by Jim Kerr

Race cars use them. So do most high performance sports car. Even some lower priced family sedans are using independent rear suspensions – but rear wheel and four wheel drive trucks and SUVs have stayed away from them until recently. There are a few exceptions: the Hummer H1 and BMW X5 and some smaller front wheel drive SUVs come quickly to mind, but for the main stream, trucks and SUVs have used the traditional solid rear axle. Now Ford is using an independent rear suspension on its popular Explorer and Expedition SUVs, and I predict other manufacturers will soon follow.

So why use independent rear suspension (IRS) on a truck? For much the same reason they are used on race cars – wheel control, stability, ride quality, and packaging. Ford took advantage of all of these attributes when designing the 2002 Explorer, and have again incorporated them in the 2003 Expedition. Lets look at how the system has improved the new Expedition.

Anybody who has driven both the 2002 and 2003 Expeditions will notice a tremendous difference in ride quality. The rougher the road, the more noticeable it becomes. With IRS, each rear wheel can act independently to bumps in the road, moving upward and back slightly to reduce the force of the impact. On solid rear axles, when one wheel moves, so does the other side. More force is imparted into the vehicle’s body.

IRS also improves vehicle control. Allowing each wheel to move separately maximizes road contact, and keeping the tires in contact with the road is a priority for optimum control. This is the reason race cars use IRS. Traction is improved for acceleration, braking, and cornering.

An often-overlooked advantage of IRS is lateral stability. Solid rear axles are mounted using leaf springs or control arms and coil springs. Both the leaf springs and control arms allow the axle to move sideways slightly in relationship to the body during cornering or evasive maneuvers. With IRS, the rear differential is mounted to the frame and separate suspension control arms are used for each side. Because of this separation and the different position of the control arms, lateral movement is greatly reduced.

So why is this so important? If you have ever wiggled the steering wheel back and forth and caused a vehicle to sway, you are experiencing lateral movement in the suspension. The more it sways, the harder it is to keep the vehicle under control. Larger, higher, and heavier vehicles are much harder to control when sudden lateral movements, such as avoiding an object in the road occur, but the stiffer lateral support of IRS helps us control the vehicle. On the 2003 Expedition, the ability to control lateral sway is greatly improved. The ride is more comfortable and driving is safer.

A big advantage of IRS has nothing to do with handling, but allows designers to improve vehicle packaging. With a rear differential mounted to the frame rather than moving with the wheels, the body to differential clearance can be greatly reduced. The driveshaft doesn’t have to move up and down over bumps, so driveshaft tunnels in floor pans can be made much smaller, providing more interior space.

The Explorer and Expedition have used the IRS design to enhance interior space even more. Although the independent rear drive axles do move up and down with the wheels, they move much less where they pass through the frame than a solid rear axle does. Ford took advantage of this and redesigned the frame so the axles pass through tubes welded into the frame. With this design, the frame can be made lower in the area of the rear axle and so can the floor pan of the vehicle. This is a big advantage when designing the interior of the vehicle for maximum cargo space or room for rear passenger’s feet.

The only major disadvantage of IRS is higher manufacturing cost because of the greater number of parts. IRS can be as strong as solid rear axles and are able to support as heavy a load if designed for it. Better ride, better handling, vehicle stability, and more interior room are significant advantages of IRS. Look for more large vehicles with it in the future.

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