Big Test

Mighty Merc

Story: Brian Cowan | Photos: Gerald Shacklock - Video: Trevor & Hayden Woolston

Location: The Saddle Road, across the Ruahine Ranges. Time: Nightfall, early winter.  Conditions: Heavy drizzle, turning into steady light rain as the climb progresses.


Payload: Fabricated metal bridge beam – 22 metres long, by 2.2m high….and weighing 50.5 tonnes. Holy rows of eight Batman, this sounds like a job for a REALLY serious heavy-haul combination!


And that’s what we’ve got Robin – in the form of Tomoana Warehousing’s Mercedes-Benz Arocs SLT 8x6 and its associated TRT trailers.


The truck – currently the only one of its type in the country – has been purpose-designed for this sort of work. Even so, it is really being put to the test tonight.


While the beam isn’t the heaviest (nor the longest) of the more than 20 that Tomoana has already carted from Eastbridge in Napier to Wellington’s Transmission Gully motorway project, Merc driver Dave Harris admits that these conditions are as challenging as any he’s encountered on this run. And he’s concentrating...every metre of the way.


Through the tightest bends on the uphill slog from Woodville, he’s keeping the big tractor unit as high on the road as he can, searching for the less-wet patches of seal to ensure the best-possible traction for the three driving axles. Even so, we have a couple of heart-stopping moments when the tyres slip, very briefly, before regaining grip.


The potential pucker-factor is even higher after the summit, as we begin the steeper descent towards Ashhurst. In this case it’s the possibility of too much braking force that keeps Dave’s focus keen.


The Arocs is fitted with Merc’s new turbo retarder clutch – an option that integrates the torque converter starting clutch for the truck’s 16-speed PowerShift 3 AMT with a hydraulic retarder.


Developed by Voith, the unit combines compact dimensions and a weight around 80 kilograms less than conventional systems to deliver stupendous stopping grunt – up to 750 kilowatts/1005 horsepower in the higher of its two settings...and that’s on top of a three-stage engine brake.


In simple terms, the retarder should theoretically be able to steady an Arocs SLT loaded to its rated maximum GCM of 250t on a one-in-three slope. On this trip the Tomoana combination tips the scales at a bit under 108t, so we’re probably good for one-in-seven!


That’s on a dry road however, which is why Dave is taking it very cautiously as the rain settles in....much to my relief, as his passenger. He expresses a bit of disappointment at the conditions: “In the dry, I’ve been able to come down here with 20t more weight, a couple of Ks quicker and a ratio higher, and not touch the service brakes once, and I’d have loved you to see that.


“This retarder is magic. However, under these conditions it could easily lock the wheels. There’s no thrill in doing it wrong, and with the loads we’re carting, if it does go wrong it can be truly spectacular.” Thankyou, Dave….I’m happy with unspectacular!


The NZ Truck & Driver team will be leaving the Arocs at Ashhurst, while the truck and its three attendant pilot vehicles carry on southwards. It’s now fully dark, so no point in trying for more photos, and the rest of the run to the works site is relatively easy.


But the hours we’ve spent with Dave and his new Merc have certainly been instructive, beginning around 1pm when we meet the truck and the Tomoana team at the yard of NSB Infrastructure in Napier, where the beam has been sandblasted and freshly painted and is now being readied for the trip. The structure hasn’t moved far from its origin – it was built at Eastbridge, a block away....




We venture into the realm of the heavy haulers with this Trevor Test, when we catch up with Tomoana’s Mercedes-Benz Arocs 3363 AS.

Its heavy in every way, with a truck and trailer combined tare weight of 57 tonnes and a loaded weight today of a modest 107 tonne. It’s rated with a 33t GVW and 250t GCM.


Making up the weight is the giant Mercedes-Benz 8x6 tractor unit, hooked up to a TRT gooseneck trailer with three rows of eight and a rear electronic self-steer trailer with four rows of eight.


The bridge beam that we’re carrying today is not actually one of the bigger ones – weighing-in at just over 50t and destined for the Transmission Gully motorway project.


The Trevor Test kicks off in Waipukurau, on our way down to the Saddle Road, where the Arocs will cross over into the Manawatu and then carry on down State Highway One to Paekakariki.


Climbing up into the cab is very easy despite the height of the floor from the ground. There are three really good, deep, wide steps – each one slightly inset from the step below.


There are also cleverly-designed handholds: There’s a long handrail up the side of the door opening and a handhold in the side panel to the left of the steps. Once you’re on the first step, you transfer your left hand to another rail that runs up the back of the door opening.


When we pull away, the first thing I notice is the nice, clean shifting of the G280 16-speed AMT, which is set in Auto Heavy mode. In this setting it tends to hold the gears at higher revs and Dave feels it’s sluggish. We discuss this at length during the drive and it is clear that it is revving out past the maximum torque levels…and this does give the feeling of it running out of power.


First impressions are that it could upshift around 300rpm lower, keeping it in the optimum torque range. However, there is a good reason for its late upshifting – and that is to protect the drivetrain, as we are talking about a lot of torque…combined with very high weights. So the settings in the gearbox are kept more conservative to reduce strain...




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