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Garlits TalksTIRE SHAKE

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WITH PRUDHOMME, AUSTIN COIL, MONGOO$E AND MORE Text by Sam Logan Photos by Vic Moore and as Credited

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“IN MY HEYDAY, TODELIVER THEPERFECT DRAGRACING RUN WE

RACED A VERY FINE LINE,”ASSERTS DON GARLITS. “If youhad the exact right size tire forthe weight of the vehicle, andthe clutch set exactly right forthe horsepower of the vehicle,and a track surface compatiblewith the combination, you couldmake a perfect run. But if thetire was too big it would shake.If the tire was too small it wouldspin. If the clutch was set tootight, it could shake. If theclutch was set too loose, itwould over-rev the engine. Ifengine power was excessive itwould spin the tires. If enginepower was insufficient it wouldshake the tires. If the vehiclewas too light it would spin thetires. If it was too heavy it wouldshake the tires.”

What would happen if therewas a little more wheel speedthan ground speed?

“A tiny bit of tire spin used tobe okay. In my day, we hadaccess to 3,500 to 4,000horsepower, but today, if you goout and spin the tires, you losethe race. Back then it was okay tosee a little puff of blue at about300 or 400 feet because youknew the clutch had locked upand that little bit of blue told youthe tires had caught up with you,and you were now one to one. Onthis formula, the 392 engineworked the best. When we saw itpop a little blue out there at 400or 500 feet that son-of-a-gunwas on song!”

Where was the effect of tireshake most prominent?

“The shake took place just offthe starting line. The car mighthave been trying to get up on thetire, but didn’t have enoughpower to do it because the tirewas too big for the prevailinghorsepower, and the clutch was alittle too aggressive. Had theclutch been a little bit looser, or ifa little more horsepower wereavailable, or had we used a little

/ABOVE . Austin Coilexplains tire shake insimple terms. It occursfor two reasons: first,too much tire spin;second, not enoughtire spin. Also, anglingthe exit port of theheaders backwardrather than upwardgenerates moreforward thrust. Photoby John DiBartolomeo.

/ LEFT. The multi-discracing clutch used inPro Mod, Pro Nitrousand Pro Stock classeshas two adjustments:base pressure andcentrifugal pressure.The base pressure is afixed pressure appliedto each clutch spring;centrifugal pressure isadjusted by adding or subtracting 1-gramwashers to the clutch fingers.

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/Around 40years ago, theadvent of Zoomieheaders was thelatest indownforcescience. Theyincreasedtraction to therear tires. Today,that emphasishas shifted torearward-exitingheaders, whichare favored fortheir potential toreduce tire shake.Photo by JohnDiBartolomeo.

smaller tire, it wouldn’t haveshook. Today, with computer-aided data, it seems closer tobecoming an exact science.”

According to Don Prudhomme,“The biggest thing I learned whencomputers entered the racingscene was how detrimental aslight loss in engine speed couldbe. Usually due to over-aggressiveclutch clamping action, even aslight decline in engine speed of200 or 300 revs had an adverseeffect. It was imperative to keepthe revs up or the tire would likelywalk over itself.

“In today’s racing, the engineseldom goes one to one with thetire, and if it does, it’s down nearthe finish line. The tires spin forthe entire quarter-mile distance,but it’s a controlled spin. If thetrack surface is clean, you cansee the marks of evidence all theway. If it doesn’t have enoughspin, it will shake the tires.”

You might conclude that tireshake in drag racing has been aconstant threat, but not so.“When the sport first started,”insists Tom “Mongoo$e” McEwen,“both the race tracks and the tireswere poor and tire shake was

unknown. Everyone spun the tiresand the clutches were eitherdisengaged or engaged—they hadno slippage. Using our throttlefoot and the brake handle tocontrol tire spin, we drove with asmuch finesse as we could. It wasonly when the tires and tracksimproved that concerns with tireshake began to emerge.”

Curiously, traction controlhasn’t yet evolved into theperformance tool in drag racingthat has made it so conspicuousin other motor racing disciplines.Think Moto GP, for example,where until the regulationschanged at the end of 2015,racers released the clutch withfully open throttle, launched thebike and let the launch control

//In today’s racing, the engine seldom goes one to one withthe tire, and if it does, it’s down near the finish line. The tiresspin for the entire quarter-mile distance, but it’s a controlledspin. If the track surface is clean, you can see the marks ofevidence all the way. If it doesn’t have enough spin, it will shakethe tires.//

program within the ECU smoothout everything else.

Austin Coil, the man most oftenattributed with impelling JohnForce to meteoric success, haslobbied the NHRA since around2003 to adopt traction control.“Honestly, I still don’t understandwhy we can’t have it. It would savea lot of money for all participants,and it’s so easy to attain withmodern electronics. I’m convincedbeyond doubt it would producebetter racing, but regrettably, ourmain sanctioning body isvehemently opposed to suchclosed-loop control systems.”

Coil goes on to say that most ofhis fuel-car crew chief colleaguesascertain driveshaft speed at onesecond after the car leaves the

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starting line. They plan theirapproach around this, usuallysubtracting 20 degrees of ignitionlead for a mere tenth of a secondto calm the car. “Now, let’s say youhad access to a properlyadjustable traction-controlsystem, you would set thedriveshaft speed to what youknow would work for theconditions, and the computerwould take care of it bymodulating the clutch or ignitionlead or brakes or whateverrestrictive source you desire. If youwanted the driveshaft to turn at2,800 rpm after one second, that’sexactly what you’d achieve, andthe incidents of destructive tireshake would be greatly reduced.

According to Coil, “Sometimeswhen you shake, it breaks theframe or the rearend. Sometimesit causes you to smoke the tires,over-rev the motor and blow the

/When wheelie bars smack the track immediately, and the tires spin out of control, the resultis usually tire shake.

/Using the earliest centrifugalclutch he could remember, aCrowerglide, Tom “Mongoo$e”McEwen worked closely with Ram’sfounder, John Norcia, testingsintered iron clutch discs.

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“Most crew chiefs know howlong the car should spend in firstgear, and they will tune the carwith this in mind,” adds Norcia.“For example, a 2.40:1 first-gearratio will require more clutchclamping force than say a 2.60.Conversely, if you werecompeting in poorer conditions,for example, at a higher altitude,like Las Vegas, a 2.70 or 2.75 gearmight be used. In this case, as thefirst-gear ratio is lower[numerically higher] less clutchclamping force is required.”

Crew chiefs evaluate the trackcondition at the starting line. Theyascertain the quality of the rubber onthe line, the track temperature, andoften they’ll take a grip meterreading. The better the reading onthe grip meter, the better the tractionwill be. Then after the first run theywill gather the data for validation,checking how it compares with theirinitial evaluation.

THE 4-LINK SYSTEMThe 4-link dictates the way thechassis sets the rear tires on the

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SourceRAM Clutches803.788.6034Ramclutches.com

supercharger off. Often it resultsin a $20,000 expense. Adoptingtraction control is worthy ofserious consideration.”

Likewise, from the Pro Stockand Pro Mod scene to theSaturday night racer, RamClutches’ Pat Norcia has beenproviding useful advice on how toavoid tire shake where possiblefor 30 years. Astute racersperceive the car as an entirepackage, which includes the tires,suspension, gearing (bothtransmission and rearend), andthen decide how much clutchclamping force is required topropel the load forward.

Beyond the car’s necessities,track conditions have to beascertained: the tracktemperature and atmosphericconditions—the air quality. Thebetter these elements can beunderstood as a unit, the betterthe performance.

“Ideally, the tires should beoperating in a controlled spinthrough first gear, says Norcia. “Ifthe car starts off too slowly andthe tire is glued to the race track,the centrifugal action of theclutch exerts control at high rpm,and because it has slipped toolong and the car’s not movingsufficiently, tire shake is likely.

“Alternatively, if wheelie barsengage the track immediatelyand the tires spin out of control,usually they go into tire shake.But probably the most commoncause of tire shake is excessivelaunch rpm, improper clutchclamping force or not enoughfirst-gear ratio.”

A first-gear ratio that is too highnumerically produces a mechanicaladvantage over the tire. Forexample, if you release the clutchpedal and the tires begin spinningand cannot gain traction—theycannot hook to the track—tireshake is the likely consequence.This event is easily recognized inthe acquired data, which will showthat the clutch locked and the caraccelerated through first gearunusually quickly, in a time periodthat was too short.

race track. Its differing layout helpspromote or suppress tire spin. “If thetrack is in top condition and thetraction is good, you can inducewheel spin by skillful 4-linkadjustment. Conversely, if the trackis mediocre, adopting a moredownward angle of the lower barcan introduce a little more traction,”explains Norcia.

More traction can also be realizedby using a wider spread (greaterdistance between top to bottombar mounting centers) on the rearaxle housing. Typically, the greateramount of horsepower available,the greater the spread of the 4-link.

SHOCKS AND DAMPERSWheelstands are inefficient atlaunch in these classes, andsuccessful racers use that energy tomove forward. The front suspensionstroke on modern drag cars isrelatively short, often no more than1 inch during upward travel. Moreimportantly, the extension rate ofthe front shocks is very stiff, whichslows the lift of the front end andaids in maintaining traction to therear tires.

The enduring challenge in racing has always resided infinding a competitive tune, andthe educational path can be a lot of fun, so are the prospects of winning.

/Steve Matusek, engineer, racer andproprietor of Aeromotive, says he once had arace car that shook the tires when hechanged gears. Photo courtesy of Steve Matusek.

/First used in fuel cars in the ’60s, sinterediron is a powdered mix of different metalsbonded to a steel clutch plate, and thenbaked in a furnace.

/Crew chiefs gather to scrutinize trackcondition at the starting line, examine thequality of rubber on the line and check track temperature.

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