Building a Pinewood Car
The 6 Stages of
(The ability to go down the track without touching the sides)
(Maximise the Speed)
(Make sure you can get to the end of the track)
|Smooth, Round Wheels
(Gives you a smooth run down)
Wheels & Lubrication
Painting & Finishing
(This is the least important but there is nothing like having a car that looks the part)
Testing & Tuning
You may already have an idea as to what your car should look like when you're done but in order to take this idea to a completed form you should have a plan. Take a little time to sketch out your idea on paper. If you haven't decided what you want to do you may want to check out some possibilities. Draw your design at full scale so you can transfer the profile and shape to the wood block later.
What kind of car do you want to build? Well, there are several basic types of car classifications cars that are fast, cars that are fast to build and then there are character cars. Character cars are cars that model other types of cars or objects. Remember that a highly decorative car with characters decals and other trim will not be as aerodynamic as a plain car. The sleek low profile designs will tend to have less wind drag and therefore faster. The fast car is usually not a handsome car. Don't limit your design ideas but we'll talk about the plain, more aerodynamic designs and remember, you can paint car just about any way you you'd like.
You can use any of the templates included
here for your car if you wish or design your own
Now that you have a design idea it's time to transfer the profile (side of the car) and plan view (top of the car) to your block of wood. The block included in your kit is usually close to 180cm in length but may vary a little shorter or longer. Be careful to measure the final overall dimensions of the finished car to ensure that your design does not violate the racing specifications.
Use scissors to cut out your car drawing and trace the outline onto the side of the wood. If you prefer, you may find it just as easy to copy or duplicate your lines on the wood directly. Use a dark pencil or ball point-point pen so that the lines are easy to see when you're cutting.
When cutting out the car shape stay as close as possible to the lines. The more accurate you are now the less you will have to sand later.
Sanding the wood body will eliminate any of the saw blade marks as well as any small blemishes in the wood surface. If you have access to a motorised belt-disc sander your work will be quickly done but for most of us a sheet of sandpaper and a block will do just fine. Start by using a rougher sandpaper (220 grit) and wood or rubber block on the rough portions of the wood car body. Gently smooth the edges and corners of the car while using a little more pressure on the flat areas. When you have the smoothed switch to the finer (400 grit) paper. It will provide an excellent surface for your final finish.
If you drill holes for your weight and then
fill them you may need to sand these also
Your finished wood block along with the, wheels, axles and trim will not usually weigh much over 75 grams while the finished car is allowed to weigh in up to 150 grams (or 100 grams in the Restricted 100 Division). Don't even think about skipping weight addition if you want to be race competitive. The weight of your car overcoming friction is what will allow to you to win over other cars. You must make gravity work for you. Your car must overcome both breakaway friction and minimise air resistance and it will do this by being as heavy as allowed while presenting the smallest profile to the air-stream. That's why we wanted the low and skinny body design.
There are two basic approaches to adding weight to a derby car. The easiest is to attach pre-drilled and shaped lead weights to the outside of the car. Some of the commercial varieties are cast such that they provide a tapered shape and break-off ribs that permit convenient adjustment to overall weight after the car is assembled. It is best to attach this type of weight to the bottom of the car so the centre of gravity may be kept low. If you use this type of weight on the bottom of your car ensure that the weight doesn't hang down too far. It may not be obvious until race-day but the weight could drag on the track guide. This could prevent the car from moving off the starting line. Mortise or "hog out" a void in the wood on the underside of the car and then attach the weight inside the void.
The other method for adding weight involves the installation of weight internal to the body so that there is no additional wind resistance. This may be only a small advantage but it just might make the difference of a winning millimetre or two at the end of the track. Most car profiles will be narrower at the nose and provide little space for adding lead internally. There is an advantage in placing the weight in the back. The front wheels perform the function of guiding or steering and the less weight on these wheels the easier the car corrects itself when it strikes the guide strip. Fewer and shorter contacts with the guide strip mean a faster car.
Each internally weighted car will have a little different cavity placement based on the wheel/axle position and amount of wood available to accommodate the weight. The hole or cavity for the lead weight must be large enough to accommodate the weight you using. You will need fewer holes for lead than you will for other materials. Plan on drilling at least 2 or 3 holes of 8-9mm diameter at a depth of 30mm each. Experience has shown that holes drilled from the side or back tend to work the best. Locate and drill the holes being careful not to drill all the through the wood. Also make sure that you are leaving enough wood around the hole to provide a margin of safety in your drilling operation.
There are many things that you might use to add weight to the car but you will find that lead will probably be used most often. It is the heaviest practical element available for its volume, it works easily and is commonly available in a number of forms. As options to lead you could try steel in plate, tubular forms or even common bolts. Other metals may be used but just as steel you will find them difficult to work and sometimes awkward to attach or insert.
Lead is toxic and should be handled as little as possible.
Use gloves and never put your hands in or near your mouth after handling it.
Always wash your hands thoroughly after handling lead.
Do not use mercury at all! It is toxic, difficult to handle
and not allowed under the Rules
Weigh your car on accurate scales. Most household scales are not very accurate. If you have access to calibrated scales compare a known item weight on the calibrated scales to your home scale indication for that same item. Mark this reference for use later. Allow for scale inaccuracies by not adding to exactly the maximum weight. It is better to be slightly light than have to have to remove weight on race day. Consider also, while you may an accurate scale the ones used on race day may not. It could be weighing items heavier than they actually are!
Weigh your car body, wheels, axles and any other parts that will be on your car all at once. This weight is usually less than 100 grams. Now, with your car lying with the weight holes facing up carefully add your weight until the weight is just 10-15 grams lighter than the maximum allowed. Allow enough space in the holes so that you can add filler material in the next step.
Once that you have got the correct amount
of weight you are ready to seal the hole(s) in your car body.
There are a number of materials that you can use to cover the
weight holes in car body. If you are in a hurry and want to
ensure a good seal try using automobile body putty. This type of
filler material is a two-part mix that sets in 15 minutes. You
will need only a small amount but it works very easily and may be
sanded, drilled and painted. Standard wood fillers that don't use
a catalyst will take longer to harden (sometimes overnight) and
may need to be applied with several thin coats. Apply the filler
so that it may be sanded down smooth to the original wood
surface. You'll want to recheck you total car weight at this
Check whether the class you have selected allows you to move the axle slots. Only the Open class allows you to do this
For Open Class Only. It has been discovered over the years that cars with a longer wheel-base can be faster than shorter wheel-base cars. With this in mind you may want to consider relocating the two axle slots in the car block toward the ends of the block.
There are none so important as the wheels. They must be mounted so they run free, track straight, and be able to be lubricated. Don't paint your car with the wheels on. If possible avoid putting the wheels on at all until you have finished the painting.
You must ensure that the wheels roll smoothly, in a straight line and roll very easily. There are things to check and fix on each of the wheels. First, the wheels must be perfectly round. Some moulds may produce slightly out-of-round wheels which are slower than others. To check for this put the wheel on an axle and spin it. It should turn with the outside surface at a single reference point never varying. The run-out or the wheel movement along the axle axis should also be minimal. There isn't much you can do to correct a bad wheel. The wheels are or much better quality in recent years and bad wheels have been greatly reduced.
Check the wheel for burrs on the running surface of the tire and hub areas. These need to be freed of any extra plastic residue or moulding marks.
The nail type axles that come in the Pinewood derby kit must be used in the construction of your car. These provide no bearing surface so there is friction between the plastic wheel surface and the metal axle. Since this friction reduces speed we need to minimise the contact surface area, make the surfaces smooth and lubricate the mating surfaces. It is against the rules to machine the plastic wheel and these procedures usually require a lathe or other tools. That still leaves the axle open to "play with". The following steps suggest things you can do with simple hand tolls to improve the performance of the axles.
Axle Burr Removal
First, the heads of the nails used as axles in the kit will often have a old or casting mark in two places just where the head attaches to the shaft the nail. Remove this web of metal with a file being careful not to gouge or scratch the running surface of the shaft. This will prevent the axle from grinding the plastic hub area and slowing down your car. This is usually best done with the axle chucked up in a drill press or drill motor that is secured into a stable position.
Optional step. This step can be performed before actual polishing but is designed for those creating "the ultimate" racing machines. It's not necessary for the average racer. Use a fine flat file to reduce the overall diameter of the axle. To do this, chuck the pointed end of the axle into a drill press or drill motor that has been secured with a vice or clamp. Place the file against the rotating axle and apply even pressure while moving the file slowly. Do this until the area within 12mm of the head is smaller than the rest of the axle body. The more metal that is removed the less contact surface available to create friction. The drawback to removing too much metal is that the axle becomes weaker and will not tolerate being dropped or withstand rough handling without bending. This is often a trial and error procedure with much testing required to result in a fast turning wheel.
The axle can be finished to a high lustre by following the steps detailed here. First, mount the axle in drill motor chuck exposing the head and the first 20mm of the axle. Secure the drill so that it doesn't move. Now cut a piece of 400-600 grit wet or dry sandpaper to a strip approximately 12mm wide and 10 to 15 mm in length. Wet the surface of the sand paper with water or light machine oil, start the drill and loop the sandpaper over the axle and pull the paper back and forth like a shoe polish cloth. Work the paper until the metal is smooth in the wheel running area (next to the head of the axle). This usually takes about a minute for each axle. Now, using either pumice paste or metal polish in a soft cloth (like a tee-shirt), start the drill again and press the cloth and polish compound into the axle with a slight movement back and forth. This will also take about a minute. The finished axle will be very smooth and bright in appearance
The type of lubrication is usually restricted at most races to dry lubricants but there are great advantages to using the right lubrication. By the same token there is harm in using the wrong lubricate. First, we should discuss what it's all about.
The wheel should turn on the dry axle without any undo force but the friction between the two parts will quickly act to slow it down. It's this friction that you would like to eliminate. While we can't eliminate friction completely it can certainly be reduced. An automobile uses steel roller or ball bearings to reduce friction on its wheels but our car isn't permitted to use them. We can only lubricate what we already have. A lubricant is any agent that provides a reduction of friction. While there are many types of lubricants many will either not work on light-weight parts or are not formulated to work with plastics. Petroleum products such as motor and household oil may soften the plastic wheels. They could, after a time, fail to turn at all. This is not the surprise you'd like on race day. Other liquid or aerosol lubricants include a spray-on Teflon, WD-40, CRC and 3 in1 oil.
The most common and successfully used
lubricants are the graphite formulations and Graphite-Moly
blends. They provide a very smooth plating of microscopic spheres
that greatly reduce rolling friction. Plain graphite is available
in hardware stores and some variety stores. When installing your
wheels fill the axle hole of the wheel while capping the other
side. Gently push the axle through the wheel. Do this several
times and spin the wheel to help distribute the graphite through
the running surface. A good test of the wheel, axle and the
lubrication is a spin test. While holding the wheel in the axle
in a horizontal position spin the wheel with a flick of your
finger. It should spin freely slowly coming to a stop after 20 to
30 seconds. If it didn't spin that long - take a close at your
wheel clearance, axle finish and lubrication. Correct the
problems then test them again.
The bare wood surface will act much like a sponge when your paint is first applied and it will take several coats of paint to seal and finish the wood. A better approach is to apply a wood sanding sealer to the wood. This acts like a primer coat for the wood and provides a good base to apply the colour finish paint.
Prepare a place to paint your car that will be out of the house while you are painting and out of the reach of young children while your car is drying. You may either paint one side at a time waiting between coats or suspend the car on a string with a nail in the axle slot and paint all of it. Brush or spray the sanding sealer on the car with a complete coat and wait for it to thoroughly dry. After it is dry, sand it with 400 grit wet or dry sandpaper. You will find that the finish is smoother if you use a wet-sanding process. Sand until the sanding-sealer is smooth but not through the sealer to the wood.
You are now ready for the finish colour coats of paint. The best and smoothest finishes will be had with a spray paint but brush-on paint will not affect the overall speed of the car either. Use fast drying enamels and avoid using different brands on top of each other. Above all don't use lacquer paint on top of enamel paint. Your paint will wrinkle and bubble. If you get a run in the paint, let it dry and sand it smooth. Re-coat it later. You can achieve a very, very smooth finish if you wet-sand between coats with 600 grit wet-or-dry sandpaper.
If you are going to apply decals and detail
work now is time to do this type of work. If you are careful, you
can apply a clear coat of finish over the decals to seal them.
Dont use too much clear-coat at a time or youll
wrinkle the decals.
The guide strip on a pinewood derby track will keep the cars on the track and prevent them from hitting each other. This strip is necessary but each time your car's wheels hit it the car slows down a little. This is where wheel installation becomes important. If the car runs straight it will less often hit the guide strip.
There are a number of little tricks to consider in this stage of the car building. First, while you must run all 4 wheels they all don't necessarily have to touch the track surface. If each wheel has rolling resistance don't roll all of them. Simple. Usually, the best one to elevate off the track is one of the front wheels. Second, to prevent additional rolling resistance install the axles at an angle to the body so that wheels ride the end of the axle not against the car body. Install your wheels so that there is clearance between the body and wheel and ensure that the car body surface has a hard finish (No washers though) next where the wheel hub might touch the body.
Test roll the car so that you are satisfied that the car rolls in a perfect line. Put the car on a flat board or other smooth surface that has a straight line scribed for reference. Lift the board so that the car begins to roll. It should roll very close to the line. If it doesnt, then a front end alignment is required. Slightly bend the wheel axle(s) to correct the drift.
Another test using a long smooth surface is to check for tracking or wheel alignment. Draw a straight reference line on your surface and place the car on the surface with the wheels on top of that line. Now elevate the surface to the rear of the car to start the car rolling. Your car should roll along that line if it is tracking straight.
Now that you have finished construction and initial wheel alignment of your car you will want to test and re-test your car until you're sure that you have reached the best that the car can do. This will involve reviewing the last few steps in the construction phase of the car and verifying those details. Go back to any of the previous steps if you feel the car isn't right. Then you might just decide to use another car for comparison. Remember you can't race last years car in this years races
Using a smooth board or table, evaluate how soon the car(s) start to roll. To judge the rolling resistance and the initial breakaway friction resistance place your car(s) on the surface than slowly elevate one side until they just start rolling. The lower the angle the lower the friction and better your car.
Forward or "R" for Race
You will find that sometimes for reasons
that you can't explain some cars will run faster when its running
backwards. Maybe it's the weight position, alignment, wheel
placement or other more obscure reason but for whatever reason it
is a fact that most cars will run slightly faster in one
direction than in another. Please understand that while there is
a slight difference it may not be enough to make a huge
difference in the long run. Just the same, if there's difference
you're just as well to take advantage of it. Run your car against
another car and try racing it both frontward and backwards to see
which is faster. Maybe that "R" on the gear shifter is
for RACE in Reverse.
Its too bad that you dont have
a chance to race your car every day but that makes it all the
more special when you do race. We have to remember what this is
all about. Fun! Of course we have spent a
considerable amount of time building this car and we expect to do
well b-u-t so did a lot of other racers. At the
end of all the racing there will one car declared the fastest.
It may or may not be yours. It may not be your fault or something
you have control but sometimes luck may in the end help determine
the winner. This is where your sportsmanship will come into play.
Sometimes it's hard to be a good loser but remember you are in
good company and you will have done your best!
Content of this page
courtesy of Pinewood Derby Super Site
Content © 1999 Larry Bosworth. Modified & re-published with permission