Concepts Suspensions

Of primary concern to four wheel drivers when dealing with suspension is articulation. Articulation refers to vertical wheel travel, i.e. how far up and down the suspension allows each wheel to move. The better the articulation, the better the chances of the wheel remaining in contact with the ground, thus maintaining positive traction. Articulation is largely defined and affected by the type of axle (solid or independent), and the damping system (coil or leaf sprung).

There are two basic types of axle systems - independent and solid axles. Each have merits, and each have their drawbacks. Discussions as to which is better often lead to argumentative stalemates, so I will try to present an objective description of both. Note that many modern off-roaders and leisure 4x4's employ a combination of the two - solid axles at rear, and independent at front.

Solid Axle Suspension

Solid axles (referred to by some as solid beam or live axle) mean suspension systems where the wheels are joined to an axle which consists of a solid beam. This has some interesting effects when driving both on- and off-road.

For the sake of this discussion we'll limit ourselves to the front wheels only, although it obviuosly applies to both the front and rear axle. When driving a vehicle equipped with solid axles on the highway, and an irregularity or bump is encountered with, say, the left wheel, that wheel will lift to go over the bump. Because the axle between left and right wheels is solid, the entire front of the vehicle will also lift up. The result is that solid axles make for a very irregular, and consequently "hard" ride on the highway.

Off-road, however, the same effect can be magnified, due possibly to larger bumps, in our hypothetical case the ubiquitous rock. When the front left wheel encounters a rock, and the front right wheel continues on even ground, the front left wheel will rise, raising the differential and the body of the vehicle with it. This has the desired effect of raising the differential out of harm's way. Consider the image at left.

Another interesting effect of solid axles is that when the front left encounters a rock, and the front right falls into a hole or depression, a solid axle vehicle is less likely to loose contact with the ground. This is because the solid axle and vehicle wieght force or "push" the front right down, more often than not maintaining contact with the ground, and subsequently maintaining traction on both wheels. The obvious benefit here is off-roading (such as rock crawling), whereas high-speed, on-road driving can be bone-jarring.

Independent Suspension

Independent suspensions consist of an axle "joined" in the middle by the differential. The two parts of the axle (left and right of the differential) can therefore move independently of each other, hence the name. When encountering a bump or irregularity with the left wheel when driving at speed on the highway, the wheel will lift over the bump, as expected. However, as the left side of the axle is independent of the right, the differential and consequently the body of the vehicle will stay more or less at the same elevation. The result is a more regular or "smoother" ride on the highway.

Of course this has a more dramatic effect when off-road. When encountering a large rock with the left wheel, the wheel will lift to go over the rock, but the differential will not lift as much, and possibly stay in harm's way, resulting in scraping the diff over the obstacle. The benefit of independent suspension therefore is on high-speed on-road driving, or high-speed, cross-country driving such as the Paris to Dakar rally. Slow-speed rock-crawling is not as effective as with solid axles.

Coil and Leaf Sprung Suspensions

The difference between coil-sprung and leaf-sprung suspensions is pretty straight-forward. While slightly more complex and newer, coil-springs (or just "coils") offer superior vertical wheel travel than their leaf-sprung counterparts. Leaf springs are simpler in design, and have been around longer.

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