Pinewood Derby Stories and Photos from Maximum Velocity
If you have done any pinewood derby research on Google, or looked at derby products for sale on eBay, then I'm sure you have run across statements such as:
- "Tungsten Disulfide (WS2) has an extremely low coefficient of friction of 0.03 -- lower than that of Teflon, Graphite, or Molybdenum Disulfide"
- "Molybdenum Disulfide (MoS2) has a lower coefficient of friction than graphite. This means (brand name removed) is a better lubricant than graphite."
If you are looking for better derby performance, then statements such as these are quite intriguing, especially when accompanied by a graph comparing the coefficient of friction of each substance (see Figure 1).
Figure 1 - Lubricant Comparison from BryCoat, Inc.Graphic from BryCoat, Inc. web site
When viewing statements and graphs such as these, it pays to take a careful look at the details. In Figure 1, the range of the data is between one and three hundred thousand PSI. This is an extremely high pressure condition. Most importantly, note how the coefficient of friction of the three materials appears to be the same at the low end of the data.
An important question to ask oneself at this point is, "What happens as the pressure reduces further, especially at a pressure that would occur in a pinewood derby car?" Clearly, for MoS2 or TS2 (or any other lube for that matter) to be of value, it must have a lower coefficient of friction than graphite at the pressure found in pinewood derby cars. So, let's test these lubes in that pressure range.
BACKGROUND The coefficient of friction is the ratio between the mass of an object and the amount of force required to move the object. Normally, the coefficient of friction is indicated by the Greek letter "µ". So the mathematical formula is:
F = µM or µ = F/M
Where F is the force required to move the object, µ is the coefficient of friction, and M is the mass (weight) of the object(1).
As an example, if you were to push a heavy box on a rough road (asphalt) and then on smooth sidewalk, pushing it on the sidewalk will be easier because the µ of the sidewalk is lower. If you then pour oil on either surface, it will be easier to slide the box because the oil has lowered the µ.
The coefficient of friction can be measured in any of several ways. For our purposes, we are going to use the "Tilted Plane" method. In this method, an object will be placed on a smooth surface which can be incrementally tilted. At some point, the object will slide on the surface. The angle at which the object slides is called the "friction angle", and is notated as ø. We can then calculate µ as follows:
µ = tan(ø) (2)
EXPERIMENT SETUP The main piece of equipment for the experiment is a tilting apparatus (Figure 2). It consists of a piece of glass on a tilting frame, which is moved when the crank winds the string around a rod. An angle measuring device shows the current tilt angle.
Figure 2 - Tilted Plane Apparatus
The sliding blocks are also critical components. Eight blocks of Delrin (3) were machined to the same dimensions, and provided with two smooth surfaces. Delrin is not as consistent in density as some other plastics, so there was a slight weight variance between the blocks. So, the lightest block was found, and then small holes were drilled in the sides of the other seven blocks until all of the blocks weighed the same. The resulting weight of each block was 2.05 ounces. This is the typical load for one of the rear wheels on a pinewood derby car.
EXPERIMENTAL PROCEDURE Three lubes were tested: TS2, MoS2, and Max-V-Lube Graphite. Two blocks were used for each lube, and the last two blocks were used without lube as a control.
The glass plate on the tilting apparatus was first cleaned thoroughly with a glass cleaner. This cleaning was repeated each time the lubricant was changed.
A quantity of each lubricant was placed on a sheet of clean newsprint. One side of a block was then rubbed on the lubricant until thoroughly coated (Figure 3). The block was then placed on the glass and slid back and forth to distribute some of the lube onto the glass. The block was then recoated with the lube and placed on the uphill side of the glass. The glass was then slowly tilted until the block slid to the downhill side of the glass. The tilt angle was then recorded. This test was repeated five times for each block.
Figure 3 - Lubricant Application
EXPERIMENT RESULTS After all of the tests, the high and low angle measurements were removed and the remaining three were averaged. µ was then calculated for each of the lubricants and for the control test. The results are shown in Figure 4.
Figure 4 - Coefficient of Friction for Pinewood Derby Lubricants
As shown by the data, both MoS2 and TS2 have a higher coefficient of friction than Max-V-Lube graphite at 2.0 ounces.(4) Thus, these lubricants will be less effective than graphite as a pinewood derby lubricant.
(1) Oftentimes, F(normal) is used instead of M in the equation. These are essentially the same.
(2) If you are interested in why this is true, there are several good discussions of this on the Internet. Just search for "measure friction angle" on Google.
(3) A brand name of an engineered plastic.
(4) In a previous test using this apparatus, I compared many of the top graphite brands with MoS2 and TS2. In all cases, graphite had a lower µ than MoS2 or TS2. At that time I also tested "Dry White" Teflon lube. Not surprisingly, the µ of "Dry White" was higher than MoS2 or TS2. Note also that spin tests were performed on all of these lubes with compatible results.