Thank you, Derby Talk

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loren.phillips
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Thank you, Derby Talk

Post by loren.phillips »

To the Derby Talk community:

We had our pack pinewood derby race on Saturday, 9 Feb 2013.

Daniel is a Webelos II this year. So it was his last time to race. He crosses over at the Blue and Gold dinner in March 2013 and will be joining the Boy Scouts.

Daniel’s car never lost a heat. He won 1st place in Webelos II den, went on to finals, and won pack champion. He carried home 2 trophies.

Daniel and I owe a lot of thanks to the community of dedicated individuals on this forum. You have been our knowledge base for over 2 years. We have carefully studied what you have written about building a pinewood derby car. And we are grateful to be a part of this forum and share our story.

Our pack has a relatively new (4 years old) aluminum track (BestTrack). The track has 4 lanes. Each car runs a heat in each lane. The track surface is very smooth. The entire track is about 40 feet long.

Details of this year's car are below.

Wood Block: A block with about 18 identified grain lines running the length of the block was used. This was the densest block I could find. If I remember correctly, it weighed about 4.80 oz out of the box. All sides of the block appeared square using the tri square. No light was seen between the tri square and the block.

Construction: The car is a 3/16 inch thick flat board design with notched center (looks like a shovel with square ends and a thin narrow section connecting them). The thin narrow rectangular section is ½ inch on the top and bottom, and 3/16 inch on each side. The surface on all sides was sanded to 2000 grit. Painted, and coated on 3 sides with a thin coat of 5 minute epoxy for strength and smoothness. Left front wheel was raised up about 1/32 inch; it did not touch the track. The front of the car was 1 9/16th inch wide and beveled to a sharp edge, with the bevel on the top. This directed air flow to the top of the car, and not underneath. The underneath of the car had 2 coats of hard nail polish and was subsequently sanded with 1000 and then 2000 grit sandpaper. It was very smooth.

Wheelbase: extended. Axle holes were marked exactly 5/8 inch from the end of the block for both front and back wheels. Axle holes were drilled into the raw block with the Pro Body Tool and a tri-square was used to get the tool as close to square with the block as possible. The tool was secured to the block with a strong clamp. A pin vise was used to manually drill the holes, drilling about 1/8 inch at a time and pulling the bit out to get rid of the shavings. To check the rear holes, two drill bits were inserted into the newly drilled axle holes, and a tri square was used to check if the holes were perpendicular to the block. Based upon visual observation, the holes appeared to be exactly 90 degrees to the block. The front 2 holes were drilled and checked in the same manner. They also appeared to be 90 degrees to the block.

Center of mass: 5/8 inch in front of rear axle.

Ballast: 54 Tungsten cubes (6 rows of X 9 cubes, 3/16 inch cubes) were used for the primary ballast. While off the car, the cubes were glued together in a tight, rectangular shape (6X9 cubes shape, one cube high) using a 1/64 inch thick piece of balsa. The balsa was sanded down until it was barely visible; just enough was left to hold the cubes together.

One side of the rectangle (the side that had 9 cubes) was just about 1 ¾ inch. This side was oriented on the 1 ¾ inch side of the rear car body and seemed to fit perfectly on the car.

The ballast was glued to the top of the car. Others on this forum have said that this is not a good idea, but it has worked for us. The ballast was about ¼ inch vertically from the axle plane. I knew if I did it this way I could get the ballast centered over the rear axles. With this ballast added, the car was 3/8 inch thick at its thickest point. (If there is another car building opportunity in the future, weight pockets will be cut in the car body and the primary ballast will be at the axle plane).

A very small tungsten fishing weight was placed on the very back of the car to bring the weight to exactly 5.0 oz.

Wheel lateral runout: This was phase 1 of wheel testing. Wheel lateral runout was determined by the wobble test. Used an initial batch of 25 wheels (bought in 2011-2012). Prepped an axle by de-burring and polishing to 3000 grit, applied graphite to axle, put wheel on axle, chucked wheel/axle assembly in non-spinning drill, spun wheel with “finger spin test”, observed spinning wheel for wobble as it slowed down. Sorted wheels into 4 piles – no, small, moderate, and large wobble. Only the wheels in the no and small wobble pile advanced to phase 2.

Wheel radial runout: This was phase 2 of wheel testing. Made a wheel concentricity gauge (thanks to Fat Sabastian). Used it to measure wheel radial runout. Wheels used showed a range of 0.001 inch to 0.007 inch. Chose wheels with radial runout of 0.003 or less. Wheels used has runout of 0.003 (raised front), 0.003 (DFW), 0.002 (left rear) and 0.001 (right rear).

Wheel Bore Diameter: Measured bore diameter on all wheels (entire batch of 25 wheels). Almost all wheel bores measured 0.0970 inch. One was 0.0960, 2 were 0.0975 inch. This measurement was not used as a selection criterion for wheels that go on the car. However, after final wheel selection was made, bore diameters on all 4 wheels that went on car measured 0.0970 inch.

Wheel bore and hub polish. Wheel bore was polished using Sporty's bore prep method. Adhesive-backed 4000, 6000, 8000, and 12,000 micromesh plus 0.050 inch diameter carbon fiber rod were used. I carefully placed the micromesh on the rods, placed the rods in a drill at low speed (about 600 rpms) and inserted the wheel bore on the spinning micromesh and moved them up and down on the rods for about 30 seconds per grit. After the 12,000 grit, I placed a drop of 0.05 micron alumina slurry on the 12,000 grit rod, and put a final polish on the bores. The bores were extremely shiny.

The small diameter wheel hub (the part of the wheel that touches the car), and the large diameter wheel hub (the part of the wheel that touches the rail) were polished with 4000, 6000, 8000, and 12,000 grit micro mesh.

Axle shape and polish. About 30 axles were initially tested.

The sharp edges of the 4-sided point were removed with a file. Axles were then chucked in a drill. Axles were de-burred with a finger nail file while chucked in a drill but the drill was not spinning.

Axles were placed in a drill and spun at very slow speed by depressing the start trigger very slightly. A dial indicator was placed next to the spinning axle and a reading on the indicator was taken in an attempt to quantify the wobble. Wobble was measured at a place along the axle where the wheel will spin.

Wobble ranged from 0.003 inch to 0.009 inch. Axles with 0.005 inch or less wobble were kept.

Axles that were kept were placed in the Pro Axle press, and an attempt was made to straighten them by striking the press lightly with a hammer while rotating the axle through 3, 6, 9, and 12 o’clock positions.

Axles were re-checked for wobble using the dial indicator. Axles with 0.003 inch or less were kept. All axles used had 0.003 inch wobble.

It was not possible to determine if the observed wobble was (1) because the axle was not straight or (2) because the axle was out of round (ovoid) where the measurement was made, or (3) due to drill chuck runout (which was measured as 0.003 inch).

The underside of the axle head (the part that touches the wheel) was slightly beveled using a small triangular file. The diameter of the axle head was not reduced. The axle head/shaft joint was deepened so that the joint has a slight groove or depression in it.

The axle shaft (where the wheel rides) and the underside of the axle head were polished with progressively finer grit sandpaper (400, 600, 800, 1000, 1500, 1800, 2000, 2400, 3200, 3600, 4000, 6000, 8000, and 12,000 grit). Axles were then polished with diamond polishing paste (1, 0.5, and 0.25 micron) using a leather cloth and a small amount of water. Final polish was accomplished with 0.05 micron alumina slurry and a leather strip. No scratches on the axles or underneath the head were observed with a 20X hand lens.

Axles were cleaned between grits by rubbing with a cotton cloth soaked in 91% alcohol. After prep, axles were kept submerged in 99% isopropyl alcohol until final assembly a few days before the race.

Rear and Front axle camber and Toe. Camber was set at zero-no camber. The goal was to build a straight runner. We wanted the car to go as straight as possible. We do not do well bending axles. All wheels had no toe in or toe out.

Alignment: Alignment board was 4 feet long and about 8 inches wide, with a
centerline and 0.5, 1, 1.5, and 2 inch marks on either side of the centerline. All axles on the car were marked with a sharpie.

The “no front wheels skid test” was conducted first. Ballast was added to the car. A small tack was placed under the front of the car. The rear wheels were put on the car, and both rear axles were initially set to the 12 o’clock position.

Each wheel was aligned separately.

The left rear was rotated to 3 o’clock. The car was run down the alignment board 3-5 times. The drift at 4 feet was recorded on a data sheet for the left rear 3 o’clock position.

The left rear was rotated to 6 o’clock. The car was run down the alignment board 3-5 times. The drift at 4 feet was recorded on a data sheet for the left rear 6 o’clock position.

The left rear was rotated to 9 o’clock. The car was run down the alignment board 3-5 times. The drift at 4 feet was recorded on a data sheet for the left rear 9 o’clock position.

The best left rear wheel position was recorded and the left wheel was set at this position.

Then the right wheel was aligned. The procedure was the same for the right wheel. The best right rear wheel position was recorded and the right wheel was set at this position.

Fine-tuning the left and right wheel followed the same procedure until the best possible alignment was obtained. The goal was to get the car to go as straight as possible down the board. The final result of the front wheel skid test was that the car went consistently about ¼ to ½ inch to the right of the centerline at 4 feet.

The 2 front wheels were then put on the car and the procedure was repeated for the entire car. Again, the final result was that the car went consistently about ¼ to ½ inch to the right of the centerline at 4 feet. It would not go right down the middle.

Lubrication: Krytox.

Both the axles and the wheel bores were thoroughly cleaned with 91% isopropyl alcohol and dried with a paper towel. Dirt, lint, and dust were blown from the axle and wheel bore with canned air. They were then inspected with a 10X hand lens to make sure they were as clean as possible and nothing would interfere or clog up the axle/wheel bore assembly (no hair, dirt, dust particles, etc.).

Gloves were worn for wheel/axle assembly to keep skin oils from interfering with the car’s performance. One drop of Krytox on the axle, put the wheel on the axle, insert the axle/wheel assembly on the car, and spin the wheel a few times. Oil is so wonderful. No mess. I think (but I am not positive) that we were the only ones in the pack to use oil. The rules allow it.

Weight: 5.0 ounces exactly.

Weight distribution. 3/16 inch tungsten cubes were placed on the top rear of the car in a tight group. They were adjusted to give the car a 5/8 inch center of mass. The weights are centered in the rear above the rear axle. They were glued in place with 5 minute epoxy. See ballast above.

Aerodynamics. The very thin body provided excellent aerodynamics. The beveled front end helped. A thin layer of 5 minute epoxy on 3 sides of the body and the nail polish on the bottom enhanced the aerodynamics by providing smoothness and strength.


Critique of our car’s performance is below.

(My role in the race was car stager at the start line. I also pulled the start lever to send the cars down the track. So I had an excellent vantage point for observing the cars as they traveled down the track. My wife was at the finish line and had a good vantage point for observing how far Daniel’s car was in front or behind the other cars at the finish line).

On each heat that Daniels’ car ran in, I noticed that the front end of the car wobbled several times on the flat before it reached the finish line. The wobbling started on the flat (perhaps 5-6 feet after the curve) and continued to the finish line. I am not sure what caused the wobble. I suspect that it was one or a combination of 2 reasons: (1) we could not get the car to run perfectly straight down the alignment board, and/or (2) we deliberately did not build a rail rider.

My wife said that Daniel’s car won most heats by about 2 car lengths. The closest a car came to Daniel’s was about 1 car length. So I think Daniel’s car won by A LOT.




Discussion of the race itself is below.

There were trophies for 1st, 2nd, and 3rd place in each den (5 dens), and 1st, 2nd, and 3rd place for pack finals. Therefore, there were 18 trophies to award.

The electronic timer on the track broke on the first heat of the race. We had no spare. The derby race organizer got the timer to display 1st, 2nd, 3rd, and 4th place for each of the 4 lanes, but it would not record a car’s time in seconds or mph. The organizer elected to use a points system for determining winners, rather than speed. 1st place got 1 point, 2nd place got 2 points, 3rd place got 3 points, and 4th place got 4 points. All cars ran once in each lane. There are 4 lanes on the track.
Therefore each car ran 4 times for the initial race. Lowest overall score wins. We awarded 1st, 2nd, and 3rd place for each den (3 places X 5 dens = 15 racers), and these racers advanced to the finals.

There were some ties. Ties were broken by a single elimination race to determine who would be the winner. This was where the discussion about sportsmanship in the pack meeting the night before became very important.

Since I am Cubmaster, I spent the “Cubmaster’s Minute” (during the pack meeting the night before the race) talking about the purpose of the pinewood derby. My notes came from a power point presentation that was put together several years ago by a major contributor to this forum. I won’t name names. OK, it was Stan Pope. I found it on the internet and thought it was perfect for the occasion. I discussed the meaning of sportsmanship, how winners and losers should act, and all of the reasons to have a pinewood derby race. I talked about how the race allows the scout to explore and learn about family relationships (it gives them an opportunity to discuss anything with their parents while building their car; it allows them to learn who their parents are, to get closer to them, etc.). I talked about personal achievement (this is their car; they have worked hard to build it, be proud of the accomplishment whether you win or lose, etc). I talked about having fun (both while building the car and while racing it). The scouts were very attentive and I think it sank in.

Some of the single elimination races were very close (perhaps less than ½ inch separated the winner from the loser). However, no one complained that the race was unfair. I noticed no one being upset that they did not win. Everyone seemed content with the results. So, 1st, 2nd, and 3rd place winners in each den advanced to the finals.

For the finals, each car had 4 heats to run. Same points procedure. No ties in the finals. Daniel’s car took 1st place (pack champion) in the finals.

Daniel and I have come a long way over the last 2 years. However, there are still many things to learn. I am still an avid reader of Derby Talk, and much of what we know is due to the Derby Talk community. I owe this community a lot. Thank you, Derby Talk.
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Stan Pope
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Re: Thank you, Derby Talk

Post by Stan Pope »

Way to go!

Good detail on all aspects. :bigups:
Stan
"If it's not for the boys, it's for the birds!"
Speedster
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Re: Thank you, Derby Talk

Post by Speedster »

That car sounds FAST !!!! Why not enter it in the Mid America Race coming up in May.

Might we see a picture of the car?
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