I’ve noticed that there’s a lot of misinformation floating around out there about the Ford 9″ axle and what can be done with it. Some people are of the mindset that the 9″ is inherently weak because the outer diameter of the ring gear is .75″ smaller than the outer diameter of a Dana 60 ring gear. This is not the case. I won’t go into all of the different factory cases and applications, because there are already hundreds of websites and articles dedicated solely to that.
All modern differentials utilize “hypoid” gears. A hypoid gear set is essentially a cross between a spiral bevel gear and a worm gear. Hypoid gears are a type of spiral bevel gear whose axis does not intersect with the meshing gear. This means that as hypoid gears rotate, the teeth “slip” against each other.
Spiral bevel gear:
Now that the basics are out of the way, why is the 9″ remarkably strong when compared to some of the physically larger axles that are produced? Well, the amount that the teeth slide against each other can be directly correlated to the design of the gear set. The largest contributing factor is how far the pinion gear sits above or below the centerline of the ring gear. The farther the offset, the more the gears slip. This is referred to as a “high hypoid” design. Now, while designs like this tend to produce more heat and rob more power during operation, there is an extremely important silver lining. High hypoid gears generally have more contact area between the ring gear and pinion gear. This is because higher hypoid designs lean more toward the worm gear end of the spectrum. While it may not seem like a big deal in the grand scheme of things, it surely is. When comparing tooth overlap and cross sectional area of each tooth, a Dana 60 (depending on ratio and exact gear design) has only 50%-70% of the contact area that a 9″ does. In addition, the 9″ incorporates a third pinion support just like the revered 14 bolt full floater. Do I have your attention?
While the third bearing might seem insignificant in the grand scheme of things, it is EXTREMELY important. Deflection is what causes most major ring and pinion carnage. The third pinion bearing is placed on the opposite side of the pinion head when compared to the two traditional pinion bearings. Because of this “double shear” style of support at the pinion head, deflection is greatly reduced. Although it can never be completely eliminated, deflection control with a third bearing incorporated into the pinion is superior to any “standard” two bearing setup.
Ford 9″ differentials can be used with a variety of axle sizes. Beware, as carrier bearing sizes are generally specific to the spline count of the carrier. 9″ carriers can be had in 28, 31, 33, 35, and 40 spline varieties. Because of size constraints, the only 40 spline carriers available are spools.
Third member housings are also available in a variety of configurations and materials. Nodular iron and aluminum dominate the scene. There are several different styles of pinion supports available as well, some stronger than others. Fairly new to the game is a company named True Hi 9. They are the only manufacturer of high pinion 9″ housings. Currie has a similar claim, but their differentials use Ford 8.8 gears. Also, True Hi9 differentials use a “load bolt” that presses against the back of the ring gear to further reduce deflection.
Because of the high hypoid design, and tremendous aftermarket following of the 9″, a wide variety of gear ratios are available, all the way from 2.86:1 to 7.33:1.
Since the Ford 9″ has been the axle of choice for desert racers and high power street rods for years, there are an incredible number of housing manufacturers. Some of the most popular in the rock-crawling world are Spidertrax, Diamond Axle, and Crossed Up Off-road. However, if one chooses to venture outside the rock-crawling scene, the possibilities are endless, and there is almost no limit on the money that can be spent on a housing.
Since anything smaller would essentially be a waste of the strength of a 9″, most people run a 9″ differential and use everything else from a Dana 60. Dana 60/70 35 spline shafts will fit directly into a 35 spline 9″ carrier with no modification. This means that the tremendous aftermarket support that the Dana 60 receives in terms of axle shafts, knuckles, etc., can be combined with the following of the 9″.
Although arguably weaker than a full floating setup, a semi-floating axle is cheaper to build and generally weighs less. Building a semi-floating 9″ rear can be done with most, if not all, of the spline counts available. Bearing cups and retainer plates are available from a number of sources, as are the correct bearings to support the axle shaft.
There are essentially three ways to build a full floating 9″ rear. One way is to use a pressed-in spindle that is compatible with a Dana 60 hub. Another is to use a custom flange that mimics the bolt pattern of a Dana 60 knuckle. This allows a person to weld the flange to the end of the axle tube, and then bolt a traditional Dana 60 front spindle onto it. The last way is to adapt a unit bearing to the end of the axle tube. Spidertrax sells the components to do this and also sells F-350 unit bearings that have been re-drilled to many different patterns. In the case of a full-floating 9″ rear, custom axle shafts must be ordered. They have to be splined on both ends, and a drive slug is placed between the axle shaft and the internal splines of the hub.