Wednesday, January 30, 2008

Repair Guide: Rear Suspension

When the vehicle has a front engine and front drive, the rear wheels are usually suspended independently from a rear crossmember by arms that go to the back or trail the mounting points. This is called a trailing arm independent rear suspension. Wheel spindles are attached to the two trailing arms, which extend rearward from mounting points on the body where they are attached with rubber pivot bushings. A crossmember is welded between the two trailing arms just behind the pivot bushings. This crossmember, working with the two trailing arms, provides an anti-sway-type stabilization for the rear suspension.

Many trailing arm suspension designs use coil springs surrounding vertical shock absorbers. The shock absorbers are attached at points inboard of each spindle and extend upward to rubber isolated mounting points above the rear wheel wells in the body. Other systems use separately mounted shocks, springs, and sway bar.

Rigid Rear Suspension

The suspension system for rear drive vehicles must provide up-and-down wheel movement as well as provide the space for drive axles. For many years, American vehicles have used a design called the rigid rear suspension system to solve this problem. This suspension system is designed around the rear axle housing. There are two basic disadvantages to this system, however. First, because both rear wheels are connected rigidly to the axle housing, movement up or down by one wheel affects the other. Second, this design also has a large amount of unsprung weight.

There are two basic designs of rigid rear suspension systems. One uses a leaf spring to spring and locate the axle housing. The other uses a coil spring and some type of control arm assembly.

Two rubber bushed lower control arms mounted between the axle assembly and the frame maintain the fore-and-aft relationship of the axle assembly to the frame. Two rubber bushed upper arms control driving and braking torque and sideways movement of the axle assembly. The rigid axle holds the rear wheels in proper alignment.

The upper control arms are shorter than the lower arms, causing the rear axle housing to rock and tilt forward on compression. This rocking or tilting lowers the rear drive shaft to make possible the use of a lower tunnel in the rear floor pan area. The upper arms control drive forces and side sway.

The coil springs are located between brackets on the axle tube and spring seats in the frame. They are held in the brackets and spring seats by the weight of the car and by the shock absorbers, which limit axle movement during rebound. Ride control is provided by two shock absorbers angle mounted between brackets attached to the axle housing and the rear spring seats.

The rear axle housing is attached to leaf springs by U-bolts. The spring front eyes are attached to the frame at the front hangers through rubber bushings. The rear ends of the springs are attached to the frame by the use of shackles, which allow the spring to change its length while the vehicle is in motion. A stabilizer bar is used to control body sway. Control arms are not required with leaf springs.

Independent Rear Suspension

The high poor handling characteristics of the rigid rear suspension system has led to the development of a number of independent rear suspension designs for rear drive cars. Basically, these designs are the same as the short, long arm; MacPherson strut; and trailing arm systems described previously. The main difference is providing for the mounting of the differential and drive axle assembly. The suspension system shown below allows each rear wheel to move up and down independently of the other. Two large trailing lower control arms support the wheel assembly. The differential is mounted to the frame and is sprung weight. The movement of the control arms is controlled by coil springs and shock absorbers between the control arms and the frame.

Another type of rear independent suspension uses a MacPherson strut attached to a lower control arm to get independent action of each wheel.


1. My mechanic says my car needs ball joints. Please explain.

Ball joints are a part of your vehicle's suspension that connects the steering knuckles to the control arms. A ball joint is essentially a flexible ball and socket that allows the suspension to move and at the same time the wheels to steer. Cars and trucks without strut suspensions typically have four of them (one upper and one lower on each side). Cars and minivans with strut suspensions have only two (one lower ball joint on each side). Some front-wheel drive cars also have ball joints on the rear suspension.
Ball joint locations in front end

Like any other suspension component, ball joints eventually wear and become loose. Excessive play in the joint can affect wheel alignment and tire wear. Loose joints can also cause suspension noise (typically a "clunking" sound when hitting a bump).

WARNING: If a ball joint fails, the suspension can collapse causing a loss of control. So don't put off having a bad set of joints replaced.


Joints should be inspected before they're greased (since grease takes up some of the slack in the joint). Ball joints are pretty easy to check, but each type requires a different inspection procedure. Use the wrong procedure and you'll get misleading results. The procedure that needs to be used depends on the location and loading of the joint:

* LOWER LOAD CARRYING ball joints are found on front- and rear-wheel drive vehicles where the coil spring or torsion bar is on the lower control arm. You'll also find them on the rear suspension of 1985 & up FWD Buick, Cadillac, Pontiac & Oldsmobiles, too.

Joints with built-in wear indicators (most GM and Ford RWD cars, rear joints on the FWD GM cars, and GM RWD vans, S10 & S15 Blazer) must be checked with the full weight of the vehicle on the tires on the shop floor or on a drive-on style ramp -- not with the wheels up or the suspension supported by jack stands.

No measurements are required if a joint has a wear indicator because internal play is indicated by the position of the grease fitting boss. The boss protrudes about .050 inches on a new joint. As the joint wears, the boss recedes into the housing. The joint is considered "good" as long as you can see or feel the edge of the boss protruding from the housing. But if the top of the boss is flush or below the housing, it's time to replace the joint.

On lower load carrying ball joints without a wear indicator, the joint is checked in the unloaded condition with the wheel raised off the ground and the lower control arm supported by a jack stand. A dial indicator is then used to measure play in one of two directions: sideways (horizontal or radial play) or vertically (axial or up-and-down play). The direction to measure depends on the application (refer to a manual for the exact specs).

Sideways play is measured with the indicator positioned against the inside of the wheel rim near the joint. The wheel should be pushed in and out by hand to check sideways play, and lifted with no more than 25 lbs. of force to check vertical play. Many joints allow up to .250 in. of sideways (radial) play, but some allow no play or only .015 in. of play. Always refer to the vehicle manufacturer's specs.

Vertical play is measured with the dial indicator positioned against the knuckle stud nut (Ford & GM) or the joint housing (Chrysler). A joint that has more than .050 in. of vertical play doesn't necessary require replacement because the specs range from zero play to as much as .125 inch of play.

The most common mistake that's made here is to use too much pressure on a pry bar or to insert a pry bar between the control arm and knuckle rather than under the wheel. Pry hard enough and any joint may appear to be bad.

* LOWER FOLLOWER NONLOADED ball joints are found on two kinds of applications: RWD cars where the spring is over the upper control arm, and vehicles with MacPherson strut suspensions. On both applications the lower joint is checked with the wheel raised off the ground hanging free (no stand under the lower control arm). Rock the wheel in and out by hand. A good joint should show no movement.

One exception here is 1978-80 Omni & Horizon which allows up to .050 inch of sideways play. Another exception is Chrysler FWD minivans and FWD cars ('81 & up). On these applications, the lower joint has a wear indicator grease fitting. Joint play is checked with the wheels on the ground rather than raised. If the grease fitting can be twisted with your fingers, the joint needs to be replaced.

* UPPER LOAD CARRYING ball joints are found on vehicles where the spring or torsion bar is on the upper control arm. Like the lower follower nonloaded ball joints, the upper joints are checked in the unloaded condition with the wheels off the ground -- but with a wedge or block between the frame and upper control arm to support the upper arm. On most applications, any movement calls for replacement. But on some Fords, up to .250 in. of radial play is allowed.

* UPPER FOLLOWER NONLOADED ball joints are also checked with the wheels off the ground but with the lower control arm supported. Any movement usually calls for replacement.


Any joint that exceeds the vehicle manufacturer's maximum allowable wear needs to be replaced. The greater the amount of wear, the greater the urgency to replace it.

Ball joints are often replaced in complete sets, or at least in matched pairs on both sides (both lowers or both uppers). This is because the joints on both sides of a vehicle usually have the same amount of wear. If one is bad, the other usually is too. Load carrying ball joints usually wear out before ones that don't carry a load, so it may only be necessary to replace the loaded joints instead of the complete set.

Replacing a set of ball joints requires separating the control arms from the steering knuckles, a job which can be difficult depending on the design and age of the vehicle. At the very least, it usually requires a special "ball joint fork" tool to loosen the ball joint stud from the knuckle. If this sounds like more of a a job than you want to tackle, let a professional do it the work.


2. I feel a high speed shimmy in the steering wheel. What's causing it?

A high speed shimmy is usually caused by a wheel that's out of balance or a bent wheel.
The first thing to check for would be a bent wheel. Raise the front of the vehicle off the ground and rotate each wheel by hand. If you see any sideways or in and out movement of the wheel, it is bent and needs to be replaced.

WARNING: Although some people claim they can straighten bent wheels, doing so is risky -- especially with aluminum alloy wheels. Replacement is the safest option (but also expensive).

If you don't see any sideways movement in the wheel, it doesn't necessarily mean the wheel is straight. There may be just enough sideways runout to cause a shimmy, but not enough to see. To find this kind of problem, you'll need a dial indicator. More than about .050 inch of sideways runout can be enough to cause a problem.

If the wheels seem to be straight, have the balance of both wheels checked (or rebalanced). If that fails to cure the shimmy, you may have some kind of tire problem due to defective belt alignment or tire construction. Other causes may include loose or improperly adjusted wheel bearings, insufficient caster alignment (check and readjust alignment as needed), or a worn steering damper (on trucks or other vehicles equipped with a steering stabilizer).

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