This chapter looks at muscle cars with rear suspension systems using coil springs with two upper and two lower control arms. These cars include GM intermediates built in 1964–1977 (A-Bodies), 1978–1988 (G-Bodies), 1994–1996 Impala SS cars (B-Bodies), and 1979–2004 Ford Mustangs (Fox-Bodies).
Rear-wheel-drive cars with rear coil springs and two upper and two lower control arms were designed to give a combination of maximum comfort, stability, and traction. Unfortunately for the performance enthusiast, stability and traction gave way to comfort.
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General Motors and Ford factory four-link suspensions are quite similar to a four-link style suspension in a tube-chassis racing car, except it is not adjustable and the control arms are not parallel from front to rear. The two upper control arms are angled with the two front mounts farther apart than the two rear mounts. The two lower control arms are angled–just the opposite with the two front mounts closer together than the two rear mounts. These opposing angles act like a track locator, keeping the rear end centered under the car.
With soft rubber bushings in each end of the control arms, a quiet suspension with a soft ride was achieved. These control arms are what hold the rear axle housing in the car. The front of the two lower control arms are attached to the frame rails and the rear of the two lower control arms are attached to the axle housing. The front of the two upper control arms are attached to a crossmember welded 90 degrees to the frame rails and the rear of the upper control arms are attached to the rear-end housing. When one of these cars launch it tries to roll the pinion upward and roll the rear end out of the car. The upper control arms keep this from happening. As it attempts to roll the pinion up, the upper control arms (which are attached to the crossmember) pull on the crossmember, preventing this from happening. Race cars with a lot of torque can and do flex this cross-member and eventually break it due to the flexing from repeated pulling and releasing. A piece of steel only flexes so many times before it eventually cracks and breaks.
GM factory 4-speed cars came equipped with triangulation braces. These bars should be added to any GM rear-wheel-drive car with rear coil springs, two upper control arms, and two lower control arms. These 4-speed bars attach to the same bolts used at the front mount of the upper control arms down to the same bolts used at the front mount of the lower control arms. These stiffeners were appropriately named 4-speed bars since cars equipped with 4-speed manual transmissions launched much harder than those with automatic transmissions. With today’s automatic transmissions and high-stall torque converters, an automatic-transmission car launches as quick and almost as hard as a manual-transmission car.
The 4-speed bars triangulated the two front upper control arm mounts with the frame attempting to eliminate the crossmember flex providing stiffness because of being attached to the frame. Although these 4-speed bars were better than none at all, they were just a bent piece of steel, which was not strong enough under race conditions. The factory 4-speed bars also had a slotted hole in one end for ease of installation that allowed for some crossmember movement.
An upgrade to consider is to replace the control arm rubber bushings with polyurethane bushings by Energy Suspension. These greatly improve the handling of a car, whether it be hard cornering or just aggressive driving. All bushings in the upper and lower control arms, plus the two in the axle housing itself, must be replaced—for a total of eight. For ease of replacement, remove one arm at a time, complete the polyurethane bushing installation, then reinstall that arm before moving on to the next arm.
Control Arm Upgrades
I have never seen an upper control arm boxed-in from the factory, but it would be a good idea for a performance application. Be careful not to allow the steel boxing plate to interfere with the suspension travel or movement.
Boxed factory control arms with polyurethane bushings are also formed from thin mild steel and not rigid enough for heavy-duty drag racing applications. When it’s time to upgrade, I suggest using tubular chrome-moly upper and lower control arms.
Using chrome-moly tubing and chrome-moly 3/4-inch adjusters, these arms have strength not found elsewhere. Even the heim joints are 3/4 inch for strength and they are bushed down to the bolt size needed. By having adjustable upper control arms, you are able to adjust pinion angle and chassis preload.
All GM cars respond well to no-hop bars, which get their name from the fact that they eliminate wheel hop by causing the chassis to lift and push down harder on the tires. By raising the rear of the upper control arm and changing its angle, the car plants the rear tire harder and eliminates wheel hop. If the upper and lower control arms are extended farther forward, they eventually intersect. This imaginary intersection point determines whether the car lifts or squats in the rear when leaving the line.
Stay away from the cast-iron no-hop bars that have been on the market for years. They worked well with yesterday’s low-horsepower cars, but they are too tall and can cause the upper control arm to hit the bottom of the trunk floor over the rear wheels on high-horsepower cars. By raising the rear of the upper control arm too far (shortening the imaginary intersection point to close to the rear end) the car hits the tire too hard. You can over power a tire causing it to wrap up like a rubber band. When it can’t wrap any more it unwraps (like a spring) allowing the tire to unload and lose traction.
Because of this relatively radical action, the taller no-hop bars work some of the time at some tracks, but not all of the time at all tracks. For maximum strength correct-height (shorter) no-hop bars are made from 3/4-inch-thick laser-cut plate steel, not cast iron. They also have two mount holes to provide different mount points and further adjustment, although most racers only use the bottom hole.
Using no-hop bars requires a different adjustable upper control arm, since the angle is different.
On some of the earlier GM 12-bolt rears, the upper mounts were 1½ inches farther apart than the later, more typical axles. In order to use the early rear you would have to make some offset upper control arms. With my no-hop bars and adjustable upper control arm sets, you can install one of the early 12-bolt rears into a later car. The upper control arm normally goes to the outside of the no-hop bar, but for this application the upper control arm may be moved to the inside of the no-hop bar, which is 3/4 inch different. Doing this on both sides makes up the difference necessary to complete the installation (3/4 inch + 3/4 inch = 1½ inches).
You must install no-hop bars at an angle because of the center section design. There is some slop in the hole that the locating ring of the no-hop bar locates to, where the upper bushing used to be. This is not a problem. When you tighten the bolt holding the no-hop bar, it is not able to move. If you want to decrease some of the slop, you can keep the steel sleeve around the outside of the bushing, cut it narrower (to match the width of the casting it came out of), and insert it from the opposite side before you bolt the flat plate in place. It sounds complicated, but it’s simple once you see the parts involved.
No-hop-bar suspensions should only be used in cars equipped with a transmission brake or cars leaving the starting line at or near engine idle. That rules out turbo cars that need to spool up the turbo to get the engine speed (RPM) up where the turbo makes good boost unless the car has a transbrake.
Without a transmission brake, leaving the line with the engine RPM raised very far above idle while holding the brakes causes the car to lift in the rear before launching. The car loses part of the full suspension travel needed to plant the tires as hard as possible. What happens is that the car has a harder time transferring weight from the front to the rear (pitch rotation) since the rear of the car has already risen (transferring weight to the front of the car). If you wish to leave the line above idle, you
can experiment with the RPM level at which your car starts to lift in the rear by raising the idle a couple hundred RPM at a time and checking to see that the car does not lift in the rear at all when you put the car into gear from neutral. Keep doing that until you reach either the RPM you want and the car has not risen in the rear, or until the car starts to lift in the rear, in which case you need to lower the RPM.
Tubular lower control arms should come cross drilled with tubes welded in place to mount a sway bar for non drag race applications (see Chapter 10). Sway bars should not be used in drag race applications. Fixed-length lower control arms are recommended for applications up to 450 hp. Once cars approach 600 hp, adjustable lower control arms are recommended.
Adjustable lower control arms allow the rear axle housing to be properly squared in the car’s chassis, which allows the car to roll more easily and therefore accelerate faster down the quartermile. They also allow the axle to be moved forward or rearward, which sometimes allows for a larger rear tire to be installed. If you move the car’s rear axle in either a forward or rearward direction, be sure to check your driveshaft for proper end play. A minimum of 3/4 inch to a maximum of 1 inch of driveshaft end play needs to be maintained.
A car with heim joints in both the upper and lower control arms needs the axle to stay perfectly centered under the car. Diagonal braces exist for the lower control arms which act like a track locater that keep the housing centered. The track locaters attach to the lower control arm (approximately halfway back) and angle toward the center section. They attach to clamps bolted around the axle tubes as close to the center section as possible.
In testing with cars pushing 1,000 hp or more, quarter-mile ETs were reduced by up to .3 second using this setup with the diagonal braces. The reason for this reduction in ET was that the rear axle in the car now stays in a straight line while the car moves down the track instead of swinging side-to-side under the car. This same track locator can be used on lower control arms with holes in them for rear sway bar mounting.
Ford Mustangs seem to respond better with what is referred to as lift bars. These lift bars work by lowering the rear mount point of the lower control arms. By contrast, GM cars respond better to raising the rear of the upper control arms using no-hop bars. From the factory, both the Ford and GM cars had different lengths and angles on their respective control arms and thus different imaginary intersection points.
Control Arm Adjustments
Once all four control arms are in place, it is time to adjust the upper control arms. Solid lower arms are built to the same length as the factory arms they replace. If you are using adjustable lower control arms they should be installed at the same length as the factory arms. If you wish to move the rear axle forward or back, both lower control arms must be set to the same length whether it be longer or shorter. If you discover that it is necessary to change them for alignment purposes, you need to reset the upper arms again. Do not install the diagonal track locater arms until all four control arms are set to the length you want.
If you are not using no-hop bars, both upper control arms should be adjusted to the same length as the factory arms they replace. If you are using no-hop bars you should remove one upper control arm at a time to install them.
To adjust the upper control arms and pinion angle, please refer to Chapter 3 under the heading “Pinion Angle” on page 28.
Now set the preload by shortening the passenger-side upper control arm and then locking those jamnuts into place. These cars usually drive to the right. Shortening the passenger-side upper control arm builds preload into the suspension and makes the car leave straight. For cars in the 400-hp range, I suggest one full turn (or six flats, as the adjuster has six sides). Higher-horsepower cars may need up to two turns (or 12 flats). Make a pass to evaluate the changes. If the car still turns toward the right, shorten the passenger-side upper control arm farther. If it turns to the left, lengthen the passenger side upper control arm. Make changes in one or two flat increments at a time. Too big of a change does not allow you to discover the best-possible setting.
It is preferred to use double-adjustable adjusters, meaning no bolts need to be removed to make adjustments. Just loosen the jamnuts to make adjustments as the adjuster has left-hand threads on one end and right-hand threads on the other end. Turning the adjuster is all that’s required to lengthen or shorten a given control arm. Be sure to look at the threads when loosening the jam-nuts so that you are not accidentally tightening them.
By comparison, single-adjustable arms require one end of the control arm to be removed to make adjustments. This makes the tuning process more difficult, especially with the upper control arms, since they’re difficult to access and can be hard to remove and reinstall.
As discussed in Chapter 3, the best way to be certain of what is happening is to mark the steering wheel with a white or contrasting stripe at 12 o’clock (with the front wheels straight) and videotape your launch. After reviewing the video it is very apparent whether the steering wheel is turned right or left to correct a less-than- desirable launch.
Cars without adjustable upper control arms need a stronger and/or taller spring on the rear passenger side than on the rear diver-side to set preload. Air bags can also accomplish this in cars with 400 hp or less. Normally, about 5 psi of air pressure in the driver-side air bag or no air bag at all on the driver’s side, and 25 psi of air in the passenger-side air bag is enough to make the car launch straight. If the car turns right upon launch, add air to the passenger-side air bag. If the car turns left upon launch, remove air from the passenger-side air bag.
Both of these solutions (using a stronger spring on the passenger’s side or using air bags) on four-link suspensions with non-adjustable rear upper control arms compensate for preload and partially get rid of wheel hop. But they leave no method of setting pinion angle. Cars with adjustable upper control arms should not use either of these methods, as the preload is controlled with the adjustable upper control arms. Cars with adjustable upper control arms should use springs with equal strength on both sides.
Cars with no-hop bars should lift in the rear. Cars with no-hop bars and adjustable upper control arms should use rear springs of equal length and strength.
A car that lifts in the rear should use taller and lighter weight springs in the rear in order to compensate for the fact that they are lighter in weight and in strength.
A car that has lighter-tension springs lift farther and faster than a car that has heavier-tension springs. As you remove 100 pounds from a lighter spring, it lifts farther and faster than removing 100 pounds from a heavier spring.
Never use ladder bars with a factory style four-link rear suspension. The upper and lower control arms (if extended forward) eventually reach an imaginary intersection point, which is the pivot point of the rear end. Ladder bars add a second pivot point (at the front of the ladder bar mount). The two opposing pivot points bind up the rear suspension. Therefore, they do get rid of wheel hop; however, the rear suspension becomes bound up and stiff and can’t lift or squat. This does not allow any weight transfer (pitch rotation).
If you stick with the factory style four-link set up and these discussed additional modifications, it will not be necessary to alter your original car to achieve runs well into the 7-second-range quarter-mile ET.
Written by Dick Miller and Posted with Permission of CarTechBooks