Friday, April 26, 2013

Weighting Materials: So many choices

What do you use to add weight to your car? Certainly the answer to this question is not "none!" For without adding weight to the car, it will not weigh-in at the maximum allowable weight (typically 5 ounces), thus it will not reach peak performance. Generally 2.5 to 3.5 ounces of weighting material is required to bring the car to the maximum weight.

So what do you use for weighting material? I have seen people use a variety of materials such as lead, no-lead weight, pennies, nuts and bolts, fishing weights, etc. Virtually any material with a non-trivial density can be used. But there are advantages and disadvantages to the various materials.

In this article we will look at several factors that affect the choice of weighting material: density, malleability (ease of shaping), cost, safety, and availability. Density will be given in grams/cubic-centimeter. A lower density number indicates that the weight will take up more space on the car, while a higher density number indicates that the weight will use less space on the car. In addition, the density of each weighting material will be compared to the density of lead.

LEAD

Lead has been the traditional weighting material since the inception of the pinewood derby. At a density of 11.34, lead is quite dense when compared to other possible weighting materials. The relatively small amount of lead required for weighting provides the car builder greater flexibility in the car design.

An added benefit of lead is that it is very malleable. With most materials the car builder must create holes or cavities in the car body to fit the shape and size of the weighting material. However, with lead the builder can create any size hole/cavity and then shape the lead to fit the hole.
Another nice feature of lead is that it can be easily drilled. Thus, the weight of the car can be easily reduced at the weigh-in by drilling out of a portion of the lead.

But as you probably know, lead has a downside. If used improperly, lead is toxic, so care must be taken when using this material. At our house, we abide by the following rules:

- Wash your hands after handling lead (and keep your fingers away from your face).
- Keep lead away from food, water, and food preparation areas.
- Collect and properly dispose of any lead pieces.
- Do not sand, saw, or file lead. Lead particles will be created which cannot be easily collected.
- Do not melt lead. The fumes are toxic, and the lead can pop and/or splatter during the melting process causing eye and skin injuries.

In summary, lead is clearly a winner in density and malleability. But be aware of the risks and safety precautions before choosing to use lead as a weighting material.

Lead Density - 11.34
% Of Lead Density - 100%
Malleability - Very good
Cost - $0.75 to $1.15 per ounce
Safety - Caution advised
Availability - Some brick and Internet-based hobby stores including: Maximum Velocity

ZINC

Zinc (or 'No-lead') weighting material is available in a variety of shapes. The material has a density of about 6.30, which is 56% of the density of lead. Thus, almost twice as much no-lead material is required versus lead.

No-lead weight is quite hard, so it cannot be shaped. To use no-lead weight a hole/cavity must be created to fit the shape of the weight. Also, since no-lead weight is very hard, don't plan on using a drill to reduce the weight of the car at the weigh-in.

The weights sold at hobby shops under the brand name "PineCar" are zinc (BSA branded weights sold at scout shops are also zinc). Several of these products are intended for mounting underneath the car with screws. Be aware that if the weights are attached directly to the bottom of the car, the 3/8 inch clearance specification will likely not be met. To ensure adequate clearance, a pocket must be milled into the bottom of the car to accept the weight.

No-lead Weight Density - 6.30
% Of Lead - 56%
Malleability - Very poor
Cost - $1.16 to $1.75 per ounce
Safety - Okay
Availability - Brick and Internet-based hobby stores including: Maximum Velocity

PENNIES

Pennies are a common weighting material, as they are very cheap and readily available. Generally, a hole is drilled into the bottom or back of the car (13/16" diameter), and pennies are glued into the hole. How many pennies do you need? That depends on when they were minted. Prior to 1982, pennies were comprised of 95% copper and 5% zinc. After 1982, the composition changed to 97.5% zinc and 2.5% copper (during 1982 both types were minted). Since copper is more dense than zinc, the older pennies are heavier. Assuming that you use pennies minted after 1982, 11 pennies is close to 1 ounce.

Although pennies can be bent if needed, they are not easily shaped. Safety is not really an issue with pennies, but you should always wash your hands after handling money.

Penny Density - 7.17 (after 1982), 8.80 (before 1982)
% Of Lead Density - 63% (after 1982), 78% (before 1982)
Malleability - Poor
Cost - $0.11 per ounce
Safety - Okay
Availability - Your pocket or coin jar

STEEL

Nuts and bolts (and screws and washers for that matter) are also commonly used as weighting material. A typical use is to insert screws into the bottom of a car to fine-tune the weight. Assuming galvanized steel (most common), the density is about 7.70, so it is a bit more dense than pennies. Steel cannot be shaped, and safety is not an issue.

Steel Density - 7.70
% Of Lead Density - 68%
Malleability - Very poor
Cost - About $0.30 per ounce
Safety - Okay
Availability - Hardware Store

TUNGSTEN

Tungsten - Tungsten is a very dense metal. At a density of 170% of lead, tungsten is the densest material that is practical for use on pinewood derby cars. Tungsten is non-toxic so it is gaining increased usage in weighting applications where lead is not appropriate. For example lead has been banned in many streams, so tungsten is often substituted for lead weight on fishing flies. Tungsten is very hard, so it cannot be shaped or easily drilled.

Click on the link below for more information on tungsten, including a chart comparing the density of tungsten with zinc and lead:
http://www.maximum-velocity.com/tungstencylinders.htm

Tungsten Density - 19.3 (essentially the same as pure gold)
% Of Lead Density - 170%
Malleability - Very poor
Cost - $2.95 to $4.99 per ounce
Safety - Okay
Availability - Maximum Velocity

OTHER METALS

Other metals exist with greater densities; however, they are either very expensive or very rare. Both gold and platinum have a higher density than lead (170% and 189% respectively), but you would end up with a car that should be locked in a safe!

SUMMARY

What do you use for weighting material? As you can see, there are a variety of choices, so pick the material that best meets your needs.


Weight Comparison Chart

From Pinewood Derby Times Volume 5, Issue 15

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