The rack sits behind the engine and transmission. The steering arm comes off the back of the knuckle (known as rear steer). The problem with this is that on a rear wheel drive car, the transmission and engine need to be right where the rear steer rack is. Here is how a similar setup works for a front engine, rear wheel drive car:
Notice how the the steering arm comes off the front of the knuckle here. This puts the rack much further forward. On a mustang the rack is under the sump which has a bump in it to make room for the rack:
Here are some solutions that won't work:
Use the rack from the mustang and just swap the left and right knuckles on the focus to make them front steer.
I really liked this idea for a few minutes until I realized that the ackerman would be reversed that the car would be undrivable (outside wheel trying to turn a tighter turn than the inside wheel).
Stay with rear steer, just put the rack below everything
This is a no go because the rack will be way below the scrub line of the car which means if you run something over the rack will get hit which is wicked bad news. Even if this wasn't an issue, having the rack really low will cause horrible bump steer.
Here are some solutions that might work:
Just move the rear steer rack way forward, but still let it rear steer the knuckle.
The rack can be moved forward a bit. In Carroll Smith's book "Engineer to Win" he describes just that. It will cause the rack ratio to get quicker at large steering angles, which is fine. It will also increase ackerman. (I think I will discuss the implication and magnitude of this in a future post) I have to assume there must be a reason nobody moves the rack forward to solve this. There are a couple I can think of: The tie rod forces increase as the rack moves forward, and the steering ratio gets slower (at small steering angle) and becomes nonlinear. Also there is a very limit range of motion in the rack ball joints.
Use the mustang knuckle on the focus strut and control arms
If it fits or can be made to fit. Also will have to think hard about the effect on handling. The focus frame rails are much wider than the mustang.
Swap left and right knuckles and build something to move the tie rod outboard to the correct location for ackerman steering.
Does caster and king pin inclination / scrub radius get messed up?
Take the whole mustang suspension over to the focus
The mustang does not use coil overs like the focus. The springs rest on perches:
So the perches have to go too, or make new ones. Again the Focus rails are much wider., but I think the track width is the same. So the perch would be interupted by the frame rails leading to some crazy fab.
Find some car with a front steer knuckle, or whole suspension that fits or nearly fits.
Junkyard scavanger hunt. Seems unlikely, but might be worth a quick check.
Buy a mustang II front suspension.
These are used by hot rodders everywhere because they are self contained and weld right to frame rails of various widths. The black beam in the image below represents the frame rails. would have to cut the fender where it meats the frame rail. Would have to work out the geometry on thisto ensure it would fit on my wide frame rail spacing and height.
Here are some solutions that will work:
Make the focus knuckle into front steer
Kugel makes a piece that bolts on that moves the steering arm knuckle forward of the knuckle. This is less than ideal, and the price is especially not ideal (~$700). This is how all the v8 focus builds I have found handle this. This is the fail safe if I don't find something better.
For reference, here is an article about how different front suspensions and power steering work:
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