Detroit Metro Soap Box Derby

DMSBD Tech Tips
Wheel & Lane Swap and Timer Differential
-by Ian Carsten

We use the wheel & lane swap and timer differential in an attempt to make soapbox derby racing as fair as possible. In the early days of derby, racers used whatever wheels they chose. But some wheels roll much easier than others. As a result, races were won or lost depending on wheel quality. Standard wheel sets were introduced to address this problem. This helped, but even standard wheels can vary too much. We swap wheels to eliminate the effects of wheel differences.

Just as some wheels can be faster, one of the lanes may be quicker. That can happen because the slope is slightly greater in one lane, particularly near the start. This can also happen when one lane has a rougher surface than the other making it slower. A car could win because it ran in the faster lane. We use the lane swap to eliminate lane dominance as a factor in derby racing.

Initially, judges at the finish line did their best to determine which car crossed first. Although it works if the margin between cars is large, a human judge can't see the small differences common in derby racing. And how could a judge determine the winner in the better of two runs when each of the two cars won one of their runs down the hill? That's why we determine wins with timers that record differences as small as 1/1,000 second.

The 2-4-2-Wheel Swap

In a wheel swap race your car always rolls on two wheels from one set and two wheels from another. If one of the original wheel sets were faster, separating them into two new sets changes that relationship. There are two ways wheels are swapped. The most common procedure is called the 2-4-2-wheel swap. The name comes from the two wheels that are swapped before the first phase, the four wheels exchanged between phases, and the return of two wheels at the end of the heat.

We'll illustrate this with an example. Since Sally and Joe have been paired to race each other, they've both put their cars on a support, such as a milk crate. That places the wheels above the ground so they can be changed easily. Joe and Sally randomly draw labeled ping-pong balls to determine which wheels to swap. Sally reaches into a coffee can held by the official and draws out a ball. Since the can was held up too high for Sally to see into, she doesn't know which wheel she's chosen until she gets the ball out of the can.

The four balls in the can are labeled RF, LF, RR, LR corresponding to the wheel positions right front, left front, right rear, left rear. Then Joe reaches in and draws the second ball. Let's suppose Sally drew RF and Joe picked LR. Sally pulls the retainer pins securing her right front and left rear wheels, removes the wheels, and hands them to Joe. He does the same, handing his RF and LR wheels to Sally. Sally puts Joe's two wheels on her car and he puts Sally's on his. Joe and Sally have used random selection to temporarily create two new wheel sets. One set stays in lane one and the other stays in lane two. Only the cars and drivers switch lanes.

We'll assume Sally is in lane 1 and Joe has lane 2. When they reach the finish line, the nose of Sally's car cuts the infrared light beam across her lane. That starts the timer. Joe's car cuts the beam across his lane .025 second later stopping the timer. So Sally has a .025-second advantage in phase one. This is what is meant by "timer differential."

The cars are loaded crosswise onto the return trailer high enough to change wheels. Sally removes all four wheels—the lane one set—from her car and hands them to Joe. He takes the lane two set from his car and gives them to Sally. Joe puts the lane one wheel set on his car while Sally installs the lane two set on hers. The cars and drivers return to the top of the hill and place their cars on the starting plates. This time Joe is in lane one and Sally is in lane two. Suppose Joe wins phase two with a timer differential of .023 second. Since Sally's .025-second win in phase one is .002 second faster, she wins the heat by .002 second.

Now the final part of the wheel swap takes place. Sally removes Joe's two wheels from her car and gives them back to him. He removes Sally's two wheels from his car and gives them back to her. Each racer returns to the pits with their own wheels. (Derby racers place their names on each of their wheels to identify them.)

The result described in this example is common and suggests both cars and driving skills are closely matched. It also suggests lane one and/or the lane one wheel set is slightly faster. But it doesn't matter. The beauty of the wheel & lane swap and timer differential is it removes wheel and lane differences and human judgment from the process. The race is as fair as we can make it.

2-4-2 Progressive Wheel Swap

Occasionally there are claims that some unscrupulous racers have selected sluggish wheels or "doctored" one or two wheels to slow them in an attempt to gain an unfair advantage against their opponents. Supposedly this involves intentionally bending one spindle upward or introducing crossbind that would normally slow a car. The sluggish wheels are placed on the altered car where they would bear less weight than the other wheels and not slow that car as much as a normally configured one. Some are skeptical that such a strategy would work as intended. Many tracks, however, use the progressive wheel swap to discourage such practices. Progressive wheel swaps are declared randomly. That makes playing games with the wheels too risky. Because if someone showed up with a "doctored" car and wheels during a progressive swap, the culprit's car would almost certainly be slower than its normally configured opponents.

With one exception, the progressive wheel swap is exactly like the conventional swap. The difference is, the final swap has each driver giving the two wheels from the set they started the heat with to the other driver. In the progressive swap, each driver returns to the pits with the wheel set the other driver started the heat with. In this way the wheel sets circulate from car to car within a division. For example, in a progressive wheel swap race, if Joe and Sally's heat were their first that day, Joe would return to the pits with Sally's wheels on his car and Sally's car would now have Joe's wheels.

The progressive swap can pose problems with drivers trying to keep track of their wheels during the course of a long race. Another problem is, a team that is eliminated early in a race may have to wait until the end to reclaim their wheels. Also, it sometimes happens that a team unintentionally leaves the race with another team's wheels.

A related article by Tex Finsterwald
Back to Tech Tips
______________________________________