lithium-Polymer battery. These
batteries are expensive, but relatively light, and have almost no
internal resistance. I found a few motors that looked to be a
reasonable size, then built a dynamometer to test and compare them.
This approach wasn't very consistent, so in the end I simply picked
the motor that sounded and felt the strongest.

So far, this wasn't nearly complicated enough for me. I figured that
this drag car ought to look like one, so I decided to hide the
drive train, switches, etc. into a drag car model so that it would be
a "sleeper". The model I chose was the Stone, Woods, and Cook Willys.


Model Box Photo


My Car

The start switch is hidden behind the grille. When it is race time, a
pin fits through one of the holes in the grille and into the switch,
and rests against the start post on the track. There is an arming
switch underneath the car just in front of the rear bumper. There was
also a cutoff switch underneath the car, so that when the car falls
down onto the stop strip at the end of the track, the power is cut
(well, that was the idea).

Quite a bit of work was required to transform a model kit that was
intended to be stationary into a rolling, self-powered, self-guided
vehicle. Here are photos of the chassis, showing the motor and gear
layout. The silver rectangle is the battery pack. The gears are out
of a broken DVD player.


Top of Chassis


Bottom of Chassis

I should mention that the track is not set up until the night before
the day of the race (after the impound). So, testing is very
difficult. But I did get the coordinator to allow a couple of
practice runs. On the first run, the tires had too much traction and
lifted the front end when they caught on a track joint, resulting in a
derail. But that run put enough graphite on the tires to reduce grip
enough so that didn't happen again. The next run was 1.998 seconds,
on 49 feet of BestTrack!

Video of a Run (WMV format)

On race day, the car got a lot of interest for looking good, though
some thought that it was "just a plastic model." No one expected a two
second run, and it was easily faster than the other Outlaws, all of
which were gravity powered. The run times gradually increased, not
because of the battery, but because of graphite buildup on the tires.
Going fast was not a problem with this car, but getting it stopped
was. I seriously underestimated the speed at the end of the track,
and didn't have a good way to stop the car. At the end of the fourth
run, one of the guides that keeps the rubber wheels from contacting
the center rail broke, and the car could not stay on the track after
that. I took the car out of the race instead of needlessly damaging
it further.

This year I'll probably reuse the motor and battery, but forgo the
fancy body in favor of a much more robust frame, and I'll build a
stopping device to fit onto the end of the track. I may also switch
to front wheel drive to eliminate the possibility of wheelies, and I
can then clean the graphite off of the tires after each run to
maximize traction and maintain consistent times.

Stephen Parks

Read More at: Pinewood Derby Times Volume 9, Issue 4

The Car Showcase is a regular feature of the Pinewood Derby Times Newsletter. To subscribe to this free e-newsletter, please visit:
www.maximum-velocity.com/subscribe.htm

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