02 03 Pinewood Derby Stories and Photos from Maximum Velocity 04 05 15 16 19 20 21 22 23 24 25 26 27 28 31 32 33

Lubricant Testing

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

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.

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)

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.

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

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

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

(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.

Read More at: Pinewood Derby Times Volume 8, Issue 10

A feature article is a regular part of the Pinewood Derby Times Newsletter. To subscribe to this free e-newsletter, please visit:

(C)2010, Maximum Velocity, Inc. All rights reserved.
35 36 37 38