23 Agustus 2009
Independent vs Solid Axle
Sumber :

The debate between Independent suspension and solid axles is a holy war that has raged for years. The truth of the matter is that each design has properties that are both good and bad for different things. For rock crawling, the solid axle offers so many advantages that it's barely a question; very few serious rock rigs even consider using independent suspensions.

Independent Suspension

There are a few ways to do it, but the basic idea of an independent suspension is to make each wheel move up and down separately from any of the others. This reduces the amount of motion in the suspension, which contributes to a more comfortable ride, and helps improve traction on relatively flat surfaces at high speed.

Solid Axles

A solid axle is exactly what it says; a single, inflexible housing that extends from one wheel to the other. If one wheel goes up, the one on the other side goes down. While this can make the ride a little more bumpy on the street, it helps get traction at low speeds by improving the chances your tires are touching. Almost more importantly, a solid axle has fewer moving parts and almost always has a stronger, less exposed design. In short, a solid axle has fewer potential points of failure.

Suspension Travel

To make the comparison, let's consider my solid-axle YJ and a fully independent Hummer (H1). The course includes a large boulder, about twenty inches tall, located on the side of the trail and narrow enough that only one tire will ever be on the boulder at a time. This is a common thing on many trails, so it's perfect for our example.

In our example, our Hummer drives up until the front passenger side tire is on the rock. The ground beneath all three other tires is level, however, because the front tire is on the rock, the vehicle chassis is no longer parallel to the ground. The rock on the passenger side is compressing the suspension, as well as lifting the chassis. On the driver's side, the increased weight because of the angle is causing some compression on that side as well, which actually increases the angle between the chassis and the ground. In the rear of the vehicle, the angle is also compressing the driver's side suspension, but there's no rock to push up on the passenger side. Because there's no force pushing either up or down on the passenger side tire, it remains in a neutral position, several inches in the air.

Next, let's consider my YJ. I approach the rock the same way, and pull my front passenger tire up onto the rock. Like the Hummer, my passenger side suspension compresses. Unlike the Hummer, my solid axle pivots around the differential, so as my passenger side suspension compresses, the driver's side tire is actually pushed down, decompressing the suspension on that side. This reduces the angle between my chassis and the ground. Moving to the back tires, we see that the driver's side rear axle is being pushed up because of the angle of the chassis. Again, the axle pivots around the differential, and my passenger side tire is pushed down. In fact, my rear passenger tire is being pushed down against the ground, despite that side of my vehicle being "light". Unlike the Hummer, all four of my tires are in contact with the ground, affording me better traction. It's also significant to note that my chassis is at a smaller angle to the ground than the Hummer; I'm not as close to my roll-over angle as the Hummer.

Before we dismiss the independent suspension completely as an undesirable rock-crawling system, there is one thing to consider; modern suspension designs allow for a hydraulic, adjustable suspension. A well designed adjustable independent suspension could overcome most of the suspension-travel flaws in the independent suspension design. By the same measure, however, a hydraulically adjustable suspension would be equally capable of providing the same advantages in a solid-axle set-up.

Potential for Damage

One of the best attributes of an independent suspension in an off-road vehicle is the ability to move the differential up and out of harm's way, increasing center clearance. Unfortunately, the necessity for moving parts to allow for the relocated differential introduces an inherent weakness. In a solid axle set-up, the weakest link is almost always the CV joint in each steering axle shaft. The need for flexibility not only exposes the shaft, but it adds a number of small moving parts. In an independent suspension, every axle shaft, even the non-steering ones, have not one but two CV joints. This not only introduces more potential failure points, but it exposes even more of the axle shaft. Even the best independent suspensions are more prone to failure than a solid axle equivalent, so obtaining equal strength means greater weight.

Solid axle designers have come up with an interesting technique for lifting not just the differential, but the entire axle, up relative to the wheels. This design, known as Portal Axles, uses a set of gears at the outer end of the axle shafts to position the center of the wheels several inches below the center of the main axle shaft. While this does introduce an additional level of complexity, the advantage is that doesn't need flexibility and can be completely enclosed in a solid housing. Another great advantage of portal axles is that the portal gears can be set up at unequal ratios, increasing the final gear ratio to provide even more torque and slower speeds. The Mercedes Benz Unimog stands out as an example of the advantages of a portal axle.

The conclusion, then, is that while independent suspensions may be desirable for high speeds, in the world of rock-crawling, the solid axle is the clear winner.