Steering racks are not the stuff of Mustang passion, at least not until your Mustang goes wandering or you're building a track car that needs the last word in precision. Then you need the expertise of chassis experts who have been studying Ford steering to keep their customers headed where they want to.
You need Maximum Motorsports, in other words. Which is exactly where we headed when we noticed the directional control in our '91 Mustang daily driver had degenerated into something more akin to stirring walnut-laced oatmeal than precise guidance. What we found was a subject far more detailed than we had guessed, but with a few clear-cut choices for the enthusiast once past all the theory.
To keep this discussion within practical limits, we'll confine ourselves to '79-'04 Mustang steering. These are the Fox and SN-95 cars that typically need attention. The '05-'11 steering is mechanically similar but already incorporates the improvements developed during the Fox and SN-95 era, plus the later steering racks mount completely differently and offer no interchange possibilities.
Let's start at the beginning. All Fox and later Mustangs use rack and pinion steering. This means the steering wheel is attached to a shaft passing through the firewall and connected to what we all casually call the “steering rack” or simply “the rack.”
No matter what you call it, the rack assembly converts the rotation of the steering shaft into left-right motion. That left-right movement is connected to the front wheels via tie rods screwed to each end of the rack. The rack assembly also provides the steering gear ratio, plus controls and applies the hydraulic power assist from the power-steering pump.
A close-up view of the rack...
A close-up view of the rack and pinion intersection shows the helical-cut gears pinion gears intersecting the straight-cut gears on the rack. The ratio between the rack and pinion gears defines the steering ratio, and by varying the spacing of the teeth on the rack, Ford builds in variable ratio steering. This also means changing steering ratios means changing the rack assembly. In this photo, we turned the rack upside down so the gears would be easier to see.
We laid out this steering...
We laid out this steering rack housing and steering rack to illustrate the fundamentals of how the power steering applies force. Note that on the rack there is a fence or piston just a little left of center. On the rack housing you can see the hydraulic lines entering the housing on either side of where the fence on the rack rides. The other end of the hydraulic lines run to the input shaft (pinion) area of the housing, where the spool valve lives. As you turn the steering wheel, the spool valve ports hydraulic pressure to one side or the other of the rack fence, thus pushing on the rack.
At left is a Fox pinion with...
At left is a Fox pinion with straight-cut teeth; at right is an SN-95 pinion with helical teeth. The SN-95's helical teeth are inherently more precise, allowing less backlash, and thus improved steering feel and precision. The four orange lines running around the Fox pinion's collar are O-rings; the SN-95 uses the same O-rings but they weren't installed for this photo.
Inside the rack assembly are the pinion, which is the small gear at the bottom end of the steering shaft, and the steering rack, which is a horizontally mounted steel shaft with a series of teeth ground into it. The pinion gear meshes with the steering rack's teeth, which is where the steering gear ratio comes from (how much the steering wheel turns compared to how much the front wheels turn). So far, so easy.
The real fun begins with the hydraulic power steering used on the majority of Mustangs. Here pressurized fluid is piped from the power steering pump to a control valve built into the steering rack. This hydraulic servo valve, typically called the spool valve due to its design, directs fluid into the rack assembly where it applies pressure against one side or the other of a fence built onto the rack. The fluid pressure thus assists the driver in moving the rack against the tire's traction.
The devilish details lay in the intricacies of the spool valve. We auto enthusiasts are normally not familiar with spool valves as there's only one on a car--the one inside the steering rack. It's a reasonably complex device, and we never see it because spool valves don't require maintenance and we don't hot rod them. But--and this is the whole point in swapping steering racks--there are delicately machined details in the spool valve that give the steering rack assembly its characteristic steering feel.
To come right to the point, even after installing the best tires, new tie rods, urethane bushings, stiffer springs, a sport steering wheel, and all-metal intermediate steering shaft, there's still more precision to be gained by swapping a Fox steering rack for one of several SN-95 racks.
We're about to take a detailed look at the spool valve for those who want to know the hows and whys of its operation. There are a lot of words, so skip ahead if you just want the practical considerations, but you'll understand much more about your Mustang's steering if you wade through.
At the steering wheel end...
At the steering wheel end of the pinion shaft, the Fox uses splines and the SN-95 employs a triangle set of two flats. Accommodating the spline/flat difference is the only bolt-in consideration when swapping steering rack assemblies between Fox and SN-95 cars. All other dimensions are identical.
Power steering feel and response...
Power steering feel and response is largely determined by the fine machining found in the spool valve assembly. Here the pin is pointing to one of the tiny edges machined into the edge of a spool valve groove machined into the pinion shaft.
Looking inside the collar...
Looking inside the collar that forms the outer portion of the spool valve shows the machined slots that correspond with the similar slots in the pinion shaft. As the pinion and collar rotate relative to each other, these slots vary the volume of power steering fluid directed to the rack.
It's tough to understand how...
It's tough to understand how the power steering spool valve works, so a few extra photos should help. Let's start with the input shaft. As seen here, the steering wheel end is to the left and the right end has two flats machined into it. The grooves around the shaft near the right end are part of the spool valve; they're the ones with the tiny machined edges.
Here's the pinion shaft with...
Here's the pinion shaft with the long, thin torsion bar--the "T-bar"--extending toward the camera. The near end of the T-bar carries a hole to accept a roll pin used to tie the T-bar to the input shaft.
OK, here's the input shaft...
OK, here's the input shaft in our left hand and the pinion shaft in our right. The T-bar slides into the hollow input shaft, and the flats on the end of the input shaft mate loosely with corresponding flats inside the pinion shaft.