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Tech Tips – Suspension

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SUSPENSION


ARTICLE COURTESY OF GOODGUYS GOOD TIMES GAZETTE – FEBRUARY 2005 ISSUE (PAGE 166)

 BASIC IFS ALIGNMENT CAMBER REFERS TO THE LEAN OF THE TIRE AS VIEWED FROM THE FRONT.  POSITIVE CAMBER HAS THE TOP OF THE TIRE LEANED OUT.  WHICH LEADS TO BETTER STRAIGHT-LINE STABILITY.  EVER YOU CAR TRAILER WILL WANDER IF OVER LOADING HAS BENT THE AXLE TO WHERE THE CAMBER GOES NEGATIVE. POSITIVE CAMBER GAIN GEOMETRY INCREASES CORNERING ABILITY BY LEANING THE TIRE INTO A TURN LIKE A BICYCLE.  IT IS ACHIEVED BY HAVING THE UPPER ARM GOING DOWNHILL TOWARD THE CENTER OF THE CAR.  FRONT STEER CAMAROS (70-81.)  MUSTANG II, AND INDY CARS CLEARLY EXHIBIT THIS CONCEPT.

CARS WITH THE UPPER ARM GOING DOWNHILL TH THE BALL JOINT INCLUDE REAR STEER CAMAROS (67-69), NOVA (68-74), AND 64-72 CHEVELLES.  THE TIRE WILL LEAN AWAY FROM THE TURN, COSTING TIRE FOOTPRINT AND LIMITING THE ULTIMATE CORNERING ABILITY WHEN DRIVEN HARD.  THEY ARE OFTEN ALIGNED WITH NEGATIVE CAMBER TO COUNTERACT THIS DESIGN, AT THE EXPENSE OF FASTER TIRE WEAR.  PENSKE RACING PUT A TALLER IMPALA STATION WAGON/ CORVETTE SPINDLE IN THEIR TRANS AM CAMAROS DURING ’67-’69 IN ORDER TO GET POSITIVE CAMBER GAIN LONG WITH BIGGER DISC BRAKES.  A SPECIAL UPPER CONTROL ARM AND STEERING ARM MADE THE SWAP WORK.  SEVERAL COMPANIES MAKE A SIMILAR KIT TO PUT THE TALLER FRONT STEER CAMARO SPINDLE ON THE ’64-’72 CHEVELLES, SHOWING A 20% IMPROVEMENT IN SKID PAD NUMBERS.

SPINDLE SWAPPING FROM ONE INDEPENDENT SUSPENSION TO ANOTHER IS A PRACTICE THAT OFTEN LEADS TO TROUBLE.  THE BASIC ELEMENTS OF SUSPENSION DESIGN MUST BE CLEARLY UNDERSTOOD  BEFORE SUCH A THING IS ATTEMPTED.  PROPER CONSIDERATION OF ACKERMAN PRINCIPLE, CONTROL ARM ANGLES, AND ANTI-BUMP STEER GEOMETRY MUST BE MADE TO PERFORM A SUCCESSFUL SWAP. YOU NEED TO KNOW HOW TO LAY OUT THE INSTANT CENTERS IN ORDER TO DO ALL THAT.  EXCELLENT COMPUTER PROGRAMS EXIST, AS WELL AS BASIC DRAWINGS IN SUSPENSION DESIGN MANUALS.  DON’T THINK THAT ANY SPINDLE WILL FIT ANY SUSPENSION!

CASTER IS THE INCLINATION OF THE KINGPIN, OR LINE THROUGH THE BALL JOINTS AS VIEWED FROM THE SIDE.  THIS IS DONE BY HAVING THE UPPER PIVOT POINT SLIGHTLY BEHIND THE LOWER.  NOT BY SETTING THE ENTIRE CROSSMEMBER AT AN ANGLE.  THE IDEAL SITUATION HAS THE LOWER CONTROL ARM PIVOT AXIS LEVEL AT FINISHED RIDE HEIGHT AND RAKE, WITH THE UPPER ARM RAKED DOWN AT THE BACK FOR ANTI-DIVE (WHICH WE’LL GET INTO SHORTLY.)  MOST MODERN CARS USE ABOUT 1 1/2 DEGREES POSITIVE CASTER, WHICH USES THE WEIGHT OF THE CAR TO STRAIGHTEN THE WHEELS. MORE CASTER INCREASES THIS EFFECT, WHICH IS WHY BONNEVILLE CARS RUN 12-15 DEGREES POSITIVE CASTER.  MUSTANG II IFS IN PARTICULAR BENEFITS FROM 3-4 DEGREES CASTER FOR THE TIGHTER ROAD FEEL, WHEN YOU HAVE A POWER RACK TO COMPENSATE FOR THE GREATER EFFORT OF LIFTING MORE  WEIGHT.

MANY EARLY IFS CARS SUCH AS ’39-’57 CHEVY, ’49-’53 FORD, ’49-’54 MERCURY, AND ’39-’56 MOPAR WERE ALIGNED WITH NEGATIVE CASTER, WHICH USED THE WEIGHT OF THE CAR TO TURN THE WHEELS BEFORE POWER STEERING BECAME COMMON, BUT CAUSED A LOSS OF HIGH SPEED STABILITY.  TODAY’S ROADS, RADIAL TIRES, AND POWER ASSIST WILL NEED POSITIVE  CASTER FOR A BETTER FEEL, BUT DON’T EASILY GET THERE.  WE NEED TO MOVE THE UPPER PIVOT BACK ABOUT 1/2″ TO 1/4″ TO GET RIGHT.  SPECIAL UPPER ARMS FOR ’55-57 CHEVY ARE AVAILABLE, BUT THE OTHERS ARE TOUGHER.  THE FORDS AND MERCS CAN  HAVE THE UPPER ARMS “CHEATED” BACK BY ADVANCING THE UPPER ARM SHAFT IN THE BUSING, THUS MOVING THE PIVOT BACK.  THE MOPAR AND GM ARE TOUGHER, AND WILL REQUIRE MODIFICATION TO THE UPPER ARM MOUNT.  YOU WON’T BELIEVE THE DIFFERENCE IN DRIVABILITY – MAKE THE EFFORT TO GET POSITIVE CASTER!

TOE IN IS ACTUALLY USED TO KEEP THE WHEELS STRAIGHT AHEAD.  ANY STEERING SYSTEM HAS A  LITTLE PLAY, AND THE TOE IN EXISTS TO TAKE UP THAT TOLERANCE.  WHERE YOU GET INTO TROUBLE IS WHEN THE TOE IN CHANGES WITHOUT THE DRIVER’S PERMISSION.  THAT IS CALLED BUMPSTEER. WHEN VERTICAL WHEEL MOTION CREATES A TOE IN CHANGE.  IT IS NOT VIBRATION FELT IN THE  STEERING WHEEL, WHICH IS GENERALLY CAUSED BY LOOSE PARTS SUCH AS U-JOINTS OR LOWER COLUMN BEARING.

WE DON’T HAVE ROOM TO GET DEEP INTO BUMPSTEER HERE, BUT THE STEERING AND SUSPENSION GEOMETRIES MUST BE CAREFULLY COORDINATED, WIDENED OR NARROWED SUSPENSIONS, SPECIAL TIE ROD ENDS, AND CHANGED STEERING GEAR ARE WARNING SIGNS.  THE LAW OF MATHEMATICS AND GEOMETRY CAN BE IGNORED, BUT NOT CHANGED.  MANY EARLY MUSCLE CARS REALLY SUFFER IN THIS RESPECT.  THEY SIT GREAT AND RUN STRONG, BUT THE AVERAGE MODERN SMALL FAMILY CAR WILL EMBARRASS IT IN TERMS OF HANDLING AND BRAKING.  IN THEIR DEFENSE, MOST WERE DESIGNED IN THE ERA OF SKINNY NYLON TIRES WITH NO REAL GRIP OF THE ROAD.  MODERN RADIALS ADD ENOUGH FOOTPRINT THAT THE DESIGN FAULTS SHOW UP QUICKLY.

FOR EXAMPLE.  PUT A ’64-’66 MUSTANG ON A LIFT AND WATCH THE WHEELS TOE OUT ABOUT 2″ AT FULL SUSPENSION DROP!  ADD SOME STICKY 16″ RADIALS AND YOU WILL LEARN ALL ABOUT BUMPSTEER RIGHT NOW!  THE REAL PROBLEM COMES WHEN ONLY ONE WHEEL HITS A BUMP, CAUSING THE CAR TO  DART UNEXPECTEDLY.  MODERN RUBBER DEMANDS MODERN SUSPENSION DESIGN, EITHER THROUGH MODIFICATION OR COMPLETE REPLACEMENT.

ANTI-DIVE GEOMETRY ATTEMPTS TO PREVENT FORWARD WEIGHT TRANSFER UNDER HARD BRAKING.  THIS IS DONE, BY HAVING THE UPPER ARM PIVOT AXIS RAKED TO THE REAR.  IT SHOULD NEVER BE LEVEL!  AT LEAST 3 DEGREES IS REQUIRED.  WHILE EXCESSIVE ANTI-DIVE WILL CAUSE SOME STRANGE CAMBER ANGLES AT TIGHT STEERING ANGLES.  I SUSPECT THIS IS WHY THE C-6 CORVETTE DESIGN HAS BACKED OFF FROM THE RATHER RADICAL 12 DEGREE ANTI-DIVE IN THE C-4 CARS.  INDEPENDENT REAR SUSPENSION ACCOMPLISHES ANTI-SQUAT GEOMETRY WITH A LEVEL UPPER ARM AND THE LOWER ARM TILTED FORWARD.

ACKERMAN DESIGN COMES FROM AN AUSTRIAN CARRIAGE MAKER WHO REALIZED THAT THE INNER WHEEL HAS TO TURN IN A TIGHTER RADIUS THAN THE OUTER WHEEL IN ORDER TO TRACK PROPERLY IN A TURN. THIS IS ACCOMPLISHED BY DRAWING A LINE FROM THE CENTER OF THE REAR AXLE, THROUGH THE SPINDLE PIVOT, AND THEN HAVING THE OUTER TIE ROD END ON THAT LINE.  THAT PUTS A FRONT STEER TIE ROD OUTBOARD OF THE BALL JOINT, AND INSIDE THE BALL JOINT FOR REAR STEER.  THIS IS  ANOTHER REASON TO BE CAREFUL SWAPPING SPINDLES.  REVERSED ACKERMAN WILL CAUSE THE WHEELS TO FIGHT EACH OTHER IN A TURN, CAUSING SKIDDING AND LOSS OF CORNERING FORCE ALONG WITH ERRATIC HANDLING.  IT WAS SO BAD ON EARLY T-BUCKETS WITH REVERSED FORD SPINDLES AND SKINNY WIRE WHEELS, THAT MANY WHEELS FAILED UNDER HARD DRIVING.  REDESIGNED SPINDLES, STEERING ARMS, AND STRONGER WHEELS SOLVED THE PROBLEM.  SIMILAR PROBLEMS RESULT WHEN MUSTANG II SPINDLES ARE REVERSED TO GET A REAR STEER RACK.  SKINNY TIRES MASK THE PROBLEM THROUGH  MINIMAL ROAD GRIP, JUST AS IN THE EARLY MUSCLE CARS, BUT MORE TIRE STILL NEEDS BETTER GEOMETRY.

BE CAREFUL.  GET EDUCATED, OR GET HELP IF YOU FEEL THE NEED TO MODIFY A SUSPENSION DESIGN. IT CAN BE DONE, BUT IT’S PRETTY EASY TO HAVE A NEAT LOOKING DESIGN THAT IS OUT TO LUNCH FROM A SAFETY STANDPOINT.  WE WANT YOU TO GET YOU AND YOUR HOT ROD TO THE NEXT GOODGUYS IN ONE PIECE!


 

THE BEST REAR SUSPENSION WE HAVE FOUND

TOTAL COST INVOLVED (TCI) REAR 4-LINK.
SHOWN HERE FOR THE 55-57 CHEVY CAR.
MANY OTHER APPLICATIONS AVAILABLE.

55-57 Chevy Rear 4-Link