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Heavy vehicle suspension testing and analysis : dynamic load sharing

By Lloyd E. Davis and Jonathan M. Bunker


When air-sprung HVs were granted concessions to carry greater mass at the end of the 1990s, Australian road authorities knew that air-sprung HVs with industry-standard (or conventionally sized) air lines between air springs did not load share in the dynamic sense. It was known at the time that concomitant increases in dynamic wheel loads from air-sprung HV suspensions as a result of ineffective dynamic load sharing had the potential to cause greater road damage than might otherwise be the case should air-sprung HVs have incorporated more dynamic load equalisation into their design (OECD, 1992, 1998). Noting that perfect load equalisation would give a LSC of 1.0 (Potter, Cebon, Cole, & Collop, 1996) LSC values (Sweatman, 1983) for steel suspensions were documented in the range 0.791 to 0.957. Air suspensions were placed somewhere in the middle of this range with LSCs of 0.904 to 0.925. This was a decade before, and referenced in, the first OECD report (OECD, 1992). In fine, the effects of poor dynamic load equalisation were published and known at the time of granting air-sprung HVs concessions to carry greater mass at the end of the 1990s. With the clarity of hindsight, the disbenefits due to higher road network asset damage may not have been recognised as having the potential to discount the societal and economic benefits of higher HV payloads. Two reports commissioned by the NTC (Estill & Associates Pty Ltd, 2000; Roaduser Systems Pty Ltd, 2002) have recommended, inter alia, evaluations of countermeasures, via testing, which have corrected HV handling problems in air-suspended HVs. The Roaduser report, p68, recommended tests to assess the effect of installing larger “pipes” or air lines so that, p31, “longitudinal air flow between axles is increased; this should improve the load-sharing capability of the suspension; in both cases where this was implemented, it was reported to fix the problem” (Roaduser Systems Pty Ltd, 2002). Further, as far back as 2000, the NTC had the recommendation put to it on p32 of the report by Estill & Associates to “investigate and evaluate ‘after market improvements’ to air suspensions” from installation of “larger diameter pipes to supply and exhaust air flow to the bag quickly and hence improve the response time of the air bag. The modification also reduces the roll and has improved stability.” (Estill & Associates Pty Ltd, 2000). Since then, the 2005 test programme funded by the Queensland Department of Main Roads (Davis, 2006b) and the 2007 test programme (Davis, 2007; Davis & Kel, 2007) comprise the only known published testing of HVs with larger longitudinal air lines since those recommendations were made. Previous work (Davis, 2006b; Davis & Sack, 2004) has shown that RFS do not load share dynamically when in multi-axle groups. That testing, in Feb 2003 (Davis & Sack, 2004), was on a semi-trailer fitted with standard longitudinal air lines (6.5mm inside diameter, 9.5mm outside diameter). The results showed that the transfer of air between air springs on the test vehicle was in the order of 3 s. Simple logic yields that if the axle spacings on a HV are 1m apart at their closest (worst case), then at 100 km/h (27.7 ms-1) the reaction time for air to start to transfer between air springs as described above needs to be in the order of 1/28 s (0.036 s) for any reasonable dynamic load sharing to occur. This value may be relaxed to about 1/21 s (0.047 s) for axle spacings of 1.3m at 100 km/h. Hence, air transfer with time constants in the order of 3 s will not load share dynamically, causing more distress to the road network than the case where air-sprung HVs have a better ability to load-share than the current fleet. Quad-axle semi trailers are being introduced to Australia. If previously the inability of air suspensions to equalise (say) 22.5 t loads across tri-axle groups resulted in unequal loadings on one axle over another for that group, the emerging scenario will be 27 t similarly imbalanced within a group of 4 axles. Arising from this, road authorities in Australia, officially or otherwise, are becoming increasingly concerned that HVs with air springs are not as sympathetic to the network asset as they might otherwise be. Recent work on tri-axle and quad-axle semi-trailers (Blanksby, George, Peters, Ritzinger, & Bruzsa, 2008) has substantiated that load sharing in air-sprung HVs with conventionally sized air lines does not occur in the dynamic sense, confirming current concerns. Davis and Bunker (Davis & Bunker, 2008b) described the testing and frequency spectrum analysis of the suspension forces, including the air springs in three HVs. In the results of that study, body-bounce forces predominated in the air spring spectra, regardless of speed. That study also showed that, as speed increased, so did the magnitudes of axle-hop and concomitant wheel forces. Maximum transfer of air from one air spring to its associated rear air spring could be seen to be an ideal situation for load equalisation. However, the practicality of the phenomenon of axle-hop requires that some imperfection needs to be introduced into the transfer mechanism to reduce the possibility of standing waves in the air spring connector exciting sympathetic oscillations in neighbouring air springs. This phenomenon has been modelled and the outputs are presented in this report. Axle inertia combined with suspension damping act to de-couple the pavement frequencies from the chassis. Alternately, another explanation in systems engineering terms is the suspension acts as a low-pass filter, isolating high-frequency road irregularities from the chassis. A result of the suspension design meeting one of its criteria in that the range of frequencies measured for the unsprung masses below the axle is not the same as the resonant body bounce frequencies. This effectively isolates the chassis as much as is possible (and therefore the payload and/or the passengers) from the harshness and vibration due to pavement irregularities. The rationale for work in this field serves both road authorities and the heavy vehicle industry. The benefits of increasing the load sharing ability between consecutive axles have been shown previously in the joint QUT/Main Roads project Heavy vehicle suspensions – testing and analysis. Better load sharing than found on most current HV suspensions would reduce wheel-forces, reduce body vibrations, lower chassis and suspension forces and provide a more comfortable ride for passengers and drivers. The benefits of these measures would be reduced road damage, reduced payload damage (especially for fragile goods) less fatigued drivers and passengers and greater life from heavy vehicle chassis, suspension and coachwork components. Other work in the joint QUT/Main Roads project Heavy vehicle suspensions – testing and analysis has postulated that an imperfect transfer of air between air springs by the use of (say) some constriction device, such as a smaller pipe, to join the connection mechanism to the air springs would be advantageous in that it would damp out pneumatic excitation of resonant frequencies in such air spring systems (Davis & Bunker, 2008b). The results and analysis in this report bear out this contention. Increasing load sharing fraction (LSF) values, implying as lessening of the constrictions between air springs are useful for reducing dynamic peak loads up to a point. After a particular value of load sharing fraction, to be determined by other research beyond the scope of this project, further increases in load sharing fractions between air springs allow axle hop and the associated ride harshness to be transmitted into the chassis of the vehicle

Topics: 091304 Dynamics Vibration and Vibration Control, 090299 Automotive Engineering not elsewhere classified, 099999 Engineering not elsewhere classified, heavy vehicle, suspension, dynamic load sharing, load sharing
Year: 2009
OAI identifier:

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