THE FIFTH AMENDMENT
Page 98
Page 99
If you've noticed an error in this article please click here to report it so we can fix it.
Fitting a fifth wheel assembly to a tractive unit is normally handled by the truck distributor, but there is no reason why other workshops or operators themselves should not carry out the work. Bryan Jarvis sees how it is done.
Fifth wheel positioning stems from the truck manufacturers recommendations based on the standard dimensions for the maximum imposed load at UK plated weights.
But these are tempered with the need to ensure adequate swing clearance between cab and trailer bulkhead while still providing optimum axle load distribution.
Where an operator requires a nonstandard artic combination for an unusual but dedicated application, the truck builder will make the necessary calculations to arrive at the most suitable position for the fifth wheel.
To find out what is involved with equipping a tractor chassis, Workshop visited Scania dealer Scantruck at Purfleet in the lea of the new Dartford river crossing.
The subject vehicle, a new Scania RI131Y1A 6x2 fitted with a lifting/trailing rear axle, is a stock vehicle that is designed for use with full-length 13.5m artic trailers. Scantruck specified a Jost sliding fifth-wheel assembly, but will fit most makes.
The fifth-wheel kit is supplied shrinkwrapped on a pallet and comes complete with electrics, air hoses (suzies) bridge plate and all other fittings and spray suppressant material, in this case Monsanto Clear Pass.
Keith Lilley, who has worked for Scantruck for the past seven years, took us through the fifth-wheel fitting operation, for which there is an eight-hour allowance.
Lilley starts by removing the suzies and electrics from the fitting kit, identifying and preparing them for pre-assembly. While VBC Dayton includes fitting instructions in the kit, Jost do not do so as yet, but says it is preparing an information sheet.
With the suzie bridge plate in the vice, the two electrical sockets are fitted and the wiring harnesses and dust covers added. All circuits except the highintensity rear fog light and the ABS system (if permanently powered) pass through one cable, allowing room for auxiliaries in the other.
Both 90° air-feed unions are fitted to the bridge plate and angled towards the vehicle's feed pipes and check valves. The two quick-release, self-sealing suzie couplers are then attached to the trailer side of the plate, which is secured to the crossmember over the gearbox with two bolts. Blanking plugs are unscrewed from the nylon air pipes — noting which one is pressurised. That one must then be secured to the red emergency-line suzie connection. The electric cables are routed inside the right hand chassis rail, carefully clipped to the main harness at intervals then fed into the large junction box inside the frame end. Using the wire stripping/crimping tool spade terminals are fitted and connections made. While all the wires are colour-coded, Scania publishes recommendations on which wires should be
connected to which terminals.
Lilley uses a Traktrail electrical test unit to check the lighting systems, and ensure that every circuit functions correctly. Connecting a trailer to carry out these checks is more time consuming but just as effective.
Next the prefabricated lead-on-ramps are placed on the back of the Scania's frame with the tail in line with the end of the rails.
After pre-setting the fifth wheel into its mid-side position it is lowered into place on the chassis using a dummy kingpin with 1.fting eye.
ThE specification sheet for the R113MA 6 x 2 R360 says that at the UK plated weights the pin should be located 115m:m behind the drive axle's centre line. This works out at 40mm between the tail of the fifth wheel plate and the front of the ramps.
Piloi: holes
Having positioned the fifth wheel and lead-up ramps all the holes are scribed out and centred before lifting the coupling back off the chassis. Pilot holes are drilled first to make it easier to get the larger drills .:hrough the chassis. These are clearance holes which should be about 17rnm in diameter for the fifth wheel mountings and the ramp's two forward brackets, and 13mm for the remaining four ramp mounting bolts.
Lilley removes all the swarf from the chassis top and sides before repositioning the fifth wheel and ramps, alligning the bolt holes in the process.
A pneumatic wrench and combination spanner is used to tighten the mounting bolts supplied with the kit. Tightening is completed with a torque wrench; the set
tings are 244Nm (1801bft) for the 16mm bolts and 98Nm (721bft) for the remaining for M12 bolts.
Jost's fifth-wheel kit also includes thermoplastic mudwings and mounting brackets which complement Scania's rigid wing stays. Adjustable stands are used to keep the wings in position while Lilley drills the top mounting holes, being careful not to hit the tyre. Once those are bolted up (using the bolts and large washers supplied) the lower holes are marked out and drilled, and the wing is secured.
When securing the rear wings Lilley checks to ensure that there is the necessary clearance to allow the axle to lift fully. To help this the securing bolts over the wheels are fitted with the heads on the inside of the wing. The rear wings must also sit below the top edge of the ramps to ensure that the trailer does not hit the wings when turning.
When fitting the spray suppressant material to the wings it must run from at least 100mm above the wheel's centre line to not more than 200mm above ground level, and it should cover the whole width of the tread.
The flaps are drilled and secured inside the wing by three bolts at the stay and four bolts at the base. This is one more than recommended but, says Lilley, when snow, ice or mud collects on it the spray suppressent doubles in weight, so that extra rigidity does not go amiss. An extra hole must be drilled in each rear flap for the small red reflector bracket.
Lilley's last task is to clean the wheelarch areas of any plastic swarf before the vehicle is signed off.