with Prudhomme and Austin Coil
By Sam Logan:
“In my heyday, to deliver the perfect drag racing run we raced a very fine line,” asserts Don Garlits. “If you had the exact right size tire for the weight of the vehicle and the clutch set exactly right for the horsepower of the vehicle and a track surface compatible with the combination, you could make a perfect run. But if the tire is too big it will shake. If the tire is too small it will spin. If the clutch is set too tight, it can shake. If the clutch is set too loose, it will over-rev the engine. If engine power is excessive it will spin the tires. If engine power is insufficient it will shake the tires. If the vehicle is too light it will spin the tires. If it is too heavy it will shake the tires.”
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What if you had a little more wheel speed than ground speed?
“A tiny bit of tire spin used to be okay. In my day we had access to 3,500 to 4,000hp but today if you go out and spin the tires you lose the race. Back then it was okay to see a little puff of blue at about 300 or 400ft because you knew the clutch had locked up and that little bit of blue told you the tires had caught up with you and you were now one-to-one. On this formula, the 392 engine worked the best. When we saw it pop a little blue out there at 400 or 500ft that son-of-a-gun was on song!”
Where was the effect of tire-shake most prominent?
“The shake took place just off the starting line. The car might have been trying to get up on the tire but didn’t have enough power to do it because the tire was too big for the prevailing horsepower and the clutch a little too aggressive. Had the clutch been a little bit looser or if a little more horsepower were available or had we used a little smaller tire, it wouldn’t have shook. Today with computer-aided data it seems closer to becoming an exact science.”
Says Don Prudhomme, “The biggest thing I learned when computers entered the racing scene was how detrimental a slight loss in engine speed could be. Usually due to over aggressive clutch clamping action, even a slight decline in engine speed of two or three hundred revs had an adverse effect. It was imperative to keep the revs up or the tire would likely walk over itself.
“In today’s racing the engine seldom goes one-to-one with the tire and if it does it’s down near the finish line. The tires spin for the entire quarter mile distance—but it’s a controlled spin. If the track surface is clean you can see the marks of evidence all the way. If it doesn’t have enough spin it will shake the tires.”
Curiously, traction control hasn’t yet evolved into the performance tool in drag racing that has made it so conspicuous in other motor racing disciplines. Think Moto GP, for example, where until the regulations changed at the end of 2015, racers released the clutch with fully open throttle, launched the bike and let the launch control program within the ECU smooth out everything else.
Austin Coil, the man most attributed for impelling John Force to meteoric success, has lobbied the NHRA since around 2003 to adopt traction control. “Honestly, I still don’t understand why we can’t have it. It would save a lot of money for all participants and it’s so easy to attain with modern electronics. I’m convinced beyond doubt it would produce better racing, but regrettably our main sanctioning body is vehemently opposed to such closed-loop control systems.”
He goes on to say that most of his fuel-car crew chief colleagues ascertain driveshaft speed at one second after the car leaves the start line. They plan their approach around this usually subtracting 20 degrees of ignition lead for a mere tenth of a second to calm the car. “Now, let’s say you had access to a properly adjustable traction control system, you would set the drive shaft speed to what you know would work for the conditions and the computer would take care of it by modulating the clutch or ignition lead or brakes or whatever restrictive source you desire. If you wanted the driveshaft to turn at 2,800rpm after one second, that’s exactly what you’d achieve and the incidents of destructive tire shake would be greatly reduced.
“Sometimes when you shake, it breaks the frame or the rear end. Sometimes it causes you to smoke the tires, over-rev the motor and blow the supercharger off. Often it results in a $20,000 expense. Adopting traction control is worthy of serious consideration.”
Likewise, from the Pro Stock and Pro Mod scene to the Saturday night racer, Ram Clutches’ Pat Norcia has been providing useful advice on how to avoid tire shake where possible for thirty years. Astute racers perceive the car as an entire package, which includes the tires, suspension, gearing (both transmission and rear end), and then decide how much clutch clamping force is required to propel the load forward.
Beyond the car’s necessities, track conditions have to be ascertained: the track temperature and atmospheric conditions—the air quality. The better these elements can be understood as a unit, the better the performance.
“Ideally the tires should be operating in a controlled spin through first gear, says Norcia. “If the car starts off too slowly and the tire is glued to the race track, the centrifugal action of the clutch exerts control at high rpm and because it has slipped too long and the car’s not moving sufficiently, tire shake is likely.
“Alternatively, if wheelie bars engage the track immediately and the tires spin out of control usually they go into tire shake. But probably the most common cause of tire shake is excessive launch rpm, improper clutch clamping force or not enough first-gear ratio.”
A first gear ratio that is too high numerically produces a mechanical advantage over the tire. For example, if you release the clutch pedal and the tires begin spinning and cannot gain traction—they cannot hook to the track—tire-shake is the likely consequence. This event is easily recognized in the acquired data, which will show the clutch locked and the car accelerated through first gear unusually quickly—in a time period that is too short.
“Most crew chiefs know how long the car should spend in first gear and they will tune the car with this in mind,” adds Pat Norcia. “For example, a 2.40:1 first gear ratio will require more clutch clamping force than say a 2.60. Conversely, if you were competing in poorer conditions, for example at a higher altitude, like Las Vegas, a 2.70 or 2.75 gear might be used. In this case as the first gear ratio is lower (numerically higher) less clutch clamping force is required.”
Crew chiefs evaluate the track condition at the starting line. They ascertain the quality of the rubber on the line, the track temperature and often they’ll take a grip meter reading. The better the reading on the grip meter, the better the traction will be. Then after the first run they will gather the data for validation, checking how it compares with their initial evaluation.
The four-link system
The four-link dictates the way the chassis sets the rear tires on the race track. Its differing layout helps promote or suppress tire spin. “If the track is in top condition and the traction is good, you can induce wheel spin by skillful four-link adjustment. Conversely, if the track is mediocre, adopting a more downward angle of the lower bar can introduce a little more traction,” explains Ram Clutches’ Pat Norcia.
More traction can also be realized by using a wider spread (greater distance between top to bottom bar mounting centers) on the rear axle housing. Typically, the greater amount of horsepower available, the greater the spread of the four-link.
Wheel stands are inefficient at the launch in these classes and successful racers use that energy to move forward. The front suspension stroke on modern drag cars is relatively short, often no more than 1 inch during upward travel. More importantly, the extension rate of the front shocks is very stiff, which slows the lift of the front end and aids in maintaining traction of the rear tires.
The enduring challenge in racing has always resided in finding a competitive tune and the educational path can be a lot of fun—so too can be the prospects of winning.
Columbia, South Carolina