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Grooved Axles
One of the major areas of debate in pinewood derby racing is the benefit of grooved axles. This technique calls for placing one or more grooves in the axle shaft near the axle head. These grooves are thought to:
1. Reduce friction,
2. Serve as a reservoir for graphite,
3. Reduce contact with a non-perfect axle surface (nail-type axles).
We'll will discuss these points a little later.
Anecdotal (and thus questionable) evidence for the benefit of grooved axles abounds. Statements such as, "I used grooved axles and won, therefore grooved axles are better than non-grooved axles", are commonly made, but cannot seriously be used to prove the benefit of grooving. In this example, any number of factors could have led to the victory; whereas the grooved axles may or may not have been of benefit (they could have actually made the car slower).
THEORIZING If grooved axles provide any benefit, there must be some scientific basis. Let's first look at the three points given above, and then consider some additional possibilities.
1. Reduce Friction - The use of grooves does not reduce friction. The amount of friction present in a system is based on mass and the coefficient of friction (F = mass x COF). The amount of surface area contact does not figure into the equation. So, when grooves are used, the amount of friction is not reduced, but instead the mass of the car is focused on smaller contact patches.
2. Graphite Reservoir - Graphite is most effective when it is thoroughly worked into the wheel bore, and any excess graphite is shed from the system. The introduction of raw graphite (from grooves or otherwise) tends to slow down the car until the break-in and shedding is completed again.
3. Reduction in Axle Contact - There may be merit in this theory. Since nail axles are not perfectly straight, round, and smooth, reducing the contact patch could be of benefit.
Here are a few more possibilities, especially as it pertains to liquid lubes:
4. Lube Channeling - Excess liquid lube tends to slow down a car (due to the viscosity - thickness - of the oil. If excess lube is used, the grooves provide space for any excess liquid, minimizing the amount of lube that is actually working to reduce friction.
5. Lube Thinning - As discussed in point 1, grooves cause the mass of the car to be focused on small contact patches. This tends to thin the oil between the wheel bore and axle (reducing viscosity), which could lead to better performance in the short term.
So, if grooves are of benefit, the reason for the improvement will likely be due to points three through five.
EXPERIMENT TIME In order to prove/disprove the benefit of grooved axles, a controlled experiment must be conducted which eliminates as many variables as possible. Under these controlled conditions, the effect of grooves on axles can be ascertained.
An experiment of this type was performed in 2003 (updated in 2004) by Bob Barga.(1) The results of this experiment generally showed that grooves were of benefit when using a liquid lube, but were detrimental when using graphite. This experiment also tested the effect of various polishing treatments. In our experiment we will focus entirely on the effect of grooved axles.
Apparatus To eliminate variables, the performance of a wheel on an axle will be measured with a weighted-wheel spin jig (see Figure 1). This apparatus consists of a machined steel ring which is sized to fit snuggly over a machined BSA wheel. The ring weighs two ounces, which is essentially the load on a rear wheel of a pinewood derby car.
Figure 1 - Weighted Wheel Spin Jig
After sliding the ring over the lubricated wheel-axle assembly, the axle is mounted onto the apparatus. A length of fishing line with a weight on one end is wound around the ring (there is a small pin on the ring not seen in Figure 1, to which the non-weighted end of the string is attached). On each test, the string is wound until the weight touches the eye hook. The weight is then released and a stopwatch is used to measure the spin time. The string is sized such that it is released from the ring before the weight reaches the ground.
To ensure consistency: - A mark is placed on the head of the axle, and that mark is oriented at the 12:00 position when the axle is mounted onto the jig, and
- The jig is shimmed such that the wheel runs on the axle without touching the axle head or the jig.
Wheels, Axles, Lubes A set of DerbyWorx Precision Pro Stock Wheels were used. Two of the wheels were used with graphite, and two with Krytox 100. The results of each pair of wheels were averaged. Two of each of the following four axles were used:
- BSA Speed Axle - Standard BSA nail axle that has been machined to create a smooth shaft.
- Grooved BSA Speed Axle (4 groove) - Same as above but with the four grooves.
- Super Speed Axle (smooth) - an oversized (0.091), nickel plated, machined axle.
- Grooved Super Speed Axle (1 large groove) - Same as above, but with a single wide groove.
One of each of the above axle types was lubed with Max-V-Lube Graphite, and the other with Kryox 100. The recommended lubrication method was used in all cases.
Pairing Referring to Figure 2, the wheels and axles were paired as shown. Thus, wheels one and two were only lubed with Krytox, while wheels three and four were only lubed with Max-V-Lube.
Figure 2 - Wheel Axle Pairing
EXPERIMENT RESULTS Each wheel-axle combination was given several "break-in" spins, and was then given ten timed spins. The high and low times were removed, and then the times were combined and averaged. The results are shown in Figure 2. Note that longer spin times mean better performance.
TEST CONCLUSIONS Here are the conclusions that I draw from these tests:
1. With Krytox 100, grooved BSA axles provide a slight benefit. This is likely due to lube thinning. With oversized axles there appears to be no difference between grooved and non-grooved axles.
2. With Max-V-Lube Graphite, BSA grooved axles provide significant benefit. This is likely due to a reduction in the contact patch. With oversized axles, grooves provide no benefit (or slightly negative benefit -- the difference in times is within the experimental variance so a solid conclusion cannot be drawn).
3. Although this was not the intent of the experiment, the data clearly shows that oversized axles perform significantly better than BSA axles.
(1) The results of the experiment are posted at: members.aol.com/StanDCmr/pwfrictn.htm
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