A Primer on Suspension Testing

Introduction

A safe vehicle must be able to stop and maneuver over a wide range of road conditions. To be able to stop and maneuver quickly there must always be good contact between the tires and the road. The suspension is responsible for keeping the tire firmly planted on the road. The suspension also provides a comfort buffer between the rough road and the passenger cabin.

There are many different types of suspensions. Each is a compromise between comfort, cost, and road holding. To meet these compromises for each vehicle, many types of suspensions have been developed. Each design shares two essential components­springs and shock absorbers.

Some vehicles have struts instead of shocks. In reality, a strut is just a shock absorber built into a suspension link. The strut is generally replaceable as one unit. In this discussion, any reference to shock absorbers will also apply to struts.

There are also some variations in the type of springs used on vehicles. There are coil springs, leaf springs, air springs, and torsion bars. Despite the differences in design, these variations all react with the suspension in the same manner. Any further reference made to springs will be general and includes any of these types.

The Spring

Springs isolate the driver from road imperfections by allowing the tire to move over a bump without drastically disturbing the chassis. If the chassis remains fairly steady then the tires are better able to follow road contours.

While springs do an excellent job of smoothing over bumps, they will keep bouncing once started. In other words, the chassis continues swaying and the tires keep hopping long after the vehicle strikes a bump. Left uncontrolled, springs give an uncomfortable ride with very poor tire to road contact. To control this undesirable behavior, a shock absorber keeps the spring from overreacting to every bump or dip and prevent excess movement of the tire and chassis.

Springs are durable items and are easily inspected. If the ride height of a vehicle has decreased excessively or a coil/leaf has broken it is advisable to replace the springs in axle sets. Consumers also often change springs to alter their vehicle's ride and handling characteristics. Spring problems are generally easy to identify.

The Shock Absorber

The shock controls spring motion by damping (absorbing) energy from the spring. A shock absorbs energy by forcing oil through valves whenever it is moved. It takes a lot of energy to push oil through the valves so when the spring is done pumping, it doesn't have much energy left to keep bouncing. Imagine running 50 yards on bare ground – you could do it and have plenty of energy to run back again. Now imagine running the same 50 yards through 6 inches of mud. How much energy would you have for a return pass?

Shocks also control the reaction of the body to road undulations. A stiffer shock tends to transmit more road irregularities to the driver but will also not pitch and roll as much as a vehicle with softer shocks. Thus shocks, like springs, can be changed to obtain a personalized ride.

Shock absorber condition is difficult to evaluate. Shocks are wear items, like air filters or tires, and do need to be replaced occasionally. The Car Care Council recommends inspection at 25,000 miles and every 6,000 miles thereafter. There are some exterior signs of a damaged shock that indicate a need for replacement, but frequently a shock absorber will have stopped working without any visible indicators.

What if the shock absorber is just worn out? It is not possible, simply by examining the shock, to tell if it is functioning. One visual indication of tired shocks is worn out and cupped tires – a very expensive diagnostic technique. The only way, up until recently, to check shock performance was a test drive or bumper jounce test – both of which are very subjective. In any case it was difficult to identify the worn part because all the shocks are working together.

All modern shock absorbers use oil to absorb energy. As the shock compresses and extends, oil must be forced through valves in the shock piston (see diagram). The valves provide resistance to the flow of fluid, which absorbs energy from the suspension. The valves let a different amount of fluid through at different shock piston speeds. That means that the shocks absorb different amounts of energy depending on how fast the suspension is moving. Thus, valves allow the suspension to provide a comfortable ride by controlling chassis movement (low piston speed) and, at the same time, provide firm contact with the road surface by controlling suspension movement (high piston speed).

There are two basic types of shock absorbers on the market today. All shocks are designed to provide comfort and safety but, like all products, some are better than others. Some shock absorber designs are better suited to handle rough road conditions without loosing effectiveness. Other differences in shock design center around the quality of components and how that affects the life expectancy of the shock.

Recently, this design has been updated by the use of low pressure nitrogen in the place of air. These are called “GAS” shocks. The nitrogen under pressure is much less prone to mixing with the oil and cause foaming. This is a significant improvement over the standard twin tube design.

The second basic type of shock absorber is called a mono­tube design. It is also a “GAS” shock because high pressure gas is used to account for volume changes inside the shock instead of a reservoir tube. The gas is pressurized to as high as 360 psi. When this type of shock is not installed, the piston rod will extend completely due to gas pressure. The gas is separated from the oil tube by a second, free floating piston. By not having air mixing with the oil, this type of shock eliminates the possibility of foaming. All the valves for mono­tube shocks are located in the piston.

There are additions to these basic designs on the market. Some vehicles have electronic controls that bypass some damping under different vehicle conditions or driver choice. Aftermarket shocks that have softer damping rates at normal travel but get firmer at extreme positions are becoming popular. These designs give a comfortable ride under most conditions but become firm when aggressive vehicle maneuvers are required. There are also very high performance shocks available that have multiple to infinite damping adjustments, allowing the vehicle owner to precisely tune suspension performance.

Why is it so important to keep good tire contact? All actions a vehicle performs – acceleration, cornering, and braking are transmitted through the tires. The tires perform all these actions through a phenomenon called friction. The most important rule of friction is that the more two objects are pressing together the higher the frictional forces between them. Therefore, it is necessary for the tire to be pressing firmly on the road at all times if the vehicle is going to brake and turn.

Even shocks that are only half as good as new can significantly impact stopping distances. The Cologne Institute for Traffic Safety tested a vehicle with partially worn shocks and then tested the same vehicle with new ones. The used shocks increased the vehicle stopping distance by 21 feet, a 23% increase! (See chart.) Twenty one feet is about 1 1/2 car lengths – more than enough distance to prevent serious injury and property damage.

The same rules apply to cornering. In an emergency situation you need as much contact between the tire and road as possible. Without maximum contact the vehicle might not be able to perform the emergency maneuver necessary to prevent injury and property damage.

The average age of vehicles on the road today is 8 1/2 years and is continuing to increase. Thus, an increasing number of vehicles are going to need shock service.

Unfortunately, customers do not often identify the need for shock replacement because the change has been so gradual that they have accustomed themselves to the degraded ride. Repair facilities can do their customers and themselves a service by identifying those worn safety components.

Shock and strut work is also very profitable. Modern Tire Dealer (1997) reports an average ticket of $169.00 at a profit of 45%. Those figures do not include the associated services that might be required, such as alignments after strut replacement.

How do you know when to replace shock absorbers? Traditional methods for determining shock replacement have always been very subjective. In other words, it was entirely left to the astuteness, biases, and preferences of the individual evaluating the shock.

The first reason for shock absorber replacement is physical damage. Damage is checked visually and could be:

• Pitted, bent, or broken rod
• Severely dented dust shield (such that shield restricts shock movement)
• Severe leaking from shock
• Worn or missing bushings (if not individually replaceable)
• Abnormally worn or cupped tires

While a few items on this list are easy to spot, others are much more difficult to quantify. Imagine a shock with oil coating it. This would seem a perfect candidate for replacement, but what if the oil was coming from somewhere else, like the power steering pump. Unless oil can be seen leaking from the shock itself, replacement is difficult to justify.

Because of the difficulty in determining the need for shock replacement with traditional methods there has long been a desire for accurate, repeatable, and scientific suspension and shock performance testing.

Until recently the technology was not available to test suspensions in a quick, user friendly fashion. There are two different types of suspension testing used today. The types are: low frequency, which tests the suspension's resistance to chassis motion; and high frequency,which measures the suspension's resistance to excessive wheel hop as well as chassis movement.

Low frequency techniques check the suspension reaction only up to 5 vibrations per second. The disadvantage of evaluating shock absorbers at single or low frequencies is that shocks have valves that change the damping at different vibration speeds. That means that low frequency tests only check the shock and suspension in a small part of their operating range. Low frequency techniques are also much less accurate due to the combined effect of the other three shocks when the whole vehicle bounces.

Manual Bumper Push Test – traditional shock testing technique.

Disadvantages – Results are based on the technician's opinion, experience, and skill. It is not a quantitative test.

Quantitative Bumper Push Testers – a portable, simple, self-powered instrument which attaches to bumper and measures rocking of the car body at low frequency.

Advantages – Portable and self-powered.

Disadvantages – To take a measurement, the user has to push down on the fender or bumper with 100 to 200 pounds of his body weight. Because each push is going to be different, these testers have difficulty with consistency and repeatability.

Automatic Drop Test – A device that drops each suspension corner from a height of about 4 inches and measures chassis rocking by weight changes at tires.

Advantages – Currently the highest evolution of low frequency testing because measuring technique is consistent.

Disadvantages – Checks only narrow range of shock performance, does not measure adhesion where it is most likely to fail, and still can't separate the performance of one shock from the performance of all shocks.

High frequency testers vibrate the suspension through the entire range of oscillations that it can react to. Currently, all high frequency testers have a plate that each corner of the car is parked on. The plate then vibrates through a range of speeds and sensors in the plate measure how much the tire is pressing on the plate at all times. During the high frequency measurement, the body does not move, therefore the other three shocks do not effect the results. The tester is simulating a bumpy road and measuring how well the suspension keeps the tire in contact with the tester. The result is given in a percent adhesion. The higher the percentage the more the suspension maintained good contact on tester. The adhesion number that is used is the lowest obtained in the test sequence.

EuSAMA Testers – The European Shock Absorber Manufacturer's Association (EuSAMA) originally defined this type of tester. Many companies produce machines that correspond to these guidelines and all measure adhesion.

Advantages – The suspension test takes only a few minutes and provides a measure of the safety of the vehicle. Each wheel is tested independently.

Disadvantages – Adhesion result is an indicator of overall suspension condition, thus safety of the vehicle. Poor adhesion is not specific enough to recommend shock replacement.

Hunter SA400 Suspension Analyzer – Identical to a EuSAMA tester in appearance and meets all EuSAMA requirements.

Disadvantage – Does not always eliminate need for visual inspection and verification of suspension part condition.

Additional features of the Hunter SA400 include:

• Evaluating individual wheel and axle balance for both adhesion and damping.
• Fully automatic testing procedure takes less than one minute per wheel.
• Automatic tests start up when both platforms are loaded with a minimum of 200 lbs.
• Test values are retained until the next test is completed. Results can be set up to save automatically in several formats.
• Print­outs can include test data, graphics, date, time, company and consumer information.
• Modular design allows side­slip meter and brake tester to be added later for a complete safety inspection lane.
• Strut and shock replacement can be recommended with confidence.
• Strut and shock performance can be accurately verified in warranty situations.
• Customers can be shown the improved damping performance after strut and shock replacement.
• Suspensions can be tuned to the desires of the vehicle owner.