Grooved Pinewood Derby Axles Friction Myth
Most will agree that the number one enemy to having a fast Pinewood Derby car is friction. Friction is a force that acts to stop the movement of two touching things… and when a Pinewood Derby car races down the track there are a lot of things touching each other. This touching is from various components of your car rubbing another component.
Usually, the biggest source of friction is the inside of the wheel rubbing the axle. When the car is racing, the inside of the wheel bore is making surface contact with the axle shaft. As the wheel rolls against the axle shaft friction is created… a lot of it.
There are several alignment, preparation and lubrication techniques designed to reduce this particular friction as much as possible. There are many opinions as to the best way to approach this problem. Any number of them may be just as effective as the other. To read some of the most effective friction reducing speed tips see the Free Pinewood Derby Speed Tips offered by Derby Monkey
Our goal is to dispel one of the theories and expose the myth regarding the reduction of the friction cause by the Pinewood Derby wheel and axle contact.
MYTH: Grooved Pinewood Derby Axles Reduce Friction
Many believe that placing one or more grooves all the way around the axle shaft will make the car faster due to the reduction of friction. It is believed that because there is less wheel bore to axle shaft contact created by the grooves that the friction will be reduced. In reality… this is simply not the case.
This type of friction on our Pinewood Derby car is largely due to the weight of the car. Let’s say a 5oz car body pushes the car axles down on the lower inside of the wheel bores. Wherever the axle shafts touch the wheel bores is where all 5oz of the car body’s weight is concentrated. This weight and surface contact is the cause of the overwhelming majority of the car’s friction.
Now, let’s say we take the very same car and install grooved axles. When the car rolls the same 5oz body is pushing the grooved axles down onto the inside bores of the wheels. Since the axles are grooved there is no doubt that there is less surface contact between the wheels and axle shafts. However, all 5oz of body weight is just concentrated over a smaller area causing the same amount of friction as before. So the same amount of friction is just distributed over a smaller surface area than the larger surface area in the earlier set up.
The result is… the amount of overall friction is the same regardless if you have grooved pinewood derby axles or not. Now, there may be other benefits from grooved axles such as the groves holding graphite. But that is a story for another day.