How To Check Car Steering And Suspension
If you suspect that you have a problem with your cars steering or suspension,or you just want a periodic check, here is a basic examination you can make.
Basic Suspension Check
Push down on top of the wing each side (individually) just once and let go, The vehicle should go down and come back up once and stay put. If the vehicle bounces up and down and appears soft this indicates a faulty shock absorber strut.
With the car jacked up at the front, place on axle stands for safety , put steering wheel in the unlocked position. Move the road wheel from side to side as if you were turning the wheels to steer. Feel for any excessive free play or movement in the steering joints ie top/bottom swivel joints, track rod ends ,steering rack and suspension bushes. Then hold the road wheel at the top and bottom grabbing hold of the tyre pushin in and out and feel for any free movement there. Any excess free play should be picked up and the offending part noticed and replaced.
Examine rubber gaiters or (boots) as they are sometimes called on the steering rack and rubber insulator bushes on the anti-roll bar.
Examine front suspension struts for leakage and split dust covers,this can be observed by looking inside the coil spring, any signs of oil around this area indicates a worn shock absorber strut and must be replaced.
Check also for broken coil springs.
A Primer on Suspension Testing
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 tyres and the road. The suspension is responsible for keeping the tyre 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.
Springs isolate the driver from road imperfections by allowing the tyre to move over a bump without drastically disturbing the chassis. If the chassis remains fairly steady then the tyres 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 tyres keep hopping long after the vehicle strikes a bump. Left uncontrolled, springs give an uncomfortable ride with very poor tyre 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 tyre 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 tyres , 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 tyres – 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.
Shock Absorber Design
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.
The oldest design still in use today is the two tube shock absorber. In this type, an inner tube holds oil which the piston moves up and down through. An outer tube holds reserve oil and air. When the piston moves up then down it draws then forces oil to the outer tube. There are usually valves on the piston, as well as, between the inner and outer tube in this design. Under very bumpy conditions, motion can mix the air into the oil in little bubbles, creating a foam. Foaming oil does not absorb energy very well. This type of shock absorber stops working, as well, when you really need it most – on very bumpy roads.
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.
An interesting variation on the monotube design is a “GAS” shock that intentionally mixes the pressurized gas with oil. Because the gas is under very high pressure the oil does not foam. The gas just travels around with the oil in little bubbles. The valving on this type of shock is located on the piston and is specially designed to handle the airoil mix. A rule of thumb for all types of shocks and struts is that “GAS” shocks are better for almost all circumstances. In addition, the larger the tubes used to construct the shock, usually the higher quality it is. Larger tubes hold more oil which then doesn’t get as hot. Cool oil retains its molecular properties longer than hot oil.
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.
The Need for Suspension Testing
Why is it so important to keep good tyre contact? All actions a vehicle performs – acceleration, cornering, and braking are transmitted through the tyres . The tyres 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 tyre 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 tyre 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 tyre 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.
Suspension and Shock Inspection Techniques
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 tyres
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.
Low Frequency Techniques
Manual Bumper Push Test – traditional shock testing technique.
Advantages – No equipment cost for this method of testing. Since it is a dynamic test, the technician observes operation.
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 tyres .
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 Techniques
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 tyre 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 tyre 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.
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