Experimental Methods

The CEU (1996) outlined three methods to evaluate the road-friendliness of heavy vehicles by subjecting them to transient excitations and analysing the response. The first is the ramp test, where a vehicle is driven at low speed (5 ± 1 km/h) over a specified ramp (with a drop of 80 mm at the end of the ramp) to induce a step excitation on the vehicle and the resulting response is analysed. The next test, known as the pull down test, involves forcing the vehicle down until it is loaded to approximately one-and-a-half times the maximum static load, suddenly released and the resulting oscillations analysed. The final test outlined is the lift and drop test, where the chassis of the vehicle is raised 80 mm above the driving axle, then suddenly released and analysing the resulting response.

Sweatman et al. (1994) devised a transient step excitation method where a vehicle is placed on blocks, begins to crawl forward and as soon as the vehicle departs the upper surface, the blocks are simultaneously removed (the blocks have ropes fitted on the side). Despite reasonable response data obtained, the authors acknowledged that the technique is not suitable for general suspension assessment. The main difficulties encountered were due to the complex procedure requiring the coordination of many people to remove the blocks from under the vehicle and also involves a great deal of time and effort to obtain “usable data” (Sweatman et al. 1994). The sprung mass acceleration, suspension displacement and wheel forces were measured and compared, with the authors noting that there is a much larger variation in the estimates of the damping ratio compared to the sprung mass frequency.

Woodroffe (1996) used a four-post shaker to simulate a number of test methods in order to investigate the road-friendliness of a trailer with various suspension types (rigid trailing arm air

suspension, four spring steel leaf suspension and a rubber walking beam suspension). A simultaneous axle drop test, where the four-posts of the shaker are displaced down by 80 mm, is considered by Woodroffe (1996) to be “a valid test for compliance” with the CEU directive. The results of this test revealed a clear distinction between the mechanical and air suspended sprung mass frequency and damping, however it was noted that there was “considerable variation” in the estimated damping ratios (Woodroffe 1996). The ramp test was replicated by sequentially dropping the axles of the trailer to simulate the test at a speed of 5 km/h. The results of the sequential axle drop test did not yield results that were “as clear and consistent as the simultaneous drop test” (Woodroffe 1996). While the sprung mass frequency estimates were in agreement between the sequential and simultaneous axle drop tests, the damping ratio estimates were not in good agreement (Woodroffe 1996). The author also attempted to drop a single-axle only; but the response of the air suspension from this test was “not readily distinguishable.”

Milliken et al. (2001) described the design and implementation of an apparatus to raise all axles of a heavy vehicle to a specified height (48, 80 and 112 mm), drop the vehicle whilst measuring the displacement response using a string potentiometer. The response was analysed using the DFT to determine the sprung mass frequency, while the damping ratio was obtained first using a weighted least squares linear fit on the logarithm of the displacement, then a modified version of the logarithmic decrement method. Milliken et al. (2001) encountered numerous difficulties with this test, some of which include the significant duration required to set up the apparatus, issues with the simultaneous release of the platforms and some issues with the use of the string potentiometer to measure the displacement. The authors found that the different drop heights did not generate “significantly different results.” It was also shown that the repeatability of the tests were within ± 7 % and ± 18 % for the damped natural frequency and damping ratio estimates, respectively. Milliken et al. (2001) acknowledged that while the apparatus is not portable, it may be installed at a permanent test facility and vehicles could be tested “within an hour.”

Davis and Sack (2004) devised a test to excite an air suspended heavy vehicle (prime mover) with an impulse by driving it at low speed (approximately 5 km/h) across a fixed pipe. The pipe used in the test was 50 mm in diameter; however no recommendation was made as to the most suitable diameter of pipe (nor was another diameter used). Results from the pipe test show the presence of the subsequent axles (2nd, 3rd and 4th) travelling over the pipe, emphasising the difficulty in obtaining uncontaminated estimates of the response from a single axle. Davis and Sack (2004) also conducted a modified version of the ramp test, with a drop height of 65 mm instead of 80 mm due to the convenience of having a drop between the warehouse slabs at the

test site. The response data analysed by the authors was the measured air pressure in the high- pressure air lines of the suspension converted to excursions of mass (40 kg incremental excursions). Only the damping ratio was estimated from these tests using the logarithmic decrement. The modified ramp test yielded a response signal that was unable to produce meaningful results in terms of the sprung mass frequency. A heavy vehicle testing rig designed to reproduce the CEU drop test was commissioned by Sweatman, where the vehicle is driven onto a platform and suddenly released (although no direct references exist detailing the testing platform). The testing platform was used to evaluate heavy vehicle suspension systems travelling on the Hume Highway in New South Wales, Australia (Blanksby et al. 2006).

The validity of the CEU and other equivalent transient test methods have been scrutinised by many authors. Cole and Cebon (1991) found that a numerical simulation of the three CEU methods produced similar results for a nonlinear heavy vehicle (for two combinations: with a trailer and then a lumped mass on the rear axle of the vehicle). The authors then investigated the amplitude of excitation by reducing the ramp height from 80 mm to 20 mm and found that the reduced drop height resulted in an increase in the sprung mass frequency estimates. Cole and Cebon (1991) explained that this increase is due to the “nonlinear, hysteretic characteristics” that are common in steel leaf springs. Further to the issues described by Cole and Cebon (1991), Woodroffe (1996) also investigated the effect of the ramp height drop and discovered that “in general, the experiments found that the damping ratio decreased with increased drop height.” This would also have an effect on the modified ramp test undertaken by Davis and Sack (2004). As has been noted throughout the various test methods discussed, there clearly exists greater difficulty in establishing estimates of the damping ratio of the suspension system than the sprung mass frequency (Woodroffe 1996; OECD 1998). Woodroffe (1996) stated that there is difficulty in establishing the road-friendliness when trucks are coupled with a trailer and that this interaction “must be eliminated to accurately assess the performance of a particular suspension.” Cole and Cebon (1991) noted a prominent flaw in the CEU directive in regards to the lack of specification about the loading of the heavy vehicle during the tests, with the authors noting that attaching a trailer does influence the estimates. The validity of the approach undertaken by the CEU was further scrutinised by Cole and Cebon (1991), stating that no known relationship exists between the dynamic characteristics of a vehicle’s suspension system and the subsequent damage induced onto roads. The approach is aimed at determining equivalency to air suspensions, which are considered to be road-friendly (Cole & Cebon 1991). The authors concluded by remarking it is “not clear why the CEU legislators have concentrated on encouraging the use of equivalent air suspensions.”

Gillespie (1985) noted that, for a quarter car model subjected to a smooth and rough road, the sprung mass mode frequency is reduced for the rough road compared to the smooth road. The author suggested this may be due to the greater suspension deflections occurring during travel over the rough road than on the smooth road; “thus it is often observed that some trucks ride better on rough roads than on smooth roads.” De Pont (1999) argued that the CEU tests can be “quite severe and may not reflect the behaviour on roads of good to moderate roughness, which are primarily the ones in need of protection.” Cebon (1993) described another issue with the CEU approach for establishing the sprung mass frequency, namely that during operation “many different natural modes of vibration contribute” and will be dependent upon the speed of the vehicle. Cebon (1993) states “there is no reason to believe that a particular transient input will excite the natural mode(s) that dominate the generation of dynamic tyre forces under highway conditions.”

Several publications since Cole and Cebon (1991) have investigated the relationship between the sprung mass mode frequency and the dynamic tyres forces generated by heavy vehicles. A common approach to describing the dynamic tyre forces was proposed by Sweatman (1983), where the root-mean-squared dynamic tyre force divided by the static force, known as the Dynamic Load Coefficient (DLC), is used. The OECD (1998), using a numerical simulation, found that a “strong relationship exists between the sprung mass frequency and DLC.” Figure 3-8 shows the relationship between the DLC and the sprung mass frequency for a vehicle with a suspension damping ratio of 0.20.

Figure 3-8: The relationship between the sprung mass mode frequency and the DLC for a vehicle with a suspension damping ratio of 0.20, from OECD (1998).