METHOD AND APPARATUS FOR TESTING SHOCK ABSORBERS AND SUSPENSION SYSTEMS

Abstract: A dynamic testing system is described for testing automotive shock absorbers and suspension systems. The testing system includes placing a vehicle (20) on a rolling road surface provided by at least one roller or pair of parallel rollers (1) supporting the front or rear wheels of the vehicle, or several such rollers or pairs of rollers supporting both the front and rear wheels (3) of the vehicle, the roller or at least one roller in the or each pair either having an irregular or eccentric surface or else being eccentrically mounted so as to impart a vertical vibratory motion to the suspension system of the vehicle when driven on that surface, positively driving the vehicle at a predetermined speed on that rolling surface, either using the vehicle's own motive power, or, preferably, by positively driving one or more of those rollers forming that rolling road, applying a controlled braking pressure to the vehicle's braking system to provide a predetermined retardation force acting on the vehicle, and whilst that retardation force is applied measuring and comparing the rotational speed of the roller(s) with the rotational speed of the vehicle wheels.

Description: METHOD AND APPARATUS FOR TESTING SHOCK ABSORBERS AND SUSPENSION SYSTEMS FIELD OF INVENTION The present invention relates to a method and apparatus for testing shock absorbers and suspension systems. In particular, the present invention relates to the in situ testing of a motor vehicle's shock absorber unit(s). BACKGROUND OF INVENTION The performance of a motor vehicle, such as an automobile or a motorbike, will be adversely affected, particularly as regards road holding ability and hence safety, if the shock absorbers and other components of the suspension system are worn or are otherwise defective. It is therefore important that the condition of the suspension components should be checked at regular intervals. There are indeed statutory requirements in the UK and other European countries for the condition of vehicle shock absorbers to be checked annually. Very often, this check consists of not much more than a visual check on the physical condition of the shock absorber and some arbitrary testing of the functional characteristics of the shock absorber by manual rocking of the vehicle. Alternatively, of course, the shock absorbers can be removed for rather more scientific bench testing, but this is time consuming and expensive, and, in any case, such bench testing does not, and cannot, simulate actual driving conditions encountered on the road by the vehicle, and so provides very little information on the in situ performance of the shock absorber or to be precise, the combined effect of all four shock absorbers, or more as the case may be. Such tests therefore bear little relation to the actual performance of the suspension system as a whole. A perfect automotive suspension system should maintain a uniform contact pressure between the wheels of the vehicle and the road surface, and an even load on the tyres at all times. In particular, there should be no tendency for the wheels to slip or lose contact with the ground when subjected to vertical acceleration, particularly when braking or cornering or accelerating. The maintenance of uniform contact pressure between the wheels and the road surface at all times is particularly important, but especially difficult to achieve, when the vehicle is travelling over a rough or pave surface where there is a significant tendency, which the shock absorbers are there to counter, for the wheel or wheels to bounce when hitting a lump or other defect in the road surface, and momentarily to lose contact altogether with the road surface, or at least undergoing a significant temporary reduction in the contact pressure between the wheel and the road surface. Wheel bounce of that kind can cause significant loss of traction when driving at speed over a rough surface, loss of grip when cornering, and loss of grip when braking. Not only that, but road holding characteristics of that kind are not a function of any one shock absorber, but rather of the complete suspension system of the vehicle. Thus a method of measuring automotive suspension performance as a whole, and in the workshop rather than out on the road, would be much in demand. So also would be a system that can measure suspension performance objectively rather than subjectively. Previously proposed methods for testing shock absorbers and vehicle suspensions in situ include systems where the wheels of the vehicle rest on vibrating plates which subject them to a vertical sinusoidal oscillation. The contact forces between the wheels and the plate are measured, either at a constant oscillation frequency or over a range of frequencies, and the results are used to derive a quantitative measure of the performance of the suspension system. For example, the minimum vertical force may be measured at the resonant frequency of the suspension system, and expressed as a percentage of the static weight of the vehicle. A disadvantage of these systems is that the measurements are carried out without the wheels rotating, so that any effects of wheel imbalance, rim eccentricity or worn wheel bearings on the performance of the suspension system are not taken into account. The present invention seeks to overcome the problems associated with the known systems. In the following discussion, the term vehicle suspension system includes a vehicle suspension system and or a sub-assembly thereof. SUMMARY OF INVENTION According to the present invention there is provided an in situ method for testing the state of a suspension system, or sub- assembly thereof, of a vehicle wheel comprising measuring the differential speeds of the wheel and a surface in contact with and supporting the wheel. In this regard, one can measure the differential speeds arising from a braking wheel or from an external torque applied to the wheel, which could be accelerating from a static or a moving state. For example, one could measure the differential speeds of a rotating wheel and a vibrating moving surface. Preferably, the differential speeds are measured by using slowing down the wheel, such as by use of a vehicle's brakes, because of greater control and the longer time for measurement. According to the preferred aspect of the present invention there is therefore provided a method for the in situ testing of a vehicle suspension system, or sub-assembly thereof, comprising: i. placing at least one wheel of a vehicle on a moving surface wherein the surface supporting the at least one wheel imparts a vertical vibratory motion to the suspension system of the vehicle when driven on that surface; ii. positively driving the at least one wheel at a predetermined speed on the moving surface; iii. applying a controlled braking pressure to the at least one wheel, such as to the braking system or to the at least one wheel, to provide a predetermined retardation force acting on the at least one wheel; and iv. measuring and comparing the speed of the surface supporting the at least one wheel with the rotational speed of the at least one wheel, whilst that retardation force is applied. The present invention also provides an apparatus for conducting the method according to the present invention. An important advantage of the present invention is that unlike the known systems wherein the measurements are carried out without the wheels rotating, so that any effects of wheel imbalance, rim eccentricity or worn wheel bearings on the performance of the suspension system are not taken into account, the present invention takes all of these factors in to account by providing a dynamic system. Preferably, the moving surface is a rolling surface provided by at least one roller and in step iv. the rotational speed of the at least one roller is compared with the rotational speed of the at least one wheel. Preferably, the least one roller has an irregular or eccentric surface or else is eccentrically mounted. Preferably, the at least one roller is mounted for concentric rotation about a horizontal axis and in step ii. the roller is simultaneously vibrated in the vertical direction thereby to cause the at least one wheel to vibrate vertically whilst being so driven. The above described method and apparatus are based on the concept of positively driving the vehicle wheels on a simulated "rough" road surface against an applied braking force and detecting any slip which occurs between the wheel and the simulated road surface, e.g. by comparing the peripheral speed of the wheel with the relative speed of the road surface, or in the case of the preferred simulated road surface consisting of a roller or pair of driven rollers, comparing the speed of rotation of the wheel with the speed of rotation of the roller or rollers, that slip being a measure of shock absorber performance, since an ideal shock absorber should, in theory, eliminate all bounce between the wheel and the road surface, and hence for an ideal shock absorber there should be no slip at all between the wheel and the road surface. The tendency of a wheel to lose its grip when driven on an uneven road surface and to slip relative to the road surface when a controlled braking force is applied is thus a measure of absorber performance. According to one aspect of the present invention there is therefore provided a method of testing vehicle suspension systems in situ which comprises placing the vehicle on a rolling road surface provided by at least one roller or pair of parallel rollers supporting the front or rear wheels of the vehicle, or several such rollers or pairs of rollers supporting both the front and rear wheels of the vehicle, the roller or at least one roller in the or each pair either having an irregular or eccentric surface or else being eccentrically mounted so as to impart a vertical vibratory motion to the suspension system of the vehicle when driven on that surface, positively driving the vehicle at a predetermined speed on that rolling surface, applying a controlled braking pressure to the vehicle's braking system to provide a predetermined retardation force acting on the vehicle, and whilst that retardation force is applied measuring and comparing the rotational speed of the roller(s) with the rotational speed of the vehicle wheels. According to another aspect of the present invention there is provided a method of testing the braking system(s) of a motor vehicle, which comprises placing the vehicle, or the front or rear wheels thereof, on a rolling road surface, automatically applying a controlled and controllable braking pressure to the vehicle's braking system to operate the brakes and to apply to the vehicle a controlled braking force, rotating the wheel or wheels of the vehicle against the applied braking force, monitoring the force required to rotate the wheel(s) against the applied braking force, and automatically adjusting the applied braking pressure so as to keep the braking force substantially constant. According to another aspect of the present invention there is provided an apparatus for the in situ testing of suspension systems comprising a rolling road surface provided by a roller or at least one pair of parallel rollers positioned to support one or more of the vehicle wheels, when the vehicle is positioned on the rolling road surface, means for actuating the braking system of the vehicle to apply a controlled braking force to the vehicle, means for driving one or more of the rollers forming the rolling road surface to rotate the vehicle wheel or wheels against the applied braking force, and means for measuring and comparing the rotational speeds of the wheel or wheels resting on the roller(s) with the rotational speed of the roller(s) , the or at least one of said rollers either having an irregular or eccentric surface or being eccentrically mounted, whereby when the roller is rotated, a vertical vibratory movement is imparted to the automotive suspension system as the wheels are rotated, and which movement is absorbed or not absorbed, as the case may be, by the suspension system of the vehicle under test. According to another aspect of the present invention there is provided an apparatus for the in situ testing of braking systems comprising a rolling road surface provided by a roller or at least one pair of parallel rollers positioned to support one or more of the vehicle wheels, when the vehicle is positioned on the rolling road surface, means for actuating the braking system of the vehicle and which means include a variable force producing means capable of varying the pressure applied to the braking system of the vehicle to actuate the brakes and to apply a braking force to the vehicle, means for driving the roller or one or more of the rollers forming the rolling road surface to rotate the wheel or wheels against the applied braking force, and means for measuring the torque required to drive said roller(s) and to rotate the wheel or wheels against the applied braking force, and wherein feed back means are provided responsive to the torque measuring means for automatically adjusting the operating pressure applied to the braking system by said variable force producing means, thereby to maintain said torque and said braking force substantially constant. According to another, and preferred, aspect of the present invention there is provided a method of dynamically testing vehicle suspension systems, or a vehicle suspension system sub- assembly comprising a shock absorber and an associated wheel, which comprises placing the vehicle or said sub-assembly on a rolling road surface provided by at least one cylindrical roller or pair of parallel cylindrical rollers supporting the front or rear wheels of the vehicle, or the wheel of the sub-assembly, or several such rollers or pairs of rollers supporting both the front and rear wheels of the vehicle or said sub-assembly, the roller or rollers in the or each pair being mounted for concentric rotation about a horizontal axis, positively driving the vehicle or said wheel at a predetermined speed on that rolling surface whilst simultaneously vibrating the roller in the vertical direction, thereby to cause the vehicle wheel or wheels to vibrate vertically whilst being so driven, applying a controlled braking pressure to the vehicle's braking system or to said wheel to provide a predetermined retardation force acting on the vehicle or said wheel, and whilst those rotational, vibratory and retardation forces are applied measuring and comparing the rotational speed of the roller(s) with the rotational speed of the wheel(s) . According to another, and preferred, aspect of the present invention there is provided an apparatus for the in situ testing of automotive suspension systems or a sub-assembly thereof comprising a wheel and its associated shock absorber, said apparatus comprising a rolling road surface providing a coaxially mounted cylindrical roller or at least one pair of parallel coaxially mounted cylindrical rollers positioned to support one or more of the vehicle wheels or the wheel of said sub-assembly, means for actuating the braking system of the vehicle to apply a controlled braking force to the vehicle or for applying a controlled braking force to the wheel of said sub-assembly, means for driving one or more of the rollers forming the rolling road surface to rotate the vehicle wheel or wheels against the applied braking force, and means for measuring and comparing the rotational speeds of the wheel or wheels resting on the roller(s) with the rotational speed of the roller(s) , the said roller(s) either being mounted for reciprocal movement in the vertical direction, whereby when the roller is simultaneously rotated and vibrated, a vertical , vibratory movement is imparted to the wheel(s) as it is rotated, and which movement is absorbed or not absorbed, as the case may be, by the suspension system of the vehicle under test, or said shock absorber and means for simultaneously reciprocating said roller or rollers in said vertical direction. In another, and preferred, aspect of the present invention there is provided for use as a test procedure an artificial road surface provided by one or more concentrically mounted, cylindrical rollers mounted for rotation about a horizontal axis, and upon which the vehicle wheel or wheels rest during the test, first drive means being provided for positively driving that roller or rollers so as to rotate the wheel or wheels of the vehicle resting thereon and against a controlled braking force applied to those wheels, that roller or rollers being mounted in a sub-frame permitting that roller or rollers to be reciprocated vertically whilst being driven by the first drive means, and second drive means being provided operable to reciprocate that sub-frame and the roller or rollers mounted therein in the said vertical direction throughout the period of the test. The main advantage of these latter preferred aspects of the present invention is that they overcome the sometimes found undesirable effect of imparting a degree of horizontal movement to the wheel, as well as vertical movement. In this regard, it has been found that concentrically mounted cam-shaped rollers, or concentrically mounted rollers with some other form of eccentric surface, and eccentrically mounted rollers with a cylindrical surface both have the undesirable effect of imparting a degree of horizontal movement to the wheel, as well as vertical movement. In other words, the direction of oscillation or vibration of the wheel has both vertical and horizontal components. Preferably the steps of monitoring the rotational speed of the said roller or rollers forming the rolling road surface, monitoring the rotational speed of the vehicle wheel or wheels, generating a pair of electrical signals proportional to those rotational speeds, comparing those two signals, and generating an output signal proportional to the difference between those two signals. Preferably the vehicle under test is fitted with an anti-lock braking system, and the anti-lock braking system is deactivated for the purposes of the test. Preferably the vehicle under test is fitted with an anti-lock braking system, and wherein said step of monitoring and comparing the rotational speed of the roller or rollers of the rolling road surface and the rotational speed of the vehicle wheel or wheels are performed by monitoring the actuation of the anti-lock braking system. Preferably the vehicle wheel or wheels are driven on said rolling road surface by a positive rotational drive to one or more of the rollers constituting that surface, and wherein the rotational torque required to drive the vehicle wheel or wheels against the applied braking force is continuously monitored and the operating pressure applied to the vehicle's braking system adjusted automatically consequent upon detected variations in the applied torque, thereby to keep the retardation force applied to the vehicle throughout the test procedure and the torque required to drive the wheels against the applied braking force substantially constant. Preferably the front and rear suspension systems are tested independently. Preferably the front and rear suspension systems are tested simultaneously and an integrated performance indicator obtained indicative of the performance of the automotive vehicle suspension system as a whole. Preferably the vertical vibratory motion is imparted to the suspension system of the vehicle under test by means of a rolling road surface incorporating one or more eccentrically mounted rollers. Preferably the vertical vibratory motion is imparted to the suspension system of the vehicle under test by means of a rolling road surface comprising one or more rollers incorporating a cam surface. Preferably the vertical vibratory motion is imparted to the suspension system of the vehicle under test by two eccentric or cam-shaped rollers rotating about a common axis and located one on each side of the test vehicle, the roller eccentricities or cam surfaces being angularly spaced one from the other thereby imparting to the vehicle suspension system vertical vibratory motions which are out of phase on opposite sides of the vehicle. Preferably said eccentricities or cam surfaces are spaced and have the same phase or are 180° out of phase. Preferably, the eccentricities or cam surfaces are movable relative to each other from being in phase to being 180° out of phase. Preferably the means for measuring and comparing the rotational speeds of the wheel or wheels of the vehicle under test and the rotational speed of the roller(s) includes means for providing an output signal proportional to the difference between the two speeds. Preferably the means for actuating the braking system of the vehicle under test comprises a variable force producing means capable of varying the actuating force applied to the braking system of the vehicle, such apparatus also including means for continuously monitoring the retardation force applied to the vehicle during the test procedure, and feed back means responsive to said monitoring means for automatically adjusting the operating force applied to the braking system of the vehicle by said variable force producing means, thereby to keep the said retardation force substantially constant. Preferably the roller(s) is or are eccentrically mounted circular cylinders. Preferably the rollers have a cam-shaped profile. Preferably the cam sections are removable. Preferably there is one or more pairs of coaxially mounted rollers whose eccentricities are out of phase with each other, and which are so mounted that during the test procedure the wheels on opposite sides of the vehicle under test rest each on a different one of said pairs of rollers so that during the test procedure the vibratory motion imparted by those rollers in the vehicle suspension system is out of phase on opposite sides of the vehicle. Preferably the eccentricities of said rollers are 180° out of phase. The eccentricities of the rollers may have the same phase. Preferably, the eccentricities are movable relative to each other from being in phase to being 180° out of phase. Preferably the phase of the rollers with respect to each other is adjustable. Preferably the rolling road surface comprises but a single roller upon which either the front or the rear wheels of the vehicle rest during the test procedure, or two such single rollers upon which both the front and rear wheels of the vehicle rest during the test procedure. Preferably the rolling road surface comprises either a pair of adjacent parallel rollers upon and between which either the front or rear wheels of the vehicle rest during the test procedure, or two such pairs of rollers upon and between which both the front and rear wheels rest during the test procedure. Preferably there is a mechanically, electrically or electronically controlled actuator for applying a controlled braking force to the vehicle's wheels, that actuator being operable to apply a controlled but variable operating pressure to the vehicle's braking system, and wherein means are provided for continuously monitoring the torque required to drive the vehicle wheels against the applied braking force, and feed back means are provided for automatically varying the pressure applied by said actuator to the vehicle braking system and to maintain the said braking force and torque substantially constant. Preferably there is means for generating a first electrical signal proportional to the rotational speed of the vehicle wheels when driven on said rolling road surface, means for generating a second electrical signal proportional to the rotational speed of the rolling road surface, means for comparing the two signals and for generating an output signal proportional to the difference between the first and second signals, and means for automatically displaying the value of said output signal. Preferably there is utilisation of a signal from the vehicle's ABS system to indicate the onset of slip between the vehicle wheels and the rolling road surface, and which comprises means for electronically processing and displaying that signal. Preferably the vertical vibratory motion is imparted to the wheel of the vehicle or sub-assembly under test by means of a rolling road surface incorporating one or more horizontally mounted cylindrical rollers mounted for reciprocation in the vertical direction, and wherein the roller or rollers is vertically reciprocated throughout the test by means of a reciprocating crank mechanism. The present invention overcomes the disadvantages of the known systems and provides a test of overall suspension performance which is related directly to road safety requirements, by rotating the wheels at the required road speed and detecting any tendency for the wheels to lose contact with a simulated uneven road surface under controlled braking. With the present invention, a perfect vehicle suspension system will ensure continuous contact between the wheels and a uniform contact pressure with the roller surface at all times, despite the roughness or eccentricity of the roller surface, that roughness or eccentricity being totally absorbed by the vehicle suspension system. For a less than perfect system, i.e. where there is a degree of bounce with or without total loss of contact pressure with the roller surface, this will be detected by a difference in the rotational speeds of the vehicle wheel and the roller under that controlled braking force, the poorer the suspension system, the greater will be the "bounce" of the wheel or wheels on the roughened or eccentric roller surface and the greater will be the difference between the roller speed and the wheel speed, i.e. the greater will be the slip between the two. Whilst the present system has been designed with a view to the dynamic testing of the complete automotive suspension system, the technique can of course equally well be applied to the separate testing of the front or rear suspension systems, or indeed the testing of the shock absorber or suspension systems associated with each individual wheel. For good results, and in accordance with the present invention, it is desirable that the retardation force applied to the vehicle (or to the individual wheel or wheels) remains substantially constant during the performance of the test. In the preferred technique, therefore, the retardation force acting on the vehicle throughout the suspension test is continuously monitored, and the applied braking pressure automatically adjusted to keep that retardation force constant. That may be achieved, for example, by monitoring the current uptake of the motor used to drive the roller(s) against the applied braking force, and automatically adjusting the pressure applied to the braking system to keep that current uptake at a constant, predetermined value. That preferred technique of continuously monitoring the retardation force applied to the vehicle and automatically adjusting the pressure applied to the braking system to keep the retardation force constant also has other applications, and forms a second and distinct aspect of the present invention. In particular it is applicable to conventional vehicle brake testing techniques and vehicle brake testing apparatus. In the conventional brake testing technique the wheel or wheels of the vehicle is or are placed on a rolling road surface and driven on that surface against a braking force applied by the operator using the vehicle's braking system. The force required to drive the roller(s) of the rolling road against the applied braking force is used as a measure of the efficiency of the vehicle's braking system. That, however, is totally dependent upon the braking pressure applied to the brake pedal, or to the hand brake, by the operator himself. A far better measurement of the efficiency of a vehicle's braking system is therefore achieved by the technique described above, that is to say by automatically applying a controlled and controllable braking pressure to the vehicle's braking system to operate the brakes and to apply to the vehicle a controlled braking force, rotating the wheel or wheels of the vehicle against the applied braking force, monitoring the force required to rotate the wheel(s) against the applied braking force, and automatically adjusting the applied braking pressure so as to keep the braking force substantially constant. Where the suspension test method of this invention is applied to a vehicle equipped with an Anti-lock Braking System (ABS) which automatically detects locking of the vehicle wheels during braking and momentarily releases the brakes in order to reduce the risk of skidding, then either that system has to be deactivated prior to the suspension test, or the ABS can itself be used to monitor the efficiency of the suspension system, since again, on a rough road surface, or on the roughened or eccentric rolling road surface used in this invention, then the ideal suspension will eliminate wheel bounce, which in turn will eliminate wheel locking and consequent actuation of the ABS, since the wheel remains in contact with the road surface at all times with substantially constant contact pressure. Thus, in the test method of the invention, the actuation of the ABS whilst the vehicle is being driven on the roughened or eccentric rolling road surface will be a measure of the efficiency, or rather the inefficiency, of the suspension system. Preferably, the means for measuring and comparing the rotational speeds of the wheel or wheels of the vehicle under test and the rotational speed of the roller(s) includes means for providing an output signal proportional to the difference between the two speeds. Preferably, also, the means for actuating the braking system of the vehicle under test comprises a variable force producing means capable of varying the actuating force applied to the braking system of the vehicle, such apparatus also including means for continuously monitoring the retardation force applied to the vehicle during the test procedure, and feed back means responsive to said monitoring means for automatically adjusting the operating force applied to the braking system of the vehicle by said variable force producing means, thereby to keep the said retardation force substantially constant. Ideally, in carrying out the above described test, the oscillatory or vibratory movement imparted to the wheel or wheels on the test vehicle or test rig should be substantially free of any horizontal components. DETAILED DESCRIPTION The automotive suspension testing method of this invention and apparatus for performing that method are illustrated in the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a suspension testing apparatus of the invention employing one roller; Figure 2 shows an alternative form of roller for use in the apparatus of Figure 1; Figure 3 is a diagrammatic representation of a second embodiment of an apparatus according to the invention; ' Figure 4 shows yet another form of roller, for use in the apparatus of Figure 1 or Figure 3; and Figure 5 is a diagrammatic representation of a third embodiment of an apparatus according to this invention. Figure 6 is a diagrammatic end view of a preferred apparatus according to the present invention for the dynamic testing of an individual shock absorber, rather than a complete vehicle suspension system. However, the principles of this apparatus and its use are the same as those for the apparatus of each of the preceeding figures. Referring to the drawings, Figure 1 is a diagrammatic representation of a first suspension testing apparatus according to the invention. The eccentric roller 1 is driven by a motor 2 at a controlled speed, which may be held constant at a selected value or may be varied during the course of the test. The speed may for example be selected to correspond to the natural resonance frequency of the vehicle's suspension, or it may be increased steadily over a range. The vehicle 20 is mounted so that its wheels 3 are resting on the roller 1 while the vehicle is prevented from moving in a forward or reverse direction by a restraint 4. The eccentricity of the roller 1 is such as to impose a vertical oscillating motion on the wheels of the vehicle which is representative of that which would be experienced in driving over a bad road surface. Tachometers 5 and 6 monitor the rotational speeds of the wheels 3 and the roller 1. The actuator 7, acting for example on the vehicle's brake pedal 8, is able to apply a controlled braking force to the wheels 3 through the vehicle's braking system. 18 represents an optional but preferred feed back loop which monitors the torque applied by the motor 2 to drive the wheel 3 against the applied braking force and which automatically adjusts the brake actuator 7 so as to keep the applied braking force constant. In use the wheels are spun up to the required speed by the eccentric roller and a controlled braking force is applied while the outputs of the tachometers are observed. If there is no slip between the roller and the wheels, the ratio of their speeds will remain constant. If this ratio changes, this is an indication of slip or loss of contact between the wheels and the roller. The onset of slip may be used as an indication that one or more shock absorbers, or other components of the suspension system, are faulty. Alternatively, the percentage change in the speed ratio under controlled braking may be used as a quantitative measure of shock absorber or suspension condition. The tachometers 5, 6 may be mechanical, optical or magnetic. Their outputs may be fed to an electronic comparator 9 which enables the change or percentage change in speed ratio to be computed and displayed automatically. The comparator may incorporate an audible or visual alarm to indicate when slip occurs or when a preset percentage speed ratio change has been exceeded. In. Figure 1 the front wheels of the vehicle are shown resting on the roller. If it is required to test the rear suspension the vehicle is merely repositioned so that the rear wheels rest on the roller. If the vehicle is fitted with an anti-roll bar, cross-coupling via the anti-roll bar between suspension components on opposite sides of the vehicle could distort the measurements or, alternatively, require the anti-roll bar to be uncoupled while measurements are made. This problem may be overcome by subjecting the wheels on opposite sides of the vehicle to vertical sinusoidal oscillations which are equal in amplitude but opposite in phase. Figure 2 represents in part, in diagrammatic form, an alternative embodiment of the invention in which this is achieved by positioning the wheels on two rollers 10, 11 so mounted that their eccentricities are out of phase with each other; the phase difference should preferably be 180°. The angular positions of the rollers 10, 11 may be adjustable with respect to each other so that the test may be run with the rollers in phase or out of phase with each other. Figure 3 represents in part yet another embodiment of the invention in which the vehicle wheels rest between rollers 12 and 13, which are both eccentric and which are mechanically coupled together so that they rotate in the same direction. This embodiment avoids the need for the restraint 4 shown in Figure 1. Figure 4 represents in part, yet another embodiment of the invention in which the roller 14 on which the wheels rest has the form of a cam whose profile may be selected to impose various preferred types of motion on the wheels; for example a step function of vertical movement. The cam profile may be integral to the roller or may be in the form of bolt-on sections 15 which may be changed as required. A number of bolt-on sections may be attached to the roller at different positions on its circumference. Figure 5 represents yet another aspect of the invention in which rollers 16, 17 are provided enabling both the front and rear suspension systems of the vehicle to be tested simultaneously. The rollers 13, 14 may be taken to represent any combination of the roller types 1, 10, 11, 12, 13 or 14 described above. Referring to Figure 6, the apparatus shown comprises a triangular main frame 101 in which is mounted, for rotation concentrically about a horizontal axis 102, a cylindrical surface roller 103 driven by a belt drive 104 from a motor unit 105. A pivotally mounted rigid radius arm 106 maintains the axis 102 of the roller 103 at a fixed distance from the motor unit 105. Roller 103 is mounted at its opposite ends in bearing blocks 107 mounted in turn on resilient or spring mounted support blocks 108, which permit the roller 103 to be reciprocated in the vertical direction by means of the crank 109 eccentrically driven by a second motor unit 110. Together, the crank 109 and the radius arm 106 constrain the vibratory or reciprocatory movement of the roller 103 to the vertical plane. Resting on the roller 103 for the purposes of the test is the vehicle wheel 111 connected to the shock absorber 112 under test, in this case an individual shock absorber mounted in the triangular frame 101, the assembly being stabilized by the independent suspension unit 113. For the purposes of performing a dynamic in situ vehicle suspension system test, which is the preferred application of the method and apparatus of this invention, the shock absorber 112 and suspension system 113 will be mounted in the vehicle 114 itself, and resting via its wheels 111 on the roller 103, and as shown in phantom in the drawing. The roller 103 can be replaced by a pair of parallel rollers, both mounted on the . resilient block 108, and supporting the vehicle wheel (and the vehicle) between the nip of the two rollers. Also the apparatus may be constructed for the purpose of testing one suspension unit only, e.g. the offside or nearside suspension unit at either the front or the rear of the vehicle, or the front suspension only, or the rear suspension only, or the complete vehicle suspension system, in which case two units such as shown in the drawing will be provided spaced one from the other and supporting both the front and rear wheels of the vehicle. The test procedure is carried out substantially as previously described, and need not be described in detail here. Basically the test comprises driving the roller 103 at a constant speed via the motor unit 104, thereby driving the wheel or wheels 111 at a constant speed against an applied braking force applied via the vehicle's braking system, or in some other convenient manner when a single separate shock absorber is under test, rather than in situ in a vehicle. Simultaneously the roller 103 is reciprocated vertically by the crank 109. This imparts a vertical vibratory movement or "bounce" to the wheel 111 as it is driven on the roller 3, that vibratory movement being absorbed, or not as the case may be, by the shock absorber 112. As previously disclosed, the efficiency of the shock absorber is measured by comparing the rotational speed of the wheel 111 with that of the roller 103. With a 100% efficient shock absorber, there will be no bounce of the vehicle wheel 111 on the surface of the roller 103 and hence no difference in speed. The more inefficient the shock absorber, the greater will be the bounce of the wheel, and the applied braking force will be more effective, thus causing slip between the wheel and the roller and a consequent variation in rotational velocity of the wheel compared with that of the roller. As indicated above, the technique of this invention can also be used as a measure of braking efficiency. In vehicles fitted with Anti-lock Braking Systems (ABS) , a signal may be derived from the ABS unit to indicate the onset of slip. In another implementation of the invention this ABS unit signal is used as an indicator in place of the tachometers previously described. Other modifications in the apparatus described above and in the method of performing the invention will be apparent to those skilled in the art without departing from the scope of the invention herein described and hereinafter claimed.

Claims: CLAIMS 1. An in situ method for testing the state of a suspension system, or sub-assembly thereof, of a vehicle wheel comprising measuring the differential speeds of the wheel and a surface in contact with and supporting the wheel. 2. A method for the in situ testing of a vehicle suspension system, or sub-assembly thereof, comprising: i. placing at least one wheel of a vehicle on a moving surface wherein the surface supporting the at least one wheel imparts a vertical vibratory motion to the suspension system of the vehicle when driven on that surface; ii. positively driving the at least one wheel at a predetermined speed on the moving surface; iii. applying a controlled braking pressure to the at least one wheel, such as to the braking system or to the at least one wheel, to provide a predetermined retardation force acting on the at least one wheel; and iv. measuring and comparing the speed of the surface supporting the at least one wheel with the rotational speed of the at least one wheel, whilst that retardation force is applied. 3. A method according to claim 2 wherein the moving surface is a rolling surface provided by at least one roller and in step iv. the rotational speed of the at least one roller is compared with the rotational speed of the at least one wheel. 4. A method according to claim 3 wherein the least one roller has an irregular or eccentric surface or else is eccentrically mounted. 5. A method according to claims 3 or claim 4 wherein in step i. the at least one roller is mounted for concentric rotation about a horizontal axis and in step ii. the roller is simultaneously vibrated in the vertical direction thereby to cause the at least one wheel to vibrate vertically whilst being so driven. 6. A method of testing vehicle suspension systems in situ comprising placing the vehicle on a rolling road surface provided by at least one roller or pair of parallel rollers supporting the front or rear wheels of the vehicle, or several such rollers or pairs of rollers supporting both the front and rear wheels of the vehicle, the roller or at least one roller in the or each pair either having an irregular or eccentric surface or else being eccentrically mounted so as to impart a vertical vibratory motion to the suspension system of the vehicle when driven on that surface, positively driving the vehicle at a predetermined speed on that rolling surface, applying a controlled braking pressure to the vehicle's braking system to provide a predetermined retardation force acting on the vehicle, and whilst that retardation force is applied measuring and comparing the rotational speed of the roller(s) with the rotational speed of the vehicle wheels. 7. A method of testing the braking system(s) of a motor vehicle which method comprises placing the vehicle, or the front or rear wheels thereof, on a rolling road surface, automatically applying a controlled and controllable braking pressure to the vehicle's braking system to operate the brakes and to apply to the vehicle a controlled braking force, rotating the wheel or wheels of the vehicle against the applied braking force, monitoring the force required to rotate the wheel(s) against the applied braking force, and automatically adjusting the applied braking pressure so as to keep the braking force substantially constant. 8. An apparatus for the in situ testing of suspension systems comprising a rolling road surface provided by a roller or at least one pair of parallel rollers positioned to support one or more of the vehicle wheels, when the vehicle is positioned on the rolling road surface, means for actuating the braking system of the vehicle to apply a controlled braking force to the vehicle, means for driving one or more of the rollers forming the rolling road surface to rotate the vehicle wheel or wheels against the ■ applied braking force, and means for measuring and comparing the 0 rotational speeds of the wheel or wheels resting on the roller(s) with the rotational speed of the roller(s) , the or at least one of said rollers either having an irregular or eccentric surface or being eccentrically mounted, whereby when the roller is rotated, a vertical vibratory movement is imparted to the 5 automotive suspension system as the wheels are rotated, and which movement is absorbed or not absorbed, as the case may be, by the suspension system of the vehicle under test. 9. An apparatus for the in situ testing of braking systems 0 comprising a rolling road surface provided by a roller or at least one pair of parallel rollers positioned to support one or more of the vehicle wheels, when the vehicle is positioned on the rolling road surface, means for actuating the braking system of the vehicle and which means include a variable force producing 5 means capable of varying the pressure applied to the braking system of the vehicle to actuate the brakes and to apply a braking force to the vehicle, means for driving the roller or one or more of the rollers forming the rolling road surface to rotate the wheel or wheels against the applied braking force, and means 0 for measuring the torque required to drive said roller(s) and to rotate the wheel or wheels against the applied braking force, and wherein feed back means are provided responsive to the torque measuring means for automatically adjusting the operating pressure applied to the braking system by said variable force 5 producing means, thereby to maintain said torque and said braking force substantially constant. 10. A method of dynamically testing vehicle suspension systems, or a vehicle suspension system sub-assembly comprising a shock absorber and an associated wheel, which comprises placing the vehicle or said sub-assembly on a rolling road surface provided by at least one cylindrical roller or pair of parallel cylindrical rollers supporting the front or rear wheels of the vehicle, or the wheel of the sub-assembly, or several such rollers or pairs of rollers supporting both the front and rear wheels of the vehicle or said sub-assembly, the roller or rollers in the or each pair being mounted for concentric rotation about a horizontal axis, positively driving the vehicle or said wheel at a predetermined speed on that rolling surface whilst simultaneously vibrating the roller in the vertical direction, thereby to cause the vehicle wheel or wheels to vibrate vertically whilst being so driven, applying a controlled braking pressure to the vehicle's braking system or to said wheel to provide a predetermined retardation force acting on the vehicle or said wheel, and whilst those rotational, vibratory and retardation forces are applied measuring and comparing the rotational speed of the roller(s) with the rotational speed of the wheel(s) . 11. An apparatus for the in situ testing of suspension systems or a sub-assembly thereof comprising a wheel and its associated shock absorber, said apparatus comprising a rolling road surface providing a coaxially mounted cylindrical roller or at least one pair of parallel coaxially mounted cylindrical rollers positioned to support one or more of the vehicle wheels or the wheel of said sub-assembly, means for actuating the braking system of the vehicle to apply a controlled braking force to the vehicle or for applying a controlled braking force to the wheel of said sub- assembly, means for driving one or more of the rollers forming the rolling road surface to rotate the vehicle wheel or wheels against the applied braking force, and means for measuring and comparing the rotational speeds of the wheel or wheels resting on the roller(s) with the rotational speed of the roller(s) , the said roller(s) either being mounted for reciprocal movement in the vertical direction, whereby when the roller is simultaneously rotated and vibrated, a vertical vibratory movement is imparted to the wheel(s) as it is rotated, and which movement is absorbed or not absorbed, as the case may be, by the suspension system of the vehicle under test, or said shock absorber and means for simultaneously reciprocating said roller or rollers in said vertical direction. 12. An artificial road surface for use in the in situ testing of vehicle suspension system comprising one or more concentrically mounted, cylindrical rollers mounted for rotation about a horizontal axis, and upon which the vehicle wheel or wheels rest during the test, first drive means being provided for positively driving that roller or rollers so as to rotate the wheel or wheels of the vehicle resting thereon and against a controlled braking force applied to those wheels, that roller or rollers being mounted in a sub-frame permitting that roller or rollers to be reciprocated vertically whilst being driven by the first drive means, and second drive means being provided operable to reciprocate that sub-frame and the roller or rollers mounted therein in the said vertical direction throughout the period of the test.

Inventors: TRUELOVE RICHARD SAVILL, WHITTINGTON KEITH RICHARD, TRUELOVE, RICHARD, SAVILL, WHITTINGTON, KEITH, RICHARD
Applicants: TRUELOVE RICHARD SAVILL, WHITTINGTON KEITH RICHARD, TRUELOVE, RICHARD, SAVILL
Application year: 1993
Main classification: G01M17
IPC codes: G01M17