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4WD - is chassis or unibody better?


Long, long ago the argument over separate chassis or unibody construction was settled for cars – it was unibody by a country kilometre.

But decades after the world’s car makers abandoned the chassis for almost all cars; it’s still a big factor for anyone considering a 4WD.

The advantages of unibody – or monocoque – construction are significant for cars. A unibody car is lighter than one with a body-on-chassis, it’s easier to build-in today’s crash-crush requirements and it can have a lower centre of gravity for better handling.

However, for 4WD wagons that might be bought to work in extreme conditions and off-road, the separate chassis has prevailed and, until relatively recently, dominated.

4WD Ute available in New Zealand continue to be exclusively chassis-based.

The advantages of a chassis are in the strength and relative simplicity of its architecture: thumping great steel rails and cross-members able to offer a high degree of protection to the mechanical components, and on top of which the body can be bolted.

“Bolting the body on” is important in the Ute world, where a variety of cabs and tray sizes are sold, usually on the same chassis.

But in the wagon segment, the separate chassis, or ladder chassis as it’s often known, has been swept up by the changing tide of safety requirements, fuel economy objectives and technology.

There are two schools of thought about the separate chassis and safety legislation.
Some 4WD manufacturers, such as Toyota and Suzuki, say they can engineer a perfectly good chassis with the required “crushability” while retaining its inherent strength.

Others, however, have chosen to change to unibody, which can be configured to achieve a remarkable degree of crushability and passenger compartment integrity.

Indeed, unibody design is now so sophisticated that windshield and rear window glass often make an important contribution to overall structural strength.

Those advantages, coupled with a usually lighter vehicle and the attendant fuel economy benefits, make unibody a very tempting choice for engineers charged with designing new 4WD wagons.

Huge advances in computer aided design (CAD) have accelerated and eased the move to unibody. CAD has made them easier to design well, optimise for easy production and then modify at new-model or facelift time.

There’s a recent twist to the story as some manufacturers, Land Rover and Mitsubishi included, have gone to a hybrid structure that is unibody, but integrates the basic architecture of a traditional chassis.

These bodies are very complex and would have been unlikely were it not for CAD. In some cases, they lose the weight advantage of a “simple” unibody.

Unibody and separate-chassis 4WD wagons are likely to coexist for some time. For example, when the next Toyota Land Cruiser appears here late this year, it’ll still be on a separate chassis and will likely have a 10-year model life.

So should you favour one type of body over the other when looking for a new or used 4WD wagon?

In most cases, no. Just choose the model that appeals most. If towing heavy loads is an issue, you may find that some chassis-based wagons have a higher braked capacity.

And if you’re interested in going bush-bashing and mud-plugging with 4WD clubs, a separate chassis vehicle is much easier to seriously modify – like lifting the body to allow tyre sizes the manufacturer never even dreamed of.

It’s also possible that a separate chassis wagon will be easier and cheaper to repair after an accident, and to fix rust damage.

On the other hand, unibody vehicles often handle and ride better and are easier to get into and out of because they’re usually lower.

Auto Trader New Zealand