WEIGHT A MINUTE - Derby Tech - October-December, 1990

By George Brower


Many Derby people spend hundreds of hours building a racer, and yet spend about 60 seconds deciding where the weight placement should be. This weight a minute decision often means placing the weight where it can be reached fast, rather than placing it where the car can become fast. I want to show you how the importance of weight placement and how to obtain proper placement.

Everyone knows that the heavier a racer is the faster it is. no big secret here, but why it is faster is important. A heavier racer is faster because it has more potential energy (PE). This PE is converted into kinetic energy, the stuff that makes your racer move. Therefore, the more PE you obtain through weight placement the faster you go.

The formula for determining potential energy is: PE = H x W. The total amount of PE that your racer has, is found by multiplying the height by the weight. It sounds simple enough, but wait a minute. The height is the vertical distance that your racer's center of gravity (CG) on the starting ramp is above the CG when your racer touches the finish line. This vertical distance times the racer's total weight is the PE. Even though two different racers have the same total weight they usually have different amounts of PE. Why? Because of the height of the racer's CG is different. One racer has the weight falling a greater distance than the other (Figures 1 & 2).

Figure 1

This shows that the lower the CG, the more PE there is to propel the racer. Through this ramp transition of 90 degrees, the low CG falls 1/2 inch farther than the high CG.

Figure 2

The lower the CG on a transitional track surface, the more the PE to propel the racer. The high CG falls 1.82" while the lower CG falls 2.17".

When you lower your cg's weight or when you move weight to the extreme tail of a racer you move the cg up a higher vertical distance from where it will be when the racer is at the finish line. This happens because the starting gate area slopes more than the finish line area. If the ramp or hill slope did not change there would be no advantage (Figure 3).

I was involved in a number of hill tests using different weight placement. The results were converted for the Fort Wayne and Akron hills (figure 4).

Fort Wayne Hill: The ramp transition angle is about 4.75 degrees. Since this transition takes place almost immediately, it becomes a major factor in your racer's speed. Being tail heavy and having a low CG are very important. Not only do you get the big push off the ramp, but you get a push throughout the track because of the concave hill. If you moved your racer's cg lower by one inch, your gain would be 11.68 inches for the ramp transition and .82 inches for the hill transition. The overall gain for just one time down the hill is 12.5 inches. You want to be as tail heavy as allowed and get your CG as low as you can.

Figure 3

When there is no ramp or track surface transition, there is no advantage for a low CG. The vertical distance between a low and a high CG is 1/2" at the start, and the same 1/2" at the finish.

Akron Hill: The transition at the ramp is virtually zero. Being tail heavy and having a low CG does no good off the ramp. A low CG will help later. There is a slight advantage to be gained from the concave hill. The hill is a 16 percent slope at the start and a 1 percent slope at the finish. This amounts to only .40 inches per foot pound of energy gained. If you could move your racer's CG lower by four inches you would be only 2.4 inches faster. Each inch you lower your CG, you will increase your speed by .6 inches. It is very difficult to lower your CG by four inches. if you can lower it by one inch you have done a good job. Your weight should be balanced and the CG as low as possible.

Now let's take a minute and discuss how to obtain proper weight placement. As we have seen up to this point, you get the most speed out of weight placement, the weight should be low, towards the rear and to the maximum allowable.

The weight of the driver plus the weight of each part of the racer determine the total weight. Likewise, each of the elements should he considered in where you place your CG. Yes, where you place your CG. I know that you can't place it anywhere you want, but I want to say that you have some control in where it goes, and you probably have more control than you think. Consider the weight of everything that goes down the hill as a challenge in planning for the best CG.

Driver: What can you do about the driver? Not a lot, but here are some. The driver adds weight that should be placed low. therefore, plan to lower the driver by using a minimum thickness floorboard and having a minimum ground clearance. The lighter the driver the easier it is to get your weight low. Make sure that the driver's clothing, especially the shoes, are lightweight. I dread mentioning a diet or exercise to a kid, but they are a consideration.

Racer elements: By now, you see that if you design your racer with something like light tops you can add more weight to the bottom. By the same token, use light sides, use little fiberglass and use the floorboard to attach your hardware. Speaking of floorboards, the junior floorboard should be heavy, but as previously mentioned thin. Use the chart provided for a comparison of some weights of wood (Figure 5). Design your racer to have a minimum wheelbase. The back of the rear wheels should be even with the tail using steel wheels. This helps move your CG back. Lead weight should be placed low. Lead weight on the floorboard should be thin. The thinner it is the lower it will be. In the senior division you can get it as low as your ground clearance. Lead placement is the final step in getting your racer's total weight, tail weight and CG weight where you want them. And yes, place your adjustable weight low, and where it can be reached fast.

I hope this article helps you appreciate potential energy's importance and how to get more through weight placement. Now you can plan your racer's design and construction with weighty elements to consider. Items like the driver and your racer's elements take on additional importance. You may want to take longer than a minute on weight planning and placement.

Weight Placement Adjustment Fort Wayne (former NDR national site) Steel Wheels - 250 lbs. - Inches of Improvement Akron - Plastic Wheels - 236 lbs.- Inches of Improvement
Lower CG one inch 12.50 .60
Add one pound tail heavy (unbalanced) 1.7 -.10
Add one foot/pound of energy to PE 10.68 .45
Move CG one inch to rear 19.28 1.42

Figure 4

The inches of improvement for a junior racer would be slightly more than the amount listed in Figure 4. The -.10 slower for being tail heavy for each pound would be slightly smaller.

 

Ash

0.58

Aspen

0.37

Balsa

0.17

Basswood

0.35

Birch

0.56

Capirona

0.85

Cedar

0.43

Cherry

0.49

Cottonwood

0.39

Cowbaril

0.72

Douglas Fir

0.47

Elm

0.48

Fir

0.38

Goncalo Alves

0.86

Greenheart

0.88

Hemlock

0.43

Hickory

0.69

Karrie

0.7

LaPacho

0.92

Lignumvitae

1.08

Mahogany

0.45

Maple

0.52

Oak; Laurel

0.6

Oak; Live

0.84

Oak; Northern Red

0.6

Oak; White

0.64

Pine; White

0.35

Pine; Longleaf

0.57

Pine; Ponderosa

0.39

Pine; Sugar

0.35

Poplar

0.41

Redwood

0.39

Rosewood

0.75

Teak

0.62

Walnut

0.53

Figure 5

Specific gravity of different types of wood. A specific gravity of 1.00 would be equal to the weight of water. As you can see there is a wide range, from balsa .17 to lignumvitae 1.08, which will sink if placed in water.


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