Roll cou­pled com­bos – the pros and cons

Owner Driver - - Your Say - Arnold McLean Keirav­ille NSW 2500

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.

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