Tips for a Faster Derby Car

  Pack 631, Larkspur, Colorado 

 

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Tips for a Faster Derby Car

from:  the Cub Scout Grand Prix:  Pinewood Derby Guidebook.

TIPS FOR A FASTER CAR

These factors can help determine how fast a Pinewood Derby car can be:

 

  1. Maximize weight.  The car should be as close to 5 oz. as possible.  The car then has the highest potential energy that is permitted.  See "Add Weights" at the end of this information packet for theories on adding weight.

 

  1. Reduce friction in the wheels.  The friction of the wheel on the axle can be reduced by polishing or machining the axle as shown below.      

 

3.      Reduce the weight of the wheels, if possible.  Machining material out of the wheel interior will help the car accelerate faster when the gate opens  because there is less weight in the wheels for the car to have to rotate.  The wheel must remain solid.  See the above diagram.

 

 

  1. Raise one of the front wheels ("lazy wheel").  If one of the wheels is raised, the drag of the wheel is eliminated.  Adjust steering after the wheel is raised.  Remember to maintain proper height over the track or the car may drag.

 

  1. Steering alignment.  Make sure wheels are straight to minimize collisions and rubbing with the guide strip (lath) on the track.  Pre-drill holes as specified on page 6 of these instructions prior to shaping the body.  Then, after the body is shaped and decorated, mount the wheels.  Check that the wheels rotate smoothly, and give the car a test push on a flat, smooth surface.  If the car rolls easily and straight with a slight push, the axles should be "locked in place".  This will maintain the alignment of the wheels after many races.  To lock the axles, apply 5 minute epoxy in the axle slots, allowing the glue to flow around the axle (and the sharp tip of the axle nail).  Obviously, keep the glue away from the wheel.  A good practice is to carefully apply glue to within 1/4" of the edge of the car.  This will secure the axles permanently, but will not interfere with the wheels.

 

           These are suggestions only, not race requirements.

 

           Some of these tips require delicate work to avoid breaking parts.

 

TIPS ON ESTIMATING WEIGHT

Note: 1 ounce = 28.375 grams, or g

Your kit consists of the factory made wheels, axles, wood body, instructions, and number decals.  You may  use pieces of steel, screws, nails, lead, lead tape, or other materials as desired to weight your car.  The maximum allowed weight of the completed car is 5 ounces (141.9 g).  The 4 wheels and 4 axles weigh 16 g.  The pine block in the kit, because of variations in the moisture content of the pine and the pine itself, weighs between 97 to 136 g, the median weighing about 118 g.  Depending upon how much pine is removed to shape the body, most cars will need between 1 to 2 1/2 ounces of weight to bring them up to 5 ounces.

To estimate how much weight will be needed, you can use the following technique:

1.  After the Scout designs the car on the attached graph paper, count the squares inside the car body and ratio as in the following example.  There are 140 squares total.  Assume the pine block weighs 118 grams.

So after the excess wood is removed from the block, the wood body should weigh about 75.9, or 76 g.  Add the weight of the wheels and axles:  76+16=92g.  Subtract the weight of the car from the maximum allowed: 141.9 - 92 = 49.9g.  So we need to add 49.9, or about 50 grams of weight, or 1.75 oz.  You could add 1.5 ounces of steel weights, and then fine tune the weight of the car during weigh in. 

For reference, here are some weights of steel wood screws and steel flat washers:  

3/4"x6= 0.9g;   1"x6= 1.5 g;   1/2"x8= 1.0 g;   3/4"x8= 1.5 g;   1"x8= 1.9g;   1/4" steel flat washer = 3.3 g;   With the kit:  1" x 1" x 1/8" steel = 14 g;  1" x 1" x 1/4" steel = 28 g

 

2.  Of course, shaping the sides will also remove wood; estimating this is more difficult than the procedure for the side view of the car since the cross-sectional area usually varies greatly as you go from the front of the car to the back of the car.  Rarely will shaping the sides remove more than 2-3% of the estimated weight from the step above.  This usually amounts to about 2 grams.

 

3.  If you add decorations such as a windshield, driver, etc., you can estimate the weight by using a density of wood relationship.  If the object floats, assume it has the density of wood, so a cube 1/2" on a side will weigh 1 gram.  If it sinks, estimate its density as 2 grams per 1/2" cube.

 

4.  Alternatively, you can buy Pinewood Derby weights from the Scout Shop.  These weights are safe, as they are made of zinc, not lead, but they are not as dense as steel (steel = 7.87g/cc, zinc = 7.13 g/cc).  Lead weights, if used, should be handled by an adult to minimize the potential for lead dust inhalation or ingestion (hand-to-mouth pathway) by children.

 

DESIGNING THE CAR

The Scout should use Graph paper to design his car.  The dimensions of the wood block and the locations of the wheels on the graph paper are the same as in the kit.  The paper can then be cut out and traced onto the pine block.  Be sure to leave enough wood around the axle slots; if it is too thin, the pine can split.

* More information is available from Cub Scout Grand Prix:  Pinewood Derby Guidebook.