A Short Course In Choosing Or Building The Right Axle
Out on the trail you'll see axle extremes-big-tire rigs with spindly little axles, and others with monster-sized axles. The little axles would probably run from those big tires, screaming in terror, if they could. Those massive axles weigh as much as a V-8 engine, are three times stronger than they need to be and cost big bucks. Neither extreme is particularly elegant or efficient. There's got to be a "Three Bears" just-right point. And there is. We're going to show you how to find it.
The axle in your 4x4 is caught in the middle of a tussle between driveline torque (see glossary below for unfamiliar terms) and the grip offered by your tires. As long as the tires lose grip before the torque load exceeds the breaking point of some part in the axle, you're fine. The problem is that tire grip is 50 percent of the trail-performance equation (traction + clearance = trail performance) and it is grip that makes difficult 'wheeling possible. Fortunately, grip is in short supply on most trails. That's why you see rigs with smallish axles surviving.
Grip starts with the ground, or driving surface and is measured as a Coefficient of Friction (CoF). It's really more than just the ground, because some tires will provide better grip on a particular surface than others. Perfect grip is expressed as a 1.0 CoF. Actually, some racecar tires on special pavement can reach 1.5 or higher. Normal pavement and normal tires usually achieve a CoF of between 0.6 and 0.8. Rock may have about the same CoF as pavement. Hard dirt runs from 0.3 to 0.4. Mud can run from 0.1 to 0.3, while ice can run from a fraction above 0 to 0.15.
Vehicle weight plays a big part in this. More weight on a tire equals more grip on a hard surface, but it takes more drivetrain torque (and strength) to move more weight. Plus, heavy rigs are a liability in soft ground, so weight definitely is a double-edged sword. Bottom line, more weight equals more traction on a hard surface (to a point), but it then becomes more possible to exceed the torque capacity of the axle. Keeping the tires aired up, by the way, can act as a fuse. An aired-up tire will slip sooner than an aired-down tire 98 percent of the time. You won't make every obstacle, but you aren't as likely to break axle components. Tire grip is also expressed as traction torque, or the amount of torque the tire/ground combo can support. Traction torque is an equation that factors the weight of the tire, the CoF and the tire radius. Increase or decrease any of these values and you increase or decrease traction.
Axle-torque loads increase with tire diameter. This is because a larger-diameter tire makes more traction torque at the same weight than a smaller one. Going from a 31- to a 37-inch tire increases the torque load on the axle by about 20 percent because the tire can hold 20 percent more torque. That's independent of the increased loads that may come from other factors, such as lockers or really sticky tires. Another element is the rotational inertia, or flywheel effect, from the added rotational mass of a big tire. The larger the tire, the more torque it takes to start it into motion and the more it takes to slow it down. Additionally, four-wheeling tires and wheels are usually pretty heavy. This is mostly a negative issue when tires are spinning because it can multiply momentary or "spiked" torque loads. This added weight also is what makes bigger tires harder to stop when you've got them out on the road.
Most of you know that the traction available to an open differential is essentially limited to the amount of grip one tire can supply. That's the primary reason for lockers and limited-slips, so that when one tire on an axle loses traction, the other can still move the vehicle. That's great for getting through an obstacle but it may be tough on the axle. Normally, a 4x4 shares the total traction torque required to move the vehicle between all four tires. Weight transfer (see Vehicle Dynamics, below) or poor ground surfaces may reduce the traction of tires at one end or one side of the vehicle and increase it at the other. In rockcrawling situations, you may have a weight-transfer situation where one tire has a whole lot of the total weight and is providing 90 percent of the tractive effort. A locker will allow 100 percent of the axle torque to go to that one axleshaft, at least to the limit of grip for that tire. If the grip exceeds what that one axle can stand, the shaft breaks. A rig with an open diff or even a limited-slip will long since have given up, spun the lightly loaded tire and saved the axle. The bottom line is that if you run a true locker, your axle strength needs are greater than if you run an open diff or a limited-slip-but if you do run a locker, your rig's ability to conquer terrain is greater.
The aspect of vehicle dynamics that causes many broken axle parts is weight transfer. When your vehicle climbs, descends or is tilted on any axis, weight is transferred from the high side to the low side. That unloads the high tires, reducing grip, and loads the low ones, increasing grip. Bear in mind that the distance between the tires, whether that's the wheelbase length or axle width, has an effect on weight transfer. More distance equals less transfer at any given angle, and vice versa.