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The function of coil springs is to support the vehicle at its desired height, to maintain proper alignment angles and to absorb road shock transmitted through the tires and shock absorbers.
Constant flexing in normal service gradually weakens the suspension system, which in turn allows the car to sag. Once this sag becomes pronounced, the entire car is affected. Steering components are forced out of position, altering steering linkage geometry. What has been lost from the original coil spring can only be completely restored by installation of new coil springs. This will restore the vehicle to the correct height.
Most steering linkages systems are designed so that the tie rod assemblies connect to the center link or rack at some angle. On these vehicles, with the steering linkage in front of the spindle, a sagging suspension will force the tie rod out-ward and cause a “toe out” condition (Figure 1). If the linkage is positioned behind the spindle on these vehicles, the outward force of the tie rods caused by the drop in suspension height will cause a “toe-in” condition (Figure 2).
The steering arm connection for vehicles equipped with rack and pinion steering have the same design characteristics and functions incorporated in parallelogram steering. Thus a change in vehicle height will have the same effect on toe change with either linkage system.
There are some linkage systems that are designed with all components approximately horizontal. A decrease in suspension height will cause these changes to be reversed, pulling the tie rods inward and produce the “toe-out” condition in (Figure 1) and the “toe-in” in (Figure 2).
Because “toe-in” has a great effect on tire wear, riding height is critical to tire life.
Incorrect riding height also affects the camber angle. As the vehicle begins to sag, the angles of the control arms will change. This change in suspension geometry and camber angles will greatly affect the lateral scuff or in-and-out movement of the tire as it rolls over road irregularities. The suspension system is designed to minimize this in-and out movement of the tire; however, the extreme lateral scuff caused by the change in geometry will cause premature tire wear (Figure 3).
A sagging suspension means that tires wear faster, some steering control and handling is lost, headlight aim is adversely affected, and vehicle appearance is marred.
Coil Spring Inspection
A quick overall visual inspection will detect any obvious sag from front to rear or from side to side. Upon visual inspection under the car, look for the two ends of the control arms out of level, damaged or worn rubber bumpers and shiny or worn spring coils. All indicate weak coil springs.
Before visual inspection or suspension height measurements can be accomplished, the vehicle must be on a level surface, tires must be full and there should be no passenger or luggage compartment load. Beginning at the rear bumper, jounce the car up and down several times, proceed to the front bumper and repeat, releasing during same cycle as rear jounce.
More accurate inspection will reveal less obvious side-to-side sag by measuring heights at specific points on each side of the suspension system. If bumper is bent or damaged, compare ground-to-fender measurements taken at center of wheel cutout in fender. Allowable height difference should be no more than for ground-to-bumper measurement.
For accurate front-to-rear comparison, measurement points should be checked against manufacturer’s recommendations for the specific model, after determining that the vehicle is properly unloaded of abnormal cargo.
If inspection reveals a weak coil spring on either side, both sides must be replaced. Never attempt to avoid replacement by hiding the effect of a weak spring with any kind of insert, block or so called stabilizer. An insert will raise the vehicle height, but at the expense of handling and comfort. An insert virtually disables the coil it separates, and eliminates its ability to absorb shock. Further, the insert concentrates the stresses on a single point of the coil rather than distributing these forces throughout the spring. This simply accelerates fatigue and failure of the spring. Overload and air shocks are equally unsuitable as a means for overcoming sagging coil springs. These devices are designed to maintain proper vehicle height in overloaded conditions only.
Installation Tips
Be sure the spring compressor is firmly seated on spring before fully compressing spring.
Do not put fingers between coils when handling compressed spring.
Note the compressed height of spring being removed in order to compress the new spring to that same height.
If the spring needs to be arced to be removed, lay on a flat surface and note the amount of arc to compress the new spring in a similar fashion.
Check the springs for driver’s side sticker after removing from box.
Check the spring seats for wear.
If spring has an epoxy coating on the outside, use a spring compressor (this will prevent damage to the coating)
Note the end configurations of the old spring in the location that they are installed so that the new spring may be installed in the same manner.
Check the spring insulators and replace if necessary.
Note the condition of shock absorbers and replace as required. Many times the shock must be removed to service the spring providing a good time to replace the shocks as well.
Check the condition of bushings and other related suspension components since replacement may be easier with spring removed.
Inspect control arm bumpers and replace as needed.
Check out all the steering and suspension parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on why you should replace coil springs, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photo courtesy of Wikimedia Commons.
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