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22nd June 1995, Page 32
22nd June 1995
Page 32
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Keywords : Truck, Aerodynamics

o aero kits save you money? Last

year CM came up with some definitive answers when we took an MAN 38-tonne tractor around the Millbrook test track with, and without, the manufacturer's own cab streamlining kit (CM 8-14 September 1994). Our results confirmed that these kits do earn their keep—at 55mph and 60mph fuel consumption was down by 9.5% and 12% respectively.

A properly designed aero-kit can also work wonders on 17-tonne rigids: witness the much-publicised Argos Freighter with slippery bodywork developed by aerodynamics guru Val Dare-Bryan, who was also behind the original TNT aerodynamic artic and the ERF EC cab. Five years ago the Argos Freighter showed a 18% fuel saving running back-to-back with a standard 1718 Freighter box van around our Welsh middleweight test route (CM12-18 April 1990).

So much for the big stuff. But can operators of 7.5tonners expect the same benefits? To find out we went back to MilIbrook with an MAN 8.153F box Van.

There's been a lot of speed-limiter legislation over the past three years, but none of it has affected the humble 7.5-tonner. The 56mph EU limiter rules only apply above 12 tonnes GVW and, despite rumbles to the contrary, there are no official plans to extend limiter law downwards to the 7.5-tonne breakpoint.

As a result "non-HGV" rigids are still allowed to do 70mph on the motorway and 60mph on dual carriageways (see panel, below right). The wisdom of allowing a 7.5tonner to travel at 70mph (113km/h) is debatable; especially if it's in the hands of a driver who's never handled anything bigger than a family saloon. But plenty of operators claim to need the faster motorway journey times that a 7.5-tonner can provide, and they need to control their fuel bills just as much as operators of heavyweights. The faster you go, the more wind resistance you have to overcome and we were keen to see what savings could be achieved at 70mph.

• TEST PROCEDURE

On a normal CM roadtest we're at the mercy of the elements. The wind can (and does) blow from any direction around our 360km middleweight test route, making it unsuitable for any kind of definitive aero test.

That problem doesn't apply to the circular, banked Millbrook test track where we completed 25 two-mile laps with and without the aero equipment fitted.

Last year our 17.322 tractor had the luxury of a MANCATS computer-controlled limiter providing easy speed control around the Millbrook circuit. With our fully laden 7.5 tonner we had to rely on the accuracy of the test driver's right foot to maintain a constant speed: arguably this is closer to real-life and our average speed figures for each run show a pretty high level of consistency.

We then repeated the with-and-without comparison at 56mph to find out what fuel benefits might be achieved by holding a 7.5tanner down to the EU motorway speed limit for heavier trucks.

As before we measured fuel consumption with a flow meter but we also carried out some 0-70mph sprints to assess the effect of aerodynamics on acceleration.

Our 8.153F was the same truck which CM road tested last summer (CM 440 August 1994) and with 60,000km on the clock it was more than run-in.

• AERODYNAMIC EQUIPMENT

MAN launched its aerodynamic body kit range at last year's IRTE show. The system on CMs L2000 is a bit of a pick'n'rnix. The top GRP cab deflector and side collars are standard MAN accessories (similar systems are available for M90 rigids and F2000 tractors). The top deflector fixes directly on

to the cab roof via captive nuts and costs £367 (ex-VAT); the collar costs £351.

Our 8.153F also had GRP side and rear valances supplied by Ipswich-based Kuda (UK) which makes aerodynamic equipment for a wide range of trucks. The side valances are bolted and rivefted directly on to the body frame or a supporting framework of galvanised channel—a hinged section gives access to the battery. Kuda charges £195 per body metre plus fitting.

MAN Truck & Bus(UK) is happy to supply either a full kit or top deflector only, leaving an operator to add a collar at a later stage. "The majority of saving comes from the blade" it says, "because it's getting rid of the flat-fronted truck. The blade probably produces 60-65% of the benefit."

But Glenn Whatling of Kuda (UK) warns that if you don't fit the collar you retain the vortex between the cab and the body which creates drag.

Ideally the truck should penetrate the air smoothly, pushing it aside into a bow wave which then quickly falls back along the sides of the vehicle as the undisturbed boundary layer, flowing smoothly along the body until it reaches the back of the truck.

For the record the base body on the MAN truck was a 5.5x2.75m (18x9ft)JC Payne GRP dry freight box with well-radiused front corners.

111 TEST RESULTS

An effective aero kit can reduce a rigid's frontal area drag by up to 50%. In other words it would need 50% less horsepower to go through the air at a given speed. "A quick rule of thumb is that if you can achieve 50, saving in drag on a rigid it equates to 20, saving in fuel," says Val Dare-Bryan.

So can aerodynamic equipment reduce 7.5-tonner's fuel bill? Judging by our result the answer is yes, although we weren't able t match the returns delivered by streamline thoroughbreds like the Argos Freighter.

As before we've based our results on actua fuel used, rather than overall improvements ii mpg, as the latter can paint a more flatterinl picture.

At 70 and 56mph the aero-kit reduced fut consumption by just under 12%; not surpriE ingly the saving was (slightly) more prc nounced at 70mph.

Converting our fuel saving into an "rnp improvement" gives slightly higher figures c 13% and 12.7% at 70 and 56mph, but opera tom thinking of buying aerodynamic equic ment will calculate their payback periods a the actual fuel saved.

Although the improvement in fuel econom; was fairly consistent at 56 and 70mph, at th higher speed we could actually feel th difference.

With the top cab deflector, collars and aid valances in place the 8.153F had no troubl holding a steady 70mph lap after lap after lar even when running into a headwind.

Without the aerolcit it was a different truck To start with it took us an extra lap's "run-up to get up to the required speed; then we had t keep the pedal to the metal most of the time t overcome the extra wind resistance. The lad of an aero kit also made the MAN a lot mor skittish in crosswinds with more noise insid the cab.

Smoother aerodynamics certainly helpe the MAN accelerate up to 70mph—on averag the non-aero truck took 20 seconds longer t hit the motorway limit.

While the final figures more or les matched our expectations of a factory devel oped system, we're not sure why the L2000' 70mph constant-speed fuel economy shouli be so far behind the 19.5mpg (14.51ft/100km) i delivered on the 64-mile M4 section of our reg ular roadtest route last August. We assum there was more opportunity for freewheeling on the motorway than we had realised, and maintaining a constant 70mph on the Millbrook bowl meant driving with the engine under full load outside its solid green economy band for much of the time.

• CONCLUSIONS

Exaggerated claims from aerodynamic equipment makers have put many operators on their guard, and with good reason. There is no doubt that an aero kit can reduce fuel bills, but the scale of that saving will vary enormously.

Prime candidates would be high-mileage just-in-time operators running rigids on the motorway at up to 70mph. But if your 7.5-tontier barely gets into top gear and you specialise in short runs the payback period might be longer than your ownership of the truck. That said, Dare-Bryan warns: "It's too easy to make sweeping statements. Good aerodynamics can work at slower speeds—not least as local delivery operators tend to keep their vehicles longer—

but it's important to look at it very closely."

Actual savings will depend on an individual truck's specification and oper ation. But taking our the 8.153F as an example, if our non-aero rigid clocks up an industry average of 80,000km a year at 90km/h (56mph) it would consume 13,3241it (2,931gal) every year. With an aero kit this could be reduced by 1,504 litres which, at a nominal pump price of 55p/lit, represents a saving of around £827 a year. This is more than the cost of the basic MAN cab deflector and collar kit so the operator could expect to be in profit within 12 months.

Before you get carried away, few vehicles complete their annual mileage at a steady 56mph! Many 7.5-tonners spend their time on stop-start urban work, where aerodynamics have least effect, and this would have a dramatic effect on the payback period which will be much longer (see operator experience, right).

Talking with hauliers about aerodynamics it's clear that you get what you pay for. Specially designed and windtunnel-tested systems like those used on the Argos Freighter, and by TNT on its own artics and rigid trucks, are optimised to deliver the very best airflow over those vehicles bodies. A truck manufacturer's system is designed to work as part of the cab shape and can't always be expected to deliver optimal aerodynamics with every type of body on the market There are exceptions, however: notably ERF's adjustable aero system and the set-up on the Volvo FH12.

Similarly, an "off-the-peg" system offered by a proprietary aero kit supplier is bound to be a compromise if it has to suit a wide variety of cabs.

Perhaps the most sobering thought to come out of our test is what can be achieved by simply slowing down. Our 56mph "limited" nonaem 7.5-tanner may have taken 10 minutes longer to complete its 50 miles than it did at 70mph. But it used 4.9 litres less fuel to do it. So if speed isn't of the essence—and how many operators really need to run their 7.5tanners at 70mph?--why blow money out of the exhaust pipe?

As one operator told us: "You can fit all the devices you like on a truck but it still comes back to the driver's right foot."

L by Brian Weatherley

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Organisations: European Union
Locations: Ipswich

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