Axle 101: Solid Reasoning Or Independent Thinking?Posted in How To: Suspension Brakes on March 28, 2017
Some debates are never ending and will rage until time stops. Ford or Chevy. Good versus evil. Old or new. Chocolate or vanilla. Donkey or elephant. Which is best can lead to arguments that are difficult to win, although they may be simple to answer from your perspective. Really, who likes evil or would rather drive a vanilla-flavored Ford over a double-chocolate Chevy? These debates occur in the off-road world as well as at the ice cream stand, and many are settled by opinions as opposed to cold hard facts. Still, what works for you may not be the answer for someone else, especially when he or she is trying to accomplish a different job than you.
One debate that has and will continue to rage is the never-ending query: What’s better off-road, the solid axle or independent suspension? Spoiler alert, this article is going to tell you, and were not going to wait until later, we will tell you now. The answer is—much like in other realms of debate—not as simple as just stating either A or B. For us, if you want to go fast, independent almost always trumps solid, and when going slow over really rough ground with big tires, solid axles are king.
Now that you know—and assuming you want to keep reading—we’ll tell you when and where each type of off-road suspension is better and why both excel in the real world despite the ongoing debates (that’s why making a one-size-fits-all decision doesn’t work). The truth is both types of suspensions can be built to excel in almost any off-road environment, but not all suspension/axle types are necessarily easy or inexpensive to build.
What is it: Solid AxleSolid, or beam axles, are just that: a solid beam that connects right and left wheels, front or rear, to one another. Of course there are a couple of different ways of doing this, but the basic idea in a 4x4-driven axle is the same. There is a solid axlehousing that houses gears, oil, bearings, and axleshafts with hubs, brakes, and wheels on the ends. The beam is then connected to the frame via a couple of different methods: leaf springs, link suspension with coil springs, or rarely some combination of those two. Front solid axles are slightly more complicated because they also have two pivot points that allow each wheel to steer right or left while still transmitting the engines power to the wheels. The idea is as simple as an old wooden toy car or a Conestoga wagon. The wheels are connected to the axle, which in turn is connected to the frame or chassis of the vehicle. It’s not high tech, but follows a couple of rules that never seem to fail us. For one, solid axles follow the KISS (keep it simple stupid) rule and solid axles also follow another of our favorite rules, one we will call the “If it ain’t broke, don’t fix it” rule. To that end, the design is old, but it’s simple, and it works pretty darn well off-road, so, generally speaking, it’s easiest to not mess with that.
Solid Axle Strengths-Solid axles are simple and durable, time-tested and reliable if not antiquated. As a 4x4 drives over rough terrain and one tire gets pushed up by a bump or rock, the tire on the other end of the solid axle gets pushed down. That can help keep the tires in contact with the ground and preserves traction for both tires to a point. That does not occur to the same extent in vehicles with independent suspension (as the name implies), although weight transfer does occur it is through the chassis, which also occurs in solid-axle rigs.
-Solid axles are heavy. That’s bad when you are going for efficiency but great when you are going for durability and keeping a vehicle’s center of gravity low.
-Solid axles have few to no joints where the direction of transmitted power has to be changed as the suspension cycles, only when steering on a driven front axle.
Solid Axle Drawbacks-Solid axles are heavy, adding to a vehicles weight and when manufacturers go to extreme measures to make them lighter they tend to become a shadow of their former selves. These light-duty versions tend to bend and crack when those of us who push them, push them.
-Independent suspensions also generally allow for more ground clearance as the differential can be tucked up high and the driven axleshafts can go down to each wheel when at ride height.
What is it: Independent Suspension-Each wheel and tire per axle is attached directly to the frame using one, two, or more control arms. The axle’s differential housing, containing the axle gears, bearings, oil, and differential are also affixed directly to the frame. This design allows each tire to move independently from the chassis and the other tire on that axle.
Photo By Harry Wagner
Independent Suspension Strengths -Independent suspension systems generally offer lower overall weight and less unsprung weight. Unsprung weight is weight that moves with the tires/wheels rather than with the chassis of a vehicle. Moving weight requires energy and controlling weight is difficult as that weight increases. Therefore, suspensions that have to react quickly to surface changes do better with less unsprung weight. This is true both on- and off-road.
-Independent suspensions also allow for more control over the suspension geometry as the suspension cycles. Changing arm length ratios can help keep the tires of a vehicle parallel with the ground as the suspension cycles and or help impart a steering input as the suspension droops or compresses. Therefore, you generally see independent suspensions favored in motorsports where the suspension and axles move up and down rapidly and control of the axle movement is critical. Shocks control compression and rebound, and the lighter the load, the more effect a shock has over the control.
Photo: Mike Lee Austin
Independent Suspension Drawbacks:-Independent suspensions are inherently more complex and therefore push the KISS rule to the side. That’s not to say that they are weaker. More that there is added potential for wear points and failure points. Most independent suspensions require at least two axle joints per side where the direction of power from the engine has to be changed, and that has to occur in multiple planes for steering axles. More pivot points allow for more points of wear and potential damage that can stop a rig in its tracks.
-Adding suspension lifts to most independent suspension designs is more complex than with solid axles as more mounts and brackets have to be made.
-Most of the joints on independent suspended axles use a boot to keep the joint lubricated and protected from dirt and water. These boots wear and crack and are easily torn if they come in contact with road or trail debris.
-In attempts to keep things light weight, most axlehousings in modern independent suspension systems use cast aluminum for their differential housings instead of cast iron. With a little research we can’t definitively say that aluminum is not as strong when it comes to serving as a differential housing, but we can say that the anecdotal evidence exists. It could be that these housings are optimized for weight and lack the necessary metal to keep them together, but it seems to be an issue. We have seen several IFS aluminum front differential housings from a number of manufacturers fail when the going gets tough. This is especially true when larger tires are added and/or traction improving devices are added, but we’ve even seen a couple aluminum front differential housings fail in stock 4x4s. Locking differentials, lower gearing, and larger-than-stock tires increase loads that can contribute to a catastrophic failure in the differential housing. Some vehicle manufacturers like Toyota and GM in the Hummer H3 even opted to use a cast-iron differential housing in the front of their vehicles. Toyota switched back to an iron diff after a few years of using an aluminum housing, and the Hummer H3 used an iron version of a previously aluminum front differential housing. Our conclusion is to pay attention to the materials used and the reputation of any differential you plan on using and abusing off-road.
Photo by Chang Ho Kim