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Three Wheel Cars - Are They Faster?

At Maximum Velocity, our extended wheelbase car kits and blocks are drilled such that when installed, one of the front axles will be slightly raised above the other three axles. Several times during the pinewood derby season I get a call from a customer complaining that their kit was mis-drilled. I then explain to them that the raised wheel is part of the design and will make the car go faster.

It probably seems silly to build a four-wheeled car, but then lift one front wheel so the car rides on three wheels. Maybe it is silly, but the real questions is: Does it make the car faster? Today's article will explore this topic and provide some data that will answer this question.

WHERE IS THE ADVANTAGE?
How could a raised wheel provide an advantage? The initial thought is that since one less wheel is on the ground, the friction on the track (and between axle and wheel bore) would be reduced. However, this isn't true. The amount of friction present is based on the weight applied and the type of surface (coefficient of friction). The amount of surface area is not relevant. Since the weight of the car would be shifted to the three wheels on the ground, the frictional surfaces would be unchanged.

So where would the advantage come from? The advantage is threefold:

1. Quicker start - When the starting pin falls, the car must start rolling so some energy must go into starting the wheels spinning. By lifting one wheel, the amount of energy needed to start the wheels spinning is reduced by 1/4 (By the way, based on the same principle reducing the weight of the wheels will also provide a quicker start).

2. Alignment simplification - The alignment of only three wheels needs to be adjusted instead of four. Oftentimes, just the front 'steering wheel' must be adjusted.

3. Elimination of weak wheel - Oftentimes, one of the wheels will not spin as nicely as the others. By placing the weak wheel in the raised position, the weak performance won't affect the car.

SHOW ME THE PROOF

Okay, actions do speak louder than words, so let's look at some tests. We made several test runs using one car. Five runs were made with four wheels on the ground, then five more runs were made with the wheel raised. This sequence was then repeated. In addition, both BSA and Awana wheels/axles were tested. As seen in the data and graph, using a raised wheel gave a consistent advantage over four wheels on the ground.




BSAAvgDelta% Improved
4 wheels3.1183
3 wheels3.07060.04771.53%
AwanaAvgDelta% Improved
4 wheels3.0989
3 wheels3.03260.06632.14%

TIPS FOR RAISING A WHEEL
1. Make sure the center of gravity (the front-to-back balance point) of the car is behind the midpoint of the car. This ensures that the raised front wheel stays off the ground.

2. You can raise either front wheel (we always raise the left wheel, but that is just arbitrary, probably because I am right-handed!).

3. Axle holes - Drill the raised hole 1/16 inch higher than the other holes. Don't go higher or you will increase of the risk of the raised wheel climbing over the center lane guide.

4. Axle Slots - Use the Pro-Body Tool II to deepen the slot on one side. Another option is to angle one front axle to get a wheel off the ground. Avoid angling the axle so far that the wheel touches the car body.

5. The raised wheel must spin freely. At some point it will likely touch the center lane guide. If the wheel doesn't spin freely, it will act as a brake (bad news).

CONCLUSION
Especially when using axle slots, it is often quite difficult to get all four wheels to touch the ground evenly. In fact, many times people end up with three-wheeled cars by accident! Don't let your car design be accidental. Unless prohibited by the local rules, raise one front wheel to get a little extra dose of performance.

From Pinewood Derby Times Volume 4, Issue 14

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