Drag Racing with Jim Hand – Part 9: Reaction Time & Rollout
In an earlier part, we discussed reaction time, and indicated that it is measured from the green light switch closure until the front tires leave the starting line. That is technically correct. However, if your local track’s clocks indicate a perfect light as .500, the reaction time is measured from the closing of the switch for the last yellow light until the front tires leave the starting line. If your track measures a perfect light as .000, it’s timers are measuring from the green light activation until your front tires leave the starting line. Both measurements tell you the same information, and your task is to obtain the quickest and most consistent R.T. possible.
Recall that elapsed time and reaction time are totally separate entities, but are closely intertwined in drag racing. Your E.T. will stay the same on a particular run whether your R.T. is .550 or .900, while your R.T. can be .550 each run, but the E.T. may change. To win regularly in bracket racing, your E.T. must be consistent and predictable, regardless of whether it is 14 seconds or 18 seconds and your R.T. must be reasonably good on every run.
Reaction time is the result of several factors. First is how you “read” the lights – that is when you give your car the command to leave. The next consideration is where in relation to the starting line that your car begins its acceleration. Finally, the rate at which your car accelerates after it receives your “go” command is of interest. None of these is more important than the other, and all must be factored in when practicing your driving. We recommended that you always stage to the point where the “stage” light barely lights. This places your vehicle the furthest from the actual starting line, which helps E.T., but most importantly, it provides a positive line. It takes a given time for any vehicle to begin to move after you command it. Accordingly, if you stage at a different position each run, it will take a different amount of time to reach the starting line and your reaction time will vary each run. Additionally, your elapsed time will vary because your car has a different distance to travel before the starting line is reached, and this causes your car to be running at a different speed when it actually crosses the starting line.
The ideal starting signal for a driver is to see one light and go! That is the idea behind the various types of electronic driving aids. If enough electronic controls/delays can be added to your car, you can literally leave on the first yellow light. That eliminates having to guess when to leave in the light sequence. I do not favor the use of electronics for street type car racing, and do not plan to discuss it further. However, anything we can do to find an identical time to leave each run will make us more consistent drivers. For example, several years ago, I could obtain good R.T.’s by leaving in about the middle of the third light. Obviously, doing that consistently during bright daylight and darkness was very difficult at best. An analysis of my situation indicated that leaving at the beginning of the third light would provide much more consistent results. However, that would also cause red lights. If I could keep the wagon in the lights longer after I gave the go command, I could leave slightly earlier (beginning of the third yellow).
This is a good time to talk about “rollout”. Rollout is the actual distance your car must move from the staged position until the starting circuit for the E.T. timers is activated. Obviously, the staged position is a significant factor, but the front tire diameter also has a fairly large effect. If you have smaller diameter tires, you will need to drive slightly further forward to turn on the “stage” light, and the rear portion of your smaller tire will reach the starting line quicker when you “leave”. That would indicate that you would need to leave later on the light sequence to avoid redlighting. Conversely, if your front tires were larger diameter, the reverse is true, and you could leave earlier on the sequence. I took advantage of this rollout variance by switching from P205 X 75 to P225 X 75 front tires. Presto, I can now leave at the beginning of the third yellow light! Understand that actual rollout distance is unimportant, but the overall combination of staging position, rollout, and acceleration capability of your vehicle are vitally important in obtaining good and consistent R.T.’s.
A word about front tires. In my opinion, the use of the skinny race style front tires on a street vehicle is extremely dangerous, and does not provide any measurable advantage at the drag strip. These tires simply do not have the load carrying capability for a normal street vehicle, and they positively do not have the braking capability needed for an emergency stop. The slight weight loss they may provide is more than offset by the quicker ET provided by a larger diameter tire. Concerning rolling resistance supposedly reduced by such “race” tires, most are bias belted, and radials of any diameter and moderate width (75 to 80 aspect ratio) will provide as good or better rolling resistance. Radials can be inflated up to the maximum rated pressure (listed on the side of the tires) for racing, and this reduces the rolling resistance even further. An added disadvantage of small diameter front tires is that they tend to lower the front end which will always adversely effect rear wheel traction. The vehicle should set level or slightly lower in the rear for best weight transfer.
Readers, I need some help from you. I have almost completed a review of the subjects planned for this series. Please suggest specific subjects you would like discussed, or discussed in more detail. Also, any questions or comments about drag racing will be considered for publication. This series was intended to primarily benefit folks with little or no experience in drag racing, and for owners of stock or near stock vehicles. However, input from all readers interested in the sport will be considered. I am not qualified to respond to inquiries about driving aids such as delay boxes, crossover boxes, throttle stops, transmission brakes, etc., nor the use of nitrous oxide or super-chargers for performance enhancement, and will not attempt to answer same. I do have some limited comments about several of these subjects. NOS is a relatively easy and safe way of increasing performance providing that it is correctly installed and used, and the engine is never over revved! It is strongly recommended that a rev limiter always be used with NOS, and that it be set at a conservative RPM. Be aware that NOS is virtually useless in normal bracket racing because it makes the vehicle very inconsistent in E.T. The bracket racer who regularly loses the first or second round of eliminations and decides to install $600-800 of electronic driving aids usually becomes an occasional winner only until the better drivers are forced to do the same. After all drivers spend the same amount of money to become competitive, the order reverts to about what it was before the first guy installed the stuff. In other words, electronics generally does not make a good driver out of a so-so driver. That is why I would like to see electronic driving aids barred in all bracket classes for stock, street, and real street/strip vehicles, and I think the trend is finally heading that way.