02 03 Pinewood Derby Stories and Photos from Maximum Velocity 04 05 15 16 19 20 21 22 23 24 25 26 27 28 31 32 33

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.

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

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.

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.

A 32 foot aluminum Freedom Track was used with a Judge Timer. For
each run the car was staged in the left lane.

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-
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).

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 bushings. 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

Figure 3 - Experimental Results

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

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

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 ā€œIā€ 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.

Figure 5 - Two-Wheeled Car with Bushings

Read More at: Pinewood Derby Times Volume 7, Issue 14

A feature article is a regular part of the Pinewood Derby Times Newsletter. To subscribe to this free e-newsletter, please visit:

(C)2010, Maximum Velocity, Inc. All rights reserved.
35 36 37 38