Drawbars: Rare birch coming out of hiding?
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HEREAS at the moment at 32 ns many operators today are stricted by weight rather than atform length, this situation uld change at 38 or 40 tons. Though in extensive use on a Continent, the drawbar has where near that popularity in a UK and so many operators 3 unfamiliar with the pros and ns of rigid and trailer as part of ransport system. There is cernly a lot more involved than arely hooking a trailer on to a back of a rigid.
VBG of Kettering runs a conRaney service to operators lo are thinking of moving into 3 drawbar field. Though the mpany makes the couplings, d thus has a declared interest that particular aspect, no commendations are ever 3de as to make of lorry or 3ke of trailer. VBG likes to ream n 100 per cent neutral on this erely quoting dimensions :her than make.
VVhen called in, the first thing VBG asks, according to Tom le the company's drawbar ecialist, is what the operator wits to carry — usually he has 'eady come up with a rough ecification of his own.
auite often this will evolve )und two equal body lengths
say, 24ft. VBG would then c, does he need 2 x 24f1 bodies does he merely need a total of It of load space? Not quite the -ne thing.
-laving sorted out the basic rameters, VBG will supply a tailed written spec which, as antioned earlier, is totally imrtial as to make of chassis or iler. As well as going into fine tail on dimensions, the report II also cover the small points. For example, it will recommend changing the square number plate on the rear of the prime mover for an oblong one. Why? Very simple — the square type projects downwards too far and is likely to be knocked off by the drawbar as the outfit goes round a corner.
A small point, but it does indicate VBG's attention to detail.
The VBG recommendation might go against some of the operator's first thoughts. For example, he might want to go to a system using dollies which might allow use of existing semi-trailers. So at this point it might be worth explaining the various types of drawbar which can operate within the same overall concept.
Categories It is possible to subdivide the drawbar concept into three distinct categories. The first is a full drawbar trailer on which the front axle steers, and then there is the single axle or close coupled twin-axle trailer with a fixed A-frame. To these must be added the dolly converter.
This consists of a separate axle and A-frame equipped with a fifth wheel. There is no great advantage to this system unless the operator needs to be able to interchange regularly with semitrailers. It is viable in this case but otherwise it wastes weight and is difficult to handle. On top of this the dolly needs plating and is almost as expensive as a full trailer.
Again it is possible to have two types of dolly; one has a non-oscillating fifth wheel and a hinged A-frame, while the other has a fixed A-frame and an oscillating fifth wheel. The first one acts as a drawbar until the brakes are applied, when the brake torque tends to cause a nosedive with the only thing keeping the semi-trailer on the fifth wheel being the bottom rim of the kingpin.
With the fixed A-frame and oscillating fifth wheel, the latter and the kingpin are then doing what they were designed to do. The brake torque doesn't just disappear, though; it merely moves further up the outfit to the jaws of the prime mover where it sets up a vertical oscillating force.
In certain traffic conditions it is perfectly feasible to get a vertical force of over four tons acting on the drawbar eye so it is essential to use the right type of coupling (that is, a load-bearing type).
The standard drawbar coupling allows too much vertical movement, but this motion is cut down to a minimum with the load bearing type. Although the dolly is technically capable of being used in a drawbar system, it must be with the right coupl ing and even then VBG is n particularly enthusiastic unle the semi-trailer interchange ab ity is essential, By far the most popular ar most efficient system is the ft drawbar with a steering fro axle. The operator is not buyir extra weight or complexity and is, in theory, easy to handle. include the words "in theory as this is where the subject drawbar geometry comes in.
Confusion Drawbar geometry cove' every aspect of the coupling i cluding the cross-member position on the forecarriag hinge system and so on. There no set standard in Europ though there are signs that th is changing at least as far as tt drawbar eye diameter is col cerned with most people turnir to the ISO standard of 50mi (1.97in).
Currently, there are anotht seven national standards inclut ing the Swiss (34mm pin use with'a 35 or 40mm eye), the Ge
an 38nirn pin with 40mm re), the French (50mm pin with 5rnm eye) and so on.
In the case of the coupling iws these should never be lounted in a conventional cross lember whether the coupling is I-chassis or underslung. The Dnventional cross-member of a gid just isn't designed for the 3rt of loading it is likely to get. his does not of course apply to factory machine specifically inmdecl for drawbar operation ut to a conventional rigid which 3 being converted to the 'stem,
rawbeam So it is essential to replace the tandard cross-member with ne designed for drawbar loadig and VBG is so conscious of le pitfalls that to avoid confu sion the drawbar cross-member is called a drawbeam.
Currently, about 90 per cent of drawbar operators go for an underslung coupling in preference to the in-chassis type, and there are certain well-defined reasons for doing so. The in-chassis mounting is a big contributory factor to poor handling as by definition it means a short Aframe which makes for nervous handling as the trailer tends to react too quickly.
With the underslung frame it is possible to use a longer drawbar and still stay within the overall length limit.
The optimum length of an Aframe as far as Tom Dee is concerned is in the region of 1.8 to 2.7m (about 6 to aft). The inchassis coupling prevents this as most operators want to maximise the load space by having a narrow gap between the rigid and trailer. A good rule of thumb to apply to this problem of coupling location is that the further away the coupling goes from the rear axle, the more likely is the tail to wag the dog.
It is a fact of life that most roads are not exactly billiard table flat so with the underslung coupling there is the problem of vertical articulation to think of. Angling the rear of the chassis avoids the drawbar fouling on the frame and it also gives the driver easy access to the coupling.
Use of the in-chassis coupling also leads to problems with drawbar axis height. The rear coupling height is so different from the front coupling that it is virtually impossible to attach the trailer to the front for nosing in. The lower height of the underslung A-frame makes this variation far less of a problem.
One of the worst drawbar combinations from the handling point of view is that of little trailer/big lorry. The ideal is a short prime mover and a long trailer, the combination of which will "handle like a dream" quote Tom Dee.
Brake conversion It is of course perfectly fee ble to convert an existing sc rigid to drawbar operation. F example, many two-axled chE sis, limited to 16-tons solo by and U regs, are plated up to tons gross train weight.
Assuming the power-t weight ratio and driveline spe fication are man enough for t job, the only thing to do is rebuild the standard braki system up to that of a tracti unit. As well as adding the exi line, this usually means char ing the park brake set up a putting in extra tanks.
One very good recomrnenc tion which Leyland makes on t subject of drawbar conversion that at least two axles of the ric should be equipped with spri brakes as one axle might not able to cope. Spring brakes the trailer are a good idea Tom Dee's opinion, as the cc compares favourably with 1 maintenance and renovati necessary to get a mechani park brake through the anni test.
With long wheelbase chess putting in extra relays will h( in getting a better response tir on the trailer, Although ma operators check on the driveli suitability of their vehicle befc conversion, many of them forç to see whether the compresc will have enough puff to cope whether it will be running f out all the time with the additi of the trailer.
Most drawbar operators are the own-account sector a many of their vehicles have return empty. An empty draw' outfit can be difficult under br, ing so load-sensing is essenti
It is advisable to have all air lines (which should not coiled suzies as for an antic) I between rigid and trailer to Ii mounted couplings via an t derslung route. This kee everything out of the way duni cornering.
A lot of damage can be dc to the coupling if the relat height of the eye and jaw is t far out. It is of course possible have someone standing there hold up the A-frame to line it but that rather implies twodrawbar operation.
VBG markets a drawbar I ance kit which is spring-loac to maintain the coupling hek automatically before, during a after coupling and uncoupli without driver attention. A scr adjuster is incorporated to ei ble the device to have an aim universal application.