TABLE OF CONTENTS
Rollout and reaction time —
more important than you might think
by Wayne Scraba
Rollout is a familiar word in drag racing. Even if you only occasionally race your car in brackets, it's a term that could save you countless red-lights. It also can improve the vehicle's elapsed time and help if you're late. Sound interesting? It is. Best of all, most rollout improvements are free of charge — or at least cheap.
How the system works
To chart your progress as you stage your car, corresponding lights are affixed to the Christmas Tree. The pre-stage bulb tells the driver that his or her car is about to enter the stage beam — which, in essence, is the dragstrip starting line. When the front tires of the car pass completely through the stage beam, the timers begin. If you start too soon, the evil red-light comes on and you go home early.
According to Jerry Bickel, "Increasing the rollout distance of a car gives the racer advantages that can make the difference between winning and losing. The added distance means that a little more time will elapse before the light beam reconnects. This makes it less likely that the driver will red-light, and the car gets a running start to cover the quarter-mile. The length of the track is also shortened slightly by the increased rollout distance.
"On the downside, increasing the rollout distance of a car will also increase your recorded reaction times. If your reaction times are good, you should increase the rollout distance as much as the rules allow." More on this topic later.
"No matter what class, the driver with a better reaction time has a better chance of winning the race. Reaction time, as a number on the e.t. slip, measures the driver and the vehicle performance at the starting line. The driver reacts first, with a foot off the clutch, a finger off the transbrake button, or a foot off the brake pedal. The response to this action is the vehicle reaction time, or rollout. Variables that influence reaction time include engine rpm, clutch or converter slippage, tire spin, engine performance, front tire diameter, suspension setup, staging technique, and any combination of the preceding.
"The movement of the tire out of the stage beam is what stops the reaction-time clock and starts the e.t. clock. The stage beam must be blocked by the vehicle tire at the beginning of the Tree sequence, and the beam must be unblocked the instant the green light comes on."
What parts of the vehicle can you change to influence reaction time? As mentioned by those at Port-A-Tree, engine rpm, clutch or converter slippage, tire spin, engine performance, front tire diameter, suspension setup, and staging techniques affect overall reaction time. One of the easiest mechanical things to change is the front tire diameter.
According to Jerry Bickel, tire rollout is critical. "There are only two ways to legally increase the rollout distance: Use larger diameter tires or stagger the front wheels," he said. "Race sanctioning organization rule books [NHRA's included] allow a certain amount of wheelbase stagger. This means that one tire (usually the right front) is positioned slightly behind the other. Because the light beam is broken by the front wheels at the starting line, wheelbase stagger increases the rollout distance by the same amount."
Unless the car in question is a purpose-built drag machine (dragster, altered, Funny Car, Pro Stocker, and the like), it's extremely difficult (if not impossible) to change the front tire stagger. In cases such as this, the best bet is to physically change the overall diameter of the front tires. As you can imagine, the front tire diameter and, to a certain degree, tire pressure play a serious role in determining rollout.
By increasing the size (diameter) of the front tire, rollout is increased. For example, most of today's street cars are fitted with low-profile tires on all four corners, and these tires are substantially shorter than an old-fashioned 78-aspect ratio tire. By adding a tall tire, such as a 78-series or a dedicated drag race front tire, rollout is increased.
How much can rollout change? Take a look at the chart below from Goodyear. For the sake of comparison, only non-dragster tires are listed.
Note that the rollout measured above is the total rollout of the tire. In essence, that measurement is how far the tire moves in one complete revolution. The chart does, however, show just how much rollout difference there is between various sizes of tires.
Running starts and red-lights
If front tire pressure is increased, the overall diameter of the tire is increased, which in turn increases rollout (at least in theory). Conversely, if the front tire pressure is reduced, the rollout decreases. I should point out, however that a number of racers have tested this theory, and when kept within safe levels, it's often difficult to see much change by raising or lowering tire pressure. You'll also find that excessively lowering the front tire pressure can have an adverse effect on handling, especially at the big end of the dragstrip. Practically speaking, you're probably better off playing with front tires of a different diameter.
When using a tire diameter advantage on the starting line, be absolutely positive that you bump your car slowly into the pre-stage and stage beams. This is called shallow staging — a process that works well with street-strip vehicles (or footbrake cars) that do not have outstanding vehicle reaction times. If you drive right through the beams, the advantages of more rollout are tossed out the window.
In the end, you may find that by "adjusting" the rollout, you will be capable of cutting better lights (i.e., reaction times decrease) while at the same time lowering the elapsed times (i.e., the car now has a running start).
The price? In the big picture, next to nothing. Down the road, we'll take a close look at varied (and precise) starting line rpm, "adjustable" travel transmission brake switches, and other reaction-time factors. In the meantime, think long and hard about rollout. A simple front tire swap can often spell the difference between going home early and enjoying the spoils of competition.