There was a time when the Ford 9-inch axle was synonymous with strength. It was the must-have upgrade, not only for four-wheelers but for hot rodders and drag racers, too. It was plentiful, virtually bulletproof, and it made ring-and-pinion swaps relatively simple, thanks to its drop-out centersection.
Yet during the last few years, the four-wheeling landscape has changed, thanks to the growth of events like competitive rockcrawling and tough-truck-style competitions. Suddenly what used to be strong enough isn't anymore. To tackle the sheer rock faces and treacherous man-made obstacles that would have been considered impassable until recently, a rig must now be equipped with massive tire-and-wheel combinations. (Remember when a 35-inch tire was considered tall?) Suspensions now have to travel through a range of motion that would make a desert racer proud. And a transfer case can't just have standard low-range anymore; it's got to have gears low enough to let a rig walk right up a tree.
Taking the brunt of all this beef are the axles. More unsprung weight, increased operating angles, and higher torque loads all take their toll on the equipment that sends power to the wheels. So in addition to 39.5-inch tires, custom coil-spring suspensions, and 4:1 crawler gears, we're seeing a lot of 1-ton axles-Dana 60s and 70s, GM 14-bolts-under 4x4s that left the factory with 1/2-ton, or lighter, chassis ratings.
And typically, what competitors do to set up their vehicles is imitated by four-wheelers everywhere. So now, even casual 'wheelers are considering hacking up their fenders, doubling up their transfer cases, and building custom axles for a weekend trip to the boonies.
All that stout gear is interesting, but it may be overkill for certain kinds of 'wheeling. Or as Ray Currie of Currie Enterprises put it, "What's used in competition isn't necessarily right for real-world four-wheeling." Now, there's nothing wrong with strength, except for the big hit it puts on your wallet. But consider that you may not need to spend thousands (or tens of thousands) of dollars on a 1-ton axle setup if all you're doing is putt-putting through the woods and climbing the occasional slickrock on the way to your favorite fishing hole.
To Currie, the average four-wheeler doesn't need to emulate the competitive rockcrawlers if he's running tires shorter than 37 or 38 inches, a stock or mildly modified engine, and either a stock or low-geared transfer case, like an Atlas II. This is where our old friend the 9-inch axle comes back into the picture. A 9-inch in the rear of such a vehicle, matched with another 9-inch in front or even a Dana 44 would be just fine-strong, reliable, and not too pricey. "The 9-inch delivers the most strength per dollar," Currie told us. (You can get away with a lighter axle like the Dana 44 in front, Currie told us, since the torque loads on the front axle usually aren't as great as they are on the rear.)
Axle Strength
Before we get into the particulars of modern 9-inch design, let's talk about what makes an axle "weak" or "strong." To begin with, take a look at the photos comparing the relative sizes of the ring-and-pinion gears commonly used in 4x4s. The 9-inch is so named because its ring-gear is 9 inches in diameter. As you can see, it's just plain beefier than a Dana 30 or 44, and also beefier than the GM 10- and 12-bolts. It is also significantly smaller than the ring-gear in a Dana 60. The simple rule of thumb here is, the bigger the ring-gear, the greater its resistance to breaking.
Ring-gear diameter is one...
Ring-gear diameter is one measure of strength, so is ring-gear thickness, and the size of the teeth in the gear. Again, as a general rule, the thinner the ring, or the thinner the teeth, the weaker the gear. These three ring-and-pinion sets-(from L to R) Dana 44, Ford 8.8-inch, and Ford 9-inch-are all about the same diameter. But notice how much stouter the Ford gears are compared with the 44.
How do you break a ring-gear? Any differential, even a 9-inch, is vulnerable to high torque loads. Send too much power too quickly to the ring-and-pinion and you'll force them apart, which can lead to broken teeth on either or both gears.
To combat that, Ford designed the 9-inch with three pinion support bearings instead of two. An additional pocket bearing, with a retainer built into the gear case, supports the pinion's head and keeps it from twisting away from the ring-gear under torque load. This pocket bearing has been a major contributor to the 9-inch's reputation for being bulletproof.
The other weak link can be in the axles themselves. While the shafts sometimes break, where front axles are concerned it's more often the U-joints and yokes that tie them together that are vulnerable. High torque loads can snap them, as can extreme operating angles. Now, there's nothing inherent in the 9-inch's design that can prevent U-joints from snapping, even in a custom-built 9-inch frontend, but it can easily be upgraded with bigger axleshafts and heavier-duty U-joints.
Spline count is just one component of shaft strength. Probably more important on front axles is the beefiness of the U-joints, where the inner and outer shafts meet, as this will more likely break before the shaft gets sheared. At the bottom is the 297X U-joint from a 3/4-ton truck. This used to be considered heavy-duty equipment before the Super Duty F-450 axles and U-joints (top) began showing up on hard-core rigs. If you're considering going up in spline count, be sure to upgrade the U-joints, too.
 From the side, you get an...  From the side, you get an even better idea of how much thicker the Ford 9-inch is, on the right, compared with the Dana 44. |  Axle diameter, usually referred...  Axle diameter, usually referred to by the shaft's spline count, is another measure of an axle's strength. All of these axleshafts can be installed in a Currie 9-inch. Spline counts (from top): 28, 30, 31, 35, and 40. |  This photo illustrates the...  This photo illustrates the difference in pinion height between a standard 9-inch (left) and a Currie High-Pinion (right). Moving the pinion up 3 1/2 inches can make a big improvement in driveline angles, especially in short-wheelbase vehicles. The trade-off is that the high-pinion axle isn't as strong when used as a rear axle. |
High-Pinion
While upgraded axleshafts and U-joints are important parts of strong axles, they are only the tip of the iceberg when it comes to 9-inch modifications. Since the basic 9-inch axle design has been around since the mid-Fifties, engineers have had a lot of time to tinker with it and add their own custom touches to make it better suited to a number of specialized tasks. The reverse-rotation, or high-pinion, 9-inch is a good example. When they were being built by Ford, 9-inch axles were intended to be used as rear axles only. Junkyard shoppers, take note. So those who wanted the strength of a 9-inch in front had to make some changes to the differential's design. The reverse-rotation gearset, which incorporates a ring-gear with a spiral cut in the opposite direction of the standard ring-gear, was born of that process.
As a side benefit for four-wheelers, the pinion in a reverse-rotation axle is positioned higher on the gear case. The pinion in a standard 9-inch sits about 2 1/4 inches below the axle centerline, while the pinion in a reverse-rotation 9-inch is about 1 1/4 inches above the centerline. Those 3 1/2 inches can make a huge difference when you're looking for ground clearance or trying to reduce driveline angles in a short-wheelbase vehicle.
The only downside to a reverse-rotation axle comes when it's used as a rear axle. It can be done, and often is, but driving it against its designed rotation costs the gearset about 20 percent of its strength, according to Currie. For that reason, he considers a 35-inch tire the height threshold for a high-pinion 9-inch being used in the rear. A front-mount high-pinion or a standard 9-inch can safely handle tires up to about 38 inches tall.
 Currie not only offers different...  Currie not only offers different pinion heights with its 9-inch axles, it also gives 'wheelers some choices in housing shape, too. The Rock Jeep TJ has skidplates welded to both the front (shown here) and rear pumpkins. If ground clearance is more important than protection, you could opt for a High Clearance housing with some material shaved from the bottom of the pumpkin. |  While a high-pinion differential...  While a high-pinion differential will help with driveline angles, it does have an impact on other components, too. In a Jeep application like this TJ, if the suspension isn't lifted at least 3 inches, the front driveshaft will run smack into the exhaust. This Jeep did have a lift, but the exhaust still had to be tweaked a bit. The spots where the tubes are free of surface rust indicate where the tubes had to be heated and bent to clear the shaft. |  Here's how a Currie High-Pinion...  Here's how a Currie High-Pinion 9-inch looks in the front of a TJ. This particular Jeep was undergoing Currie's "Rock Jeep" series of modifications, with High-Pinion 9-incher, 31-spline axles, and Detroit Lockers at both ends, plus a 4-inch lift, Johnny Joint suspension pieces and a relocated transfer case (pushed up in the frame for increased clearance). By using the high-pinion diffs in conjunction with the relocated transfer case, driveline angles were returned to nearly stock. |