Whether you're ready to order a brand-new custom axle for your Jeep, want to have your current axle rebuilt, or are still in the daydreaming stages, there are some things you should consider before making the phone call to the axle builder of your choice. Granted, we're sure the tech and sales people who answer the phones would like nothing more than to talk with you at length about how you use your Jeep, what shafts are right for you, and what your favorite color is. But in the interest of making ourselves feel useful, we're going to play the role of middle man and give you a few pointers.
While at Currie Enterprises recently, having a new rear housing and set of 35-spline shafts built for our '53 DJ-3A, we got to talking with John and Ray Currie about what information they think their customers need to supply them with in order to get the axle that's right for them the first time.
Unless you know what the ultimate goal for your Jeep is, it's going to be nearly impossible to order an axle assembly that's going to be ideal for as long as you own the vehicle. For example, if you've just started your build and are running 35s but know that some day you're going to step up to 40s, then you shouldn't go with 4.10 gears and 31-spline shafts. However, it's impossible for your axle builder to know that, so spend some time before you pick up the phone to consider things like how long you plan on owning your Jeep, how much trail time and/or street time it will see, what type of wheel and tire combination you may run in the future, and whether suspension changes may lie ahead. With this information, your axle builder can steer you in the right direction and create a package that you won't outgrow as your hobby progresses.
There are several tire-size-versus-rpm charts out there that are supposed to tell you what gear to run with what axle, but we feel like those only get you in the ballpark. Usually, if you know your vehicle's stock gearing and tire size and want to keep your engine speed close to stock, you can do a little math to figure it out. If you're running tires that are 20-percent taller than stock and your Jeep came from the factory with 3.55 gears, theoretically you'd want gears that are 20-percent deeper than 3.55s, or 4.26s. But because the number you're going to come up with probably doesn't represent an available ratio (like 4.26), you can pick the closest ratio that is available. Usually, you'd pick the ratio that's numerically higher than the number you come up with to help the engine and drivetrain compensate for the increased mass of the larger wheel and tire combination. In this theoretical case, if we were ordering a Dana 60 we'd have a choice of 4.10s or 4.56s in lieu of the nonexistent 4.26s. We'd do the 4.56s. However, if future plans call for bigger tires or less street driving, you can consider going really deep with the gears.
Perhaps more than any other factor, street driving and your personal tolerance for handling quirks will come into play when considering which type of traction device to run. All-out, off-road traction will require a locker of some type, whether it's selectable or automatic. If you're really picky about vibrations, banging, and other things, you may want to consider a selectable locker such as an ARB Air Locker that will act as an open differential on the street. If you're hardened to on-street quirks like some jerking and tire squeal and are concerned about keeping costs down, then you may think about an automatic locker. But if you're planning on making your rig for high speed or as a recreational-camping vehicle, a good compromise between streetability, traction, and price may be a limited slip like a Detroit Truetrac.
We've heard it from other axle manufacturers, so it was no surprise when the Curries repeated it, but they like to see their customers ordering a balanced axle package. In other words, it doesn't make much sense to order a Currie high-pinion centersection that's rated to 35-inch tires, then stick a 1410-yoke and 40-spline shafts into it. If your needs demand the bigger yoke and 40-spline shafts, it's probably better to order a high-pinion Dana 60 to begin with. In the same vein, you usually want to keep your axleshaft and U-joint combinations consistent in a front axle application. For example, if you upgrade to CTM axle U-joints, you should upgrade the shafts to alloy units. When trying to figure if you need 28-, 30-, 31-, 35-, or 40-spline axles, you can take the amount of torque your engine puts out and multiply it by your crawl ratio to give you an idea. Say you've got a 200lb-ft engine and a 100:1 crawl ratio, you theoretically need axleshafts capable of withstanding 20,000 lb-ft of torque. However, we've never seen the tire that can hook up that kind of force in the dirt, plus you're splitting that torque through three other axleshafts. In reality, figure that a general 31-spline shaft will pop at around 8,000 lb-ft in a destructive testing fixture, while a 35-spline shaft may go at around 12,000 lb-ft. The following chart is straight from the Currie Enterprises catalog.