Clutch actuation and control is one primary cause of traction control. here's a tidbit from a mag.
Larsen keeps pages and pages of records (all on computer now), and he can tune the car using this data. When they could run all the nitro they wanted, the tune-up was kept within one tenth of one percent of a mapping equation the team developed and uses; it is now three tenths of one percent since the fuel percentage is no longer something that can be changed.
"This equation, which Ronnie Swearigen developed while he was on the team, uses multipliers for the atmospheric changes and multipliers for the mechanical changes," said Larsen. "I've adopted some additional things to it, but it is accurate enough now that we almost always have a baseline we can go to. After looking at the information, I then bring the mechanical side up to match what the atmosphere equation, or root number, equals. It's not difficult if you do the math, you just need to have that baseline to start with. This is an objective science, just like a satellite positioning system. If you know a base number from that, you can go back to that number and be at the exact same place you started."
Lance's years of experience has made Skuza one of the few who has not suffered through multiple fires in the last decade. Once the car is set up for the track, the chassis doesn't change, leaving it to the engine and clutch to adapt horsepower to the conditions of the moment. Of course, Funny Cars are maintenance-intensive beasts, and between rounds, the team replaces all bearings, exhaust valves, pistons, and rods, but not because the latter two wear out every pass; Larsen simply wants fresh, cool parts to make sure the torque specs are correct when the engine gets reassembled. The team has eight complete short-blocks and eight sets of heads ready to go when they arrive, plus spare parts. This prevents the crew from getting exhausted by rebuilding engines at the track all night.
"When the motor is right, it's simple to make consistent, living power; the clutch is the most critical part. Of course, this is the engine-the fuel volume, spark, those things are my deal; John Stewart handles the clutch."
JOHN STEWART: The Sorcerer
Like Skuza, John Stewart is a member of an elite club; at 18, he drove his family's Top Fuel car into the Cragar 5-Second Club for dragsters; and like Larsen, he had his own business for a while, but returned to the track and helped Shirley Muldowney win her unprecedented third World Championship in 1983. Today, Stewart is considered one of the best clutch tuners in the business, and Larsen is the first to admit that Stewart's job is the toughest.
"What he does really is a black science," says Larsen. "I can go through my notes, do the math, and be confident that I can make exactly what I need in terms of horsepower. John keeps records as well, but he has to know what every part of the track is like and how much power he can let the engine apply to the back tires without smoking them. It's not so much a guess as it is the knowledge of the racing surface itself, understanding how the tires will work on it, and adapting the clutch to make them work."
When talking to Stewart, who is pretty laid back, one gets the impression he indeed has to "read the signs" to figure out just what is going to happen inside with that fat clutch can behind the Hemi.
"The first variable is the track itself; how much rubber is down on it, how does it look," says Stewart. "You can see it with your eyes, feel it with your hand, and feel how much grease is coming out of the track from the heat; I hate the grease. If the track is hot, you will not be able to use as much clutch; if it is cool, you can put more horsepower to the rear tires."
The clutch is the bag of tricks on a fuel car. It is a stack of five discs with steel "floaters" in between each one, mounted to a hub with several tall stud "towers" around the parameter. The idea is to allow the clutch to absorb some of the power so that the car does not overpower the starting line and smoke the tires, then set the clutch to lock up as the car progresses down track. Between each pass, those smoldering discs need to be serviced, and the clutch man will install new floaters, three new discs, two older discs, and resurface the rest of the unit as the engine crew does its job.
Once Stewart decides what the track will take, two variables are set. One is the amount of weight added or subtracted to the primary "fingers" which are six little arms that apply pressure to the clutch at the initial launch. This is calculated via per-finger gram weight measured by half-nuts; to take three grams off (which is the weight of one half-nut) means that you will remove a total of 18 grams from the clutch. A fair total average would be approximately 90 grams total, or 15 grams per arm, with half-nuts added or removed, based on conditions.
Meanwhile, a group of pneumatic timers that will be activated by a micro-switch under the accelerator are set as well. These timers are attached to the clutch housing via pressure-fed air lines, allowing the throw-out bearing to transfer more and more horsepower to the output shaft and rear wheels by closing an air gap between the clutch discs. Secondary fingers handle this using the applied air pressure. In the case where a track is really hot or slippery, the clutch may be slipping to some extent during the entire run.
This technology has allowed drag racers to go from 0 to 320 mph through the rear tires in a quarter-mile; without the clutch slippage, the car would instantly spin the tires right off of the starting line. As the car progresses down the racetrack, more power is applied to the rear wheels via downforce and speed. John uses a metering block with a group of five air jets (like Holley carb jets), one for each line, that controls the amount of pressure that will go to the clutch during the run, while the timers are set so that they are actuated in microsecond increments. So as the car leaves the line, timer 1 will come in at .62 seconds, timer 2 at 1.48, timer 3 at 2.12, and so on, until the car is applying all 5,000-plus horsepower to the racing surface. Tire heat and growth is also critical to making this happen.
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