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Monocoque 4X4s explained – should you be afraid?

Practical Motoring logo Practical Motoring 3 days ago Robert Pepper

© Provided by Practical Motoring Let's take the example of an ancient aeroplane, one with a wooden frame covered with fabric.  The fabric is stretched tight over the frame, and contributes almost nothing to the aircraft's strength which has to come from the wooden frame.  The aircraft  developed metal skins, stronger than just fragile fabric, and that meant the skin could contribute in meaningful ways to the strength of the aircraft, which in turn meant that the main struts in the wings and fuselage could be that much smaller and lighter, a virtuous circle.

Much the same evolution has been seen in cars.  Separate-chassis vehicles have a strong chassis to which the engine, suspension and other gear is attached, and the design is also know as frame-on-body.  In this case, the body bolts on top of that chassis, and contributes very little to the chassis' overall strength. But that's a heavy design, as that bodywork could help with the vehicle strength, but it doesn't.  One advantage is that you can change the body to whatever you like without affecting the rest of the vehicle.  Another is that there's a distinct isolation between the vehicle's engine, suspension and other components so NVH (noise, vibration and harshness) are inherently good.

Then the monocoque approach started to make its way into 4X4s too, and many wagons use the design.  Unfortunately, monocoque isn't really the right term and its use had led to some misconceptions.

A pure monocoque design would use the skin for the majority or all of its strength.  That's a problem for cars which are often damaged, because any damage to the skin would become structural.  It's less of a problem for aircraft which are dinged less often, and repaired much more diligently when they are broken.

So what "monocoque" 4x4s actually have is a chassis that is reinforced for stiffness and strength in some aspects by the body, so the body and chassis are one unit.  This is known as unitary construction, a much less sexy term than monocoque which is why it probably hasn't caught on.  Maybe 'body frame integral' or 'unibody' will enter common parlance as these are also terms of unitary construction. The main advantages over separate-chassis are extra rigidity and overall lighter weight.

a car driving down a dirt road © Provided by Practical Motoring

Now in practical 4x4 terms a unibody construction vehicle can't just have its body lifted and changed with the ease of a seperate-chassis vehicle, because the body does provide vehicle strength and rigidity.  That much is true.   What isn't true is that the unibody is some sort of weakling. It's not, there's a real chassis there, and the modern unibody is much stronger than the seperate chassis vehicles of yesteryear.

But what of the unibody's reliance on the body?  Well, the reliance isn't so much that if you ding a panel or two you've compromised the whole vehicle, or it somehow bends in the middle.  The vehicle is designed to handle that sort of problem.  And if it has recovery points, then you can be sure that those will be connected to the chassis so there's no concern about using those, any more than you would with a seperate-chassis vehicle.  If you go hooking up recovery points away from the chassis then yes, you can bend things, same as if you hooked up to the body on a seperate chassis vehicle, or even the wrong part of the separate chassis.

The unibody construction is here to stay as it offers significant weight advantages over seperate, and makes packaging the overall vehicle a bit tighter.  There's a real chassis underneath and while the body is required for rigidity and strength, most of the strength comes from the chassis, not the body, and the whole car is designed to take the odd knock without bending in half.  You need have no concern about winching or snatching provided you use recovery points mounted to the chassis.  Welcome to 2017!

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