Grand Prix Racing - The Science of Fast Pinewood Cars

# Are there advantages to lifting a wheel or two?

Lift one wheel, sure! But two?! Before the trade-offs are discussed, let's see how it's possible to lift one or two wheels.

Anyone who has tried to put four wheels on anything knows that one inevitably sticks up in the air. That's because it only takes three points for an object to rest on a flat surface. The fourth is not necessary. A car with one lifted wheel will have a wheel lifted that is farthest from its balance point. That will usually be a front wheel since most Grand Prix cars should be rear weighted to take advantage of a down-sloping track.

Lifting two wheels is a balancing act on diagonally opposite wheels. If the center of mass can be lowered beneath the axles, and placed on the diagonal line nearer the rear - that is, toward the side of rear wheel that is touching the track - it can work. Of course, the axles must be completely level and the wheels flat. There is still the problem of bobbing. Another way to lift two wheels is to wheelie using larger radius wheels that allow weight to dangle beneath them without letting the car body rub the lane median.

## What does raising a wheel do?

Wheels resting on the track are not supported by the car's axles. They do not contribute to axle drag. But they do resist spinning up because of their inertia. So raising any wheel reduces the inertial mass of the car but increases its axle friction as the wheel is now supported like the body by the axles.

If the amount of energy gained from reduced inertia is greater than the extra heat energy lost in friction, then lifting a wheel can make sense. "Can" is there because other factors like the length of the wheel base and the location of the center of mass, how level the axles are and how flat the wheels are can lead to bobbing or frequent median collisions which are worse yet.

So the question of lifting a wheel or two reduces to determining what the payoff might be and how to keep the car from bobbing: feasibility and know-how.

## The Payoff

By examining the energy relations and by running a race simulation it is shown that lifting a wheel is good and two is better on most tracks. The length of the track enters in because of the increased friction. Less rotational inertial means getting to the bottom of the ramp in less time, but higher axle friction means less coasting distance on the flat. A four on the floor car will eventually pass the raised wheelers if the track is long enough!

A race simulated using the RaceIt! program on a smooth transition between three identical cars, except for one and two raised wheels, showed these splits at the finish line:

```Effect of Lifting Wheels

Race Specification File: lifted.xml

========================= Race Standings =========================
Car Name       Time  Split Distance  Speed    Accel    Lane  Place
----------------     Starting Line    ----------------------------
Four  on Track 0.000      0.000        0.00   128.61     1      1
Three on Track 0.000      0.000        0.00   128.45     2      2
Two   on Track 0.000      0.000        0.00   128.29     3      3
----------------      Finish Line     ----------------------------
Two   on Track 2.870      0.000      154.29   -10.19     3      1
Three on Track 2.881      1.796      153.90   -10.02     2      2
Four  on Track 2.893      3.568      153.52    -9.85     1      3
```

These kinds of results can be seen in models as well.

Lifting a wheel should be faster for all Grand Prix cars and normal tracks!

## Good ol' American know-how

It is possible to lift one and even two wheels off of the track without bobbing and excessive median collisions as outlined at the top of this page. One front wheel can be lifted quite stabily if the distance between the axles is as far as possible and the center of mass is nearly over the rear axle.

Some report that these can become unstable near the end of tracks with high starting lines (about 4 feet). Evidently the increase in energy - about 40 ozin, thus speed - an extra 20 in/s, causes an increase in the force of collisions with bumps on the track and/or the lane median. This increase would be about 1/10, the same as the increase in momentum if the duration of the collision doesn't change. If the time of collision decreases, as one might expect in proportion with the greater speed, the force would increase even more, perhaps by 1/5, on the average. Is this enough to destabilize the car? That belongs to the realm of experiment.

I can not attest to the stability of diagonally lifted pairs, others assure me they work and win. If it is to be achieved, it would greatly help to lower the center of mass below the axles. Thus, a very thin car would be best. The two wheels touching the track would need to be very flat on their axles and on the track, especially the rear one. The center of mas should be placed as close as possible to the touching rear wheel. That means on the same side as that wheel on a diagonal line between the two wheels touching the track.

By cutting the front axle slot a bit wide, one can steer the touching wheel left or right while the hot glue is cooling. Adjust the "steering" by melting the glue a bit between tests. This kind of car must set on the starting line and run very straight.

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 Grand Prix Racing - The Science of Fast Pinewood Cars Copyright © 1997, 2004 by Michael Lastufka, All rights reserved worldwide.