Selecting Aftermarket Differentials to Improve Performance

There are many different types of aftermarket and original equipment differentials and technologies to choose from nowadays. What are the differences and benefits of one technology over another? With so many choices, it can be difficult to decide which one you need or want.

 


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Ultimately, you have to determine your vehicle’s performance level and purpose. Many times, your choices are limited based on your application. The more popular applications have a host of products to choose from, including four basic styles of differential: limited-slip, open, spool, and locker.

The limited-slip differentials have many subsets, and this section reviews those types. In addition, I cover the differences between passive versus active and speed sensing versus torque sensing. When possible, I group the technologies and list some of the common names. When this is not possible I simply cover the specific unit itself.

One component of the rear-axle assembly has a single input from the propshaft and two outputs to the rear wheels. The device that splits the single input into two outputs is the differential. The bevel style axle differential distributes the torque evenly between the axle shafts. This is known as a 50/50 differential because it balances the torque equally to each side, assuming there is equal surface coefficients to deliver that torque to the road. This torque split is based on the specific geometry of the side gears. Since the size or tooth count of each side gear is the same, there is an even 50/50 torque split.

 

This diagram shows the torque path through the open differential from the ring gear flange through the differential pin to the side gears. (GKN Driveline)

This diagram shows the torque path through the open differential from the ring gear flange through the differential pin to the side gears. (GKN Driveline)

 

The differential is also required to allow for speed differences experienced during normal daily driving cornering maneuvers. When you take a turn, the inside rear wheel travels on a smaller path than the outside wheel, and the differential needs to compensate for the difference in path distance traveled by the wheels.

We also need to keep in mind that the relationship of the side-gear geometry mathematically determines the open differential torque split. Since the number of teeth and therefore pitch diameter of the side gears is equal, we then have the correct geometry to yield an equal torque split. The torque must be equal on each output. If one wheel is on snow it has very low torque, but then the other wheel will also have the same low value of torque. In this rather extreme condition, the open differential is traction limited and does not deliver any more torque to the good traction wheel. The following equation represents this:

Ttotal = min {Tleft Tright} x 2

The total torque transferred is equal to the lesser wheel torque multiplied by two. If the torque is virtually zero, twice zero is just more zero. This is the negative side of an open differential. This downside can also be evident in a high-powered car accelerating with one tire more heavily loaded than the other, or even while accelerating out of a turn where the inside wheel is unloaded. This can also happen with the torque reaction from a beam axle. As reviewed in Chapter 1, the beam axle tends to lift the right wheel, thus reducing its tractive effort to the track surface.

Also, the speed relationship for an open differential is:

2 x ωdiff. = ωleft + ωright

During normal straight ahead driving, the differential case speed is the same as the left and right wheels. Therefore, the internal differential gears are not moving relative to one another, but rolling as a complete assembly. While this is a little tricky to imagine, the gears inside the differential are not rotating relative to the differential case during normal driving conditions. This is why the gears are straight bevels as opposed to spiral gears like the ring and pinion hypoid set. The straight-bevel gears are normally noisy as the gears tumble relative to one another, but this only happens during turns and the above extreme traction conditions. Most drivers cannot discern the differential gear noise since there is always other noise in the vehicle.(as discussed in Chapter 4.)

 

This chart illustrates common performance characteristics of an open differential. The left-wheel torque is on the vertical axis and the right-wheel torque is on the horizontal axis. The slope of the line is at 45 degrees, which signifies the 50/50 torque split. The negative values in the lower-left-hand corner cover what happens during deceleration and braking events.

This chart illustrates common performance characteristics of an open differential. The left-wheel torque is on the vertical axis and the right-wheel torque is on the horizontal axis. The slope of the line is at 45 degrees, which signifies the 50/50 torque split. The negative values in the lower-left-hand corner cover what happens during deceleration and braking events.

 

Remember that of any two gears in mesh, the smaller is always called the pinion. Some people refer to the pinions as spider gears because they run around the side gears like spiders on a web during differentiation events such as turning corners.

Remember that of any two gears in mesh, the smaller is always called the pinion. Some people refer to the pinions as spider gears because they run around the side gears like spiders on a web during differentiation events such as turning corners.

 

Most differentials have two pin-ions that mesh with the side gears. In order to increase the torque-carrying capacity, some differentials utilize three or four pinions meshing with the two side gears. The only three-pinion differential that I am aware of is from the Ford 10.5-inch axle used on one-ton trucks. Most differentials that need additional torque-carrying capacity in a small package utilize a four-pinion-style differential.

One last important item to cover is that, during normal driving conditions, an open differential is more than adequate. It is only when we have excess torque that we are trying to transfer, or wheel loading that is uneven, that the shortcomings become apparent. Limited-slip differentials were invented to address these deficiencies.

 

Limited-Slip Differentials

Based on traction limitation due to excess torque, limited-slip differentials and locking differentials were developed. There are many different types available, and each variant attempts to be the best of both worlds by acting like an open differential in turn maneuvers, but approaching full lock during excessive speed differences across the wheels, such as in low-traction events.

There are two potential means for the torque to be transferred through a limited-slip differential. The first path is just like an open differential: from the ring gear flange to the differential pin to the pinions, and then to the side gears. The second path is when the side gears alone cannot resist the torque or excessive speed across the differential. In this situation, there is a parallel path typically through a clutch pack to help the differential gears.

 

The common path of power through the side gears is shown in purple on this schematic. The secondary parallel path is shown in blue through the clutch pack. (GKN Driveline)

The common path of power through the side gears is shown in purple on this schematic. The secondary parallel path is shown in blue through the clutch pack. (GKN Driveline)

 

This chart compares the open-differential performance (light blue) with a limited-slip differential with a torque bias ratio of 2.5:1. The performance of the curve can be more aggressive depending on whether the differential is in drive or coast mode.

This chart compares the open-differential performance (light blue) with a limited-slip differential with a torque bias ratio of 2.5:1. The performance of the curve can be more aggressive depending on whether the differential is in drive or coast mode.

 

These limited-slip devices are characterized by their performance behavior when there is a speed or torque difference across the two axle shafts, and subsequently across the differential. Otherwise, they typically behave just like open differentials when there is no speed or torque difference across them.

The limited-slip differential tries to equalize this difference by taking speed from the faster wheel and sending it to the slower wheel. For example, one wheel is traveling at 20 mph and the other is at 2 mph. The limited slip tries to equalize the speed to 11 mph per wheel. The mechanism that performs this speed equalizing process is what separates one limited-slip differential from another. During these speed difference events, I like to describe the performance of the differential by the term torque bias ratio (TBR). Torque bias ratio is simply the higher wheel torque divided by the lower wheel torque. It can also be thought of as a torque multiplier across the differential.

TBR =Thigh ÷ Tlow

The wheel with low torque is typically unloaded (or on a slippery surface like snow or ice) while the wheel with higher torque has more load or better traction available. Typical values for TBR are in the 1.1 to 4.0:1 range for most differentials (see “Spools and Mini-Spools” and “Lockers” below).

In theory, a purely open differential has a TBR of exactly 1.0:1. In reality there is some internal friction on the differential gears sliding relative to one another (the pinions to differential pin), and therefore even an open-differential provides a bias ratio of at least 1.1:1. Depending on the side gear and pinion design, open differential TBR can go as high as 1.3:1 or so. The torque bias ratio is a good measure of how effective the differential is at improving the total tractive effort of the vehicle.

Passive vs. Active

Most differentials covered here fall into the category of passive differentials. This means that the device is purely mechanical and does not have any electronic control or feedback. The differential in this case is just responding to a predetermined trigger event, such as excessive wheel slip, and engages to try to limit that wheel slip to the extent possible by the device. Passive devices work well for poor-traction-enhancement conditions.

Conversely, an active differential has electronic control and can be activated independently of wheel speed or torque. These devices can be preemptive and engage before excessive wheel slip occurs. These are typically found on high-end modern vehicles and offer additional benefits over passive devices.

 

Here, an open differential is compared to a preloaded differential. A spring applying pressure to a clutch pack provides the preload. Notice that the performance curve has the same slope as the open. However, it is offset by the amount of preload that has been established in the clutch-pack-and-spring system.

Here, an open differential is compared to a preloaded differential. A spring applying pressure to a clutch pack provides the preload. Notice that the performance curve has the same slope as the open. However, it is offset by the amount of preload that has been established in the clutch-pack-and-spring system.

 

The open-differential performance curve is again illustrated in light blue. The differential with an example torque bias ratio of 2.5:1 is in red, while the differential that has a torque bias ratio with preload is in light green.

The open-differential performance curve is again illustrated in light blue. The differential with an example torque bias ratio of 2.5:1 is in red, while the differential that has a torque bias ratio with preload is in light green.

 

The main benefit is the ability to control vehicle dynamics and enhance yaw stability. Yaw is the term used to describe the rotation of the vehicle around its vertical axis. Some prefer to call it fishtailing, pushing, or plowing. These devices can also integrate with the vehicle anti-lock-brake and stability-control systems. (More on active devices later in this chapter.)

Torque Sensing vs. Speed Sensing

The terms “torque sensing” and “speed sensing” refer to the trigger event that the differential device is reacting to. If there is a torque difference across the wheels, and subsequently the differential, the torque-sensing differential will bias torque to the high-torque wheel. This can be accomplished through a clutch pack or even mechanical friction.

In contrast, a speed-sensing differential responds to a speed difference across the differential device, and tries to balance the speed across the two wheels.

Differential Preload

The last piece of this puzzle is the idea of differential preload. In Chapter 4, I discussed how to rebuild typical clutch-plate-style limited-slip differentials. For a limited-slip, the initial preload, or break-away torque, allows power application when one drive wheel is on ice or in the air. Preload is actually a force applied across the side gears, which is usually derived from clutch plates and a spring that resists the relative motion of the side gears to one another.

The differential acts like a locked differential until the preload torque is overcome, then a speed difference is allowed. The value of the overall preload must not to be too high, or wind-up and tire scrub will be experienced during low-speed cornering maneuvers.

We can also combine the torque-biasing effect and preload in a differential. One method to achieve this is to use the side-gear separating loads to apply an additional force on the clutch packs. (This is covered in more detail later in this chapter.)

Cone-Style Limited-Slip Differential

Limited-slip differentials use a cone-shape friction surface on the back of the side gear to provide both preload and torque bias. The most common is the Auburn Gear differential, which has springs between the side gears that provide preload force to the conical surfaces of the differential case. Separating forces from the differential gear’s rotating during speed difference events apply additional loads to the differential case for increased locking action.

 

8)This sectioned Auburn cone-style differential clearly shows the conical shape on the back surface of the side gears (gold). This is reached by the inner surface of the differential housing. The Auburn Pro and High Performance series are offered. The main difference being the Pro Series has a higher bias ratio (around 3.5:1) than the High Performance Series (2.5:1). (Auburn Gear)

8) This sectioned Auburn cone-style differential clearly shows the conical shape on the back surface of the side gears (gold). This is reached by the inner surface of the differential housing. The Auburn Pro and High Performance series are offered. The main difference being the Pro Series has a higher bias ratio (around 3.5:1) than the High Performance Series (2.5:1). (Auburn Gear)

 

An actual sectioned differential is shown so you can see the tapered ring between the side gear and housing. (GKN Driveline)

An actual sectioned differential is shown so you can see the tapered ring between the side gear and housing. (GKN Driveline)

 

This schematic shows the torque transfer through the SuperLSD. The tapered ring (yellow) is between the side gear and the inner case wall. This unit also utilizes preload springs. (GKN Driveline)

This schematic shows the torque transfer through the SuperLSD. The tapered ring (yellow) is between the side gear and the inner case wall. This unit also utilizes preload springs. (GKN Driveline)

 

The SuperLSD by GKN is another cone-style limited-slip differential on the market, but it is mainly for OEM applications. This is similar to the Auburn unit, but actually has an additional frictional element between the side gear back face and the differential housing. This additional frictional element is called a tapered ring.

Flat-Clutch-Plate-Style Limited-Slip Differential

The traditional flat-clutch-plate-style differentials are what can be found in most OEM axles. (A typical application was discussed in Chapter 4.) These are also referred to as multi-plate differentials. There are a couple of twists on these designs, but let’s cover the common types first: Dana Trac-Lok, Ford Traction-Lok, GKN TL, Moroso Brute Strength, and Eaton Posi. They are all very similar in function and performance. Let’s talk about torque bias ratio and pre-load of these units.

The performance curve on the graph is offset by the amount of preload from the open differential reference line (green). Also, the performance curve slopes change to correspond to the changing torque bias ratio. The blue lines are labeled as a left-hand turn, which means that the outside wheel, in this case the right wheel, is traveling faster than the inside or left wheel.

 

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Some slight variations to the multi-plate limited-slip differential design produce even greater torque transfer during slip events. The main twist is the addition of a split housing with a ramp mechanism, and that can apply a greater thrust force on the clutch as compared to just the side gears. These units can also have a set of Belleville washers that allow for preload tuning when set up properly. Some of the units that are available are the Dana Powr Lok, Chrysler Sure Grip, and GKN Lock O Matic (LOM).

 

This graph of the typical performance provides actual test data of a multi-plate differential. This differential had approximately 25 ft-lbs of preload torque as measured across the shafts. The TBR range varied from 1.7 to 2.5 during the test cycle.

This graph of the typical performance provides actual test data of a multi-plate differential. This differential had approximately 25 ft-lbs of preload torque as measured across the shafts. The TBR range varied from 1.7 to 2.5 during the test cycle.

 

Here is the completely assembled unit. Again, notice the traditional style spiral wound preload springs.

Here is the completely assembled unit. Again, notice the traditional style spiral wound preload springs.

 

This is an exploded view of the differential internal components of a typical multi-plate limited-slip differential. The Eaton, GM, and Dana units use this style of compression springs; the Ford unit uses an Sshaped spring. (Eaton Corporation)

This is an exploded view of the differential internal components of a typical multi-plate limited-slip differential. The Eaton, GM, and Dana units use this style of compression springs; the Ford unit uses an Sshaped spring. (Eaton Corporation)

 

Here is a cutaway of the cam-style multi-plate differential. The flats are machined into the differential pin, while the pin reacts to the triangle-shaped pocket. One last interesting piece is the inner housing in chrome that actually moves outward to apply the clutch pack. (GKN Driveline)

Here is a cutaway of the cam-style multi-plate differential. The flats are machined into the differential pin, while the pin reacts to the triangle-shaped pocket. One last interesting piece is the inner housing in chrome that actually moves outward to apply the clutch pack. (GKN Driveline)

 

On a typical G80 LSD, the small cylindrical piece in the differential window is the flyweight mechanism. The engagement pawl is the larger sickle-shaped piece above it and the governor lockout mechanism.

On a typical G80 LSD, the small cylindrical piece in the differential window is the flyweight mechanism. The engagement pawl is the larger sickle-shaped piece above it and the governor lockout mechanism.

 

This schematic illustrates the apply forces from the cam mechanism. You can see how the input torque is used to separate the cam-shaped housing. The cam angle determines the separation force and how much is applied to squeeze the clutch packs.

This schematic illustrates the apply forces from the cam mechanism. You can see how the input torque is used to separate the cam-shaped housing. The cam angle determines the separation force and how much is applied to squeeze the clutch packs.

 

The Eaton Command Traxx, G80 (the GM option code), or the gov. lock (short for governor lock) is a passive, speed-sensing limited-slip differential, which contains a set of flyweights like those found on a distributor timing advance mechanism. These fly-weights are driven off any speed difference across the differential. When a speed difference (typically above 100 rpm) occurs, the flyweights centrifugally advance and engage a pawl mechanism that stops the flyweights.

The flyweight mechanism then engages a counterweighted pawl mechanism that overcomes the detents in between the differential side gear and the cam plate. The detents in the cam plate are designed to ensure that the device does not unintentionally engage. This pawl mechanism engages a cammed clutch system that squeezes a clutch pack to lock the differential.

There is a reaction block located in the center of the differential pin to transfer the axial force of the cam plate to both clutch packs located on both side gears. It incorporates carbon-fiber-based friction material and a governing mechanism to allow for lock conditions at speeds lower than 20 mph. Above 20 mph, the device performs like a traditional open differential. If there is a lock condition (below 20 mph) and the torque is continuously applied (i.e., wide-open-throttle condition), the device remains locked until there is a torque reversal.

Fully Active Limited-Slip Differential

A fully active limited-slip differential is only available in a high-end luxury vehicle, such as Mercedes, Jaguar, Land Rover, and Porsche. The technology is an electronically controlled limited-slip differential (eLSD). These devices have full torque control across the differential. This total control allows the differential to act as an open or a locker, or anything in between. This allows the OEM the ability to actually command torque across the differential. The differential is even intelligent enough to report back to the electronic control the actual torque transferred.

The most common device of this type is the GKN Electronic Torque Management (ETM) system. It has an electric motor that drives a ball ramp through a gear set to actuate a clutch pack. The gear set multiplies the torque of the electric motor, which allows for a low-torque motor. The ball ramp translates rotational motion into axial motion. This axial motion is used to apply force to compress the clutch pack, and therefore transfer torque. In order to reduce the electric load during an extended lock-mode application, a brake is integrated into the motor drive system. The motor fully applies the clutch and the electric brake holds the motor output in place.

ECTED Differential

The Auburn Gear Electronically Controlled Torque Enhancing Differential (ECTED) is an electronically controlled torque-biasing differential. The basic function begins with the activation of an electromagnetic coil. This coil is fixed to the differential and has a bearing mounted on its inside diameter that supports the rotating differential case. The engagement of this electromagnetic coil applies a magnetic pull force to a cone clutch. The powdered metal cone clutch is pressed into a ductile iron cup system that engages a ramp ring.

The ramp ring has a ramp angle that multiplies the coil force. Amplification of the coil force allows for a smaller coil and less electrical current draw. Next, there is a reaction ball ramp device with a ramp angle and a series of balls that amplify the applied force further. The final result is a very large load on the clutch surfaces, which transfers the torque.

 

On this sectioned ETM eLSD, you can see the electric drive motor, gear reduction, and ball ramp on the upper left side. The differential and clutch packresemble those of a traditional mechanical system, except that the clutch pack is all on one side of the differential. This clutch pack does not rely on side-gear separating force to engage and is fully controllable. (GKN Driveline)

On this sectioned ETM eLSD, you can see the electric drive motor, gear reduction, and ball ramp on the upper left side. The differential and clutch packresemble those of a traditional mechanical system, except that the clutch pack is all on one side of the differential. This clutch pack does not rely on side-gear separating force to engage and is fully controllable. (GKN Driveline)

 

In this exploded view, the clutch is to the right of the electromagnetic coil (light blue). The cone clutch (gray) is on the right. It actuates the ramp ring (yellow) to the left of the cone clutch, which then activates the ball ramp mechanism. In turn, a load is applied on the clutch pack through the side gears and thrust block. (Auburn Gear)

In this exploded view, the clutch is to the right of the electromagnetic coil (light blue). The cone clutch (gray) is on the right. It actuates the ramp ring (yellow) to the left of the cone clutch, which then activates the ball ramp mechanism. In turn, a load is applied on the clutch pack through the side gears and thrust block. (Auburn Gear)

 

The engagement and disengagement of the device is fully controllable. The device builds on the cone-type clutch technology that Auburn Gear has been supplying for quite some time. It is probably the first electronically controlled clutch-pack-based limited-slip differential on the market. Some may contest the limited slip designation, as this device basically just has an on/off mode. But since there is a clutch pack, the device can be engaged at any time and the total torque transferred is controlled by the torque capacity of the clutch pack.

 

Open Differential

Open differentials are by far the most common differential encountered in production vehicles. They have great on-road manners and performance, and work well for most applications. They typically do not offer any appreciable resistance to speed or torque differences in driving conditions. As mentioned earlier, the TBR is between 1.05 and 1.3:1. They are also the simplest, and therefore, most cost effective for the OEM to outfit the vehicle from the factory.

As discussed earlier, there are some trade-offs to this simplicity. One is that the open differential has little place on a performance-minded vehicle. The traction-limited short-coming quickly becomes evident by the ever-present single-wheel peel during hard acceleration.

One major consideration with the open differential housing is that it is required if you want to install a miniature spool.

 

Spools and Mini-Spools

The extreme opposite of the open differential is the spool, which is basically a large bracket for the ring gear and does not have any differential gears at all. Some folks even use the term “Lincoln Locker,” which refers to the days when hot rodders would take an open differential and arc weld the gears to one another as well as the differential case with a Lincoln-brand arc welder.

 

The spool resembles a needle-and-thread type of spool in shape, but there are no side or pinion gears. Consequently, it does not have the round section required to house them. The spool does not have a means to access C-style axle-retention clips. Since there are no access windows to install these washers, the axle shafts must be retained at the wheel ends.

The spool resembles a needle-and-thread type of spool in shape, but there are no side or pinion gears. Consequently, it does not have the round section required to house them. The spool does not have a means to access C-style axle-retention clips. Since there are no access windows to install these washers, the axle shafts must be retained at the wheel ends.

 

Here is a comparison of an Ultra Lite spool (left) to a traditional spool (right). Not only is the small one on the left specific for racing, it has been lightened as much as possible to further reduce weight. This is the ultimate in weight reduction and torque transfer possible for the differential.

Here is a comparison of an Ultra Lite spool (left) to a traditional spool (right). Not only is the small one on the left specific for racing, it has been lightened as much as possible to further reduce weight. This is the ultimate in weight reduction and torque transfer possible for the differential.

 

The block-style mini-spool kit replaces the internal gears of an open differential. While this kit reuses the factory-style C-washers to retain the axle shafts, it is possible but not recommended to do this. Instead, C-clip eliminators are strongly recommended. (Auburn Gear)

The block-style mini-spool kit replaces the internal gears of an open differential. While this kit reuses the factory-style C-washers to retain the axle shafts, it is possible but not recommended to do this. Instead, C-clip eliminators are strongly recommended. (Auburn Gear)

 

The one-piece-style mini-spool is designed for differential cases that are split like those found on the Ford 9-inch. This replaces all of the internal gearing of the open differential. (Auburn Gear)

The one-piece-style mini-spool is designed for differential cases that are split like those found on the Ford 9-inch. This replaces all of the internal gearing of the open differential. (Auburn Gear)

 

By rigidly constraining the wheels to one another, the spool is best suited for straight-line-only driving. This makes the spool a favorite for purpose-built drag cars and off-road trucks. The spool has no means to disengage, and is not meant for on-road usage. The rigid connection provides constant speed to both wheels independent of the road surface or tire loading conditions.

Miniature spools serve the same function as a normal spool, but the actual approach is a little different. The main benefit of the mini-spool is to your budget. They are typically 40 percent cheaper than a full spool. With a mini-spool, the kit includes new internals to replace the factory side and pinion gears, eliminating the differential functionality. As mentioned above, you need a factory-style open differential case to remove the internals from and replace with the mini-spool parts. One drawback of the mini-spool is that it still depends upon the strength of the factory differential case and differential pin.

 

Lockers

Lockers and spools are very similar in the fact that they rigidly constrain the differential outputs to go the same speed. Therefore, the wheels are mechanically locked to each other. The only difference is that the driver typically selects the locker engagement while the spool is always mechanically locked.

The selectable locker is a device that acts like an open differential during normal driving conditions and when the driver selects the locked mode. It replicates a spool by constraining both axle shafts to rotate together. This is the best of both worlds and very common among off-road drivers.

 

This OEM-style locking differential features a dog clutch that can be engaged and disengaged across the differential. The electromagnetic coil (lower right) electronically actuates the dog clutch. (GKN Driveline)

This OEM-style locking differential features a dog clutch that can be engaged and disengaged across the differential. The electromagnetic coil (lower right) electronically actuates the dog clutch. (GKN Driveline)

 

This is an exploded view of the four-pinion Eaton ELocker differential. The electromagnetic coil on the right engages the locking pin mechanism when the driver selects the lock mode. (Eaton Corporation)

This is an exploded view of the four-pinion Eaton ELocker differential. The electromagnetic coil on the right engages the locking pin mechanism when the driver selects the lock mode. (Eaton Corporation)

 

These are sketches of the function of the dog clutch mechanism. The upper left one shows the dog clutch in the open position, which provides the traditional open-differential performance. The lower left one illustrates the dog clutch initial engagement. The one on the right shows how the cam angle on the back side of the dog clutch helps wedge and hold the dog clutch closed. (GKN Driveline)

These are sketches of the function of the dog clutch mechanism. The upper left one shows the dog clutch in the open position, which provides the traditional open-differential performance. The lower left one illustrates the dog clutch initial engagement. The one on the right shows how the cam angle on the back side of the dog clutch helps wedge and hold the dog clutch closed. (GKN Driveline)

 

The driver actuates the locking-differential mechanism on the aftermarket ARB Air Locker. An  onboard air compressor, which must be installed to supply the air pressure required for engagement, triggers a pneumatic piston.

The driver actuates the locking-differential mechanism on the aftermarket ARB Air Locker. An onboard air compressor, which must be installed to supply the air pressure required for engagement, triggers a pneumatic piston.

 

In keeping with the earlier discussion about bias ratio, since the locker can send all torque to one wheel when it is locked, it has an infinite torque-bias ratio. There are a few of these available from OEMs and the aftermarket.

Passive Quasi-Lockers

Passive quasi-lockers are limited-slip differentials that are typically not clutch-plate based. Since they are not clutch-plate based, they can at times be aggressive in their engagement and disengagement. Some of these units actually make a clicking or ratcheting noise during turn maneuvers. The clutch-plate-based units that were discussed earlier have the ability to absorb speed differences and gradually engage. This category of differentials use positive-engagement-style dog clutches. This positive engagement and subsequent disengagement can be harsh feeling in the vehicle.

Even the names of these units help to get the point across about the performance to be expected. If a clutch-plate-style device does not meet your requirements, then one of these should.

 

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The Eaton No Spin and Detroit Locker are basically the same device. The only difference is that the No Spin replaced the factory differential internals, and the Locker is a complete differential assembly. Similar to mini-spools and spools, these devices do not use traditional bevel-style differentials. Instead they use a more aggressive, positive engagement from a dog clutch. The earlier versions of these units were so aggressive that tire scrub was a given behavior during turn events. The newer versions are much better behaved, and some even refer to them as Soft Lockers. They are still based on the same speed-sensing principles, it’s just that engagement modulation is non-existent.

 

Here is an exploded view of the Detroit Locker. The bevel gear differential has been replaced with a dog-clutch-style arrangement. These units do not have clutch plates to slip, and therefore provide 100 percent torque transfer. (Eaton Corporation)

Here is an exploded view of the Detroit Locker. The bevel gear differential has been replaced with a dog-clutch-style arrangement. These units do not have clutch plates to slip, and therefore provide 100 percent torque transfer. (Eaton Corporation)

 

This exploded view of the No Spin illustrates just the internals of the Detroit Locker that fit into the OEM differential case. (Eaton Corporation)

This exploded view of the No Spin illustrates just the internals of the Detroit Locker that fit into the OEM differential case. (Eaton Corporation)

Here are just the internals of the EZ. You must re-use the OEM differential case. You can see the springs that allow the device to disengage and act like a well-mannered open differential. (Eaton Corporation)

Here are just the internals of the EZ. You must re-use the OEM differential case. You can see the springs that allow the device to disengage and act like a well-mannered open differential. (Eaton Corporation)

 

These are all of the parts that come with a typical EZ Locker. You can see the tooth profiles and the slight chamfers to help eliminate nicking of the teeth corners during overrun conditions. (Eaton Corporation)

These are all of the parts that come with a typical EZ Locker. You can see the tooth profiles and the slight chamfers to help eliminate nicking of the teeth corners during overrun conditions. (Eaton Corporation)

 

This completely assembled unit appears as it would inside the differential case. You can see the oval-shaped hole for the differential pin. This oval shape spreads the couplers apart to lock the device.(Eaton Corporation)

This completely assembled unit appears as it would inside the differential case. You can see the oval-shaped hole for the differential pin. This oval shape spreads the couplers apart to lock the device.(Eaton Corporation)

 

An exploded layout of the Eaton Gearless differential is shown here. The added friction-based clutch pack typically provides a smooth and positive locking effect. (Eaton Corporation)

An exploded layout of the Eaton Gearless differential is shown here. The added friction-based clutch pack typically provides a smooth and positive locking effect. (Eaton Corporation)

 

The Eaton EZ Locker is similar but has some minor tweaks. It still retains the dog clutch engagement of the other units, but has an oval-shaped ramped housing that forces the shaft couplers outward to engage. Under light torque conditions, the dog clutch couplers can ratchet over one another and mimic the performance of an open differential.

The Richmond Gear Powertrax and Lock Right are similar to the above units. The Lock Right is more aggressive and you may experience clicking sounds during differentiation events.

The Eaton Detroit Gearless Locker is a combination of the concepts of the above lockers and a clutch pack system. It is similar to the ramp- and cam-style multi-plate units but it does not have any traditional bevel gears. There are shaft couplers that are engaged through the clutch pack. The clutch pack is engaged from the ramp mechanism, which makes this unit one of the more popular lockers to use for street-driven vehicles.

 

Helical Technology

There are several suppliers marketing similar-style helical devices. Among them are Torsen, Quaife, Guest, Keen and Nettlefolds (GKN), American Axle & Manufacturing (AAM), and Eaton/Traction Technology (TracTech). Some typical factory-installed applications are the GM F-cars (Camaro/Firebird) and Mazda Miata. Because these devices all have a similar function, they are discussed together. They are referred to as helical gear limited-slip devices based on examination of the structure. In normal driving conditions, these devices function like a traditional open differential. Although this is an ideal situation for normal straight-line driving conditions, variable road surfaces, such as loose gravel, snow, and ice often require torque biasing to one of the drive wheels. During these road conditions, helical gear differentials respond instantly and shift power to the wheel with the most traction, avoiding a wheel-spin condition or momentary loss of traction. This is accomplished through a series of helical (worm-shaped) gears in place of the traditional bevel gears of an open differential.

When torque is applied to the worm gears, the side gears thrust in an axial direction and the pinion gears thrust radially and axially into their pilot bores in the differential case. The magnitude of the thrust forces is controlled through adjustment of the helix angle (side-gear axial thrust) and pressure angle (planet gear radial thrust).

The helical gears can be thought of as the planet gears that are orbiting the side gears in this style differential. Friction is generated between the gears and housing, creating resistance to differentiation; the degree of resistance is proportional to input torque. This light frictional resistance is then multi-plied by the ratio of the worm gears to the side gears. Speed difference is not required to create torque transfer. Torque-sensing differentials remain “locked” until a torque imbalance across the differential overcomes the internal friction.

 

This cutaway illustration of the helical-style differential shows that all of the gears have a curved or helixed tooth profile. This helix produces some of the internal friction under load, which allows a torque difference across the outputs of the differential.(GKN Driveline)

This cutaway illustration of the helical-style differential shows that all of the gears have a curved or helixed tooth profile. This helix produces some of the internal friction under load, which allows a torque difference across the outputs of the differential.(GKN Driveline)

 

The Detroit Truetrac by Eaton is a helical or worm-style differential, and this exploded view of the differential and the helical gears clearly shows this particular differential design. (Eaton Corporation)

The Detroit Truetrac by Eaton is a helical or worm-style differential, and this exploded view of the differential and the helical gears clearly shows this particular differential design. (Eaton Corporation)

 

Here is an illustration of the helical pinion or planetary gears and the thrust and axial forces they exhibit. These forces react against the other gears and housing to create the torque-sensing and bias effect of this device. (GKN Driveline)

Here is an illustration of the helical pinion or planetary gears and the thrust and axial forces they exhibit. These forces react against the other gears and housing to create the torque-sensing and bias effect of this device. (GKN Driveline)

 

The blue lines are the torque-biased and preloaded differential performance; the red lines are the torque-biased performance.

The blue lines are the torque-biased and preloaded differential performance; the red lines are the torque-biased performance.

 

In the drive or forward direction, the side gears thrust in one direction. In the reverse or coast direction, the thrust force is in the opposite direction. This allows the helical differential the ability to have different friction washers on either end of the gear and therefore change the torque bias ratio from drive to coast conditions.

Typical torque-bias ratios for helical differentials are in the range of 1.7 to 3.0:1. There are some more aggressive units available that have a TBR as high as 4:1. These devices combine a helical differential with a clutch pack to provide preload. The Torsen T2R is one of these devices; autocrossers typically use them in their vehicles to control inside wheel spin when cornering under power.

We need to also keep in mind that torque-sensing differentials are trying to bias torque across the differential. If the torque is very low on one wheel and the preload and/or torque bias is not sufficient to get the vehicle in motion, the device performs like an open differential. In that situation, it is quite helpful to lightly apply the brake and throttle at the same time. The brake force is biased across the differential to the tire with good traction.

The original Torsen T1 crossed-axis helical (non-C-clip style) differential was used in the High Mobility Multipurpose Wheeled Vehicle (HMMWV, a.k.a. Humvee) and the idea of applying the brakes lightly while accelerating in extreme conditions is quite abnormal. So if you have a helical-style differential in your vehicle, you need to understand this less-than-intuitive driving technique when taking off from a stop in slippery conditions, so your vehicle gains adequate traction.

 

Viscous Control Technology

Viscous coupling is not available in the aftermarket today but is found on some high-performance sports cars, such as the Dodge Viper and BMW M series, so I feel that I should discuss it. The Harry Ferguson Limited Research Department discovered viscous control in 1954. They found that a child’s toy called “Potty Putty” (also commonly known as “Silly Putty”) resisted motion when placed under a shear stress, but would return to its normal state when the shear force was removed.

Basically, it was a quantity of thick silicon oil, which would conform to the shape of the container it occupied, such as a child’s hand. When the child would throw the material on the floor, it would resist the shear stress internal to the fluid upon reaching the floor and actually bounce like a rubber ball. The engineers and scientists at Ferguson realized that they could combine this nature of the silicon fluid across a differential, and provide viscous control.

A viscous coupling is a mechanical speed-sensing device without external control and is therefore a passive device. It has a sealed housing filled with a predetermined quantity of silicon fluid. Inside the housing are two sets of interleaved plates. These plates have connections similar to a traditional limited-slip differential clutch pack’s friction and reaction plates. One set of plates is attached to the outer drum via a spline connection, and one set is attached to the inner shaft via a spline. The viscous coupling plates, however, rely on shearing of the fluid between plates, as opposed to friction as with a traditional clutch pack.

When the outer drum rotates at a different speed than the inner shaft, the plates shear the viscous fluid. This shearing of the fluid causes a resistive shear force to be exerted on the opposite plate. This basically causes a torque transfer between the plates based on the relative speed across the plates. Based on this type of architecture, the device is said to be a speed-sensing device. The engagement profile of this type of torque transfer device is said to be digressive. This digressive nature is based on the non-Newtonian behavior of the silicon fluid.

 

This graph compares the viscosity of different weights of silicon oil to temperature. A reference line of 10W30 engine oil for comparison is shown. The engine oil and typical hydrocarbon-based fluids show a much more dramatic viscosity change when exposed to temperature extremes. The silicon oil used in viscous devices has a much flatter, and therefore consistent, viscosity with temperature change.

This graph compares the viscosity of different weights of silicon oil to temperature. A reference line of 10W30 engine oil for comparison is shown. The engine oil and typical hydrocarbon-based fluids show a much more dramatic viscosity change when exposed to temperature extremes. The silicon oil used in viscous devices has a much flatter, and therefore consistent, viscosity with temperature change.

 

Here are just the internals of the EZ. You must re-use the OEM differential case. You can see the springs that allow the device to disengage and act like a well-mannered open differential.(Eaton Corporation)

Here are just the internals of the EZ. You must re-use the OEM differential case. You can see the springs that allow the device to disengage and act like a well-mannered open differential.(Eaton Corporation)

 

This graph illustrates transferred torque on the vertical axis, and slip speed on the horizontal axis. The upper line (green) shows a typical digressive curve. As the speed difference increases, the rate of torque-transfer-increase declines, which is illustrated by the curve flattening out. This progressive curve (blue) is shown as a comparison.

This graph illustrates transferred torque on the vertical axis, and slip speed on the horizontal axis. The upper line (green) shows a typical digressive curve. As the speed difference increases, the rate of torque-transfer-increase declines, which is illustrated by the curve flattening out. This progressive curve (blue) is shown as a comparison.

 

This cutaway of a viscous device acts across an axle differential. Notice the large number of viscous shear plates on the right side of the part. These are sealed from the rest of the axle components so they are separated from the highly viscous silicon fluid. (GKN Driveline)

This cutaway of a viscous device acts across an axle differential. Notice the large number of viscous shear plates on the right side of the part. These are sealed from the rest of the axle components so they are separated from the highly viscous silicon fluid. (GKN Driveline)

 

This is a schematic cutaway of the ViscoLok differential. You can see the viscous shear pump on the left that applies the piston (green) to compress the clutch pack. (GKN Driveline)

This is a schematic cutaway of the ViscoLok differential. You can see the viscous shear pump on the left that applies the piston (green) to compress the clutch pack. (GKN Driveline)

 

42

The actual hardware has been cut away to illustrate the concept. Just as above, the pump mechanism is on the left with the clutch pack between the pump and side gear.(GKN Driveline)

 

43

This differential shows the combination of helical and viscous technologies. Notice the helical portion on the upper right and the viscous portion on the lower left. Again, unfortunately, this technology is not available in the aftermarket today. (GKN Driveline)

 

While viscous coupling was first used on four-wheel-drive (4WD) vehicles in the 1970s, it became the preferred method for 4WD in the 1980s. Many vehicles, such as Mercury Mountaineers, GMC Safari Vans, Lamborghinis, and Fiats, have used this technology. This technology was applied across differentials as a means to have a behavior that was not related to the torque output of the device. Therefore, the driver would not be required to apply the brakes lightly when one tire was on ice and the other on asphalt during an acceleration event.

The viscous differential uses pure fluid shear to balance speed differences.

ViscoLok is another kind of viscous technology that uses viscous shear in combination with a clutch pack. It is found on the 2009 SRT10 Dodge Viper and BMW M series. The ViscoLok has the fluid shearing of silicon fluid, but actually pumps the fluid to an apply piston. That apply piston compresses a clutch pack. The performance characteristic curve of the viscous pump allows it to behave with a progressive characteristic.

Another differential combines the helical and viscous technologies. This hybrid-style device is both speed- and torque-sensing. The speed-sensing portion comes from the viscous coupling mechanism; the torque-sensing portion is generated from the work of helical gears. This seems like the best of all worlds and the product performs quite well. Unfortunately, it is limited to OEM applications only at this time. But you can see that the OEMs and suppliers have spent quite a bit of research and development on technologies to find the best possible solutions. There are other exotic and scarce devices in the marketplace, but since they are quite rare, I will not review them.

Gerotor Pump Style

Since I am discussing speed-sensing, progressive limited slip differentials, let’s review the history of the gerotor pump-style devices. Developed by Asha Corporation World Rally Championship racing, this differential uses a mechanically actuated gerotor style pump that is driven by a speed difference across the axle shafts. This concept is utilized by Dana Corporation with the Hydra Lok limited-slip differentials.

The Gerodisc technology, as it is commonly referred to, is a self-contained hydro-mechanical limited-slip differential, which consists of a hydraulic pump, an apply piston, and a set of friction-based clutches. The hydraulic pump is a gerotor style pump. When there is a speed difference across the two pump rotor elements, the gerotor pump begins turning and fluid flow is developed. This flow is directed to an apply piston that in turn presses against a series of interleaved clutch packs.

 

44) Here is a cutaway of the initial designed gerodisc style limited-slip differential. The gerotor pump (black) is just left of the ring gear. The apply piston (blue) is to the right of the pump, and the clutch pack is next to that. The typical bevel differential is also used with this type of device.

44) Here is a cutaway of the initial designed gerodisc style limited-slip differential. The gerotor pump (black) is just left of the ring gear. The apply piston (blue) is to the right of the pump, and the clutch pack is next to that. The typical bevel differential is also used with this type of device.

 

On this production eGerodisc limited-slip differential, you can see the electrically controlled solenoid valve on the cylindrical portion off the right side of the differential case. Unfortunately, the pump, piston, and clutch pack are enclosed and cannot be seen.

On this production eGerodisc limited-slip differential, you can see the electrically controlled solenoid valve on the cylindrical portion off the right side of the differential case. Unfortunately, the pump, piston, and clutch pack are enclosed and cannot be seen.

 

The greater the speed difference, the greater the hydraulic pressure, and therefore the greater the amount of torque transferred. These devices follow the speed-sensing progressive curves. The curve can be shifted up or down during the design phase depending on the vehicle requirements. As the speed difference increases, the torque delivered is nearly proportional. The curve is actually progressive and the ease of engagement at lower speed differences helps to smooth out the engagement feel of the device. This actually helps to protect the device from high-speed partial-engagement situations.

Since the device is purely mechanical, there is also a temperature-compensating bi-metallic valve that adjusts fluid flow based on the temperature of the fluid. This helps ensure adequate performance over different fluid temperatures and viscosities.

This technology only transfers torque when there is a speed difference across the differential. In normal vehicle operating conditions, it performs just like an open differential. However, it may take excessive wheel spin before the device transfers torque because the pump needs to have a speed difference to pump fluid and the performance curve is progressive.

This technology has been adapted by Eaton Corporation to allow for electronic control. An electrically controlled hydraulic valve has been added to the device and allows modulation of the device during speed difference events. This device was in production on the 2005 Jeep Grand Cherokee axles.

 

Torque Vectoring

If you think that the eLSD is cool, torque vectoring is the ultimate in torque biasing. Torque vectoring ideas were originally developed for race car applications, and are now featured on select BMW and Audi models. The idea is similar to the eLSD, plus it combines electric clutch control with an additional gear set per wheel. This allows the system to speed up and slow down the wheels relative to one another. Even in a straight line, the vectoring units can speed up one side and slow down the other to force the vehicle to turn. This gives the OEM vehicle dynamics guys a really powerful tool to control the behavior of the vehicle.

 

A sectioned torque vectoring rear axle has an actuation that’s similar to the eLSD and has additional gearing on each end. This gearing is selectively engaged and over speeds or under speeds the wheels when required. You may have also noticed that this axle does not use traditional tapered roller bearings but actually tandem ball bearings from the Schaeffler Group in order to reduce bearing drag. (Schaeffler/Joe Palazzolo)

A sectioned torque vectoring rear axle has an actuation that’s similar to the eLSD and has additional gearing on each end. This gearing is selectively engaged and over speeds or under speeds the wheels when required. You may have also noticed that this axle does not use traditional tapered roller bearings but actually tandem ball bearings from the Schaeffler Group in order to reduce bearing drag. (Schaeffler/Joe Palazzolo)

 

Summary

I have provided a huge amount of information to try to answer the question, “Which differential should I put in my car?” This sounds like such a simple question but is difficult to answer. First you need to look at what is available for your application. At times, this limits your choices. Then you need to decide why you are replacing the factory differential. Is it in need of repair? Does it have some annoying behavior that you experience while driving the vehicle?

Be careful. Each differential has its pros and cons. I hope that after reading this chapter, you are now armed with more information about the different devices and can further research your specific application and performance goals to get the right differential.

 

Written by Joe Palazzolo and Posted with Permission of CarTechBooks

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