GARLITS TALKS TIRE SHAKE

WITH PRUD­HOMME, AUSTIN COIL, MON­GOO$E AND MORE

Drag Racer - - Contents - Text by Sam Lo­gan Photos by Vic Moore and as Cred­ited

With Prud­homme, Austin Coil, Mon­goo$e and More

“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 was too big it would shake. If the tire was too small it would spin. If the clutch was set too tight, it could shake. If the clutch was set too loose, it would over-rev the en­gine. If en­gine power was ex­ces­sive it would spin the tires. If en­gine power was in­suf­fi­cient it would shake the tires. If the ve­hi­cle was too light it would spin the tires. If it was too heavy it would shake the tires.”

What would hap­pen if there was 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 4,000 horse­power, 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 400 feet 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 500 feet 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 was 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 com­put­eraided data, it seems closer to be­com­ing an ex­act sci­ence.”

Ac­cord­ing to 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 200 or 300 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.”

You might con­clude that tire shake in drag rac­ing has been a con­stant threat, but not so.

“When the sport first started,” in­sists Tom “Mon­goo$e” McEwen, “both the race tracks and the tires were poor and tire shake was

//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.//

un­known. Ev­ery­one spun the tires and the clutches were ei­ther dis­en­gaged or en­gaged—they had no slip­page. Us­ing our throttle foot and the brake han­dle to con­trol tire spin, we drove with as much fi­nesse as we could. It was only when the tires and tracks im­proved that con­cerns with tire shake be­gan to emerge.”

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 of­ten at­trib­uted with 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.”

Coil goes on to say that most of his fuel-car crew chief col­leagues as­cer­tain drive­shaft speed at one sec­ond after the car leaves the

start­ing 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 drive­shaft 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 2,800 rpm after one sec­ond, that’s ex­actly what you’d achieve, and the in­ci­dents of de­struc­tive tire shake would be greatly re­duced.

Ac­cord­ing to Coil, “Some­times when you shake, it breaks the frame or the rearend. 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 30 years. As­tute rac­ers per­ceive the car as an en­tire pack­age, which in­cludes the tires, sus­pen­sion, gear­ing (both trans­mis­sion and rearend), 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 that the clutch locked and the car ac­cel­er­ated through first gear un­usu­ally quickly, in a time pe­riod that was 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 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 4-LINK SYS­TEM

The 4-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 4-link ad­just­ment. Con­versely, if the track is medi­ocre, adopt­ing a more down­ward an­gle of the lower bar can in­tro­duce a lit­tle more trac­tion,” ex­plains 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 4-link.

SHOCKS AND DAM­PERS

Wheel­stands are in­ef­fi­cient at 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 to 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 are the prospects of win­ning.

Photo by John DiBar­tolomeo. / LEFT. The multi-disc rac­ing clutch used in Pro Mod, Pro Ni­trous and Pro Stock classes has two ad­just­ments: base pres­sure and cen­trifu­gal pres­sure. The base pres­sure is a fixed pres­sure ap­plied to each clutch spring; cen­trifu­gal pres­sure is ad­justed by adding or sub­tract­ing 1-gram wash­ers to the clutch fin­gers.

/ ABOVE . Austin Coil ex­plains tire shake in sim­ple terms. It oc­curs for two rea­sons: first, too much tire spin; sec­ond, not enough tire spin. Also, angling the exit port of the head­ers back­ward rather than up­ward gen­er­ates more for­ward thrust.

/Around 40 years ago, the ad­vent of Zoomie head­ers was the lat­est in down­force sci­ence. They in­creased trac­tion to the rear tires. Today, that em­pha­sis has shifted to rear­ward-ex­it­ing head­ers, which are fa­vored for their po­ten­tial to re­duce tire shake. Photo by John DiBar­tolomeo.

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

/Us­ing the ear­li­est cen­trifu­gal clutch he could re­mem­ber, a Crow­er­glide, Tom “Mon­goo$e” McEwen worked closely with Ram’s founder, John Nor­cia, test­ing sin­tered iron clutch discs.

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

/First used in fuel cars in the ’60s, sin­tered iron is a pow­dered mix of dif­fer­ent met­als bonded to a steel clutch plate, and then baked in a fur­nace.

/Steve Matusek, en­gi­neer, racer and pro­pri­etor of Aeromotive, says he once had a race car that shook the tires when he changed gears. Photo cour­tesy of Steve Matusek.

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