Last issue, we took a look at how the Christmas tree and the overall starting system works at a drag strip. Port-A-Tree’s Al Smyth went through the layout. Smyth also discussed how roll out and reaction time are intertwined. This time around, we’ll look at the car, and how you can work with (by swapping parts or during the build, staggering the front wheels) to change rollout. Check it out:
WHat parts of the vehicle can you change to influence the reaction time? As mentioned by Port-A-Tree, engine RPM, clutch or converter slippage, tire spin, engine performance, front tire diameter, suspension set up as well as staging techniques have an affect upon the overall reaction time. Even the location of something like the two-step button (steering wheel or a foot operated switch) can have an affect upon reaction times. One of the easiest mechanical things to change is the front tire diameter. Jerry Bickel points notes that tire roll out is critical: “There are only two ways to legally increase the roll out distance: Use larger diameter tires or stagger the front wheels. Race sanctioning organizations rulebooks allow a certain amount of wheel base 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, wheel base stagger increases the roll out distance by the same amount.”
Unless the car in question is a purpose built drag machine (dragster, altered, funny car, Pro Stocker, etc.), then 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 roll out. By increasing the size (diameter) of the front tire, the roll out 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 example or a dedicated drag race front tire, the roll out is increased. How much can it change? Take a look at the following chart from Goodyear (for the sake of comparison, non-dragster tires only are listed here):
Size Overall Diameter Overall Roll Out
23.0X5.0-15 23.0-inches 72.2-inches
24.0X5.0-15 24.0-inches 75.4-inches
25.0X4.5-15 25.0-inches 78.5-inches
26.0X4.5-15 26.0-inches 81.7-inches
27.0X4.5-15 27.0-inches 84.8-inches
28.0X4.5-15 28.0-inches 88.3-inches
It should be pointed out that the roll out measured above is the total roll out of the tire. In essence, that measurement is how far the tire moves in one complete revolution. The above chart does, however, go to show just how much roll out difference there really is between various sizes of tires. Port-A-Tree’s Al Smyth reminds of something people might not consider: “Roll out is actually two things – the diameter of the tire and the other is the time it takes to actually roll out”. It’s something you have to remember.
Running Starts & Red Lights…
The bottom line is, an increase in the front tire diameter provides the racer with greater insurance against viewing the red “eye” on the Christmas tree. In simple terms, the tall front tire will roll further without un-blocking the stage beam — effectively fooling the starting system. In addition, increased roll out allows the car to literally take a running start (albeit a short running start measured in inches) when the tree comes down.
Tire pressure can have an effect upon rollout, but plenty of racers have tested this tire pressure situation, 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 upon handling, especially at the big end of the dragstrip. Practically speaking, you’re probably better off playing with different diameter front tires.
When using this 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 foot brake cars) that do not have outstanding vehicle reaction times. If you drive right through the beams, then the advantages of more roll out are simply tossed out the window.
In the end, you may find that by “adjusting” the roll out, 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, not that much. A simple front tire swap can often spell the difference between going home early or enjoying the spoils of competition.