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
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.
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
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
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.
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
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