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Suspension performance testing
The suspension system, while not absolutely essential to the operation of a motor vehicle, makes a big difference in the amount of pleasure experienced while driving. Essentially, it acts as a "bridge" between the occupants of the vehicle and the road they ride on.
The term suspension refers to the ability of this bridge to "suspend" a vehicle's frame, body and powertrain above the wheels. Like the Golden Gate Bridge hovering over San Francisco Bay, it separates the two and keeps them apart. To remove this suspension would be like taking a cool dive from the Golden Gate: you might survive the fall, but the impact would leave you sore for weeks.
Think of a skateboard. It has direct contact with the road. You feel every brick, crack, crevice and bump. It's almost a visceral experience. As the wheels growl across the pavement, picking up a bump here, a crack there, the vibration travels up your legs and settles in your gut. You could almost admit you were having fun, if you didn't feel like you were gonna toss your tacos at any second. This is what your car would feel like without a suspension system.
In the interests of road safety, it is logical to include in periodic roadworthiness tests an inspection of vehicle suspension performance. The results of tests with a prototype machine are presented and a specification proposed for a valid suspension test.
Demonstrations organized by the European Shock Absorbers Manufacturers’ Association ( EuSAMA) in many countries have drawn attention to the importance of correctly functioning shock absorbers. In the United Kingdom it is anticipate that the Department of the Environment will include a specific shock absorber check in the MOT Test with effect from January 1977.
Of the machines currently available for testing shock absorbers without removing them from the vehicle, there is no real consensus of opinion concerning their validity to evaluate suspension safety objectively. But it is felt that possible more stringent legislation on European periodic vehicle tests in the future will demand a form of objective testing on equipment that is incapable of erroneous interpretation.
Since its formation in 1971 EuSAMA has realized the imnportance of the problem, and initially charged its technical sub-committee with the task of examining and analyzing the various test machines then available. Two basic types of machine were offered at that time for diagnosing faulty shock absorbers. These were:
Machines which lift up the wheels on an axle by about 100 mm and then let them drop. The subsequent displacements of the body on each side are recorded and the results compared with preset values for the particular vehicle and the suspension position, front or rear. Such a machine simulates a step input and records the subsequent body movements (see Fig 1).
Machines which measure wheel movements induced by the exitation of the suspension through a frequency scan from above resonance frequency to zero, applied by means of a spring-supported platform under the tyre. Results are recorded in the form of wheel displacement against time. While passing through the wheel bounce resonant frequency the maximum amplitude is obtained and this is compared with preset values for the particular vehicle and the suspension position front or rear (see Fig 2).
A third machine, introduced later, measures phase shift induced by the excitation of the suspension at a constant frequency and stroke, applied by means of a vibrating platform under the tyre. The phase shift between the moment of excitation and the force-reaction is recorded and the result is compared with preset values for the particular vehicle and suspension position (see Fig 3).
These systems have three fundamental drawbacks:
A: The actual damping is compared with the original damping the limit being a certain degradation in comparison with the original performance. The original performance, however, can already be marginal.
B: The problems of limit setting, namely by whom should the limits be set and what are the criteria they should about? At present there is hardly any relation between set limits and acceptable performance in practice.
C: The practical problem of various limits for different vehicle types and their suspensions. This requires comprehensive reference manuals that need continuously updating.
Despite these fundamental drawbacks, examples of the ? widely used test machines were put through their paces by the Automotive Engineers Laboratory of the University of Ghent, as well as by several EuSAMA members. As expected, the first conclusion is that no test method which does not include dismantling the shock absorbers from the vehicle is able to furnish information concerning the shock absorber alone, and it is in fact the whole of the vehicle suspension system that is tested. This can be considered as a positive aspect of testing, since the whole of the suspension should be in good condition for safety; although the shock absorber is the component most likely to deteriorate with use, other defects such as incorrectly inflated tyres, broken springs or seized ball-joins should if possible be diagnosed.
Of the other factors which influenced test results it was found that all machines gave results that were much affected by shock absorber temperature. In the case of the drop type testing machines, defects in shock absorbers caused by high frequency excitation could not be detected. With the frequency scan type of machine, approximately constant force input implies a big difference in results between vehicles with soft or hard suspension, so that changes in springs from normal to heavy duty (which the operator may be incapable of identifying ) can considerable affect the result.
Each make of machine had its own characteristics, but as the basic test principles were considered to be unacceptable these details will not be presented here.
After due consideration the technical sub-committee advised the General Assembly of EuSAMA that although the existing machines, when correctly operated, could help to diagnose many faulty shock absorbers, a responsible association could not authorize such equipment as the parameters measured were not considered technically representative of any particular aspect of roadworthiness.
Taking brake testing as an example, it was noted that test machines give a direct reading of braking efficiency as a percentage of g without the need to identify vehicle type or to use reference manuals. Similarly, minimum braking performance levels can be set for all automobiles irrespective of model, so that a customer knows immediately if his brakes need attention, Some machines show brake imbalance, but do not indicate which component is faulty.
Applying the same principles to vehicle suspension, it should be possible to propose a test which furnishes a direct reading as a value or preferably as a percentage, to indicate whether a suspension is considered satisfactory from the viewpoint of safety. Moreover, this must be achieved objectively, that is to say without need of any identification, interpretation or reference to manuals by the test operator.
The technical sub-committee therefore looked for a parameter which could be considered a suitable criterion of safety in relation to vehicle suspension. As stated earlier, there is only one component normally subject to deterioration with use—the shock absorber. So the role of the shock absorbers must first be defined. These have two functions to perform: to damp the movement of the vehicle body on its springs and to control wheel bounce.
The permitted movement of a vehicle body on its springs is very much a matter of taste, and it is largely in the control of such movement that a sports shock absorber differs in damping characteristics from a shock absorber aimed at optimum comfort. The movement of a body on its springs does, of course, materially influence roadholding but in reality few ordinary drivers are capable of reaching the limits of the modern car in this respect, so the value of body damping is relatively unimportant for safety measurements. In any case, most drivers of a vehicle with poor body damping will quickly limit their speed and manoeuvres to the vehicle’s handling capacity.
Wheel bounce, on the other hand, is a measurable phenomenon and the dangers of vehicles with uncertain wheel contact are well known. Both cornering and braking performance are well known. Both cornering and braking performance are limited by tyre anherence to the road; this is dependent on the vertical wheel contact as well as the tyre’s own properties.
A parameter which permits the objective measurement of one aspect of roadholding, and therefore of vehicle suspension safety, was thus isolated but it was still necessary to be able to express it in terms that could be readily interpreted.It was proposed, therefore, to measure the minimum remaining vertical contact force between tyre and road under a given excitation at wheel-bounce frequency and to express it as a percentage of the static wheel load. Such a possibility was discussed at a meeting between the technical sub-committee and Dr Verschoore of the University of Ghent. A general concensus of opinion in favour of such a test was reached, though some members expressed doubts concerning the possibility of measuring this parameter in practice, as well as doubts concerning the results Aparamet。.
At a later date the sub-committee was informed that a prototype machine of German origin, using approximately the principle outlined above, had been submitted for evaluation to the University of Ghent. After certain recommended modifications had been performed, tests by both the University of Ghent and a member company of EuSAMA demonstrated the possibilities of such a test, and amply justified the technical sub-committee’s decision concerning the parameter to be measured.
Details are given below of the tests performed and the results obtained on a prototype machine, developed by Maschingfabrik Koppern & Co, Hattingen, West Germany, and presented by courtesy of S A Monroe International, Brussels, Belgium.
The machine (see Fig 4)
Wheel movement is induced by excitation of the suspension through a frequency scan from about 25 Hz to 0, applied by a platform under the tyre, moving with a fixed stroke of 6 mm. One wheel is tested at a time. Results are recorded in the form of
Minimum dynamic wheel load *100% Static wheel load
The tester’s analogue read-out showed deviations from the maximum dynamic force indicated on the oscilloscope.
Test readings are compared with the impressions of an experienced test driver because no scientific test method for roadworthiness has yet been approved. The final determination of roadworthiness and vehicle comfort is still done by vehicle manufactures by the subjective assessment of one or more experienced test drivers.
The test method outlined below will indicate in nearly all cases whether a vehicle suspension is roadworthy or not. Nevertheless, a visuall inspection of the suspension elements is recommended in addition to the performance test, as incipient failures can sometimes be detected visually before performance deteriorates. Secondly, the test is of the vehicle suspension, wheel by wheel, and will indicate only whether there is a fault; it will not locate the fault, though a skilled operator may be able to diagnose certain defects from the test read-out.
Obviously there is a requirement to design a machine able to detect when a certain percentage of static friction is exceeded. Development work in this area is still required.
While there have been enhancements and improvements to both springs and shock absorbers, the basic design of car suspensions has not undergone a significant evolution over the years. But all of that's about to change with the introduction of a brand-new suspension design conceived by Bose -- the same Bose known for its innovations in acoustic technologies. Some experts are going so far as to say that the Bose suspension is the biggest advance in automobile suspensions since the introduction of an all-independent design.
How does it work? The Bose system uses a linear electromagnetic motor (LEM) at each wheel in lieu of a conventional shock-and-spring setup. Amplifiers provide electricity to the motors in such a way that their power is regenerated with each compression of the system. The main benefit of the motors is that they are not limited by the inertia inherent in conventional fluid-based dampers. As a result, an LEM can extend and compress at a much greater speed, virtually eliminating all vibrations in the passenger cabin. The wheel's motion can be so finely controlled that the body of the car remains level regardless of what's happening at the wheel. The LEM can also counteract the body motion of the car while accelerating, braking and cornering, giving the driver a greater sense of control.
Unfortunately, this paradigm-shifting suspension won't be available until 2009, when it will be offered on one or more high-end luxury cars. Until then, drivers will have to rely on the tried-and-true suspension methods that have smoothed out bumpy rides for centuries.