Secrets of the Ford 9-Inch Rear Axle

Even though it was developed more than 60 years ago, the Ford 9-Inch is the rear axle of choice throughout the American high-performance world. Here’s why. 

 

When the Ford Motor Co. unveiled its 1957 vehicle line in October of 1956, in the press materials there was only brief mention of a new rear axle assembly for its cars and light trucks. Engineered in-house and produced by the company’s Sterling Axle Plant on Mound Road, which had opened only a few months earlier, the axle proved to be a winner—beyond anyone’s wildest dreams, actually. At the time, no one could have foreseen that today, more than 60 years later, the Ford 9-Inch is ubiquitous all across the American racing and performance scene.

The exploded diagram above reveals many of the design features that made the 9-Inch so popular:

+  The carrier housing is a front-loading dropout type, also known as a “banjo” or “pig” style, which is far more mechanic-friendly than the more common Salisbury/Spicer design in which the differential carrier loads into an integral axle housing from the rear. Here, backlash and pinion-depth adjustments are quick and easy, and gear ratio changes can be accomplished in minutes.

+   The pinion-shaft assembly is carried in a separate, detachable sub-housing (a cartridge, as some describe it), which simplifies adjustments even further and allows  beefy, large-diameter bearings and yoke.

+   The axle shafts are secured in the housing with sturdy retainer plates at the housing ends, rather than with C-clips inside the carrier, a setup that is not terribly safe or suitable for serious racing use.

 The ring gear (crown wheel in the Queen’s English) is a generous 9.0 inches in diameter, which allows the axle to withstand extreme torque loads and lent the rear axle its familiar name. Ford also manufactured axles of this design with 8.0-inch and 9.38-inch ring gears for various applications, and at one time or another, the axle family has been used in virtually every U.S. car and light truck platform produced by the company between 1957 and 1986.

 

So by fortune or design, the 9-Inch checks a number of important boxes for high-performance use. And when we dig a little deeper, we can see even more significant advantages, starting with a property called hypoid offset, above. In the hypoid gearset, introduced by Packard and Gleason Gear Works in 1926, the pinion gear is offset from the ring gear’s centerline, rather than centered as on a conventional spiral-bevel gearset. The result is a sort of bevel/worm gear hybrid, combining both meshing and sliding action between the gear teeth, and the increased contact area produces a stronger, quieter gearset. (Hypoid axles also allow a lower driveshaft and flatter passenger floor, surely the main reason they were embraced by the American car industry.)

In most U.S. passenger car drive axles, hypoid offset is generally in the 1.25-in. range (top left gearset). But on the Ford 9-inch (lower left gearset) the offset is much greater: 2.38 inches. This provides an even longer, deeper tooth contact (yellow arrow). The increased contact area does come at some cost: greater friction, more heat (often requiring a differential cooler), and a small but significant increase in mechanical loss— around two percent. In most applications, racers find the sacrifice is more than worth it. But it’s surely no coincidence that the 9-Inch was discontinued on production cars when fuel efficiency became a prime concern.

One more advantage of the 9-inch worth mentioning, as indicated by the red arrow above: Unlike most every other unit of its class, the Ford carrier includes an extra journal on the nose of the pinion to support a bearing set deep in the case, which stabilizes the gearset against deflection and allows a shorter, more compact pinion shaft.

With all these valuable attributes, the Ford 9-inch is far and away the favorite of the American high-performance scene, from street rodding to NASCAR, and it has been for decades—despite the fact that Ford hasn’t offered the unit in a production vehicle since 1986. Every component, down to the last spacer and seal, is now available in the performance aftermarket. Like a small-block Chevy V8 or a Fender guitar, an entire 9-inch axle can be assembled without a single original factory part. Specialist suppliers including Strange, Mark Williams, and Moser Engineering (shown above) offer a complete range of components and assemblies for every conceivable purpose.

It may seem a little odd that one of the top racing series in the world depends on a major component that was developed more than 60 years ago, but it’s true: Every car that runs in NASCAR Cup today has a Ford 9-Inch rear end under it—yes, even the non-Ford entries. As a result, the NASCAR teams have amassed vast inventories of 9-Inch assemblies, as shown below, in every gear ratio you can imagine, for tracks from Martinsville to Talladega.

But the way we hear it, that may be changing soon. Reportedly, NASCAR will ditch the venerable 9-Inch on the next-generation Cup car due in 2022 and adopt a sequential transaxle similar to those used in the Australia Supercars Championship. Still, we know that the Ford 9-Inch will be around the performance world for decades to come.

9 thoughts on “Secrets of the Ford 9-Inch Rear Axle

  1. The Ford 9″ rear is now made in a high pinion set up for off road use.
    And now you can get them made from aluminum. Amazing

  2. Nine Inch, most over rated thing ever. Heavy, eats power and burns the oil bad!
    You have to throw away everything Ford to get the strength including the axles. And even then the drag is a killer. I would have said 10% drag over the GM 10 bolt.
    Comparison ,, 3.55 GM v 3.55 9″ and you can feel the drag,, and smell the thing as well. And 35lbs heavier! The only reason is like many,, needing the shorter gear sets available,, 3.7 3.8. 4.11.This in a road race tintop.
    I used vegetable lim slip, And changed it every meeting. these days I believe synthetic oils help keep the oil from frying as much
    I broke Ford axles so used GM ones with no issue at all.
    It is easier to set up pinion depth once you have a box of shims, but I can guarantee they will drip through the stacked shims.
    At one stage I had 3 of 9″ here, Galaxy and Ranchero cruisers and the race car. Every one had a puddle under them.
    As for noise? I have never heard a dead quiet one and I have owned probably 20 351 Falcons
    They were probably ok under a low speed F100 or a big yank tank but even then they break. The reason Ford went the equally heavy but stronger Dana in F trucks

  3. I enjoyed your little jab with the lead pic. The Ford 9-in rear end is in the Toyota NASCAR show chassis they have at NAIAS every year.

  4. Never had any trouble with the 9 inch. I have built lots of cars and trucks over the years and the 9 inch has proven over and over to be the toughest set up I have ever seen.

  5. in 10 years of racing in the 60s we blew up 5 gm rear axels and went to the ford stock 9 inch and ran 6 years on the same axel. adjusted the backlash 1 time in that time. One hell of a tough rear axel.

  6. Many oval tracks, in entry level divisions that don’t allow a quick change, a 9″ is still mandatory because of the inherent dangers of having a wheel and axle assembly bouncing around a race track. Different gear ratios have become cheap, and with a nodular iron casting, virtually bulletproof. If you only do weekend local racing you only need one, maybe two gear sets. The lower pinion shaft allows for lower weight distribution of the whole car while keeping the driveshaft at the proper angle. Less wheel hop on deceleration, less rear axle wrap up in acceleration.
    Way too many things that are too important in racing . . . unless you want to spend only a minute in the car at a time, and don’t want to make turns. 😁

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