How differentials work

By: Paul Tuzson - Story & Photos

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diffs diffs

How a differential works can be confusing, but not anymore...

From Unique Cars #319,Jan 2011

When we assembled a nine-inch rear end that was destined for the drag strip, it was a full-spool unit in which both axles were solidly linked together. For that reason, it didn’t really qualify as a differential because there was no capacity for individual rotation of either axle. Great for keeping a high-powered car straight and true on the strip, not much use on the road where corners are involved.

Almost everyone knows that a differential allows both rear (or front) wheels to turn at different speeds when turning a corner. There is, however, confusion about how differentials work, particularly the limited-slip differential (LSD). For instance, people take the term ‘single-wheeler’ to mean a differential that drives using only one wheel. This is incorrect. All differentials apply equal driving torque to both wheels at all times. Some people will find that jarring but it’s absolutely true.

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1. Supa-Trik dismantled this rare, all-original factory nine-inch LSD, with virtually no wear, for us. We’ve covered adjusting the position of the case and engagement pattern before

Wheels are obviously attached to axles and the external splines on the inner ends of those axles are mated with the internal splines on vertical side gears in the differential assembly. Between both side gears, and in constant mesh with them, are pinion gears. These pinion gears (often called spider gears) are mounted on shafts. Importantly, the pinion gears can rotate freely on the shafts. The ends of the shafts protruding through the pinion gears are fixed into holes in the differential case.

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2. This is the relationship between the main drive pinion (A), ring gear (B) and differential case (C). This hole in the case (D) is one of four that hold the differential pinion gear shafts

The differential case has bolted to it the ring gear, which is turned by the main drive pinion meshed with it.

As the ring gear and attached differential case are rotated, the shafts carrying the pinion gears rotate with it, end-over-end, so to speak. Because pinion gears are meshed with the side gears, the side gears also turn with the differential case. These, in turn, drive the axles and wheels. All these components are always in-mesh so driving force, or torque, is always applied to the axles.

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3. Note how one side gear (A) can turn independently of the other because the differential pinions (B) pivot freely on their shafts. The triangular arrow on the end of the shaft represents the driving action of the differential case

Now, even while the differential case is rotated by the ring gear, the pinion gears within it can still turn freely on their shafts. However, when a car is travelling straight, the differential pinions don’t turn. They remain stationary on their shafts and do just one thing – transmit the rotation of the carrier to the side gears to provide driving torque to the wheels. Simple.


4. Here, you can see the independent rotation of a side gear and pinions (pink dots) with the assembly set in the case. Clearly, when the case turns, it rotates the entire differential assembly yet, even as it does so, differential action between the side gears can continue

Differential pinions do turn on their shafts when a car turns a corner. The outside wheel, axle and side gear have to travel faster than the differential case is rotating. Conversely, the inside wheel, axle and side gear must turn more slowly than the case.

The reason the side gears can turn at different speeds to allow this is because the differential pinion gears with which they’re meshed are free to turn on their shafts.


5. Throughout this article we've used original Ford technology, differential case cover (A), LSD clutch steel plate (B), LSD clutch friction plate (C), side gears (D), differential pinions (E), pinion shaft (F) and pre-load springs (G)

Specifically, during a right-hand turn, the right-hand side gear turns more slowly than the differential case while the left-hand side gear turns faster. The pinions between both side gears rotate clockwise on their shafts (viewed from the outer ends of the shafts) to allow this to happen. Yet, even while this is happening, the case as a whole is still turning and carrying the shafts and pinions around with it, effectively driving the side gears through the pinions even though the pinions are rotating on their shafts. That, in a rather complicated nutshell, is how a differential works.


6.Interior detail of a NoSlip Detroit Locker from Eaton

So far we’ve described an open differential or single-wheeler. A single-wheeler works perfectly well – until one wheel loses traction.

When one wheel connected to an open differential loses traction, it simply spins while the wheel with grip remains stationary. However, both wheels still receive equal torque through the rotating case, pinion shafts, pinions and side gears.

But because the wheel with grip has very high resistance to moving, it tends to stay stationary. Consequently, the side gear attached to it becomes a sort of continuous pivot point for each of the pinion gears.


7. If you carefully think about the actions of teh pinion gears in a Truetrac, you'll see how differential action occurs. Truetrac centres are also made with four pinions (per side)

The differential case still pulls the pinions around with it but because the side gear connected to the wheel with grip remains stationary, each pinions rolls around it while the other side of each pinion continues to drive the wheel without grip.

The torque applied to each side gear by the pinion gears is equal but it can do more work on the side without traction because resistance is so much lower on that side.


8. An assembled Truetrac from the driver's side


The lack of grip at one wheel is really a lack of friction. An LSD solves the problem by creating, within the differential case, the friction needed to keep each wheel turning. In the classic Ford nine-inch, friction plates and steel intermediate plates similar to those used in auto transmissions are used.

In addition to having a side gear splined onto the left axle, a clutch hub is also splined onto the axle. Circular friction clutch plates are splined to the outside of the hub and intermittent steel clutch plates set between them.


9. This is an E-locker designed for four-wheel drive vehicles. We've head of them being used in performance cars but haven't seen one

These steel plates have ‘ears’ protruding from their outer edges that fit into matching cut-outs on the inside of the differential case cover.

When the axle splined to the clutch hub turns at the same speed as the differential case, the plates are stationary in relation to each other. When the axle turns faster or slower than the case, the friction plates and intermittent steel plates slide against each other in opposite directions.


10. The central triple-coil, pre-load spring pack sits in the middle of the one-piece pinion shaft assembly in a Borg Warner diff

If a wheel loses grip, friction between these plates limits the degree of slip between both axles and ensures that some available drive torque is applied to the wheel with grip.

Borg Warner

The Borg Warner diff is very common and used in huge numbers of Australian cars. It also has pinion and side gears as already described but friction in its LSD is created by cones that bear against matching surfaces machined into the inside of the differential case and cover.


11. In operation, the gears in a differential increase pressure in LSD assembles. In a nine-inch this can become intense enough to break the diff case cover, this is stronger aftermarket steel version. This view shows the other side of the cover and position of LSD clutch assembly and side gear

In both LSDs described, springs in the centre of the differential pinion gear mounting shaft assembly apply outward pressure against the side gear(s). This pressure is called pre-load and it determines how much force it takes to make the plates or cones begin sliding. More powerful cars need greater pre-load to prevent loss of drive but there comes a time when power outstrips the sorts of LSDs described so far.


12. The interior of the Borg Warner case (A), with a cone in place (B) and with the side gear added

Detroit Locker

The Detroit Locker is one of the most popular high-power differential upgrades. In the NoSPIN version shown, the centre driver is locked to the case/ring gear assembly by the four stub shafts extending from its outer edge. Drive teeth (‘dogs’) on each side of the centre driver engage with corresponding teeth on side clutches. The drive teeth are undercut so they lock together in a ‘monkey grip’ fashion.

Located in the middle of the centre driver is the centre cam. It has angled faces that extend past either side of the drive teeth on the centre driver. During a turn, when the outer wheel travels faster than the centre driver, the teeth on the side clutch move forward in relation to those on the centre driver and disengage from them.

Inner teeth on the side clutches ride up the ramps on the centre cam and this pushes the side clutch away from the centre driver.

As long as the wheel continues to turn faster than the centre driver, a hold-out ring keeps the centre driver and side clutch disengaged and the tops of the teeth skip across each other, which accounts for the noise heard from Detroit Lockers.

When wheel speeds equalise, the springs shown push the side clutches back into mesh. Re-engagement of the teeth can be sudden!


13. There are many different rear-end gear ratios available from both OE manufacturers and aftermarket


Like all differentials, the Truetrac has side gears splined to the axles. Around each side gear, and meshed with it, are three helical pinion gears. The pinions each side extend into the centre of the carrier and mesh with each other to form three pairs of pinions. So, the inner ends of each pair of pinions are interlocked with each other while the outer ends of each pair are interlocked with the side gears. The pinions are located in bores in the case.

Consequently, as the case/ring gear assembly turns, it carries the pinions around with it. Because they are in-mesh with the side gears, the axles turn. Friction between pinions and the differential case prevents one wheel/axle from spinning freely. The Truetrac never really locks up so it’s very smooth in operation.

So, those are the basics. Some OE diffs have a range of original parts available, some of which will handle more power than others. And of course, the Ford nine-inch has a vast array of aftermarket performance parts available.

Complete replacement aftermarket differentials like those from Eaton can also be a good idea if there’s one suited to your car.

Harrop Engineering (03) 9474 0900 brings in Eaton products but they are also available from other differential rebuilders like Supa-Trik (03) 9793 1915.


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