Automatic Torque Biasing (ATB) limited-slip differentials are a class of mechanical LSDs (Limited-Slip Differentials) that use helical or worm gear arrangements to automatically distribute torque between drive wheels based on traction conditions. ATB differentials are widely applied in performance road cars, motorsport, and increasingly in OEM platforms, where maintenance-free operation and smooth torque transfer are required.
This article explains the working mechanism, internal structure, torque bias control, and comparison to other LSD types, along with use-case recommendations for engineers and B2B procurement professionals.
Internal Structure and Mechanical Operation
Unlike clutch-type LSDs that rely on friction discs and preloaded springs, the ATB differential operates purely through gear geometry and internal torque reactions.
Key Components:
- Side Gears: Connected to each driveshaft.
- Helical Gears (or Worm Gears): Positioned between the side gears and angled relative to the axis of rotation.
- Differential Housing: Holds all gear components in alignment and transfers differential torque through the ring gear.
Working Principle:
When both wheels have similar grip, torque is evenly distributed. But once one wheel begins to slip (e.g., loses grip on wet pavement), the resistance on its side decreases. In an open differential, this leads to most torque being wasted on the slipping wheel. In contrast, the ATB LSD uses the axial forces created by helical gear thrust under load to increase resistance on the faster-turning (lower-resistance) side and bias torque toward the wheel with higher traction.
This is governed by gear mesh angle, tooth count, pressure angle, and housing support friction. The friction generated in the thrust paths—combined with the self-locking geometry of the gearset—results in effective torque redistribution.
Torque Bias Ratio (TBR)
One of the key performance indicators of an ATB LSD is its Torque Bias Ratio, defined as:
TBR = Torque on high-traction wheel / Torque on low-traction wheel
Typical TBR values:
- 2.0–2.5:1 for road use (smooth transition, minimal understeer)
- 3.0–4.5:1 for motorsports or high-torque applications
A TBR of 3:1 means that the LSD can send up to 75% of the available torque to the wheel with grip and only 25% to the one slipping.
Unlike active systems or clutch packs, TBR is not fixed—it depends on input torque, wheel speed difference, and the gearset’s behavior under load. Effective biasing only occurs when a certain torque threshold is met.
ATB vs. Other LSD Types
| Feature | ATB (Gear-Based) | Clutch-Type LSD | Viscous LSD | Electronic Locker |
|---|---|---|---|---|
|
Actuation |
Mechanical / passive |
Friction plates & springs |
Silicone fluid shear |
ECU / electric motor / solenoid |
|
Maintenance |
Low |
High (plate wear) |
Sealed (non-serviceable) |
Moderate (electrical components) |
|
Lock Ratio |
Partial (TBR-limited) |
High (up to 100%) |
Progressive |
Full lock (100%) |
|
Smoothness |
Very smooth |
Can be abrupt |
Smooth |
Abrupt |
|
Response Time |
Instant |
Instant but can wear |
Slight delay |
Variable (depending on control system) |
|
Best For |
All-weather road, light off-road, performance cars |
Track racing, drift |
Mild road use |
Rock crawling, off-road rescue |
Engineering Design Considerations
When designing or selecting an ATB LSD, attention must be paid to:
- Gear Module and Helix Angle: Affects torque multiplication and thrust force.
- Housing Friction Surfaces: Contribute to torque bias performance under load.
- Shaft Spline Compatibility: Must match OEM CV joints or axle shafts.
- Lubrication: Use GL-5 gear oils without friction modifiers; avoid limited-slip additives which can reduce TBR.
- Thermal Expansion Tolerance: Critical for maintaining backlash and preload across temperature ranges.
Contact Us for Specifications or Samples
If you’re seeking a durable, smooth, and maintenance-free LSD solution for performance or utility applications, ATB LSDs are a proven and efficient option. Contact our engineering team for product drawings, test data, or fitment inquiries.
