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Pinewood Derby Stories and Photos from Maximum Velocity
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Top Fuel Cars - Two-Wheeled Cars
(The fourth in a series of articles on cars that "stretch the rules")
Everybody (at least anybody that has done any pinewood derby car research) knows that raising a front wheel (on a rear-weighted car will improve performance. This has been proven in many experiments, including one that we published back in 2005 ("Three-Wheeled Cars - Are They Faster?" - Volume 4, Issue 14 - April 6, 2005).
But can this technique be taken to the next step; that is, can a car successfully run with two wheels off the ground, and does this improve performance? Certainly, this has been tried by quite a few people, many of which have sent me ideas for implementing such a car. So I decided to give this a shot, and try several two-wheeled car techniques.
FUNDAMENTALS
First let's consider two basic principles of three-wheeled cars:
1. On a three-wheeled car, a front wheel can be lifted with success because the car is rear-weighted. Thus, the car sits firmly on the remaining three wheels (from geometry, only three points are required to define a plane).
2. Three-wheeled cars are generally faster because one-fourth of the wheel inertia is eliminated (leading to faster starts). However, some of this wheel inertia savings is given back every time the raised wheel contacts the guide rail. Thus, a "rail-riding" technique is often employed to prevent the raised wheel from contacting the guide rail (the car is purposely aligned such that the front dominant wheel steers towards the guide rail).
Applying principle one to a two-wheeled car, generally diagonally opposite wheels are lifted (1). But for the car to successfully balance on the two remaining wheels, the weight must be balanced symmetrically around an imaginary line connecting the center point of the two dominant wheels. This is easiest to do by center-weighting (placing the weight symmetrically about the center point of the car, but rear-weighting can also be accomplished by very careful weight placement (see Figure 1). In addition, for the car to rest without rocking back and forth, wider wheels must be used (no disc-type outlaw wheels).
Figure 1 - Bottom View of Weight Arrangement on a Possible Two-Wheeled Car
Regarding principle two, to eliminate raised wheel contact, both dominate wheels must be steered towards the guide rail. This excess contact may eliminate any advantage from the raised wheels. Thus, we will also try two additional options: 1) removing the raised wheels entirely, and 2) Replacement of the raised wheels with bushings (see Figure 2).
Figure 2 - Two-Wheeled Wizard Car with Bushings Installed
EXPERIMENT SETUP
The experiment used the following equipment:
- Pinewood Wizard body
- Speed Wheels with coned hubs
- Speed Axles from Maximum Velocity
- Krytox lube
- Two bushings - These consist of bent 'Awana' axles (0.092 OD smooth axles) with filed down heads, and #4 Flanged Nylon Bushings (hardware store item). I lubricated the bushings with a drop of Krytox 100.
Car Body
The Pinewood Wizard body was initially set up to weigh 5.0 ounces with the wheels and axles, and the car was center-weighted.
Wheels/Axles
The axles were lightly polished and then lubed with Krytox 100. The wheels/axles were installed on the car (four wheels on the ground), and the axle heads were marked at the 12:00 o'clock position.
Track
A 32 foot aluminum Freedom Track was used with a Judge Timer. For each run the car was staged in the left lane.
EXPERIMENT PROCEDURE
The car was first run five times for lube break-in. Then five runs were made with each configuration. With each configuration change, the weight was adjusted as needed to maintain five ounces, and the alignment was adjusted to be optimized for each configuration.
The sequence was as follows:
1. Four wheels on car, all on ground: center-weighted. Alignment dead-on.
2. Four wheels on car, left-front wheel lifted: rear-weighted (center-weighting not possible). Alignment set to steer left. This is the baseline configuration, as it is the most commonly used.
3. Four wheels on car, left-front and right-rear lifted: center-weighted. Alignment set to steer front-end left, and rear-end right (avoid contact with both raised wheels).
4. Two wheels on car: center weighted. Alignment set to steer front-end left, and rear-end right (to keep the car on the track).
5. Two wheels on car with bushings: Alignment set to slightly drift left (I tried dead-on, but achieved better performance with a left drift).
EXPERIMENT RESULTS
Figure 3 shows the results of the test. The worst configuration was the four-wheeled car with two wheels lifted. I believe this was due to the alignment necessary to avoid raised wheel contact. The best performance was achieved with the two-wheeled car with bushing. I believe this was also due to alignment. Since minimal contact with the bushings was not overly detrimental, the near dead-on alignment produced good results.
The two-wheeled car with no bushings is a conundrum. If the alignment is set for minimal contact with the wheels, the performance rivals the bushing car. However, with this alignment, the car can easily slew sideways on the track. So, to avoid a derailment, both wheels must have a prominent toe-in. This of course degrades performance.(2)
Note that all but one configuration was center-weighted. Using rear-weighting would likely improve performance for all of these configurations.
Figure 3 - Experimental Results
CONCLUSIONS
If your race rules allow, running a two-wheeled car with bushings might prove to not only be a novelty, but also a possible winner. Also, for novelties sake, a two-wheeled car with no bushings would certainly be interesting.
If you do build a two-wheeled car, please let me know how it turns out.
(1) A design does exist for lifting both wheels on one side of the car by counterbalancing the lifted wheels with a weight outrigger. This will be a topic for a future article.
(2) Previous to this experiment, I built a two-wheeled car with no bushings using the template in Figure 1 (I hadn't thought of bushings at the time). I adjusted the alignment to prevent derailment, and tested it thoroughly on my aluminum track (which has a braking section which drops down). However, the actual competition track had a braking section which ramped up. When the car hit the braking section ramp, the angled body caused the car to immediately rotate clockwise. This resulted in the car hitting the end stop at an angle. The front of the car broke off, leaving me with just a few days to make another car. Figure 4 shows a photo of the car before it was sanded and painted - unfortunately I didn't take a photo after it was painted.
Figure 4 - Two-Wheeled Car
ADDENDUM
I wrote this article during the summer of 2007. In March 2008, I decided to go with a two-wheeled car for our outlaw race on April 18. Having learned my lesson, I avoided the angled body design and went with a capital “I” design (Figure 5). It seems to track well and is quite fast. Since the race will occur after the last newsletter for the season, I'll provide the results in a newsletter in the fall.
