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.

(1) For more information on finish line variance, see Is Your Finish
Line Providing Accurate Results?

(2) See Solenoid Start Gate: Are Races More Consistent?

(3) See Cheater Bars – Do They Work?

From Pinewood Derby Times Volume 12, Issue 11

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Maximum Velocity Pinewood Derby Car Plans and Supplies

Pinewood Derby Car Showcase – February 22, 2013

CUDA – James White


This car named “CUDA” was built for my grandson’s (Aaron Shain’s) 2013
pinewood derby races. The rear fenders are made from a portion of the
plastic bottle that the BSA wheels come in, and the cockpit canopy is
made from a slice off the side of a small shampoo bottle.

Finn McMissile – Caleb Tachick

At my son Caleb’s first Awana Grand Prix, he and Finn held their own
most of the day, but ended up around 6th or 7th. But since it was our
first year, the real goal was the design award. He came home with 2nd
place; he would have had first but his sister’s Cancer Survivor Ribbon
car took first. They never raced each other until after the event when
the track was open for fun runs. They ran neck and neck. Finn
McMissile is shown here with the BSA wheels we swapped over to for
the Home Depot race a month later, where he took 4th place — not bad
for a full bodied car.

Cancer Survivor Ribbon – Kailyn Tachick

Here is my daughter Kailyn’s 1st place Awana car in the design
category. This car was in honor of her grandma, who within the last
year underwent treatment and surgery for cancer. We thank the Lord
she is doing OK today. We did not hollow out the nose (my first year
to build), but did incorporate rear fenders, reduced midsection as on
supersonic jets, a concave tail end like a Shelby Daytona coupe, and
speed axles from Maximum Velocity. After losing her first heat(in a
double elimination race format), she continued to terrorize the
loser’s bracket to within a few heats from the end!

From Pinewood Derby Times Volume 12, Issue 11

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Maximum Velocity Pinewood Derby Car Plans and Supplies

Weight: Place it Carefully

Over the past few years I have written several articles, speed tips, and answers to questions that relate to weight position. Today, I’d like to bring this information together into one article. I realize that some of this information is a repeat, but I hope that each of you will find a few nuggets of information that you can put to good use.

TERMS

First, let’s get some terms defined:

COG: Center of gravity – the location at which an object balances in all three dimensions. In this article it will strictly refer to the lengthwise and height-wise point at which the car balances (also known as the horizontal COG and vertical COG).

Weight Position: the location on the car where the added ballast weight is placed. Note that weight position affects, but is not the same as the COG,

Wheelbase: the distance between the front and back axles.

LOCATING THE COG

The horizontal COG of a pinewood derby car can be easily located as follows: (1) set a balance stand (or a ruler on its long edge) on a table and (2) lay the car (with wheels and axles in place) on the device as shown in Figure 1. Move the car forward or backward until it balances on the ruler. This point at which the car balances is the horizontal COG.


Figure 1 – Locating the COG

HORIZONTAL COG – FRONT TO BACK

Due to track differences there is no one best horizontal COG. In fact, the horizontal COG that produces best results on one track will likely not produce the best results on another track.

There are three track types in use today (see Figure 2), with type 2 being the most predominant:

1. Continuous-slope – One continuous slope from the starting line to the finish line.

2. Ramp-flat – Starts on a slope, then transitions to a long flat section.

3. S-shaped – Starts on a slope, transitions to a steeper slope, and then transitions to a flat section).


Figure 2 – Track Types

For the continuous-slope track, the horizontal COG of the car has only a minor effect on car performance (due to uneven weight distribution on the wheels), but it is best to locate the COG near the middle of the car body.

For the ramp-flat track, best performance is obtained offsetting the horizontal COG towards the rear of the car. The actual location varies from track to track, but a good rule of thumb is 1 inch in front of the rear axle. This location should be adjusted based on the following track features:

– Length: longer tracks (over 50 feet) require a COG closer to the center of the car. Due to the offset COG, the rear wheels will bear more weight than the front wheels. On tracks with a long flat section, this extra rear friction will result in a faster speed loss.

– Smoothness: To maintain stability, rougher tracks may require a COG closer to the center of the car.

Why is the horizontal COG offset to the rear on this type of track. With an offset horizontal COG, the car will have a longer fall distance and will thus achieve a faster speed at the bottom of the hill (see Figure 3).


Figure 3 – Fall Distance Based on COG Position

For the S-shaped track, the best location of the COG depends on the length of each section. But, in the absence of track testing (see below) it is best to locate the COG similar to the ramp-flat track.

TESTING THE TRACK FOR HORIZONTAL COG

Testing for the best horizontal COG location for a given track is fairly easy if you have a track timer. Build a lightweight car without added weight, but with three dowel rod pieces sticking up out of the car (one in front, one in back, and one in the middle). Using steel washers weight the car in the front, back, and middle and compare the results. Mix and match to find the best COG for the track.

I ran time trials with this type of test car on a 32 foot ramp-flat track and found that rear-weighted cars outperformed front-weighted cars by up to one car length. Other testing results posted on the internet show similar results.

VERTICAL COG – UP AND DOWN

Let’s now move on to the vertical position of the weight. Is it better to have a high center of gravity (HCG) or a low center of gravity (LCG)? A LCG car will tend to be more stable, but it would seem that a HCG would impart more speed to the car. But is this really true?

In fact, it isn’t. Given two cars with the center of gravity at the same lengthwise location, but with one having the center of gravity low on the car, while the other has the center of gravity high on the car, the LCG car will fall a greater distance. Referring to Figure 4, note that because of the starting ramp angle, the fall distance for the HCG car is actually less than the fall distance of the LCG car. The actual difference is based on the slope angle. But on this hypothetical
track, the HCG car falls only 96.6% of LCG car’s fall distance.


Figure 4 – Effect of Vertical Weight Position on Fall Distance

Although the LCG car will attain a higher speed, due to a pendulum effect the HCG car will traverse the curved portion of the track slightly faster than the low center of gravity car. But unless the flat section of the track is very short, the LCG car will overtake the HCG car on the flat section.

OTHER CONSIDERATIONS

Here are several other factors to consider when locating the weight

Type of weight – The actual weight type does not affect the speed of the car, but does affect how easily the COG can be placed as desired. Denser weight (such as tungsten) allows much greater flexibility in weight placement than does less dense weight (such as zinc).

Length of car – Longer cars allow the horizontal COG to be moved further to the rear (it also allows a longer wheelbase which aids in stability and alignment). So it is generally best to maintain the longest possible car body.

Raised Wheel – A raised front wheel necessitates a rearward adjusted horizontal COG.

Wheel Weight – The horizontal COG must be measured with the wheels and axles installed. So, lighter wheels (such as our Outlaw Wheels) allow more flexibility in horizontal COG placement.

Wheelbase – The location of the axles affects the location of the COG. Specifically, the position of the rear axle sets a limit on rearward placement of the horizontal COG. As noted in Figure 5, simply using the axle slot closest to the end of the block as the rear axle allows a 5/8 inch rearward movement of the horizontal COG.


Figure 5 – Effect of Block Orientation on the Horizontal COG

CONCLUSION

The COG has a significant effect on the performance of the car. If possible, find out the specifics of the target track and design your car for optimal performance.

From Pinewood Derby Times Volume 5, Issue 10

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Pinewood Derby Memory – The Guest

It was Robby’s second pinewood derby and his bright yellow car was named the “Jedi Knight”. It had won his Cub Scout Pack’s pinewood derby and finished a close second to his older brother’s entry in the ‘Open’ competition that followed. But his pack was small, only eight Cubs, and the local scout council had discontinued the local District Pinewood races several years earlier (they said it made ‘too many losers out of previous winners’), so Robby was left to wonder what it must be like to compete in a BIG Pinewood competition.

Then he received a phone call from his assistant Scoutmaster, who had recently moved from another larger Cub Scout Pack several miles away. He told Robby that he thought he could arrange to enter Robby’s car in the other pack’s pinewood derby the following Saturday. He said there would be over sixty cars competing and so one more couldn’t possibly matter. Robby was thrilled to have the opportunity to compete with so many other Cub Scouts and represent Pack 400 in their race!

When his second derby day finally arrived, Robby was just busting with anticipation! The large room at the church was buzzing with nearly seventy Cub Scouts and their family members, checking in their cars, enjoying refreshments and the pre-race excitement that always seems to precede the derbies in larger packs.

Pack 199’s track was longer and a full five lanes wide! Robby watched as the cars were staged in groups and noticed that his Jedi Knight would race first in Heat #3. Then the competition began!

To Robby’s delight (and my amazement), the Jedi Knight won its first heat! Then the second, and the third!

The competition moved into hour number two, then hour number three and Robby’s car hadn’t lost a single heat!

More and more cars were eliminated, but the bright yellow Jedi Knight remained. In the last of the qualifying heats another car, a shiny black dragster design nosed him out at the finish line! But Robby’s car had enough points to be included in the championship round anyway. Everyone’s excitement grew as the derby officials lined up the five finalists for the finals. There would be three heats with only the top three finishers in each heat receiving points, then the Championship heat would be run with the three cars receiving the most points.

To our continued amazement, the Jedi Knight finished in the final group of three and would compete in the Championship heat!

As they lined up the final three cars, Robby’s yellow Jedi Knight was in lane #2. A bright red car was in lane number one, but the Jedi Knight had beaten it in an earlier heat. But wait, who was that in lane number 3? Yes, it was that same black dragster that had nosed-out the Jedi Knight in the last qualifying round! Robby’s car would race for the championship against the only car that had beaten it all day!

Before we could catch our breath, the championship heat was ready to begin! At the command of the official starter, the cars were released. Down the steep incline came the three cars wheel-to-wheel! Into the straightaway they flew as first the red car then the black dragster seemed to take a slight lead. Then as they reached the final few feet the yellow Jedi Knight in the center lane slipped past the others and across the finish line in FIRST PLACE! Everyone cheered and remarked how close all three cars had been at the finish. What a thrill for all the contestants!

But suddenly, the officials realized that there was a bit of a problem. Sheepishly, the race chairman approached me. “I have a bit of a political problem,” he whispered. “Not really,” I replied, realizing immediately the chairman’s concern. “Robby was here as a guest. He’s had a wonderful time and never expected to win, much less take home a trophy. This is Pack 199’s derby, and the trophies rightfully belong to Pack 199 scouts.”

After a quick huddle with Robby, the race chairman announced the surprising results to the crowd that never suspected that the winning car belonged to their one guest! To Robby’s delight, everyone cheered and applauded and the other contestants hoisted him on their shoulders and carried him around in a big circle chanting his name. “Robby, Robby, Robby! Hurray for Pack 400!” It was their way of saying thanks for being such a good sport as well as a gracious guest!

There was no trophy in the world that could have made Robby’s smile any bigger on that wonderful Pinewood Derby Day!

Rob Roper

From Pinewood Derby Times Volume 5, Issue 9

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Maximum Velocity Pinewood Derby Car Plans and Supplies

Pinewood Derby Car Showcase – February 12, 2013

Here are a few more ducted fan cars from our readers.

Batman – Nicholas Burgess

Batman is powered by a Maximum Velocity fan with a 7.2V LiPo battery.
We used components of a mouse trap for the front bumper, and the
internal trigger mechanism to arm the car. Batman won the no-rules
race with a scale speed of 355mph (1.5 sec on a 30 foot best track).

Gulf Jet Car – Tom Burgess

The Gulf Jet is powered by two Maximum Velocity fans with two
independent power circuits, each energized with a 7.2V LiPo battery.
The integrated bumper actuated two triggers inside the car
simultaneously to arm the fans. The jet tubes were built by covering
two paper towel tubes with synthetic wood and then sanding them down
to a smooth finish. The car was slightly heavier than the Batman fan
car, so even with two fans it came in second place in the no rules
race with a scale speed of 297mph (1.8 sec on a (30 foot Best Track).

Mega-Fan – Richard Staron

Last year I ordered one of your propeller car kits, and it was
fantastic. For this year’s derby I decided to try and go bigger. I
made a car with a 56mm fan. I was running it on five 10F, 2.7V
capacitors and a 12V battery pack (just short of the 14.8v the motor
could handle). The Mega Fan car is faster (about 0.4 seconds) than the
previous car. My design was a bit bulky; I hope to improve it for
next year.

From Pinewood Derby Times Volume 12, Issue 10

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Maximum Velocity Pinewood Derby Car Plans and Supplies

Don’t Forget the Car Parts

By Randy Davis

If you visit any pinewood derby event and view the array of cars, what
is the most popular design you will find? Almost certainly a basic
wedge, or a slight variation thereof is the most common car you will
find. When there are so many cars with the same basic look, it’s easy
for your car to get lost in the crowd.

Now, don’t get me wrong, there is nothing wrong with building a wedge.
But with very little effort you can add some pizzazz to your car so
that it stands out from the others. How do you do that? One way is
by using decals, pin striping, body skins, etc. We’ll explore this
avenue in a few weeks. But another route is by adding components to
your car. This could include engines, drivers, cockpits, pipes, and
much more. Let’s take a look at some possibilities.(1)

NON-WOODEN CAR PARTS – COMMERCIALLY AVAILABLE
The pinewood derby market offers many choices for jazzing up your car.
The major hobby vendors and others offer motors, pipes, cockpits,
drivers, and much more. I recommend avoiding metal parts as they add
weight to the car in possibly undesirable locations. Instead use
plastic parts. Here are a few cars we did using plastic parts:


R/C Airplane canopy (cut in half)


Plastic Parts from Maximum Velocity

http://www.maximum-velocity.com/standardwedgeopt12.jpg
Canopy from Maximum Velocity (painted black)


Engine, Pipes and Windshield
(I broke my rule and used metal parts on this one)

NON-WOODEN CAR PARTS – HOME GROWN

Don’t want to invest in hobby shop parts? Then you can go the do-it-
yourself route. Here are a few ideas that we came up with.


Lego Cockpit and Driver
(The opportunity for using Lego parts is enormous)


Dune Buggy Design – Roll Cage from Aluminum Rod


Speeder Design – Rocket Tubes from Brass


Wedge Design – Exhaust Pipes from Copper Tubing

WOODEN CAR PARTS
But since it is a “pine”-wood derby, let’s not overlook the use of
parts made from the block itself. There are lots of opportunities to
use the scrap wood to craft parts for the car. Here are a few ideas,
progressing from easier to harder:


Racer Design – Wooden Faring


Wooden, Faring, Side, and Front Trim


Formula 1 Design – Wooden Wings, Faring, and Side Intakes

CONCLUSION
The opportunity for adding detail to you car is limitless.
Oftentimes, very little time and talent is needed to make a big step-
change in the look of your car. So don’t let your car blend into the
pack; let is stand out by adding some car parts!

(1) Although many of the example cars are not wedges, the parts shown
could easily be applied to a wedge car.

From Pinewood Derby Times Volume 12, Issue 10

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Maximum Velocity Pinewood Derby Car Plans and Supplies

Pinewood Derby Car Showcase – February 5, 2013

Twin Cars – Dave Kaczenski

These car were built by my children, Ed and Sarah Kaczenski, and they had a lot of fun making them. Their cars ended up in the top of his or her class, and to date all of Dad’s cars have lost to Ed and Sarah.

Jeff Gordon Truck – Patrick Henderson

I am Patrick Henderson, and I am in Cub Scout Pack 124 in Indianapolis, Indiana. I am Jeff Gordon’s #1 fan so my dad and me made a Jeff Gordon Truck to race in the Pinewood Derby.

I got the decals at an Indy 500 race car show for a dollar. The man said the decals were in the wrong bin and worth more, but he let me have them for a dollar anyway. All the people there were nice and some of them gave me free Jeff Gordon stuff.

Dad and I started with the block of wood and cut it to the truck shape. I sanded it down and added fenders and the front piece to the truck. I painted the car part red and part blue to look like Jeff Gordon colors. Dad showed me how to put on the decals. I looked at one of my Jeff Gordon cars and put the decals on my truck. My dad’s real truck has a cover on the back and I wanted one on my Jeff Gordon truck. I made a cover for my truck from part of an old glove and glued the Patrick on it.

Dad showed me how to straighten and polish the axles using a drill and supplies we got from your store. It took me a long time to do the axles and my fingers got sort of tired, but they were real shiny when I got done. I put graphite on the axles and tires to make them spin really fast.

I had fun making and racing my car, and showing my car to all my Cub Scout friends. I finished third in my pack and won the Webelos at our district race and I got a trophy and two medals.

We showed my Jeff Gordon truck at the Cub Scout “Race into Scouting” show at the Indianapolis 500 this year. Everybody thought it was pretty cool. I am making a Jeff Gordon Billion Dollar car for the pinewood derby this year.

Lizard – Jeffrey & Chris Corron

This is Jeffrey’s car from last year. He chose a simple design that was easy to make. The body is a flat 1/2 inch thick plank. The body was thick enough to allow us to inlay a plate of zinc weights in the bottom of the car but not so thick to cause a great deal of wind resistance. He used a Gator Body Skin and painted the weights on top green to make them look like scales. The eyes were the finishing touch. The car ran fast; Jeffrey won the overall district championship. Often, the simplest designs are the best.

From Pinewood Derby Times Volume 5, Issue 9

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Wind Tunnel Testing

In a previous article (Volume 3, Issue 9 – The Big Debates: Aerodynamics), I shared the results of track tests using a car with a varying cross-section. The tests showed that there is an aerodynamic effect on the speed of the car, and the effect was virtually linear with regard to the area of the cross-section.

However, since aerodynamics is just one of many factors affecting the performance of a pinewood derby car, it is virtually impossible to gather any more meaningful data regarding the effect of aerodynamics using track tests. What is the recourse? A wind tunnel!

WIND TUNNEL DESIGN

Wind tunnels can be quite sophisticated (and expensive). So I chose to build my own using some clever information that I found on the internet.

Figure 1 shows the overall wind tunnel. On the left is a high velocity blower fan that I purchased at Sam’s Club. The acrylic door provides access for placing the car into the tunnel. The digital scale is used for measuring the force on the car (more on this later). The total cost to build this wind tunnel was less than $120, with the major portion being the blower fan and the digital scale (of course I had a few of those lying around).


Figure 1 – Wind Tunnel

Figure 2 shows the inside of the tunnel and the air flow grid. The purpose of the grid is to align the air flow parallel with the tunnel to minimize turbulence. The grid is especially important when using a standard bladed fan, but is likely less important when using a blower fan. The grid is made from a fluorescent light diffuser. I cut three equal-sized pieces and then super-glued them together to increase the thickness.


Figure 2 – Flow Grid

Figure 3 shows additional inside detail. The car is attached to a thread which runs over a lightweight, lubricated pulley and down to the scale. An eye hook is screwed into the bottom of the test car, and a small hook is tied to the end of the thread.


Figure 3 – Interior Detail

Figure 4 shows the scale. The board with the eye hook is glued to the scale. When the scale is turned on, it will zero. It is of course important to make sure there is no pull on the thread while the scale is zeroing. A weight (in this case a 200 gram weight) is then placed on the scale and the reading is recorded. When the fan is started, the car will pull on the thread, unloading the scale. To calculate the force on the car, the new reading is subtracted from the baseline reading previously recorded.


Figure 4 – Scale Detail

There are two additional factors to note. First, because I could not measure wind speed, the results from this wind tunnel cannot be compared with those from another tunnel. Secondly, I used Outlaw style wheels so that the cross-sectional area contributed by the wheels would be minimized. This amplifies the effect of the body shape. If standard (e.g., BSA) wheels are used, the cross sectional area is increased. This will lessen the percent difference in the cross-sectional area between the wedge and the wing, resulting in closer numbers.

TEST DETAIL

To ascertain the aerodynamic effect on various shapes, I prepared the car bodies shown below. Each car weighed 5.0 ounces and the balance point was essentially 1-1/4 inches front of the rear axle. The cross section (in square inches) of each body is listed.

Block – 2.510 – Blunt Front and Rear

Wedge – 1.580 – Blunt Front and Rear

Wedge – 1.480 – Tapered Front and 180 Degree Rounded Rear

Wing – 0.740 – Tapered Front and Blunt Rear

Wing – 0.740 – Tapered Front and 180 Degree Rounded Rear

A set of our Outlaw wheels and axles were lubed with NyOil II were used for every experiment. As you can see in Figure 2, I didn’t insert the axles all the way as I wanted to minimize any possible damage to the axles during removal.

For each test, the car was placed into the tunnel and aligned parallel with the walls. The thread was attached, the scale zeroed and then loaded, and then the wind was applied. Once the scale reading stabilized, it was recorded. During the experiment I found that the reading fluctuated, so the average reading was used. It’s interesting that the amount of fluctuation was greatest for the least aerodynamic car (the block), and was virtually zero for the most aerodynamic (the rounded Wing). I believe the fluctuation is due to wind eddies that occur as the air moves around the car. So the smoother the path, the fewer eddies occur.

TEST RESULTS

The test results are shown in Figures 5 and 6. Note that Figure 5 shows the actual force on the car (that is, the amount of wind drag) in ounces. This is the reading that affects how fast the car goes. The numbers in Figure 6 show the forces divided by the cross sections, and are in ounces per square inch. These numbers relate to the efficiency of design. Note that I tested the rounded wedge forwards and backwards. I had expected the result to be the same, but as you can see the forward facing numbers are better. I believe this is due to the importance of having a rounded rear. If the front of the wedge had been rounded instead of tapered, the forward and backward numbers may have been better.


Figure 5 – Shape Performance


Figure 6 – Shape Efficiency

INTERPRETATION

Here is my interpretation of the results:

1. The car with the least wind resistance is the rounded wing. However, the most efficient design is the rounded wedge. This design channels the air the most efficiently; however, due to the larger cross-section it would still run slightly slower than the wing.

2. A rounded rear is extremely important to fast speeds and efficient air movement. I was surprised to find that the rounded wedge performed better than the standard wing. But simply rounding the rear of wing significantly improved both the performance and efficiency.

So, if you want the fastest car, keep the cross section small, and round the rear end. This will give you the best bet for a fast car!

From Pinewood Derby Times Volume 5, Issue 9

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Maximum Velocity Pinewood Derby Car Plans and Supplies