Roll coupled combos – the pros and cons
The proposed introduction of Quad B PBS combinations grossly abuse the following gross assumptions:
1. The roll resistance of standard air suspended prime movers is invariant of the gross combination mass (GCM) and operational conditions
2. The roll resistance (or stability) of roll coupled combinations is higher than a pivoted draw bar hitched combination at similar GCM
3. The extent of off tracking, both low speed and highway speed, of roll coupled combinations is comparable to that of pivoted draw bar coupled combinations
4. Roll coupled combinations can adequately tolerate combined macro variations in road elevation, amber and curvature.
To evaluate the first assumption it is necessary to first examine the roll resistance of standard air suspensions. Now the roll resistance of air suspensions is governed by the following parameters: the air spring track, the instantaneous air pressure in the air springs, the suspension’s inherent roll resistance and the anti-roll bar contribution.
All other parameters aside, the roll resistance of an air suspension is primarily dependent on the instantaneous air pressure in the air springs. Notably, in operation air suspensions exhibit the highest roll resistance when the mean air spring pressure is maximal. Hence, in operation it is vital to maintain the air spring pressures at the highest extent possible.
Maximisation of the in service air spring pressures demands dynamic load sharing between the axles in each axle group and minimisation of frame rise, especially that of the drive axle group. Simple fact, at GML at any given location the frame rise of the proposed Quad B will be some 240 per cent and 160 per cent greater than that exhibited by a six-axle triaxle semi or tri-tri B-double, respectively!
This vastly increased frame rise extent effectively will cause the prime mover’s drive air suspension to operate on marshmallows. Hence, the roll resistance of the air suspended tandem drive of the Quad B combination, in service, will be considerably lower than that exhibited by the semi’s tandem (or B-double for that matter) drive suspension at the identical location.
The very serious implication of this deteriorated roll resistance is that the risk of prime mover loss of control on the Quad B-combination will be considerably higher than that of the semi’s (or B-double) prime mover contacting an adverse road detail. Unfortunately, computer simulations (which assume perfect smooth and flat road surfaces) of high productivity vehicles (HPVs) completely ignore the effect of frame rise and the associated deterioration in the prime mover’s drive axle group roll resistance. Hence, such simulations completely fail to predict the high risk of prime mover loss of control when HPV prime movers contact adverse road details.
Examine now the second assumption. This assumption originates from the situation where a combination first contacts and then passes over a single isolated adverse road detail (say an edge depression). Should this ‘out of phase’ edge depression pass over situation be effected at slow speed the axle groups remote from the adverse road detail counter to a large extent the roll disturbance inflicted to the combination by the axle group contacting the adverse road detail.
However, at higher operating speed the same single adverse road detail may cause the extent of roll disturbance to increase as the combination passes due to resonate phenomena. The same resonate phenomena may also occur should multiple periodic in phase adverse road details be contacted. In this situation the roll disturbance may rapidly increase in extent so much so to cause the vehicle to roll over.
Here lies a major problem exhibited with roll coupled combinations. Namely, roll coupled combinations roll as a complete unit. In comparison, with pivoted draw bar coupled combinations it is usual for only the last trailer unit to roll over. This advantage alone attracts unquantifiable safety benefit to the prime mover cab occupant/s, the prime mover and lead trailer/s. The same so associates with vastly lower insurance risk.
It is now appropriate to examine the incidence of ‘in phase’ adverse road details. Unfortunately, the frequency of these very adverse road details is rapidly increasing due to the near saturation use of B-double combinations operating at more or less identical axle group spacings. The fact is that when an air suspended axle group contacts a road or edge depression the contacting axle group immediately unloads. The load deficiency is, in turn, immediately transmitted to the adjacent axle group/s. The result is that an adverse road detail will form one axle group spacing fore and aft of the original adverse road detail. These adverse road details, obviously, include the generation of road and bridge waviness (i.e., that exhibiting relatively long wave length).
As suggested, the prime mover of a roll coupled HPV will incur a roll disturbance (hence roll-induced steering disturbance) as edge axle group in the combination passes over a adverse road detail. Subsequently, the ride in a roll coupled HPV will be vastly inferior to that of a road train combination. The more adverse and increased steering demands, in turn, will expose the roll coupled combination’s driver to higher fatigue loading. Most seriously, driver fatigue continues as the lead cause of HV accidents.
It is now appropriate to examine the paramount roll resistance consistency of the roll coupled combinations. Here it is necessary to review the foregoing brief discussion. It was made evident that a roll coupled combination operating slowly on a tarmac like surface will generate increased ‘effective’ roll resistance to the combination.
However, in service at typical operating speeds on typical local roads a roll coupled combination can, and frequently do, incur complete combination roll over, including the prime mover. Hence, roll coupled combinations in actual service experience an extreme range of roll behaviour from strongly supportive to grossly adverse.
Unfortunately, this adverse range, when acting at its worst, catches drivers out since it is near impossible to recover. The inability to recover is exacerbated by the increased frame rise extent at which the drive axle group operates. In comparison say to a double A or road train combination the driver only has the assistance or hindrance of the roll disturbances generated by the lead trailer’s rear axle group. This confirms that the roll resistance of a pivoted draw bar combination is far more consistent relative to that exhibited by the roll coupled HPV counterpart.
In regard HPV tracking our grandfathers identified that each pivoted draw bar coupled unit will either oversteer or understeer dependent on whether the draw bar length is too short or long, respectively. It simply follows that optimal length draw bars will cause the towed unit to track as per the tow vehicle, independent of speed. Extrapolating the typical camber induced 30-45mm of off tracking of B-doubles operating the Hume Highway to that expected for a Quad B, the extent of camber induced off tracking will be some 60-90mm. Notably the rear axles (or in fact the rear axle groups) of Quad B-units will be typically tracking along the unsealed shoulder. Subsequently, every time the same axles reconnect the pavement the Quad B-unit will experience a major disturbance.
Such disturbances generate high risk of loss of control, especially noting the prime mover is operating at relatively increased extent of frame rise.
Simple logic confirms pivoted draw bar coupled
“... pivoted draw bar hitched combinations exhibit far superior ... roll stability than roll coupled combinations.”
HPVs exhibit far superior tolerance to simultaneous macro variation in road surface elevation, camber and curvature than a similar length roll coupled HPV. The same reduced tolerance exhibited by the latter HPV vividly imply that roll coupled HVs inflict significantly more damage to themselves and the roads than do pivoted draw bar HVs. In regard to computer simulations of HPV PBS characteristics these again completely ignore actual road conditions. Unfortunately, real local roads are vastly removed from airport tarmacs as all HV drivers well know.
In conclusion, all HPVs must utilise dynamic load sharing air suspended axle groups, especially on their drive suspensions. In service on local roads, pivoted draw bar hitched combinations exhibit far superior and consistent roll stability than roll coupled combinations. Furthermore, ‘A’ combinations track far superior at both low and highway speed, exhibit superior tolerance to actual road conditions, incur vastly less vehicle damage and inflict vastly less road damage relative to that exhibited by ‘B’ coupled units.
In closing, it is paramount HPVs operate safely and with minimal driver fatigue on local roads by utilising the maximum extent of articulation. Roll coupled Quad B-combinations should stay where they were born: in grossly crude and over simplified computer simulations.