Off Road Truck Axleshaft Facts

The three prominent rearend axleshaft wheel-end configurations for light-duty trucks are tapered (top), flanged (middle), and full-floating (bottom), with flanged being the most common.

The three prominent rearend axleshaft wheel-end configurations for light-duty trucks are t

Design
In addition to metal grade and thickness, an axle’s profile helps determine its overall strength. Here is an overview of the popular rearend styles.

Tapered Two-Piece: This style has a separate shaft and flange. Semifloaters use a key in a keyway to mate their shafts and flanges. Nuts secure the outboard ends of the shafts to the flanges in this style.

Semifloating: This style commonly uses a one-piece flanged axle to connect the differential and wheel. Semifloating axleshafts do double-duty: They transfer torque and support the vehicle’s weight. Some semifloating axles use C-clips inside the differential to hold the shafts on the vehicle, rather than a pressed-on bearing and flange. Because the shafts’ flanges bolt to the wheels and not to the axlehousing, shaft breakage can allow the tire/wheel to move away from the axlehousing. C-clip eliminator kits for popular axles such as the Dana 35C, Ford 8.8, and Chevy 12-bolt include axlehousing ends, pressed-on bearings, and bearing retainer plates to positively retain the axleshafts to the axlehousing. DIY eliminator kits that have bolt-on housing ends tend to allow more side-loading on the bearings and shafts than weld-on ends. However, the welded ends require an alignment bar for installation to ensure that the housing tube’s centerline stays true.

Full-floating shafts come in two configurations. Frontend-style outboard splines (bottom) slide into a splined hub. Typical 3⁄4- and 1-ton applications use flanged full-floaters that bolt to the wheel hub.

Full-floating shafts come in two configurations. Frontend-style outboard splines (bottom)

Full-Floating: This rearend style is preferred for vehicles that carry heavier loads, typically 3⁄4-ton and up. As with front ones, full-floating rear axleshafts connect to hubs, which turn the wheels. The hub supports the vehicle’s weight. Should a full-floating shaft break, the hub and wheel normally remain bolted to the axlehousing. Full-floating shafts can either have splined outboard ends that key into the hubs or drive flanges that bolt to the wheel hubs.

Strength
The accompanying chart (right) gives thumbnail yield strength (the point before the metal is permanently deformed) for axleshafts of various diameters. That information can be used to calculate an axleshaft’s torque capacity using the formulas above. Radius and diameter are in inches; to convert torque capacity to pound-feet, divide the pound-inches by 12.

So, how much torque capacity does your 4x4’s axleshafts need? Maximum drivetrain output torque can be calculated by multiplying engine crankshaft torque by crawl ratio, then using a 0.85 correction factor to compensate for driveline slop. Here is the calculation for a stock ’12 JK Rubicon with a five-speed automatic.

250 lb-ft engine x 3.59 First

x 4.0 T-case x 4.10 axle = 14,719

14,719 x 0.85 parasitic loss = 12,511

12,511 = 3,128 lb-ft per shaft
4 axleshafts

Rear shafts are often quasi-equal in length; they theoretically split the load in half under normal tractive situations and when a locker or spool is engaged.

The Randy’s Ring & Pinion Differentials book, by Jim Allen and Randy Lyman, is an excellent reference for all things axle-related. It covers both OE configurations and aftermarket upgrades.

The Randy’s Ring & Pinion Differentials book, by Jim Allen and Randy Lyman, is an excellen

Shaft Selection
So what shaft is right for you? The facts and figures here will hopefully help narrow down the options. Internet forums are another possible resource, if you can find knowledgeable posters who are running your proposed setup and who attack similar terrain. Relevant posts often recommend certain manufacturers. Using these manufacturers’ tech lines can zero in on what is best now and possibly down the road, based on current and intended vehicle setups, budgets, and so on.

The bottom line on axleshafts: As with many things, price is usually proportional to quality. Higher-end shafts often involve more manufacturing/quality-control steps and use better raw materials. Many axle/drivetrain specialists offer both budget-conscious shafts (which some even label “import”) as well as premium domestic ones.

Luckily, even budget aftermarket shafts are usually significantly stronger than OE ones. Many are also backed by confidence-inspiring warranties (which, unfortunately, won’t help solve trail failures since cost of labor—which can be significant depending on axle style and where you are when it breaks—is normally excluded under the warranty). Getting the right shaft for the job will help keep you from getting shafted on the trail.