Jim wrote:
My son competed in his second Pinewood Derby a couple of weeks ago.
Last year he finished dead last. snip

We would appreciate any help or advice.

Sure - my son's first year. 1st place (64 kids) in each of 4 heats of
all 64 cars, so first over all for the troop. Way, way, fastest car
there. At superderby (top 5 from each pack - 125 cars all) 1st place
after 3 heats, 4th heat rubbed the rail, came in 13th overall. Still
just over .01 seconds for 4 heats between him & #1 overall. The cars
are all fast at regional.

The physics are simple. ALL the energy of the system at the start is
potential energy dependent solely on height & mass. You maximize these
by building the car as long as allowed (7"), car at the maximum height,
with they weight as far back as will maintain stability (cg in front of
rear axle, but close to). However, the difference in stored energy is
relatively small given the parameters, as long as you're at max length
& weight (the change in height of cg is pretty small range).

You said your sons car kept up down the slope. Heading down the slope
is the conversion of potential to kinetic energy (height to velocity).
If you're losing on the flat run it's all friction. Air friction is
negligible. My son's second year we did a football helmut (of wood) on
top of main body. Hilarious looking, wobbled, but amazingly came in
8th overall. Last year, much to my chagrin, he made superderby again
(trust me, once is enough) - but barely and got womped (took 8 hours to
get through, and was obvious this was one of the slower cars). This
year his car is an "arrow" - we haven't even assembled yet.

So what causes the difference - reducing friction. Where he (and other
top cars) all pull away is in the flat run. Ways to reduce friction,
in sort of importance (most to least):

1) trial roll the car, adjusting the axles, until it runs straight. If
it rubs against the center guide rail the entire way ain't gonna be
fast (of course, if the car isn't set up straight, out of your
control).

2) File the burr formed when the nail head is stamped. This will
definitely cause friction, and is easy to remove. A further refinement
is smoothing/truing the axles to a true round, but minor. The wheels
spin on a single area (bottom) of the nail, so as long as that is
smooth should be fine.

3) Remove the "nub" off each wheel where it was injection molded.
Helps to true the wheels as well, although I've always found them
pretty much round.

4) Lubricate w/graphite (all we're allowed) a lot, and spin the wheels
a lot, before the meet.

5) Camber the axles up a few degrees. This causes the wheel to want
to move away from the body. Wheels rubbing against body is bad.
Camber too much though and they push too hard against nail head, again
increasing friction.

6) You can shave (carefully) the profile of the wheels so they run on
an edge, not the whole surface. Note that cambering the axles should
have this effect, and over-profiling the wheels can result in
disqualification.

7) Raise one axle so car runs on 3 wheels. Not so sure. In theory,
less rolling friction (3/4 to be exact...) and no energy stored in
spinning up the 4th wheel. However, if the car wobbles at all, varying
which 3 wheels are touching, I could see this being a net negative as
different wheels would be spinning up/slowing down. Hard enough to get
the damn thing to roll straight on 4 wheels (stay on 4 wheels!!).

8) "Aerodynamics" The first car was pretty much aerodynamic - but
had "shark fins" on the back that didn't help. The 2nd was the
aforementioned football helmut. The 3rd was a "missile laucher" with a
big ass rocket out the front. I don't even remember last year's car
(thank God this is the last). Some guy's cars are professionally
finished - I mean race car lines and a paint job better than my real
car. I prime the car (spray) and my son hand paints whatever he wants.
He also spoke shaves (if needed), rasps, and sands the body before
priming (thus this year we needed a round of Bondo to fix some "design
misfeatures"). Needless to say, 6 - 10 year olds aren't too great with
hand tools and sanding is boring as hell. He used the spokeshave at 6
- it can be done - just need to supervise. They're not the greatest
looking cars, but they run okay because we pay attention to the
fundamentals - max length, weight distribution, reduce the real
friction factors (non-aerodynamic). Any drilling, etc he does. I do
the bandsawing and any chisel work (because the risk of a slip).
Obviously, a 10 yr old whose helped make 4 cars can do a lot more than
a 6 yr old on his first.