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.
