Properties of Wood - Derby Tech - February, 1986

(reprinted from an earlier article in Derby News)


The use of wood for construction of soap box Derby racers in 1976 will increase shapely. Due mainly to the rule change of no molded fiberglass racers.

There are some people including myself, that fell that the proper wood to use in the construction of a racer is one that will vibrate freely.

The vibration properties of primary interest for constructing a racer are, the speed of sound and the damping capacity or internal friction.

The speed of sound in wood varies directly with the square root of the modulus of elasticity and inversely with the square root of the density. The speed of sound in wood also decreases with an increase in temperature or moisture content.

When wood is strained, some energy is dissipated as heat. This is done through internal friction. Internal friction increases as the moisture content increases.

Clear straight grained wood will transmit vibrations (sound) faster than wood with knots and the grain running at an angle.

Therefore: The best wood to use in the construction of your racer is one that will transmit vibrations (sound) the fastest, is as dry as possible and the grain is straight.

A given racer has a certain amount of potential energy as it rests against the starting blocks. The less energy you use on your way down the hill, the sooner you will reach the finish line. Energy is used up in many ways, one of them being by the slight flexing of the racer body as it races down the course.

One way to save a small amount of energy is to use wood that has as little internal friction as possible, i.e. use wood that has a high rate of sound transmission.

There is more that should have been said about wood. To obtain a special wood, you need to know an expert who works in a lumber mill. It takes an expert to tell the difference between woods of the same species. All of these woods are not commercially available under their correct names. Noble, Grand, Pacific Silver, and Subalpine Firs are grouped the Western True Firs. They are marketed as White Fir throughout the US. These Firs are grown in California, Oregon, Washington, Montana and Idaho. High grade lumber from Noble Fir is used mainly for interior finish, moldings, siding, sash and door stock, venetian blinds and ladder rails.

The following 20 woods have the highest rate of vibration (sound) transmission of all woods. The corresponding numbers indicate the speed of sound as a comparative number only.

Fir:Noble 4070 Fir: California Red 3586
Fir: Grand 3907 Cottonwood: Black 3495
Fir: Pacific Silver 3775 Fir: White 3473
Douglas Fir: Coast 3711 Spruce: Black 3473
Fir: Subalpine 3709 Douglas Fir: Interior West 3462
Magnolia: Cucumbertree 3669 Douglas Fir: Interior North 3431
Cedar: Port Orford 3643 Spruce: Red 3420
Spruce: Sitka 3625 Spruce: Engelmann 3418
Pine: Western White 3621 Poplar: Yellow 3406
Basswood: American 3598 Aspen: Bigtooth 3389

Some other common woods:

Ash: Blue 2377 Oak: Cherrybark 3146
Balsa 3235 Oak: Live 2113
Beech: American 2576 Oak: White 2332
Birch: Sweet 3044 Pine: Eastern White 3228
Cedar: Eastern Red 1675 Pine: Jack 2933
Elm: Rock 2324 Pine: Ponderosa 2929
Hemlock: Eastern 2908 Pine: Sugar 3168
Hickory: Bitternut 2523 Redwood 3228
Mahogany 3100 Teak 2472
Maple: Bigleaf 2500 Walnut: Black 2920

Wood in the same general category such as cedar, will vary greatly as to vibration transmission. As you can see, Cedar: Eastern Red is 1675 and Cedar: Port Orford is 3643. This holds true for other woods.

Completely air dried woods can be expected to have a minimum of 15% moisture content whereas kiln dried woods will have a moisture content under 12%. 8% moisture content is about the minimum.


Return to the Derby Tech Page