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 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
© 1998-2013 Michael Wittenburg and Martin Wittenburg. All rights reserved.