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A Look At The Constantly Evolving Ford 9-Inch - Axle Evolution

Posted in Features on May 9, 2014
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Why has the Ford 9-inch axle become the go-to axle for Ultra4 racing and hardcore use? It has a lot to do with the evolution of the axle assembly from the factory-built unit, to custom-fabricated axle assemblies upgraded with aftermarket parts, to the fully fabricated axles with no factory parts in them at all.

Look under most Ultra4 cars and you will likely find some variation of the Ford 9-inch axle assembly. However these axles are far from factory units. The Ford 9-inch does have a few features that improve the assembly strength, such as a nose bearing on the pinion and large ring gear with good tooth contact, and those do make it strong enough to handle the abuse seen in off-road racing. Even though the third member of the Ford 9-inch did have some inherent strength upgrades off roaders were seeking, there still were a few issues with the factory assembly. One, the axle was a semi-float assembly limiting the strength of the axle shafts. The 9-inch was also never offered as a steering axle assembly straight from the factory.

The popularity of the Ford 9-inch axle didn’t start in the Dirt SportsNation, but when it became a staple under the back of hot rods and drag cars in the 1960s. As they grew in popularity, so did the aftermarket support. Companies like Currie Enterprises were eager to fill the need of people looking to make their axles stronger. They developed a complete product line around a redesign of the factory Ford 9-inch.

Three popular styles of Ford 9-inch housing centers, from left to right: factory-stamped housing; rockcrawling-specific fully fabricated high-ground clearance center; and an off-road-racing-specific fabricated center combining ground clearance with high fluid capacity.

In the 1990s, competitive rockcrawling was coming on strong and a new type of 9-inch was needed; one that could handle the abuse of excessive low-gear deflection of the axleshafts as well as ground-clearance issues. Frank and John Currie were pushing the aftermarket ahead when they showed up at their first rockcrawling competition in a Jeep named Rocky Road with a newly designed 9-inch with a high-pinion configuration.

Spidertrax Off Road was another company that saw the popularity of the Ford 9-inch axle in the Dirt SportsNation and applied its engineering skill to attacking the problem from a new perspective. By completely redesigning the assembly, starting with a high-clearance fabricated 9-inch housing with bearing end cups and fabricated knuckle assemblies, the design allowed a rear axle to be built using a full-float unit bearing assembly with all-new high-strength custom-made parts. Spidertrax designed their entire new product line around the DIY fabricator, as well, offering the newly designed Spider 9 axle as a kit.

These axles quickly earned the name 60-9 (the sixty-nine) axle because they were finally a true alternative to the Dana 60 axle. Spidertrax took it one step further, creating a series of products that would allow you to build a full-float rear 9-inch and using their fabricated knuckles to build a steering 9-inch axle with 50 degrees of steering angle. You can only turn an axleshaft so far before the ears on the axleshafts contact each other and cause axleshaft damage. Spidertrax solved this problem by custom machining the ends of their axleshafts in-house to allow up to 50 degrees of steering.

After designing a completely fabricated 9-inch axle kit in DIY form, Spidertrax attracted a lot of attention in the Dirt Sports Nation.

Aftermarket companies addressed other problems with the 9-inch. In off-road conditions, the center section or third member is put under a lot of stress by the gear set trying to deflect. The nose bearing holds the pinion in place against the ring gear, but the side load can overload a stock third member easily and cause severe failure. To prevent this, companies like Yukon Gear and Axle created a newly designed third member. Yukon Gear and Axle’s solution was an all-new third member made from nodular iron with increased stiffening ribs. This nodular iron third member is the one found under a lot of racecars, but the upgraded material and extra stiffening ribs are not the only added benefit. A provision for a load bolt is cast into the third member, too. A load bolt rides close to the backside of the ring gear, but not in contact with it, until the gear set is placed under a severe load. This prevents deflection and improves the strength of the third member as a complete unit.

These new nodular iron third member assemblies allowed for a complete redesign of the main part of the Ford 9-inch—the gear set. The custom third members had 4-inch side bearings for larger axleshaft clearance and an all-new 10-inch ring gear. The larger ring gear allows for more tooth contact between the ring-and-pinion and the improved surface area results in a stronger gear set. When coupled with the nose bearing on the pinion and the load bolt in the third member, it creates an incredibly strong connection between the two gears.

Upgraded third members were new to the Dirt SportsNation but not to the automotive aftermarket. Drag cars and hot rods had been benefiting from these upgrades for a long time. However, there was one issue none of these upgraded 9-inch third members solved: The third members were low-pinion axles leaving the driveshaft and entering the axle below the axle centerline.

True Hi9 created an all-new third member assembly that not only moves the driveshaft above the axle centerline, but with additional strengthening of the third member. The Hi9 utilizes a load bolt assembly similar to the low-pinion nodular iron third member. True Hi9 also created a Mega Hi9—an all-new pinion assembly with a 35-spline pinion yoke. The Mega Hi9 incorporates two load bolts located on opposite side of the pinion head where it meshes with the ring gear.

Currie Enterprises builds third members in many different configurations, ready for the end-user to abuse in the dirt.

In the axle world, bigger is often better and axleshaft spline count is one of those items. Factory 9-inches had shafts with 28- or 31-spline axles. In order to withstand the rigors of off-road racing, shaft spline counts of 35 or larger are needed. Lockers and spools to fit 35-spline axle shafts became readily available. As off-roading boundaries were pushed, though, even 35-spline shafts made of 300M material were pushed to their limits. The solution was simply to use 40-spline shafts, but this larger spline count axle does require using a large 3.250-inch side bearing third member.

Even with all the evolution of the Ford 9-inch axle over the years, things never stay stagnant in the Dirt Sports Nation. When Ultra4 cars started looking into independent front suspension systems, the center section of choice was the Ford 9-inch. This only makes you wonder how the Ford 9-inch will evolve during the next 20 years.

PhotosView Slideshow


Currie Enterprises
Corona, CA 92880
Spidertrax Off-Road
Longmont, CO 80503
Yukon Gear and Axle

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