Pinewood Derby Stories and Photos from Maximum Velocity
Pinewood Derby Myths
(An imaginary conversation between two Cub Scouts a month before the pack pinewood derby race).
Johnny: What ya' got in that baggie in your pocket?
David: These are my pinewood derby wheels. I'm "marinating" them in graphite so they will go faster.
Johnny: That's silly! What you really want are grooved axles. They reduce friction.
David: No they don't! But you can reduce friction with a raised wheel.
Johnny. I don't think that reduces friction, but I think if you angle the axles so that the wheels run on one edge, you'll reduce friction and go faster.
David: Hah, where did you hear that one? On Car Talk? Really, I think the main thing is "aerodynamism". You know, like race cars.
Johnny: You mean "aerodynamics". But that doesn't do anything at slow speeds. But we do have a super secret lube we are going to use this year that will let us win the council race.
David: Oh really! There's no such thing -- you know that "Lemon Pledge" isn't the silver bullet don't ya'?
Johnny: It's not Lemon Pledge, trust me. But we're also going to put the weight in the front of the car so that it gets moving faster.
David: That won't help. Lightweight wheels are what you need to get the car going faster. Besides, my dad is a science teacher, and he says that weight doesn't matter because of some guy named Newton.
Johnny: Well my dad's an engineer, and he says that weight does matter. We'll just see whose dad knows more after "I" win the race.
David: You mean after "I" win the race!
Well, that conversation was a bit confusing, wasn't it? Although the boys seemed confident of their "speed tips", in my mind I now have more questions than I started with. For example:
- Which of the boy's statements are true, or are they simply a myth? - Is Johnny or David's dad smarter? - Who will win the race?
I can't answer the last two questions, but I can certainly take a crack at whether any of Johnny or David's statements are fact or fiction. Over the past several years, I have run across quite a few pinewood derby myths, and have experimentally confirmed or busted many of them. So in today's article, we will review the main pinewood derby myths and label them as confirmed, plausible, or busted.
MARINATING WHEELS IN GRAPHITE Graphite reduces friction when applied to the wheel to axle contact surfaces (wheel bore, inner hub and outer hub). There is also a slight benefit to coating the inner edge and tread surface of the wheel with graphite -- see Volume 8, Issue 2 - October 15, 2008 at www.maximum-velocity.com/pinewood_derby_times_v8_i2.htm Obviously, there is no benefit to coating the non-contact surfaces such as the inner and outer walls of the wheel.
So, the question is whether immersing the wheels in a bag of graphite is more or less effective than purposefully placing the graphite on the contact surfaces. In my testing I have found that graphite is most effective when it is worked into the contact surfaces by repeated spinning (or similar). Just placing graphite on the contact surfaces is not sufficient; the graphite must be worked into each surface.
So, simply placing the wheels in a bag of graphite (even if the bag is massaged around the wheel) is a less effective technique than purposeful application of graphite.
BUSTED - One point to Johnny
GROOVED AXLES In Volume 8, Issue 6 - December 10, 2008 at www.maximum-velocity.com/pinewood_derby_times_v8_i6.htm we discussed grooved axles at length. From that discussion we learned that grooves do not reduce friction (the friction is just relocated). However, we did find that grooves can be of benefit in specific situations.
When running on BSA-type nail axles, grooves were of benefit with both graphite and liquid lubes. But when running with oversized axles, the advantage/disadvantage of grooves was indeterminate.
PLAUSIBLE - One point to David for knowing that grooved axles don't reduce friction. One point to Johnny for knowing that grooved axles can be of benefit
RAISED WHEEL Contrary to David's statement, raising a wheel does not reduce friction (the frictional forces are just increased on the other three wheels). However, there are other benefits to a raised wheel including a reduction in wheel inertia.
In Volume 4, Issue 14 - April 6, 2005(1) a raised wheel experiment was conducted which showed a significant benefit to racing with a raised front wheel (see Figure 2).
Figure 2 - Effect of Raised Wheel on Performance
CONFIRMED - One point to David for knowing that it helps, one point to Johnny for knowing that it does not reduce friction.
CANTED AXLES The term "canted" means an object set at an angle, thus "canted axles" are axles which are attached to the car at an angle. Sometimes this is done accidentally, but usually car builders cant axles in an attempt to reduce the amount of contact between the wheel tread and the track.
In Volume 8, Issue 8 - January 7, 2009 at www.maximum-velocity.com/pinewood_derby_times_v8_i8.htm a canted axle experiment was documented. The results show that up- canting the rear axles on an aluminum track may provide a very slight benefit. In Figure 3 the amount of change between the data points is within the statistical variation of the heat times, so the data is inconclusive.
Figure 3 - Effect of Canted Axles
PLAUSIBLE - One point to Johnny
AERODYNAMICS As it applies to pinewood derby cars, aerodynamics refers to the affect of air flow on the movement of the car. Johnny is correct that the effect of air flow on a vehicle does increase at higher speeds, and decrease at lower speeds. But does the effect go away at pinewood derby speeds (10 to 15 MPH average)?
In Volume 3, Issue 9 - January 21, 2004(1), an aerodynamics experiment was documented. In this experiment, the frontal surface area of a car was varied with a removable air dam. The results are shown in Figure 4. As you can see, an increase in frontal surface area does reduce performance for a pinewood derby car.
Figure 4 - Effect of Aerodynamics
CONFIRMED - One point for David
SUPER SECRET LUBE The use of an effective lubricant is critical to car performance, and there are certainly a vast number of possible lubricants that can be used. These include dry powders, liquids, and liquids that become dry to the touch.
Over the years I have tested a large number of these lubes, including but not limited to:
Liquid Krytox 100 NyOil II SB-10 Several exotic oils, both synthetic and petroleum-based
Wet to Dry Silicon Spray Emery's Slide All Several graphite suspensions Several boron nitride suspensions Spray and non-spray waxes, including Lemon Pledge
After all of these experimentation, I have found that a good quality graphite (such as Max-V-Lube) or a good quality synthetic oil (such as Krytox 100) gives the best performance.
So, is there a super secret lube? I can't absolutely confirm or bust this myth, but I don't believe there is a secret lube. If there was, it would not remain secret for very long, as it would be publicized on the Internet and be available from pinewood derby stores such as ours. There are likely other lubricants that rival the performance of Krytox 100 and Max-V- Lube, but I don't believe there are lubes that are significantly better.
UNCONFIRMED - No points awarded
WEIGHT IN FRONT The location of the ballast weight on a pinewood derby car affects the center of gravity (generally referred to as the "balance point"). The optimal balance point varies from track to track, but that position is somewhere between the center of the car and the rear axle location (1 to 1-1/4 inches in front of the rear axle location is a good rule of thumb for most tracks).
In Volume 3, Issue 14 (March 31, 2004)(1) a weight experiment was documented. In this experiment, the balance point was adjusted from the front to the back of the car. The results are shown in Figure 5. As you can see best performance was attained with a rearward balance point.
Figure 5 - Effect of Balance Point Location
BUSTED - One point for David
LIGHTWEIGHT WHEELS In recent years, lightweight speed wheels have become readily available. These wheels are weight-reduced by removing excess plastic from the interior of the wheel.
Reducing the weight of the wheel reduces wheel inertia - the tendency of a wheel to not spin. So, lighter wheels should provide a faster start for a car.
In fact, this has been proven to be true. In Volume 7, Issue 3 (October 31, 2007) at www.maximum-velocity.com/pinewood_derby_times_v7_i3.htm a wheel weight experiment was conducted. The results of the experiment show that reducing the weight of the wheels (and adding that weight to the car) does improve performance (see Figure 6).
Figure 6 - Effect of Wheel Weight on Performance
CONFIRMED - One point for David
AMOUNT OF WEIGHT While Sir Isaac Newton is credited with the "discovery" of gravity, it was Galileo that showed that the mass of the object did not affect the acceleration of the object due to gravity (true in a vacuum). However, on a pinewood derby track there are other factors at work. Most pinewood derby tracks today have a sloped section (where gravity accelerates the car), and a flat section. On the flat section, the momentum of the car, aerodynamics, and friction are among the factors that affect how fast the car slows down. Momentum is based on speed and mass, the heavier the mass, the greater the momentum. So, heavier cars tend to hold their speed for a longer period of time, and thus tend to be faster.
In Volume 3, Issue 14 (March 31, 2004)(1) a car weight experiment was conducted. The results of the experiment show that increased weight improves the performance of the car (see Figure 7). A similar test with weight beyond five ounces was documented in Volume 7, Issue 5 (November 28, 2007) at www.maximum-velocity.com/pinewood_derby_times_v7_i5.htm
Figure 7 - Weight versus Performance
CONFIRMED - One point for Johnny
CONCLUSION Well the debate between Johnny and David ends in a five to five tie. But now that you have the facts, you will be better armed for your next race (or pinewood derby debate). Good luck, and may the best car win!