Disregard the notice in blue. Grandpa had made a special version of the chassis III - and it was measurements on this one that led to this notice. The chassis III pictured below does not have this little problem. It should be roughly equivalent to an equal chassis II. (For some, chassis II is harder to build - so chassis III is a good option. The fastest car Grandpa has built, however, was still a chassis II - possibly because the "spring" constants were better adjusted than they were on his chassis III.
New notice about chassis III: Grandpa has discovered that this chassis III design has a flaw. Though it provides some independence for the wheels, it is not as good as chassis II for that. Side to side works well, but there is some effect of the opposite wheel on the other side when any wheel is pushed up. (Disregard this notice.)
Here is a description of how to build chassis III - and some examples of what others have done with it.
Grandpa's cars are not pretty. Though this car may be ugly, it has some beauty of design. Others have made some very beautiful versions, which are pictured below.
First, here is Grandpa's original version. Grandpa came in second with this car, and that's what got the analysis going - why was it slower than chassis II? It was then raced against the old chassis II, and sure enough, chassis II was faster.
The Next picture shows the shape to cut from the side. Before this cut is made, drill the holes for the axles while you have the whole stable block.
Notice that the weights are attached on soft foam, as with the older chassis. The old weight shape is probably a little better than this setup, so look back at that.
You will notice from this picture that the "knee" is above ground level. You must cut it this way. This is a very flexible chassis, and you want to be sure nothing touches the track guide as the wheels negotiate the little bumps, etc. The thickness of all the parts of this chassis make it look weak. It is quite strong. You will notice that the knee has some length to it - that's because this is a part of this chassis that needs a little extra strength. This entire cut for your block is easy on a band saw. Draw a pencil line on your block and saw it out. Leave everything a little thicker than this, and then you can sand to get the right stiffness. This cut will be more difficult with a coping saw, though it can be done. There is no need for boiling and re-shaping, as with Chassis II.
From the next two pictures, you get the advantage of this design. Each of the arms going to each wheel is very wide, HALF the width of the car. In the Chassis II design (The one that has come to be called the "FLEX," the arms go only 1/6 way across the car. They have far less lateral stability than this Chassis III design. On the car sent in by Peter Kappertz, Troop 740, Cascade Pacific Council, called "The Frog," the arms go 1/4 way across, which was an improvement for lateral stability over Chassis II. It requires lots of "milling" to make one of these. That "Frog" is very impressive to Grandpa, and it was part of the inspiration for Chassis III. It combines horizontal separation along with vertical separation to get the parts to work together. Chassis III uses this principle in such a way that no milling is needed. You need only a saw and a drill for axle holes.
Here is "The Frog": This is basically a Chassis II, but brought about the idea for Chassis III. If these arms to each wheel were thinned, this might be the best chassis of all.
This next picture is looking from the top - saw this line to within about an inch of the back of the car. Saw all the way through the chassis from top to bottom. This is a very easy cut on the band saw or with a coping saw.
And finally, this picture shows what the bottom will look like after making a little one-inch cut to separate the rear wheels. After making the top cut from the picture before this one, the bottom will look like this except for that one inch. You will see clearly what you need to cut.
Now we have all four wheels completely independent, while maintaining excellent lateral stability and very good strength. We have added no "springs," though our wooden "shape" has made our wheels quite independent.
Notes on thinness of the arms to each wheel: As you make these thinner, you will have a car that responds to lower frequency motions. As they are thicker, your car will save more energy from higher frequency motions. If they are too thick, however, you might as well have the whole block. You can get fooled by this, because you can't see the high frequency stuff, yet it is probably more important than the low frequency stuff with respect to your lost energy. You can't see it, but you can hear it! A noisy car is losing energy. You want the car to ride very quietly. The thickness shown in Grandpa's car is probably just a little too thin for best performance. Sand these arms until each wheel takes about five ounces to move up an eighth of an inch. Press each single wheel down against your postal scale to find out what you've got for each wheel. Bottom line: too thick is bad; too thin is bad.
Send questions to Grandpa - and if they might help others to see this all more clearly, I'll put them with the answers here.
Note about contests: Some rulers may decide that this car has "springs." Because there are no added parts and this is only the "shape" of your wood block, this will usually pass. They have strengthened this rule for some upper-level contests. It used to be "No added springs," then went to "No springs," and finally to "No suspension systems." Grandpa advises you to take two cars to the race - this one - and a simple one that requires little thought. Even on that car, you can use many of the principles, just not the flexible chassis.
In situating your weight, use all the principles from Chassis II. Be sure to lubricate well, and happy racing. Let me know if you win with this car.
Additional notes on wheel kits. Grandpa has found that the regular cub-scout kits with the little nails for axles are better than some of the other kits with full axles that go all the way across the car. Additionally, the full axles would have to be cut to use this Chassis. Each wheel must have its own axle. Grandpa recommends the little nails. Remove any burrs from the insides of the heads, as recommended in the main part of this report. Put the nail in a wheel, and then holding the pointed end, twist it while pulling a little against the wheel. Make sure this is smooth.
Remember, a quiet car wastes less energy, and all the energy you have is that little fall in gravity. Making noise may be fun, but it's more fun to win. A car that rolls easily and quietly on a rough surface is going to win the race.