July 17, 2002

Step By Step

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Installing an adjustable clutch pedal stop can greatly improve starting line consistency. On this run I nailed a .527 and won the race, despite my opponent's .513 light.Photo by Nathan Varney
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The almighty Christmas tree. Learning how to attack this baby will win you some races.
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With most of the interior ripped out of our project Mustang, it's easy to get a good look at the clutch pedal. To improve reaction time we'll be building an adjustable clutch pedal stop. The purpose of a clutch pedal stop is to shorten or lengthen pedal travel, thus increasing or decreasing the amount of air gap between the clutch disc and the flywheel. Limiting pedal throw has an additional benefit: Stopping the clutch pedal before it reaches full extension can prevent the clutch cable from stretching.
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Our plan was to remove the pedal, build a stop, and also weld a reinforcement plate to the floor so the stop does not deflect the weak sheet-metal floor of the Mustang.
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Mike Rozman of Crazy Horse Racing (South Amboy, N.J.) cut this piece of 1/8-inch steel from a longer piece of stock. The "X" marks the spot where he will drill the hole.
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Rozman drilled the hole where the adjustment bolt will protrude.
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Next, we located the best spot for the pedal stop and cleaned the paint off the clutch pedal so we'd get a clean weld.
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The bracket was then welded to the pedal.
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You'd be surprised how much force you can apply with your leg. To prevent the stop from flexing we welded a pair of gussets to the back of the plate.
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The next step involved welding a nut to the back side of the bracket. Once the nut was welded we fed a bolt through and had a simple adjuster. The bolt will serve as the actual stop that will contact the firewall.

Good reaction times win races--that's a fact. Be it a heads-up Pro 5.0 battle at the Spring Break Shootout or a Saturday night bracket race, drag racing winners post excellent reaction times--as well as quick and consistent quarter-mile times. Reaction time is important, that's no big secret, but let's face it, not everyone is getting the job done.

Nailing good lights is the first part of winning any drag race. And while it may seem difficult to cut consistent lights, it's really not that hard. With a little work anyone can set up a car and be consistently on the tree. Building a winning reaction time average is not some magical secret, instead, all you need is an understanding of the variables involved. Once you master them you'll be mowing the tree down.

The factors affecting how well you "hit" the tree are based around two basic things: How quickly you react to the last amber (or the flash of amber lights on a pro tree), and how quickly your car reacts once you hit the gas (or release the transbrake or clutch pedal). While the human factor is rather simple to understand, the mechanical is not. It involves staging technique (deep or shallow), front tire size, rear tire type (flexible or rigid sidewall), clutch air gap, horsepower, gearing, vehicle weight and suspension type.

Before we get into the idiosyncrasies of cutting lights, you must understand how reaction time is calculated. When a car rolls up to the line to make a run, the front tires will eventually block the pre-stage beam. At this time the pre-stage bulb (on that side of the tree) will light up. Technically speaking, the only purpose for the pre-stage beam is to identify that the stage beam is just inches away. Once pre-staged the driver will inch forward until the front tires block out the staged beam. The stage bulb will then come on. At this point the front tires will be blocking the stage beam, indicating that the driver is staged and ready to launch.

In the next moment the starting system will be activated and the amber lights will come down (assuming that our driver is racing on a sportsman tree).

As you probably know, a perfect light is a .500, but some may not know why .500 is the significant number. Five tenths of a second, or .500, is the duration of time between when the last amber goes out and the green light comes on. (On a .400 pro-tree there is four tenths of a second between the amber and the green.) So, by anticipating the green by a predetermined amount of time, you can consistently launch in the exact moment after the green comes on. In most cases this will put you in the .510-.530 reaction time range.

Now, let's back up for a second. Your elapsed time does not begin until your front tires break the staged beam; however, if you waited until the green came on you'd be stone late. That's because it takes time for your brain to signal your leg or finger (in the case of a transbrake), and another moment for the car to physically react.

Therefore, as I've said previously, it is necessary to anticipate the green coming on in order for your front tires to leave the staged beam on time. If the front tires break the beam before the green comes on, a redlight will result.

Winning drivers have learned to use a signal, before the green comes on, to tell them to go. For years, racers have relied on the last amber on the tree as the "go" signal. By doing this, you will have one half second for your brain to kick in and for your car to move.

It's important to note at this time that all humans react differently, some quicker than others, and while you may think you can try harder and react quicker, this is not recommended. Since you want to use the last amber as your signal to go, it is recommended that you react as soon as you see the light. You never want to tune yourself because you will not be consistent. You simply want to train yourself to react to the signal. Becoming good at this takes mental focus; in fact, it really has nothing to do with handling a race car at all. It's really just a mental exercise; however, under the stress of competition focus is sometimes hard to find and maintain.

Rather than try to tune yourself, we suggest that you rely on your natural reaction time and instead tune the car. Guessing, or holding back to prevent a redlight, will make you very inconsistent. Instead, tune your car to move quicker or slower. Ways to make your car quicker include switching to smaller front tires (which will break the beams quicker than taller ones), launching harder or tightening up the front struts.

To slow your reaction time, you can lower front tire pressure, install taller front tires, launch lower or reduce engine power.

One thing that is often overlooked is the clutch pedal. The stick driver leaves the line when he releases the clutch pedal and so the pedal itself becomes an integral part of the equation.

By dumping the clutch, the stick driver connects the engine to the powertrain to drive the car forward. When he steps on the clutch he mechanically forces the clutch disc to be disengaged from the engine and transmission. This occurs by forcing the pressure ring on the pressure plate rearward until it is no longer applying pressure to the clutch disc.

Racers have learned the importance of measuring the distance between the clutch disc and the flywheel in order to fine tune the amount of release necessary for peak performance. This distance is called the air gap and by manipulating the amount of clutch release (or air gap) we can lengthen or shorten the time it takes to engage the clutch, therefore effectively increasing or decreasing reaction time.

To adjust air gap on a stick shift car it is necessary to fine tune the amount of pedal throw, as this controls how far the clutch cable will pull the clutch fork. Often, the best way to do this is by building a pedal stop that will limit the throw of the clutch pedal. With such a device you can adjust your air gap and tune reaction time without going inside the bellhousing. Building a clutch stop for your late-model Mustang also has an added benefit. By limiting the clutch throw you will not stretch the clutch cable, which is known to cause premature cable failure.In just a few hours we built a super-strong adjustable clutch pedal stop for Project Stocker. We removed the clutch pedal and fabricated a unit that works quite well.

However, if you don't have access to a welder that doesn't mean you can't make your own. We've seen clutch stops bolted to the clutch pedal and they are just as effective. Additionally, we've seen stops that limit the clutch fork, but this is not recommended because this setup may cause additional stress on the clutch cable. Our unit was welded to the clutch pedal and was built from a piece of steel, a simple bolt and a couple of nuts. Total cost was less than $10.

After installing the unit we noticed instant results. The clutch stop gave us a positive feel, much like the positive stops in an aftermarket shifter, but best of all, we could adjust air gap, which has helped us cut much better lights.