Should I buy a cab-over or bonneted truck?

Australia and New Zealand are unique in the road transport world because they’re the only countries in which there’s a roughly equal split between COE (Cab Over Engine, or cab-over) and bonneted heavy vehicles. In Europe, Asia, China and Japan, almost every truck is a cab-over, while in the Americas most heavy trucks are bonneted.

The reason why heavy trucks vary in shape and size in different in parts of the world is all down to regulations and we’ve spelt that out in detail in a previous story.

So, when it comes to a decision between a cab-over or a bonneted truck, the first hurdle is considering whether your particular application requires one type or the other, regardless of your personal preference.

Every country has axle spacing regulations that are designed to protect bridge structures – hence the name ‘bridge formula’. The long spacing of axles on trucks and trailers puts predictable loadings at longer points on a bridge, in contrast to close spacing that allows more weight in one section of the bridge.

Although USA road transport regulations are quite different from Australia’s, and their upper GCM limit is a very restrictive 40 US tons on the Interstate Highway system, there are two common factors: the Yanks have relatively low legal maximum weights per axle and they also have relatively long spacing between leading and trailing axles of a combination and between axle groups.

In those two respects Australian regulations are similar and it’s important in some applications to have a very forward-positioned steer axle and the last trailer axle right at the back of the combination.

USA-sourced bonneted trucks have a choice of forward-set and rearward-set steer-axle positions, depending on the particular truck model in the range. Even US cab-overs have forward-axle positions, in comparison with European cab-overs. That made US-sourced trucks a great fit with Australian regulations, even with the advent of the first B-doubles here, back in 1984.

Cab-over prime movers were essential for the original 23-metre B-double rigs and the bridge formula favoured set-forward front axles. By that time, the K100 had virtually disappeared from the North American market, so the fact that Kenworth could build a forward-axle COE in Australia, independently of its Seattle parent, gave it the bulk of the early B-double business.

Eventually, the bridge formula dimension was relaxed and set-back-axle prime movers from Europe could compete, so KW put in longer front springs and set the steer axle back slightly, to improve ride quality. When 25-metre and 26-metre B-doubles were allowed, bonneted prime movers could compete once more.

So, why aren’t all prime movers in Australia bonneted?

Well, the relaxation of bridge formula regulations meant that European and Japanese prime movers that had set-back front axles could also couple to B-double trailers, and these makers worked hard at converting Australian truck buyers to accept their ‘vertically integrated’, one-stop-shop truck design. They also stressed their higher levels of dynamic and accident safety.

Another important marketing initiative was to provide service contracts and guaranteed buy-back schemes, so that buyers knew their costs in advance.

Easier cab access for the driver used to be an advantage of the bonneted truck, but today’s ‘staircase’ access on most COEs has negated that to a large extent. However, with older Kenworth K100s, it’s important to start on the access ladder with the correct foot, or you’re stymied when you climb up to the door.

Cab interior and sleeper space also used to be the province of the bonneted truck, with its separate sleeper module, but comparable-model, current COE and bonneted trucks are on a par. Likewise, cab-over sleepers, being on top of the engine and transmission, used to be hotter and noisier than bonneted truck sleepers, but modern ones are much more civilised.

Perceived accident safety is obvious in the case of a bonneted truck, because the engine projects out front as a ‘buffer’ for the cab and its occupants, but real-world safety doesn’t seem to be affected by that as much as overall cab integrity. Both truck types can be built to comply with ECE and Swedish cab integrity standards.

Back in the bad ol’ days, a bonneted truck had ride quality superior to that of a cab-over, because the wheelbase was longer and the driver didn’t sit over the front axle. However, modern COEs have long, parabolic front springs, or air springs that provide excellent ride quality.

Bonneted trucks with set-forward steer axles normally have a harsher ride than set-back models and COEs.

Physically doing daily checks is easier on bonneted trucks than on COEs, because all you have to do, in theory, is undo a couple of external, over-centre catches and tilt a bonnet that has torsion bar assistance. However, having to lower a bullbar complicates matters.

An under-cab check on a cab-over is more complicated, because you have to stow everything that can crash forward when you tilt the cab. However, most cab-overs don’t need a cab-tilt for daily checks, because a lift-up flap on the cab front gives access to fluid reservoirs.

On the latest COE and bonneted trucks, daily check comparisons are far less important because fluid and electrical checks are done electronically and a readout in the cab assures the driver that all is okay underneath. There’s no need to lift or tilt anything.

An issue with narrow-cab, North American bonneted trucks is the conversion to right-hand drive. Because US engines are predominantly designed for left-hand-drive installations, the turbo and exhaust system is located on the right-hand side of the engine, facing forward. Juggling steering boxes and shafts causes plenty of problems that today’s wider-cab bonneted trucks don’t have to the same extent.

Manoeuvrability is the cab-over’s principal attraction, because the overall length of a COE prime mover can be shorter than a bonneted prime mover. That means less ‘swing’ room is needed when backing into narrow laneways or loading docks and the driver is sitting virtually at the nose of the truck, where vision of the truck’s front corners is better. How many bonneted prime movers have you seen with obvious fibreglass repair work to their front mudguards?

Streamlining reduces fuel consumption as we all know and, in theory, a sloped-nose bonneted truck is slipperier than a bluff-fronted cab-over. Kenworth started the streamlined bonneted truck trend with its original T600 ‘Anteater’ and there are streamlined prime movers available from all US-based makers. Coupled to tight curtain-side trailers, that prime mover design can save on fuel.

However, streamlining the prime mover can do only so much for an overall ‘slippery’ combination, so if your task is hauling logs in a trailer skel, or earthmoving gear on a float, a streamlined pointy bit won’t help very much with fuel savings.

Also, with three- and four-trailer road trains, it’s usually more important to have a huge radiator frontal area, to keep that hard-working powertrain cool, so streamlining is way down the priority list.

The last point we need to make is the importance of styling and fashion in truck selection. You only have to consider the runaway success of Kenworth’s Legend models to see just how important this can be.

A buyer’s head may favour a streamlined bonneted truck or a cab-over with a roof spoiler and cab side extenders, but the heart steers towards a flat-fronted big bonnet, with four-poster bullbar, huge external air cleaners and fat mufflers and stacks projecting into the airstream.

At the end of the day, it’s the old adage of, ‘You pays your money and you makes your choice’.