How to Use Custom Materials
If you want to use materials other than the default materials (foamcore, wooden axle, ideal rubber tread, and plastic spring), you first must discover their physical properties and enter them into Virtual Car, so that Virtual Car can model them accurately. Here are the properties you will need to measure:
||Properties to measure:
Density (lb/cubic inch)
|Density (lb/cubic inch)
Density (lb/cubic inch)
Maximum torque (lb-in)
||Drive wheel tread
||Coefficient of static friction
You are free to choose any board material you wish, as long as you have the right tools to cut it and put holes in it. If you are planning to use an X-Acto knife for cutting, corrugated cardboard is a good choice. Other materials should be similar: they must be easy to cut, relatively thick (but not too thick), and reasonably tough. Some other ideas are: foam insulation board, or styrofoam. Or, you may invent your own board by gluing layers of thinner materials together.
The default wooden dowel material is a good all-around choice. But if you have the right tools, you may have success with aluminum rods or tubes that you can get from the hardware store, or plastic rods that you can buy in a hobby store.
You may be able to find other "springy" materials that can be used as a spring, instead of the default plastic soda bottle material. First you should try them out to see if they retain their springiness after they are rolled up onto the axle. If you find a good substitute spring material, you will have to install it into an already-built car and test it to see how much torque it provides.
Rubber bands are the best tread material. You should test your own rubber bands to make sure that the coefficient of static friction is correct for that type of rubber band.
How to Measure Thickness
To accurately measure thickness of a material, do not try to measure a single layer. Instead, stack several layers of the material together, and measure the total thickness -- then divide the total thickness by the number of layers. This will give a much more accurate measurement.
How to Measure Density
Density is mass divided by volume. Volume is length * width * thickness (for board or spring materials), or cross sectional area * length (for axle materials).
To measure density accurately, work with a large piece of material, not a small one. Find a large piece that has straight and square edges so that it can be measured easily. Compute its volume (in cubic inches) as described above. Then weigh the piece of material to get its mass (in pounds). Finally, divide the mass by the volume.
How to Measure Coefficient of Static Friction
The coefficient of friction represents the force needed to make something slide along a surface. There are two types of coefficient:
- the coefficient of static friction
- the coefficient of dynamic friction
The coefficient of dynamic friction applies to objects that are sliding along a surface. It is not important for our purposes.
The coefficient of static friction applies to objects that have not yet begun to slide.
Virtual Car needs to know the coefficient of static friction between the tread material and the racetrack surface.
One experimental method to determine the coefficient of static friction involves finding the degree of incline that makes a material begin to slide on another material. You will need a protractor, a piece of the material on which you will be racing your cars, and a square block with tread material attached to it.
Figure 1. How to estimate the coefficient of static friction
Follow this procedure:
- Find a block of any material that is flat and about 1"-2" square.
- Get a sample of your chosen wheel tread material (rubber bands are good).
- Wrap the tread material around the block so that one side is covered evenly with the tread material.
- Get a piece of the competition racetrack from your instructor.
- Place the racetrack material on a rigid movable surface such as a book, and place the tread-covered block on the surface of the racetrack material.
- Starting from a flat position, very slowly raise one end of the book until the block just begins to slide. At this point, measure the angle of the book. The tangent of this angle is the coefficient of static friction between the tread and the flooring material.
- Repeat several times and average the results.
||Angle||Coefficient (tangent of angle)|
How to Measure Spring Torque
You do not need to measure spring torque if you use the default spring material. But otherwise, you must follow this procedure. You will need:
Then follow this procedure:
- A car that you have already built
- A nail or some other sharp and strong item that you can stick into one of the wheels
- A sample of the spring material (24-36 inches long)
- A small scale, such as a postal scale or a spring scale
To finish, you will need to convert the force figures that the scale gave you, into torque figures. Torque is force * distance, so you will have to multiply the force by the distance of the nail head from the center of the wheel. For example, if the force exerted by the nail head was 0.5 pounds, and the nail head is two inches away from the center of the wheel, then the torque is:
- Install the spring in the car
- Make sure that the rear wheels are tightly gripping the axle.
- Stick the nail firmly into the wheel tread surface, so that only about a half inch of the nail head sticks out.
- Wind the spring up all the way
- Using the scale, measure the force exerted on the scale by the protruding nail.
- Let the spring unwind one or two revolutions and measure again (keep doing this until the spring is completely unwound).
(0.5 lb) * (2 inches) = 1.0 lb-in
If you were to plot the torque figures, you would see something like this:
Figure 2. Example of a Force-Deflection Curve
Finally, enter the torque figures into Virtual Car. You will also need to take note of the width of the spring that you tested, and enter that in too. This is so that Virtual Car can scale these torque figures up or down to account for wider or narrow versions of the same spring material.