Tech Tip: The Three "S's" of Suspension

Tech Tip: The Three “S’s” of Suspension

In grade school you were supposed to learn the three "R's": reading, 'riting and 'rithmatic. Maybe you did well at these subjects - or maybe you didn't. You obviously learned the reading part otherwise you wouldn't be reading this. Reading, like any other skill, is something that takes time to master. The same goes for ....

By Larry Carley
Technical Editor

In grade school you were supposed to learn the three “R’s”: reading, ‘riting and ‘rithmatic. Maybe you did well at these subjects — or maybe you didn’t. You obviously learned the reading part otherwise you wouldn’t be reading this. Reading, like any other skill, is something that takes time to master. The same goes for learning how to “read” (recognize) certain suspension problems that may indicate a need to replace worn or broken parts. What we’re talking about here are the three “S’s” of suspension service: springs, shocks and struts — and how they affect the two “R’s”: ride height and ride control.

Springs are something that should never be overlooked because they’re the foundation upon which everything else rests. They support the vehicle’s weight, and in so doing affect ride quality, ride height, wheel alignment, handling and even braking. But some technicians don’t take the time to “read” the springs because their focus is directed elsewhere.

Springs need to be inspected every so often because all springs sag with age. From the moment a new set of springs is installed in a vehicle, the springs are engaged in a lifelong struggle against gravity. Each spring typically supports a constant load of anywhere from 600 to 1,200 lbs. This force is multiplied anytime the spring is compressed as the wheels pass over a bump or extra weight is added to the vehicle. Gravity eventually wins this struggle, causing the spring to slowly lose height as it sags under the ceaseless load. The result is a loss of ride height that has an adverse effect on wheel alignment as well as the vehicle’s ability to carry loads.

One thing spring sag will change is camber. A weak spring on one side of an independent front or rear suspension will decrease camber at the afflicted wheel. This, in turn, can cause shoulder wear on the inside edge of the tire, and possibly even a steering pull away from the side with the negative camber.

Another angle that will change as a result of spring sag is caster. Only an inch’s worth of sag at the rear axle may change front caster readings nearly a degree.

Wheel loading and traction can also suffer when there are weak springs. The wheel with the weak spring won’t carry as much weight as its companions and consequently won’t hold the road as well. This can encourage wheel spin and/or wheel hop during hard acceleration, or skidding when braking on slick or marginal road surfaces.

A vehicle with weak springs will also experience more body roll and sway when cornering, and rocking when braking and accelerating. This too will have an adverse effect on traction, handling and tire wear.

Though it usually takes years for spring sag to become noticeable, it may not take as long as you think. A 10-year-old vehicle will almost always have some measurable sag if you check ride height. But some vehicles that are only a few years old may show a loss of up to an inch or more of ride height, too. A lot depends on the quality of the heat treatment in the springs as well as the weight of the vehicle and the loading it has experienced. A weekend handyman can ruin a set of springs in short order overloading a minivan or SUV with a stack of drywall or pile of landscape timbers. Passenger cars are even more vulnerable because they’re not designed to haul heavy loads. Overload the trunk with bags of cement or patio blocks and the rear springs may develop a permanent squat.

At what point does sag become a problem? When a vehicle’s ride height no longer falls within the vehicle manufacturer’s specifications, the springs need attention. Ride height specs are published in alignment manuals and electronic databases for a purpose, so use them.

When you do an alignment, one of the first things you should always measure (after checking tire pressure) is ride height. In fact, it’s not a bad idea to measure ride height on any vehicle that’s in your service bay if you suspect the springs might be weak.

Measure the ground clearance from the specified reference points on the chassis and compare them against the specs listed in the book. Or, use a ride height measuring tool that compares the distance from the center of the wheel to the lip of the fender.

Measuring is the only way to tell for sure whether or not ride height is within acceptable limits. You can’t really eyeball ride height because it’s hard to see a loss of ride height or uneven ride height side-to-side or front-to-rear unless spring sag is really bad.

After measuring ride height, you should also visually inspect the springs for damage.

Check coil springs for broken coils, cracked or damaged spring mounts, the presence of spacers or wedges (a good indication of a previous attempt to correct sag), and surface damage (damaged coatings) or signs of contact (bright spots on the spring). Many springs are coated to protect them against corrosion. A nick or scrape in the coating can allow water and road salt to penetrate the protective barrier, creating a corrosive “hot spot” that may eventually cause the spring to snap. That’s why coated springs should be handled with care, and should only be installed using a spring compressor with soft-faced jaws or fingers.

Check leaf springs for broken leafs, broken eyelets, cracked, missing or deteriorated spring grommets, and loose or damaged spring shackles.

Check torsion bar anchors for severe rusting, cracking or looseness.

If the springs are sagging, they should probably be replaced. Spacers or inserts can be used to shim a weak spring and restore ride height, but it’s a “poor man’s” fix and should only be used if spring sag isn’t excessive. The best cure is to replace the old springs with new ones.

On vehicles with torsion bar suspensions, cranking up the bars a notch or two can compensate for a certain amount of sag. But go too far and you may succeed in overstressing an already overstressed bar causing it to fail at some point in the near future.

With sagging leaf springs, the “cheap fix” is to increase the length of the spring shackles, which often succeeds in bending the springs further out of shape. A better choice here would be to replace the springs and/or install helper springs, air springs or rubber springs between the axle and frame, or overload or air assist shocks. These same suspension aids can also be used with coil or torsion springs, too, to beef up the load carrying capability of the suspension.

Most importantly, if you replace the springs, replace the shocks or struts at the same time. New ride control can increase the life of the new springs. Also, match the ride control with the new springs. If you install lowering springs, install performance ride control units to take full advantage of the new parts.

Because the damping characteristics of shocks deteriorate gradually over time, the decline in ride control often passes unnoticed until someone (like you) points out the need for replacement.

Though worn shocks and struts are often found, many motorists aren’t convinced replacement is all that necessary — so they put it off. According to one consumer survey by a leading shock manufacturer, 70 percent of people think the primary function of shocks and struts is to provide a comfortable ride. Consequently, replacement is seen as a low priority. Only 21 percent of the people surveyed recognized the fact that new shocks and struts can improve handling and ride control.

Yet the most compelling reason for replacing shocks and struts, according to the survey (56 percent), is to improve vehicle safety, handling and control. The safety aspect of ride control usually doesn’t receive much attention because few people realize its importance. But the condition of the shocks and struts does affect driving safety.

Tests carried out by the Cologne Institute for Traffic Safety in Switzerland found that “marginal” shocks (50 percent less dampening ability than new shocks) increased the straight line stopping distance by 21 feet at 31 mph (a 23 percent increase!). This occurred because the worn shocks allowed the wheels to hop rather than maintain good contact with the road’s surface. In other tests, they found that braking while cornering on slick roads with worn shocks could make a vehicle lose control. So there are valid safety reasons for replacing worn shocks and struts.

Another reason to replace weak shocks and struts is to prolong tire life and the life of other suspension components. As we said earlier, uncontrolled suspension gyrations cause constant changes in camber and toe. This scrubs rubber off the tires and accelerates tire wear. It also increases motion and friction in the ball joints, tie rods and control arm bushings too, which over time adds up to wear.

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