6.3 Backlash, Carrier Preload, Limited-Slip Differentials (LSD), and Locking Units

Backlash, Carrier Preload, Limited-Slip Differentials (LSD), and Locking Units

The differential assembly performs two indispensable functions in an automotive drivetrain: it allows the left and right drive wheels to rotate at different speeds during cornering while simultaneously transferring torque from the ring gear to both axle shafts. However, standard open differentials suffer from traction limitations when one wheel loses grip.

For the ASE A3 certification exam, technicians must understand differential carrier bearing preload procedures, case spreader protocols, open differential planetary gear mechanics, and the operating principles, testing methods, and failure modes of Clutch-Pack LSDs, Cone-Type LSDs, Helical Gear (Torsen) LSDs, and Selectable Locking Differentials.

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1. Carrier Bearing Preload & Backlash Adjustment Mechanics

Differential carrier side bearings are large tapered roller bearings that support the carrier case and ring gear inside the housing. They must be assembled with a specific bearing preload (clamping interference) to resist the massive lateral thrust forces generated by hypoid gear teeth under engine load.

+-----------------------------------------------------------------------------+
|                     CARRIER ADJUSTMENT: SHIMS VS. THREADED                  |
|                                                                             |
|   THREADED SIDE ADJUSTERS (Spanner Nuts):                                   |
|   - Turn left adjuster OUT, turn right adjuster IN ===> Increases Backlash  |
|   - Turn right adjuster OUT, turn left adjuster IN ===> Decreases Backlash  |
|   - Turn BOTH adjusters IN equal notches          ===> Increases Preload   |
|                                                                             |
|   SELECTIVE SIDE SHIMS (Cast Housing):                                      |
|   - Transfer shims from right to left              ===> Decreases Backlash  |
|   - Transfer shims from left to right              ===> Increases Backlash  |
|   - Add equal shim thickness to BOTH sides         ===> Increases Preload   |
+-----------------------------------------------------------------------------+

Threaded Adjusters vs. Selective Shims

1. Threaded Side Adjuster Rings (Spanner Nuts):
   • Used in removable carrier (third member) axles (e.g., Ford 9-inch, Chrysler 8.75/9.25, Toyota 8-inch).
   • Adjusted using a specialized spanner wrench inserted through the axle tube ends.
   • Adjusting Backlash: Loosen one adjuster and tighten the opposite adjuster an identical number of notches to shift the carrier laterally without changing total preload.
   • Adjusting Preload: Tighten both adjusters evenly by specified notches beyond zero-lash after backlash is established.
2. Selective Side Shims:
   • Used in integral carrier Salisbury axles (e.g., Dana 30/44/60, GM 10/12-bolt, Ford 8.8).
   • Shims are positioned either between the carrier bearing cone and carrier case hub (requires bearing puller) or between the bearing cup and housing bore (external shims).

Housing Case Spreader Protocol & Safety Limits

In Salisbury axles where shims fit between the bearing cups and housing, installing the carrier with proper preload requires expanding the cast-iron housing slightly with a Housing Case Spreader.

+-----------------------------------------------------------------------------+
|                        HOUSING CASE SPREADER RULES                          |
|                                                                             |
|   1. Mount dial indicator across housing opening before spreading.          |
|   2. Maximum allowable housing spread is STRICTLY 0.015" (0.38 mm).         |
|   3. NEVER exceed 0.020" (0.50 mm) under any circumstances.                 |
|   4. Spreading beyond 0.020" causes permanent plastic yield / warpage of    |
|      the cast housing, permanently destroying bearing alignment.            |
|   5. Release spreader pressure immediately once shims and carrier are seated|
+-----------------------------------------------------------------------------+

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2. Open Differential Dynamics & Power Flow

An Open Differential consists of a carrier case, a pinion cross shaft (spider pin), two differential pinion gears (spider gears), and two differential side gears splined directly to the axle shafts.

+-----------------------------------------------------------------------------+
|                       OPEN DIFFERENTIAL PLANETARY ACTION                    |
|                                                                             |
|   [ RING GEAR & CARRIER CASE ]                                              |
|                 |                                                           |
|                 v                                                           |
|   [ PINION CROSS SHAFT / SPIDER GEARS ]                                     |
|          /                     \                                            |
|         v                       v                                           |
|   [ LEFT SIDE GEAR ]     [ RIGHT SIDE GEAR ]                                |
|   (Left Axle Shaft)      (Right Axle Shaft)                                 |
+-----------------------------------------------------------------------------+

1. Straight-Ahead Driving

When driving straight ahead on uniform pavement, both drive wheels rotate at identical speeds ($N_{\text{left}} = N_{\text{right}} = N_{\text{carrier}}$). The spider gears do not rotate on their cross shaft; they act purely as driving wedges, transferring 100% of carrier torque equally to both side gears.

2. Cornering Dynamics

When the vehicle turns a corner, the outer wheel travels a longer radius arc than the inner wheel. The inner side gear slows down, causing the spider gears to rotate on the cross pin. The spider gears "walk" around the slower-turning inner side gear, speeding up the outer side gear by the exact amount the inner side gear slowed down:

$N_{\text{carrier}} = \frac{N_{\text{left}} + N_{\text{right}}}{2}$

$\Delta N_{\text{outer}} = +\Delta N_{\text{inner}}$

3. The Open Differential Traction Limitation

An open differential is an equal-torque split device: it delivers an exact 50/50 torque split ($T_{\text{left}} = T_{\text{right}}$) under all operating conditions.

• The Single-Wheel Spin Problem: If one drive wheel loses traction (e.g., ice, mud, or wet grass) and can only support $50 \text{ lb-ft}$ of torque before spinning, the differential can deliver only $50 \text{ lb-ft}$ to the high-traction wheel on dry asphalt.
• Total Delivered Tractive Torque: $T_{\text{total}} = 2 \times T_{\text{low-traction wheel}}$ If one wheel has zero traction ($T = 0$), total vehicle driving force drops to zero ($T_{\text{total}} = 0$), stranding the vehicle while the slipping wheel spins at twice carrier speed ($N_{\text{spin}} = 2 \times N_{\text{carrier}}$).

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3. Clutch-Pack Limited-Slip Differentials (LSD)

To overcome the traction limitation of open differentials, a Clutch-Pack Limited-Slip Differential (e.g., Ford Traction-Lok, GM Positraction, Dana Trac-Lok) incorporates multi-disc friction clutch packs positioned between the differential side gears and the carrier housing.

+-----------------------------------------------------------------------------+
|                 CLUTCH-PACK LIMITED-SLIP DIFFERENTIAL (LSD)                 |
|                                                                             |
|                     [ CARRIER HOUSING ]                                     |
|                              |                                              |
|         +--------------------+--------------------+                         |
|         |                                         |                         |
|   [Steel Drive Plates]                      [Steel Drive Plates]            |
|   (Tabbed to Carrier)                       (Tabbed to Carrier)             |
|         |                                         |                         |
|   [Friction Discs]                          [Friction Discs]                |
|   (Splined to Side Gear)                    (Splined to Side Gear)          |
|         |                                         |                         |
|   [Left Side Gear]                          [Right Side Gear]               |
|         ^                                         ^                         |
|         |                                         |                         |
|         +---- [ CENTRAL PRELOAD SPRINGS / RAMPS ] -+                        |
+-----------------------------------------------------------------------------+

Clutch Pack Construction & Operation:

• Clutch Plates: Consist of alternating steel drive plates (externally tabbed/keyed to the carrier case) and friction discs coated with carbon, bronze, or sintered composite material (internally splined to the axle side gear hub).
• Static Preload: Heavy central coil springs in a steel retainer block or dished Belleville spring washers exert continuous axial force, squeezing the clutch packs against the housing walls to create a baseline locking torque.
• Dynamic Torque-Ramping (Cross-Pin Ramps): In high-performance units, the differential pinion cross pin floats in V-shaped ramp pockets in the carrier case. Under heavy engine acceleration torque, the cross pin tries to climb the ramps, exerting powerful outward axial thrust against the side gears, clamping the clutch packs harder in direct proportion to engine input torque.

Friction Modifier Additive & Chatter Diagnosis:

• Chemistry: Limited-slip differentials require a specialized liquid Friction Modifier Additive (polyalkylene glycol ester / alkyl phosphate ester formulation) mixed into the hypoid GL-5 gear oil.
• The Stick-Slip Phenomenon: As a vehicle makes a slow, tight turn on dry asphalt (such as maneuvering in a parking lot), the differential clutches must slip smoothly to allow the outer wheel to turn faster. Without friction modifier, the static friction coefficient is much higher than the dynamic coefficient, causing the clutch plates to grab, release, grab, and release rapidly.
• Symptom: A severe groaning, chattering, or shuttering vibration felt throughout the rear end during low-speed turns.
• Technician Diagnostic Protocol:
  1. If LSD chatter occurs, verify fluid level and condition.
  2. Drain and refill the axle with fresh GL-5 gear oil and add $4 \text{ to } 8 \text{ oz}$ ($118 \text{ to } 236 \text{ mL}$) of OEM-specified limited-slip friction modifier.
  3. Drive the vehicle in 10 to 12 slow figure-eight turns in an open parking lot to work the additive thoroughly into the clutch pack friction facings. Chatter should disappear completely.

LSD Breakaway Torque Testing:

1. Raise the vehicle on a lift so one rear wheel is off the ground while the opposite wheel is locked solidly on the shop floor (or held with a holding fixture).
2. Place transmission in Neutral.
3. Attach a dial-type foot-pound torque wrench to a wheel lug nut adapter on the raised wheel.
4. Rotate the wrench smoothly and record the torque required to break the clutch pack loose and maintain continuous rotation.
5. Specification: Typical factory breakaway torque is $40 \text{ to } 100 \text{ lb-ft}$ ($54 \text{ to } 136 \text{ N-m}$). A reading below $25 \text{ lb-ft}$ indicates worn clutch plates or broken preload springs requiring differential overhaul.

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4. Cone-Type & Helical (Torsen) Differentials

+-----------------------------------------------------------------------------+
|                     CONE-TYPE VS. HELICAL (TORSEN) LSD                      |
|                                                                             |
|   CONE-TYPE LSD (Auburn Style):         HELICAL / TORSEN LSD:               |
|   - Tapered friction cones seat into    - Involute helical worm/planet gears|
|     machined conical case pockets       - Torque transfer via gear thrust   |
|   - High friction surface area          - ZERO clutch plates to wear out    |
|   - Non-rebuildable wear surface        - True progressive torque biasing   |
+-----------------------------------------------------------------------------+

1. Cone-Type Limited-Slip Differential (e.g., Auburn Gear)

• Operation: Replaces multi-disc clutch packs with two precision-machined tapered friction cones splined to the axle shafts. Heavy central preload springs push the cones outward into matching conical female cup seats machined directly into the carrier housing.
• Wear Characteristics: As the conical friction surfaces wear over high mileage, the cones seat deeper into the case pockets. Eventually, the differential side gears bottom out against the central cross pin, eliminating preload. Cone-type differentials cannot be rebuilt with new clutch discs; worn units require complete carrier replacement.

2. Helical Gear Limited-Slip Differential (Torsen / Quaife / Truetrac)

• Operation: Uses no friction clutch plates, cones, or preloading springs. It relies entirely on the complex geometry of involute helical pinion gears meshing with helical side gears inside precision-machined carrier pockets.
• Torque Transfer Mechanism: Under acceleration, the helical gear teeth generate massive axial separating thrust and radial tooth friction against the hardened interior walls of the carrier pockets. This mechanical friction automatically transfers drive torque away from the slipping wheel to the wheel with high traction.
• Torque Bias Ratio (TBR): Typically ranges from $2.5:1 \text{ to } 4.0:1$, meaning the differential can deliver up to $4\times$ more torque to the wheel with grip than the slipping wheel.
• Advantage: Completely maintenance-free, requires standard GL-5 gear oil (no friction modifier needed), and provides seamless, noise-free operation without clutch wear.

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5. Locking Differentials & Spools

For extreme off-road, heavy commercial, or drag racing applications where limited-slip differentials cannot provide sufficient torque transfer, Locking Differentials are utilized.

+-----------------------------------------------------------------------------+
|                        LOCKING DIFFERENTIAL ARCHITECTURE                    |
|                                                                             |
|   1. AUTOMATIC MECHANICAL LOCKER (Detroit Locker):                          |
|      - Spring-loaded dog teeth lock 100% under drive torque                 |
|      - Outer wheel overruns / disengages during coasting turns              |
|                                                                             |
|   2. SELECTABLE LOCKER (E-Locker / Air Locker / Cable):                     |
|      - Operates as a 100% Open Differential for normal street driving       |
|      - Driver energizes 12V solenoid or air piston to engage dog clutch     |
|      - Locks side gear directly to carrier case for 100% solid lock         |
|                                                                             |
|   3. SOLID SPOOL / MINI-SPOOL:                                              |
|      - Eliminates differential gears completely; splines axles together     |
|      - Restricted to dedicated race tracks; causes severe street bind       |
+-----------------------------------------------------------------------------+

Diagnostic & Operating Precautions for Selectable Lockers:

• Driveline Binding & Axle Fracture: Engaging a differential locker on dry asphalt forces both rear wheels to turn at identical speeds. During a turn, the inner and outer tires fight each other, causing violent wheel hop, tire scrubbing, extreme driveline torsion, and fractured axle shafts. Selectable lockers must be switched OFF on high-traction paved surfaces.
• E-Locker Troubleshooting: Test for $12\text{V}$ and ground at the axle housing harness connector. Measure solenoid coil resistance with a digital multimeter (typically $2.0 \text{ to } 4.5\ \Omega$). A shorted or open coil prevents dog clutch collar engagement.
• Air Locker Troubleshooting: Verify air compressor operation ($85 \text{ to } 105 \text{ psi}$ / $586 \text{ to } 724 \text{ kPa}$). Inspect pneumatic tubing for kinks, heat melting near exhaust pipes, or internal rotating seal O-ring leakage inside the axle housing.

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6. Differential Carrier & Traction System Diagnostic Matrix

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7. Differential Case / Carrier Runout Measurement

ASE Task E.12 requires measuring differential case (carrier) runout — the lateral truth of the carrier casting itself. This is distinct from ring gear runout, which is measured on the back face of the ring gear with the gear installed on the carrier.

Measurement Procedure:

1. Install the carrier on its side bearings with the specified preload (via the threaded adjusters or the selected shim stack). Many OEM procedures call for checking runout with the ring gear removed so the case flange is read directly.
2. Mount a dial indicator against the machined ring-gear-mounting flange face of the case (or the bearing piloting surface, per the service manual).
3. Rotate the carrier one full revolution and record the Total Indicator Reading (TIR). A typical specification limit is 0.002 to 0.003 in (0.05 to 0.08 mm) maximum.
4. Also inspect the side-bearing journals for scoring, check the carrier caps for spun-bearing witness marks, and check for cracks around the ring gear bolt circle.

Interpreting the Result:

• Excessive case runout means the casting is bent or the journals are damaged. The ring gear then rides eccentrically no matter how carefully pinion depth, backlash, and preload are set — and the resulting cyclic load will re-destroy a new gearset. No shim or adjuster procedure corrects a bent case: replace the case/carrier assembly and find the root cause (commonly a previously spun bearing or severe bearing failure).
• Acceptable case runout with excessive ring gear runout points to debris on the ring gear mounting flange, stretched cross-threaded ring gear bolts, or uneven bolt torque. Clean the flange, chase the threads, and re-torque the bolts to specification in a star pattern before replacing the gear set.