Project Ford Focus ZX3: Part 4

For the last few months, our Focus has been doing nothing but burnouts. At every green light, pulling out of every driveway and in every McDonald's drive-thru, our tame-looking little hatch has been leaving its mark, the mark of the dogleg. With the addition of a larger BBK throttle body and super-light FocusSport flywheel, our Focus snaps off the line like a scared cat being chased by a kazoo band. It's hard to look dignified driving a one-wheeled buzzsaw.

It's also hard to go fast. Our Focus lifts its leg and lights one off exiting every corner. It's time for a limited slip.

Quaife has had a limited-slip diff for the Focus since the car first appeared in the United States, but it's a fairly expensive part for such a cheap car. Ford's upcoming European-market Focus RS will have a factory-installed Quaife, however, and Ford's economies of scale have dropped the price to just under $600. No more excuses.

Installing a new differential is no trivial task. It requires removing and disassembling the transmission, which can cost as much as the differential itself. To be cost effective, we suggest you install a clutch, flywheel and anything else that requires the removal of the transmission at the same time. Because we already had a FocusSport flywheel, and the stock clutch seemed far from retirement, we decided to fix the Focus' granny-tuned gearbox instead.

The Focus' MTX75 gearbox is big, strong and has well-spaced gears, but the stock 3.82:1 final drive is simply too tall. Knowing that the base 2.0-liter Focus in Europe uses a shorter final drive, we rooted around in some secret Ford junk bins until we found a 4.06:1 ring-and-pinion gear. Ford Racing plans to add this ring and pinion to its catalog later this year, but the parts are already in Ford's parts network. Utter the part numbers 93ZT-7G334-BA and XS7R-7061-AB to a parts manager and flash some cash, and you should be able to get them. If you can't find a resourceful enough Ford dealer in the U.S., you may have to call one in Europe. In that case, the European part numbers are 6756205 and 1125041. Before shipping, the ring and pinion should only be about $140.

Installing either the ring and pinion or the differential requires removing the entire subframe (support the engine, remove the ball joints from the hubs, and drop the subframe and lower control arms as a unit) and then the transmission. With the tranny on the bench, you must first unbolt and remove the shift mechanism, then undo the ring of bolts holding the two halves of the transmission together. Now, with the bellhousing lying flat on the bench, gather your courage and pull the two halves apart. The input shaft should stay in the top half and the output shaft will stay in the bottom with the differential.

If you're just replacing the diff, removing the input and output shafts should reveal the differential. You'll have to swap the ring gear and the bearings from the old diff. One of the bearings should come off with a gear puller, but the other is pressed right up against the diff case and can't be removed without damaging it. Before tearing apart the transmission, order a new bearing or even two, especially if you're unsure about re-using the bearing from the other side. The differential bearings are part number S7RZ-4221-AC.

If you're doing the ring and pinion, you'll also have to pull the bearing from the end of the pinion shaft and remove all the gears from the pinion shaft. At this point, if you're still reading, you better know how to put a transmission back together. If you don't, find someone who does. As transmissions go, the MTX75 is relatively simple, but the gearbox is still one of the most complex parts of any car, and we don't recommend tackling this one without strict adult supervision.

Which is why we dumped this unpleasant job on the guys at FocusSport.

If you do tackle this one yourself, you'll get all the gears off the pinion shaft and find the pinion side bearing impossible to remove. The new bearing you'll need is part number F8RZ-7S431-BB. The one you pulled off, but don't want to reuse, is part number F8RZ-7S431-AA.

Driving it
The changes to the gearbox would go almost completely unnoticed if you've never driven a Focus. The gearing simply feels right, and nothing dramatic happens when accelerating around a corner. The Quaife works so seamlessly that unless you knew the Focus used to leave one steaming stripe on the exit of every corner, you'd probably never even know it was there. With power going to both front wheels, you can also get on the power sooner and harder when exiting a corner than you could with an open diff. The lack of wheelspin removes the air of futility from your acceleration, and the extra torque reaching the outside wheel actually helps rotate the car.

We could just tell you this, but we didn't think you would believe us, so we devised a test. Before installing the Quaife, we took the Focus to our skidpad and performed a "standing turn" test. Starting from a standstill, we accelerated as hard as possible in a 200-ft. diameter arc for exactly half of a circle, a total of 628 ft. At the launch, the car would spin both tires, then the outside wheel would get grip, the car would lurch inward and accelerate around the corner with the inside tire ablaze. The test was long enough to require a shift to second gear and some throttle modulation to keep the car on line. After several practice runs, the fastest pass was 9.4 seconds.

With the new differential and final drive in place, we repeated the test. This time the transition from initial wheelspin to cornering was smooth and uneventful. There was no tire spinning after the launch, and we could use substantially more throttle after shifting to second gear. Most impressive, though, was the time: 8.2 seconds. Next time we say a limited slip makes a big difference you'll believe us, won't you?

How Does It Work?

Enough about our tire-spinning Focus, what you really want to know is how this mysterious box of gears actually works, right? Before explaining how the limited-slip action works, let's look at how the differential part works.

We'll start at the beginning: When you go around a corner, the inside wheels have less ground to cover than the outside wheels, so naturally, they spin more slowly. The differential's job is simply to allow the two drive wheels to turn at different speeds while still staying mechanically connected to the gearbox.

Imagine, for a second, that you're the differential. When the car is going straight, the differential itself spins at the same speed as the wheels, and from your point of view as an honorary differential, all the gears inside are stationary. When the car goes around a corner and one wheel has to go faster than the other, all you'd see, as a differential, is the two axles turning in opposite directions.

With your newfound perspective, look at the diagram of the Quaife differential and imagine the sun gear in front (the gears attached to the axles are called sun gears) turning clockwise. The pinion gears around the outside of the sun gear therefore turn counterclockwise. The pinion gears from one side of the differential mesh with pinion gears from the other side, so they turn clockwise, and those pinion gears turn the sun gear on the far side counterclockwise. That, if you've lost track, means it's turning in the opposite direction from the sun gear in front, which is just what you, as a differential, want to see when you're going around a corner.

Coasting around that corner, there's no load transmitted through any of these gears, but hit the throttle and things get complicated. Look closely at what happens when the differential tries to drive the wheels. Assuming the car is going straight and there's equal grip on both wheels, the differential housing turns, which forces the pinion gears, trapped in little pockets around the perimeter of the diff, to move with it. These pinion gears are meshed with the sun gears, and because none of the gears are turning (relative to the diff housing, that is), the sun gears have to follow along, spinning at the same speed as the differential.

The gear teeth can only push on each other with a force directly perpendicular to the face of the tooth, so if the tooth is angled, so is the force on the gear teeth.

Now, the pinion gears don't rotate on a shaft; instead, they sit in tight-fitting pockets, with the tips of their gear teeth rubbing on the inner walls of each pocket. When the differential housing turns and the pinion gears push on the sun gears, the pinion gears get shoved back against the walls of their pockets. Of course, the more torque that gets applied, the harder they get shoved against the wall.

That's going to be very important in a second.

When one wheel loses grip and tries to spin, all the gears have to start turning, just as they did when we were coasting around a corner, but now that the pinion gears are being shoved against the walls of their pockets, there's some resistance. This is just the beginning of the resistance, though. In the middle, where the pinion gears mesh and the pinion on the gripping side tries to turn the pinion on the slipping side, something interesting happens.

The pinion gears are cut with helical gear teeth that mesh at an angle. When these angled teeth push against each other, the angle of the teeth causes them to pull each other up against the end of their pockets.

The combined friction of the tip of the pinion gear teeth rubbing against the pocket walls and the ends of the pinions rubbing against the ends of the pockets actually creates enough resistance to prevent the inside tire from spinning.

When designing a new application, Quaife engineers can adjust the amount of resistance in the diff by changing the shape of the gear teeth and the angle of the helical pinion gears. To see how this works, just look closely at the interface between two gear teeth, starting with gear tooth No. 1 on the left. The bottom tooth is pushing up with a force represented by the little green arrow. At the gear tooth interface, the bottom tooth pushes up on the top tooth with that same force (but now the arrow is white), and if there's enough resistance on the other gear, the top tooth will push back with the same force.

The gear teeth can only push on each other with a force directly perpendicular to the face of the tooth, so if the tooth is angled, so is the force on the gear teeth (the white arrow). In gear tooth No. 2, you can see the white arrow is now angled. This same force can be looked at as two forces, one pushing up (green), and one pushing to the side (red). As the angle of the teeth gets steeper, as in gear tooth No. 3, the amount of force pushing to the side increases. The force shoving the pinion gears into the pocket wall comes, in part, from these little red arrows.

The majority of the resistance and virtually all of the tuning happens when the angle of the helical pinion gear teeth is changed. A steeper helix translates directly into bigger red arrows shoving the pinion gears into the end of the pockets.

It's important to note if there's no resistance from the gear on top, the gears will simply turn and there won't be any red arrows at all. No red arrows mean no limited slip, so if one wheel is completely off the ground, the Quaife will act as an open differential. This is why our Focus street car, which always keeps its wheels on the ground, uses a Quaife, while our Focus rally car uses a clutch-type Kaaz differential.