Construction Tips...

The following are some additional hints and information regarding construction of fast pinewood derby cars that are not contained in my book Pinewood and Space Derbies, How to Build Cars and Rockets That Win! (which is catalog item #1). I will revise this page from time to time. Check back! As usual, always check local rules before making modifications. Some groups will allow these changes, some will not.


WHEELS


1. All wheels are not created equal.There are minute differences between wheels right out of the box and even the ones that I turn on my lathe. Some years ago I talked with the manufacturer of the Scout Grand Prix PWD kits, Beta Crafts, and found out that the wheels were produced in a 32 cavity plastic injection mold. The new manufacturer of the Scout Pinewood kits uses a mold with 16 cavities. That means that there are 16 slightly different wheels included in the kits. The easiest way to test for the fastest wheels is to use the "finger spin test". Gather up a supply of wheels - the more the better. Using a highly polished axle, place a wheel on it and give it a spin with your finger. Try to hold the axle approximately level but tilt it as necessary to prevent the spinning wheel from rubbing against the head of the axle or your finger. Count the seconds that the wheel spins using a clock/stopwatch or counting one thousand one, one thousand two, etc. Record the time. Repeat this at least one more time and preferably two more times. Calculate the average spin time. Repeat for as many wheels as you have or until you reach a point of diminishing returns. Select the 4 fastest. Simple. The reverse of this can be used to test axles. Select one good wheel. Spin it on an axle and record the time. Repeat several times. With the same wheel, test another axle Repeat for as many axles as you have or until you reach a point of diminishing returns. Select the 4 fastest. Simple.

2. Lighter wheels are faster. Anything you can do to make the wheels lighter will increase the speed of a PWD car. The reason: It takes less energy to start the wheels rotating when the car leaves the starting gate. Some years ago I tested this principle "in reverse" by loading up some wheels with lead shot to see if there was any "flywheel effect." There was none. The car was verrrry slow. Material can be removed from the tread, like the wheels in my catalog. It can be removed from the inside cavity of the wheel. It can be removed by drilling small holes in the side wall. Be careful to space the holes equally or the balance of the wheel will be affected.

3. Reduce wheel/axle friction by undercutting the inside of the wheel hub. This speed tip is similar to my "H" wheels, only instead of undercutting the center of the tread, you will undercut the inside of the wheel hub. Using a Dremel tool with a small burr (perhaps a used and discarded dental burr from your friendly local dentist or dental lab), undercut the center area of the inside of the hub by a few thousandths of an inch. Leave a small ridge at each end of the hub (about 1/16") for the axle to ride on. Made sure that these ridge surfaces touching the axle are smooth. Probably the easiest way to do this is with the Dremel drill press and some kind of jig to limit the travel of the burr vertically. The resultant cavity can be packed with my G-M lube. The cavity may tend to hold the lube during the race. Neat. I have not tested this on my track with electronic timing, but many of my customers have done it with good results. The theory is sound. Friction is the enemy of speed.

4. Burnish the inside of the hub with a pipe cleaner - the kind you get from the tobacco store, not the hardware store! Getting graphite inside the hub is a problem. How about dipping a pipe cleaner in graphite and running it through the hole? This would work the graphite into the surface of the wheel hub.


AXLES


1. Reduce axle/wheel friction by undercutting (or grooving) the axles. This is simpler but slightly less effective than undercutting the inside of the hub. Here's how: Chuck an axle in a drill motor and have on hand a small file. Spin. Starting about 1/16"from the head, file a shallow depression for about 1/8 wide". The undercut should be in the center of the wheel bearing area. Some judgment is required. If the undercut is too wide, then the wheel could "dip" down into it as it oscillates back and forth between the body and axle head. This would not be good. If the undercut is too narrow, it lessens the benefit. When the undercut is complete, polish the axle carefully. Make sure that there are no burrs left from the undercut. Note: the above dimensions are approximations based on leaving 3/32" to 1/8" clearance between wheel and body. Filing several test axles might be a good way to start. 2. There are several other axle polishing compounds besides pumice and water. What about toothpaste or silver polish (the paste kind)? If you have access to a buffing wheel mounted on a bench motor, use can apply jewelers rouge on the buffing wheel and chuck the axle in a drill motor and apply to the buffing wheel. This is perhaps the best method. Dremel also makes a cotton buffing wheel that could be used. Jewelers rouge is a commercial polishing compound. I still have a little kit of several polishing compounds which I bought at Sears and Sawbuck about 30 years ago. Perhaps they still carry it. 3. File down the edges of the chisel point on the axle.This should be obvious but perhaps is not. If you try to insert the stock Scout axles in the wheel without filing the edges, it may score the inside hub of the wheel. This is not desirable.


BODIES

1. Thin down the front of the car. Although aerodynamics is not the most important factor in building a fast PWD, it does play a role. The nose of my Slim Maxi-L car is about 3/8" - this can be thinned down a bit - to perhaps 1/4". Don't forget to round both the front and the rear - make it like an airplane wing. This will reduce the drag coefficient and cause the air to move more smoothly over and around the body.

2. Locate the rear axles as close as possible to the rear of the car.If you are using the stock Scout Grand Prix PWD with the stock location of the slots, the rear axle slot is not as close to the rear as it can be. Cut off about 7/16" of the rear of the car and glue it on the front. Or, to put it another way, the rear wheel tread should be about even with the rear of the body. This will allow more weight in the rear and a faster car. Overall length should still be 7".

3. Drill axle holes from both sides of the body.Special order long drills are available which will go all the way through the body. But I discovered years ago that long drills tend to wander a bit, especially if they hit a dense portion of wood. I would start drilling on the top side at 1/8" up from the bottom and come out on the other side at 1/16"! The drills I sell are "stubby" and designed to drill only half way through the body.

4. Drill axle holes carefully. Using a carpenter's square, examine the sides of the body to insure that it is perfectly rectangular. Some bodies are parallelograms or trapezoids! If they are, it makes the drilling of axle holes more of a challenge. Here is a way to help insure that the holes are drilled accurately. Construct an "L" shaped fence for your drill press table. Make the foot of the "L" about the same height as the width of the body. Clamp to the drill press table. Make sure that the foot of the "L" is at a right angle to the table and parallel to the drill. Now, mark your hole locations carefully and hold the bottom of the body against the foot of the "L" while drilling. This insures that the holes are parallel to the bottom of the body. This is the technique I use.


WEIGHTS & WEIGHING


1. Always use lead weights.The Scouts and Pine Car are now selling PWD weights that are not lead. I believe they are bismuth. These may be environmentally correct but they are not the best way to weight a fast PWD car. The tubular weights from the Scouts are about 60% of the density of lead. This means that you will have a very difficult, if not impossible, time getting the weight of your car in the rear. With my Maxi-L bodies, it is impossible. Rear weighted cars go faster. See the next point.

2. Move the balance point closer to the rear axle. Many of my customers have asked how I determined the 1-1/4" distance in front of the rear axle for the fore/aft balance point (or, if you prefer, center of gravity). Answer: It was determined empirically after building many PWD cars and testing them. It was not derived from some physics formula. There is no scientific "magic" to 1-1/4". At this distance the car seems to stay on the track - even a rough one. However, if you are racing on a known good track you can safely move the balance point back. One inch? Perhaps 3/4"? This will result in a gain in speed. But the trade-off is that the car is more likely to go flying if it hits a seam or other track obstruction. I have heard some ugly stories about cars ending up in the adjacent lane or even on the floor - with bent axles and broken wheels. Be careful.


TESTING


1. Build and use your own test track. This is the only way to really tell how various modifications add or detract from speed. Build a two lane track, set up one car as the standard (perhaps last year's car) and race the new modified car against it. Compare. The only problem is that the speed of cars vary from race to race. Hence the "standard" car may not be very standard. Also track lanes vary in speed - the slow/fast lane problem. Therefore ...

2. Use an electronic timer.This is the best way. My first timer was a Radio Shack stop watch. I opened the case and wired micro switches to the start/stop button connections. Mounted the micro switches in the track. Resolution was only .01 seconds (which is about 1.5 inches at the finish line). It was better than nothing, but not good enough and it did not last long. Timer resolution must be at least .001 seconds (one thousandth of a second). My current timer resolves to .0001 seconds (one ten thousandth of a second). Obviously a single lane track will suffice. (In my organizing derbies book are plans for a simple backyard track with two lanes). I my tests I run each car ten times and take the average time. Make only one modification, run the car another ten times. Record the times and the changes. At the end of the day, buy some ointment for sore leg muscles. Or, get Junior to do the car fetching for you. Believe me.


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