AXLE 101: SOLID REASONING OR INDEPENDENT THINKING? from kurplunk's blog

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.

The Jeep Wrangler is one of the last production SUVs with solid axles front and rear as of the ’17 model year (don’t forget the Mercedes G-Wagon, we almost did). The JK Wrangler’s success around the world is simply amazing. If solid axles are so antiquated, why does this thing work so well in the dirt and sell like half-price hot cakes on Sunday morning? Rumor has it that the next-generation Wrangler due in 2018 will still have solid axles. That’s good for anyone who actually wants to use the new Wrangler off-road. Having said that, the Wrangler axlehousings have several weak points with thin-wall axletubes and spindly inner knuckles that can bend under heavy use even without larger than stock tires.

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.

Ram and Ford both still sell heavy-duty 4x4 trucks with solid axles front and rear. GM got out of the solid axle 4x4 truck market in 1992, and while we love Chevys, Hummers, and GMC trucks, they are at a distinct disadvantage in durability when front locking differentials and large tires are worked hard off-road. Solid axles are generally accepted to be stronger than independent suspension and solid axle 4x4s are usually much easier to add suspension lift to and/or other performance suspension modifications.

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.

Sand cars and desert racers have embraced independent suspension for a long, long time. Many cars and trucks that go fast off-road have independent suspension front and rear. Most are fully custom so you won’t find inexpensive swappable parts in the junkyard, but the aftermarket has embraced several independent front suspension (IFS) designs found in trucks and SUVs (like this Toyota) and support them with long-travel or heavy-duty parts. Independent rear suspension (IRS) designs are also very tractable for high-speed off-road use, but few OEM production designs are up to the task of heavy off-road use. One exception might be VW Beetle IRS, which is used widely in racing and sand cars.

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.

The go-fast off-road crowd loves TTB suspension so much that it is occasionally swapped onto trucks and SUVs that had solid front axles from the factory. We’ve seen at least two Jeep SUVs, including this XJ (and a WJ), that have had Ford TTB swapped in. And while TTB has also been stripped out of several Ford 4x4s in favor of solid axles, people aren’t adding TTB just to be different. This type of independent suspension, like most, works well when hitting big bumps and going fast off-road. For rockcrawling, TTB can be built to withstand 35-inch tires, anything over that and you’re better off with a heavy-duty solid front 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.

Portal boxes can work on both solid and independently suspended axles. They use gears to lower the wheel relative to the axleshaft and thus allow the driven axle to gain ground clearance. This custom solid axlehousing with portals, built by Jesse Haines Fabrication, uses the internal parts from Hummer H1 axles, which you may or may not know, are independently suspended. Vehicles with portal axles available in the U.S. include the Hummer H1 and Mercedes Unimog. There are a handful of portal axles in the aftermarket that can be swapped onto some solid axles for Jeep, Land Rover, and Mercedes.

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.

If you Google “solid axle swap,” along with just about any 4x4 ever produced with a real transfer case, you will find information about people who have added solid axles to that truck as an attempt to improve off-road performance. In fact, some companies sell solid axle swap (SAS) kits for Toyotas, Chevys, Fords, Nissans, and many more.

-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.

Matt Lovell races this truck in the Best in the Desert 7200 class. It’s like many go-fast desert trucks with beam independent front suspension and a solid rear axle with leading arms. Up front the beams allow for lots of wheel travel and keep unsprung weight down. Out back the solid axle with long control arms handles the bumps and resists the extreme shock loads placed on the driveline from debris and jumps. Lovell built the truck himself, and he’s a nice guy—our kind of racer.Speaking of Conestoga wagons, Four Wheeler magazine is no stranger to the history of 4WD vehicles, and one of the first production 4x4s was the FWD B, like this model B-1719. These trucks were used by the U.S. and British during WWI and had two solid axles and a suspension frighteningly similar to some 4x4s built up until the late ’90s. Why? Because solid axles on leaf-sprung suspension is simple and the system works well. If it ain’t broke, don’t fix it.Innovation generally comes from places where someone tried to do something that others said couldn’t, or wouldn’t, work. We like examples of vehicles that are different and bend the rules of what others say won’t work. One longtime misled belief is that independent suspension can’t work in rockcrawling. Pat Gremillion is an off-road legend, and he used Hummer H1 parts to build an independently suspended Jeep rockcrawler that we can attest works well. Now, having said that, Gremillion’s suspension is not exactly simple, and while most could duplicate the suspension, it would probably be much easier to stick to solid axles and link suspension for your next rockcrawler.There are several solid front axles that make for viable swap candidates that you can find at a junkyard, but the Dana 60 front axle is the king. Some versions of this axle are getting hard to find, but there are several aftermarket companies that will sell you a complete Dana 60-style front axle. One of the best is Dynatrac (dynatrac.com). The company has jumped through lots of hoops to ensure that it’s current line of Dynatrac ProRock, ProRock XD, and Trail Series 60s have the best components and ground clearance available.If you have to have an independently suspended heavy-duty axle there are also several aftermarket independent centersections available. From 9-inch Ford-based centersections on up to centersections that use Dana 80 ring-and-pinions, you can build a driven custom independent front or rear axle for your 4x4. Dynatrac (shown) has a few Dana 60- and Dana 80-based centersections available as ProRock XD 60-IFS, ProRock 80-IFS/IRS, and the ProRock XD60-GM-IFS for Duramax-powered 2500 and 3500 4x4s.One drawback of independent suspension is that when it comes time to add suspension lift, many mounting points on the frame need to be lowered to maintain factory suspension geometry. That means you may need large brackets that hang down and the points for failure increase rapidly.Again, we love vehicles that bend the rules of what works and what doesn’t. OK, this Jeep isn’t a 4x4, but it does have four wheels…so is it a four-wheeler? A group of XJ enthusiasts built the XJ-R for road racing on a budget, and like other XJ and MJ-based race trucks, this thing hauls through the chicanes despite having two solid axles: one front, one rear. With the correct set up even a grocery getter with the suspension of a Conestoga wagon can handle the road.

Photo by Chang Ho Kim
credit: fourwheeler

Verne SimonsTechnical Editor, Four Wheeler Network Photographers: Four Wheeler Archives