Pinewood Derby Stories and Photos from Maximum Velocity
Canted Axles Revisited
The term "canted" means setting an object 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.
When discussing canted axles, the following questions must be asked:
1. Is there an advantage to canted axles? We will attempt to determine this with an experiment.
2. Which type of canting (up or down) would provide the most benefit? (see Figure 1) Generally, up canting is chosen as the wheel to car body contact is eliminated, and the car body is lowered. However, the guide rail contact is increased with up canting.
3. What is the optimum canting angle? We will attempt to determine this with an experiment.
Figure 1 - Canting
PREVIOUS EXPERIMENT Back in December of 2003 I conducted an experiment measuring the effect of canted axles (see Pinewood Derby Times, Volume 3, Issue 6, "The Big Debates Part 4: Canted Axles", December 10, 2003). The results of that experiment were that, in general, up-canted axles were not beneficial. These results were supported by an experiment from another person, as well as from mathematical models.
Since then, I have had many discussions with various people regarding the results of this experiment versus the anecdotal support for canting. So I decided to rerun the experiment with some key changes. The main changes to the experimental method were:
Original experiment 1. Four wheels on ground,
2. Front and back canted equally,
3. Cant angle adjusted by using multiple axle holes (axles had to be pulled and then reinserted to try a different angle. Unfortunately, this introduced experimental error.
New experiment 1. Three wheels on the ground,
2. Front dominant wheel not canted, but shimmed to cause rail-riding,
3. Cant angle of rear axles synchronously adjusted with a mechanism (eliminates error introduced by pulling axles, and using multiple axle holes).
Equipment The main equipment for this experiment is a car with an adjustable rear cant. Figures 2 through 4 show this car.
Figure 2 - Full Car
Note the fork of brass protruding out of the car. The fork is held in place by a screw into an L-bracket mounted on the car. When the screw is loosened, the fork can be moved up or down, adjusting the cant angle.
Figure 3 - Undercarriage
The fork from Figure 1 is connected to two aluminum tubes, which are press fitted into a slot milled into the bottom of the car. As the fork is moved up and down, the aluminum tubes synchronously move up and down. The axles are pressed into the aluminum tubes. (Note that the chip on the upper left is cosmetic; it doesn't affect the mechanism).
Figure 4 - Side support
To force the aluminum tubing to move up and down at an angle, the end of each tube is supported by a washer. The screws hold the washer in place (I wasn't sure if epoxy alone would hold the washer). The washer has a close fit with the tubing, so it does limit the amount of cant that can be applied to approximately 6 degrees down or up.
To set the desired cant angle, a little trigonometry is used. First, the length of one aluminum rod is measured (L). Then the up-down offset of the center part of the rod is measured (O). With this information, the following formula provides the cant angle:
Angle = arcsin(O/L)
This can also be worked in reverse to calculate the desired offset:
O = sin(desired angle) x L
Additional equipment for the experiment includes:
- 32 Foot Aluminum Freedom Track, the track was leveled with a bubble level and shims.
- Rear weighted extended wheelbase wedge weighing five ounces, front- left wheel raised
- Max-V-Lube Graphite
- Ultralight Speed Wheels from DerbyWorx
- Speed Axles from Maximum Velocity
Procedure The car was first aligned to rail-ride (see Volume 8, Issue 4), and then a few break-in heats were run. Then the following sequence was followed.
After the heats were run, the high and low times were removed and the remaining heats averaged.
Results The results of the experiment are shown in Figure 5.
Figure 5 - Experiment Results
Note the time scale on the left is in milliseconds. The results indicate that there is a very small improvement in performance by up-canting the rear axles. However, note that this is not an absolute conclusion as the statistical variation of the heat times is greater than the 3 milliseconds absolute difference in times. So, there may actually be no real improvement.
CONCLUSIONS We can certainly say that accurately up-canting the rear wheels on a smooth aluminum track is not a disadvantage, and may provide a slight advantage. However, the benefit of canting on a rough track is unknown, but may provide a more noticeable advantage.