Timed Pinewood Racing Considerations

Heat timing hardware is becoming more commonly available on pinewood derby tracks. Properly used, timing can provide excellent accuracy in identifying the fastest cars. Improperly used, it creates misunderstanding at best and severe error at worst. There are some pitfalls and misconceptions regarding timed racing that should be avoided. Here are issues that must be dealt with when considering timed racing for a competition:



For parents of competitors, few scenes are more disconcerting that to see the track operation team huddle up for two or three minutes and then continue racing without explanation. This is almost as bad as having to watch heat after heat with no indication of which car won or what is the progress of the racing plan.

The keys to resolution is communication. Three forms of communication are desirable:

Obviously Correct Operation?

Competitors and interested spectators should be able to tell if the conduct of competition is correct or not. This means that they should be allowed to see what the heat results are and to know how those results were interpreted and recorded.

Obviously, spectators can not tell the individual heat times just by watching the race. A typical race has the cars crossing the finish line at about 12 miles per hour. On the other hand, a casual observer could see that two cars finished neck and neck with about a 1/2 inch separating them. Correctly operating equipment should show a time difference for them of about 0.0025 seconds or about 0.005 seconds per inch of separation. This is an easy computation to estimate and gain a degree of confidence in the operation if they see that the times recorded have that relationship.

Similarly, should observers see that the heat winner is recorded with a time slower than the others, then they should suspect that something is wrong. On a 32 foot (28 feet start-to-finish) track, typical times are 2.4 to 2.6 seconds. Observers rightfully question a result in which the times are much less than 2.4 seconds or significantly slower than 2.6 seconds.

If information is kept hidden, then these important checks on operation are not possible and spectators have little recourse in understanding why the car that lost all of its heats received the 1st Place trophy. (Sounds impossible, but actually happened in a district's race!)

Heat Schedule

Meaningful comparison of racer times require that each racer run the same course! In Pinewood racing this means that each racer must be timed on each of the lanes the same number of times. Most race management programs produce such schedules, but may also have options to produce schedules based on other algorithms, such as Stearn's, which do not produce total lane equity.

Care is required.

Adherence to Heat Schedule

It is important that the cars actually race according to the schedule. In particular, they must occupy the assigned lanes for each heat in which they are assigned to race.

This issue is not unique to timed racing ... it is also important for points racing.

One of the real life problems with this issue is that after the heat has been run and the cars removed from the track, it may be difficult to determine which cars actually ran in the heat! One possible solution is to keep a synchronized heat photo finish record along with the heat times. (Displaying the image of the photo finish on a screen immediately following the heat is desirable, too!) Some race management software and hardware have a similar capability in which they show a section of the finish line area when the first car crosses the line, but may not manage to image all of the heat participants when competitors are severely mismatched. Those missing cars would only be significant if the wrong car were racing, since the really slow cars would be "also rans" anyway. There is a good alternative for photo-recording, but cost-effective off-the-shelf hardware is not presently available. That alternative is true photo-finish which is a still image of the finish line over time with each scan line representing about 0.0002 seconds (about 1/25 of an inch of car). The hardware to make this is available and the cost is almost right... but the right engineer is needed to bring it together and integrate it with race management software.

Track Consistency

An almost universally ignored issue is "track consistency". Everyone assumes that the track does not change during the competition. Yet it seems reasonable that over the course of an hour or two of racing the vibrations and bumps to which the track is subjected will cause the track to change enough to alter average car times by several milliseconds. Moreover, not all lanes are likely to be affected in the same way. One lane may get faster while another gets slower!

In racing within a typical pack, such changes would not affect the outcome. In the more competitive district and council racing, however, which tend to span longer times and be even more susceptible to variation, such effects would probably cause major swings in the race outcome.

One possible reason for ignoring the issue is that if such a track inconsistency is detected and observed to be large enough to swing the results, what can be done about it? How can the competition be adjusted or augmented to produce accurate results?

How to detect? One possible way is to run a small fleet of test cars a few times per lane before the competition start and again after the competition ends. Statisticians can tell us whether the two samples of times reflect the same population (the track is probably unchanged) or different populations (the track has probably changed). Cars for such tests should be of high quality (consistent times) and should be lubricated in a manner that has little or no degradation over the two sets of runs (liquid lubes.)

A byproduct of the tests is a measure of the magnitude of any track inconsistency. Comparing the test car changes to the time differences between adjacent competitors tells us if we have a probable incorrect result caused by the track. Only significant race places (the ones that get trophies, for instance) need be examined. If an inaccuracy is indicated, a short runoff might be held.

None of this is supported by any race management software that I'm aware of, but if one truly wants to make timed racing live up to its touted accuracy, he would nag the vendors until they come through with it.

Starting Gate Operation and Consistency

There are two aspects to the starting gate issue, both obvious to those who have watched timed racing run with a manually operated start gate. The first issue is false starts. A false start happens when the start gate is moved just enough to start the timer but not enough to release the cars. Then a fraction of a second later the start gate is opened. All of the cars are accorded times that are too long by that fraction of a second.

The second issue is consistent gate drop speed. If the gate drops so slowly that one or more cars stays in contact with the gate after it starts moving, then the gate is affecting times. If the gate drops at different speeds, due, perhaps, to a tiring gate operator, then the cars which are started by the tired operator will display slower times than those by the fresh, strong operator.

The "nightmare scenario" for manually operated gates is that the gate operator, if he were so inclined, could manipulate the race results in either of two ways: He could elevate a favored car by releasing the gate quickly when the favored car ran and releasing it slightly more slowly at other times. Or he could adversely affect a disfavored car (or cars) by releasing the gate slightly more slowly when the disfavored car(s) ran and quickly at other times. A slight slowing of the gate increases heat times. You wouldn't do it, but, since it can be done, you should protect yourself against such suspicion.

One solution suggested by some vendors is that the lead car starts the clock a few inches down track from the gate. The theory is that this takes the gate speed out of the timing equation. The theory is wrong... flat wrong. The speed of the gate affects the acceleration curve of the cars between the gate and the timer start sensor causing them to have different speeds when the sensor is triggered. Faster through the start sensor leads to lower heat times.

The best solution is to use a spring-loaded start gate which moves the gate away from the cars at a faster acceleration than the cars can have on that track's slope. For a 30 degree slope, that acceleration is approximately one half the acceleration of gravity. Fortunately, most track manufacturers make such gates available.

Equipment Malfunction

Some kinds of equipment malfunction causes incorrect times to be recorded. Some of these can be caught if finish line judges (human) are on duty to "sanity check" results by answering basic questions:

Few judges can perform that function well. In any case, training and testing is necessary.

Sufficient Precision

A few years ago, I heard some folks planning to "roll their own" race timing apparatus. They planned to incorporate inexpensive stop watches as the timing element. The problem is that these devices record with two decimal digit precision. Cars finishing 2 inches apart could be accorded the same time! A casual observer of the race could tell that it was clearly not a tie!

I also watched a council race finish line judge see the electronics award the race to one of the cars. The judge observed that the two cars recorded identical times with three decimal digits of precision showing. So he declared the race a tie and had the cars race again! The identical times indicate that the cars finished within about 0.2" of each other, a separation that almost all electronic finish lines can discern! Maybe there was a reason, or maybe it was written up in the "race procedures" document. Dunno.

The message is that most electronic timing circuitry are accurate well under 0.001 seconds. Some are accurate at 0.0002 seconds. Few are more accurate because of sensor variance. They could award a more precise time, but could not justify the implied accuracy.

Accuracy of Timed Race Methods

Some folks believe that the accuracy of timed racing is head and shoulders above the accuracy of all other methods. Aside from the mechanical and procedural issues discussed elsewhere, there is a source of inaccuracy in timed racing that should affect the racing plans. That is the effect of run-to-run variance for individual cars.

The effect of that variance is subtle. Lets look at it in more detail to try to understand what is the effect and what should be done to minimize the disturbance it causes.

Everyone knows that if you race your car in the same lane several times, the timer will show different results almost every time. The best cars will show smaller differences, but the differences are present none-the-less.

Another way to look at a PWD race is that it is an experiment in which each car's performance is sampled by noting the times for its runs. Sampling a car's performance by timing its runs is not like having a group of engineers measure the diameter of one bolt where the measurements will tend to be consistent to within the accuracy of the micrometer being used. Rather it is more like measuring samples of supposedly consistent bolts being produced by some manufacturing process. These samples will tend so show more differences, averaging around the bolt specifications. Similarly the time samples for various runs of the car will average around its performance characteristic.

The race schedule determines how many samples will be taken for each car and, therefore, how big a role the random nature of run to run variance will play in the results.

The way to mitigate the variance exhibited by each car is to average its time over several runs. The more runs that are averaged, the closer the average reflects the car's actual speed. But, how many are enough?

How many runs are enough? Well, it depends! It depends on how closely matched the competitors are. Suppose that the top two cars in the race are within 1/2" of each other at the finish line most of the time and that each car's several runs are within 1/2" of each other at the finish line (i.e. their heat times are within 0.0025 seconds of each other.) If each car runs twice, then the objectively slower car will win the first place trophy about 20% of the time. If each car runs four times, then the objectively slower car will win the first place trophy about 15% of the time. If each car runs six times, then the objectively slower car will win the first place trophy about 12-1/2% of the time. If each car runs eight times, then the objectively slower car will win the first place trophy about 11% of the time. If you can justify awarding the first place trophy to the wrong car 15% of the time, then in this circumstance, four runs are enough!

So, you plan for four total runs on race day. But what happens? The cars that actually enter the race are more closely matched than in the above example. The two fastest cars within 1/4" of each other. So, racing as planned, the objectively slower car will win the first place trophy about 29% of the time.

In the above analysis I used the word "percentages" to express the accuracy. In real life, we would not run the competition many times so as to observe that the slower car won so many percent of the time. What that percentage is telling us is the probability that the race will produce the undesirable result of the slower car winning the first place trophy.

Of course, the competitors don't know how error prone the process is. (At least, most of them don't know.) But I will know, and you should know!

The Attraction of Timed Racing

Why is timed racing attractive? Some thoughts, correct or not, come immediately to mind:
1. We can get "definitive" results with fewer runs, i.e. less time.
2. We can get more accurate results, i.e. the ranking produced by racing reflects the real performance of the cars.
3. We can nullify the differences between lanes.

Provided that the racing plan and equipment and track staff skill avoids the various pitfalls, some of these attractions can be realized. There are still trade-offs.

Definitive results and accurate results are not the same. Accuracy of results are degraded when the number of runs are reduced. A winner can be declared, but is the winner really the fastest car? Well, it was the fastest in the runs that were timed, but that may be the result of sampling error.

From studies using actual race times, competitor variances, and simulations we can know that for an equal number of runs, timed racing produces more accurate rankings than final standing points or quintuple elimination. This roughly means that for equal amounts of time racing, timed racing produces more accurate rankings ... but the heats per hour advantage of no-chart elimination racing offsets some of that advantage.

One in-depth study showed that the accuracy results from quintuple elimination and 4-heats-per-car timed racing are comparable in a district race setting (still assuming that the pitfalls are avoided.) In a pack setting in which the competitors are not so closely bunched, reducing the number of heats probably does not have as deleterious effect on accuracy.

One of the ways in which folks try to improve accuracy is to run in two stages. In the first ("preliminary") stage, everybody runs the same number of heats and the same number of times on each lane. Then some number of racers with the lowest cumulative times race again in "finals". Most plans that I've read show the same number of runs per car in the preliminary races as in the finals races. And, most discard the times from the preliminary races! There is no reason to believe that the finals races time samples are more representative of the cars than were the preliminary race time samples! The scheme flops!

The scheme can be saved either by increasing the number of heats in the finals or by including times from finals and preliminaries in computing the final ranking. To preserve accuracy, the number of finalists should be 1-1/2 to 2 times the number of trophies to be awarded.

What is Lost by Timed Racing

Timed Racing and other fixed schedule racing, reduce the number of heat winners as compared to adaptive methods like 3-lane Quintuple Elimination racing. The number of racers who win a heat during timed racing is in the 30-40 percent range, i.e. 60% to 70% end the racing day without even one heat win to their credit. Three-lane Quintuple Elimination racing, on the other hand, provides approximately 87% of the competitors with at least one heat win during the competition. Is this important? To most of the youngsters, it is.

Timed racing, as done by most, tends to reduce the total number of runs that each racer makes. There are two reasons. One is that the organizers are trying to keep the event duration as short as reasonable and the other is that the pace (heats per hour) for charted racing tends to be slower than for no-chart racing. Is the number of runs made important to the youngsters? For the ones with even moderately competitive cars, it is.

Last changed 12/19/2008
Copyright 2008 © by Stan Pope. All rights reserved.