Ordering Jeep Axles - Don't Waste Your MoneyPosted in How To: Transmission Drivetrain on January 20, 2008 0) (
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.
Stock YJ, TJ, XJ, and MJ axles are 60.5-inches wide from the wheel mounting surface (WMS). If your new tires are a lot larger than stock, you're probably going to hit suspension components and sheetmetal as the larger tires turn when steered or as the axles articulate. When having new axles built, it is a great time to increase your track width not only to help keep the tires out of the chassis and body but to increase the vehicle's stability at angles.
How much wider you go is largely up to you, but John Currie says that, for example, increasing the front track width of a TJ 6 inches will allow 40-inch tires to be steered fully without hitting chassis components. Knowing what wheels you're going to be running will help you dial in your exact track width because you can measure your backspacing and have the axle built to accommodate it. If you have a set of aluminum rims with a 3 1/2-inch backspacing you'd have your axle built a lot narrower WMS-to-WMS than if you planned on running stock Hummer rims with 7 inches of backspacing because the wheels with less backspacing would make the tires stick out much farther past the WMS.
The Curries stressed keeping an eye open toward the future because no enthusiast ever considers his or her vehicle truly completed. When assembling an axle, it's not that big of a deal to drill the shafts and brakes for both 5-on-4.5-inch and 5-on-5.5-inch lug patterns because the components are being machined anyway. That way, if you ever decide to swap your XJ, MJ, TJ, Unimited, or ZJ over to the bigger lug pattern down the road, it will be a simple matter of knocking out the lug studs and reinstalling them for the larger pattern. Keep in mind, though, that you can't drill a dual pattern with a five-lug and a six-lug because some of the holes would overlap.
On a front axle assembly there's a lot to take into consideration. First, John Currie told us that most stock Jeeps have about 6 to 8 degrees of caster built into the axle. That allows the tires to self-center after a corner and to track straight going down the road. However, Currie has discovered that most Jeeps with larger tires actually drive better down the road with slightly less caster (about 4 1/2- to 5-degrees) because the larger tires tend to fight each other and want to induce shimmy. Therefore, it's better to know how big of a tire you're going to run and how big a lift you're going to install so your pinion angle can be set and the knuckles can be welded on with the right caster.
If you're going to have the axle builder weld on your spring perches or suspension brackets you're going to want to know your pinion angle, or at least how much lift you plan to run. Most axle builders can get the pinion angle close armed with only the amount of lift you're running, but it's best to get under your Jeep with a cheap angle finder and get a measurement. Take your reading from the face of the yoke and with the vehicle parked on a level surface.
Most major axle manufacturers have production lines designed to turn a decent volume of axles out per day. Asking for one-off or custom items like a full truss or custom bracketry means that, in many cases, the company has to stop its production line to complete your custom order. Most are fine with doing it, but you'll need to realize that it's going to add some cost to your axle. Having stock-type brackets moved a little one way or the other on the housing probably isn't going to be that big of a deal, but if you have the skills, it may be better to do your own highly customed or one-off suspension brackets.
Trussing is something that's best left to a professional shop because as the truss is welded on, the housing tends to distort in the direction of the weld. Currie often sees customers' housings with homemade trusses that have distorted the axle ends over 1/4-inch. The company has a lot of tricks up its sleeve to ensure the centerline of the carrier and housing ends remain in alignment when a full truss is added, from pre-bending the housing to account for warpage, to using an alignment bar to install the housing ends.
Don't try to test the manufacturer's warranty just to save a few bucks. If you're repeatedly destroying components with your big-block engine and 44-inch tires, most shop owners aren't dumb and won't buy your story that your ring-and-pinion exploded into tiny fragments when your Granny shifted your rig from Park to Drive in the church parking lot. If your setup requires a gnarly axle, just pop the extra dough for a gnarly axle that's going to survive. You'll save a lot of time and aggravation in the long run for both you and your axle builder.
Conversely, don't feel like you have to act all hard-core and buy the biggest, most expensive thing on the menu if you're not hard-core. If you aren't going to like the handling characteristics of a locker and your driving style and terrain don't require a big, high-zoot axle, then don't buy one.