The 9-inch Ford rear axle is used in everything from street rods to drag racing to road racing to the Winston Cup. They've also been used in everything from the lowliest (slow) streetcars all the way up to Top Fuel dragsters. And they're used in Chevys with regularity. Unfortunately, in stock form, the Ford 9-inch housing isn't as strong as its reputation. Almost 15 years ago, pro drag racers were plagued with rear axlehousings that bent. Sure, the race teams and chassis builders fortified the housings with countless braces, supports and brackets, but the often-massive torque loads still managed to turn even the stoutest pieces into metal macaroni. In some instances, the axlehousings would bend and remain bent. In other more mysterious cases, the housings would bend during the initial launch and then snap back into form as the lap down the quarter-mile was completed-acting much like a huge, transverse spring (and this isn't exclusive to drag racing-we know of a very well-heeled and famous road race team that experienced the same dilemma).
We spoke to Chris Alston about the pitfalls of the Ford, and he elaborated on the inherent problems found in the 9-inch Ford housing: "The Ford 9-inch is a very popular housing, but you have to remember that the 9-inch hasn't been built for over a decade. That means the newest junkyard part you'll come across is at least 10 years old. Aside from age, there are three major drawbacks to the 9-inch: First, the housing axletubes are not round. Not only do they taper from 3 1/2 inches down to 3 inches, the tubes all have "flats" which are more or less squashed onto the tubes. Both of these factors force a chassis builder to custom-build every bracket because they aren't symmetrical. Finally, the OEM Ford housing face is approximately 24 inches wide. Because of this, you have to weld brackets to the face in some cars. This means you lose adjustment holes for items such as the four-link."
Another problem that's seldom discussed with Ford 9-inch housings is the rear brace. The almost-standard brace does nothing to stop downward bending on the rather flimsy Ford housing. A second brace is usually needed to eliminate downward flex. Another point to consider is the actual rear "cover" in the housing. The backplate isn't one piece; it's usually pressed into the housing. The chassis builder is forced to weld the rear cover to the housing, but that doesn't stop another problem: the hypoid action of the third member still tries to force the rear end out the back of the housing, which exerts forces on the assembly. In order to stop this, at least one of the chassis builders we spoke with, Mike Pustelny (of MPR in Detroit, Michigan) has reverted to welding a series of small tubes radially between the face of the housing and the rear cover plate. Most often called a "cage," this apparatus stops the center portion of the housing from flexing fore and aft.
With all of the cutting and pasting, the fabricator usually begins with only the skeletal remains of the stock Ford housing, and then starts from scratch. It's not hard to see that this isn't a very cost- or time-efficient way to build a rear axlehousing. There must something better. And there is. Instead of using marginal factory components, some housings are now built entirely from scratch. One of the first to fabricate an all-new housing was George Cathey of Pro Designs. Cathey's new, completely fabricated sheetmetal housings didn't rely upon marginal junkyard parts for the foundation. They didn't bend; they were brutally strong and very resistant to extreme torque loads. Just as important, they were lighter than the heavily braced Ford 9-inch housings that were standard equipment under the quickest cars. Racers being racers, the advantages became immediately clear. Soon the housings were seen under Pro Stock cars. With the decidedly different shape of the housing, the drag race media quickly came up with the term "Star Wars housings." While Cathey has moved on to other projects and doesn't build housings on a regular basis, his original housings have forged a path for several new, completely fabricated housing designs.