At Dream Factory Garage, we know that getting power to the pavement is what it’s all about. For the new generation of gearheads, understanding how your rear end works is crucial to making that horsepower count. You’ve heard the terms “Posi-Trac” and “open differential,” but how do those mysterious gears and axles actually play into getting those coveted two black stripes on the asphalt? Let’s break it down.
The Fundamental Problem: Turning Corners
Imagine you’re driving in a perfect circle. The wheel on the outside of the circle has a longer distance to travel than the wheel on the inside. If both wheels were forced to spin at the exact same speed, one would have to drag or skip, making it impossible to turn smoothly without damaging your drivetrain.
This is where the differential comes in. Its primary job is to allow the wheels to spin at different speeds, especially when turning.
1. The Standard “Open” Differential: The One-Tire Fire Creator
Most cars, especially older models and economy vehicles, come with an open differential. It’s designed for smooth, everyday driving.
- How it Works (and Fails): Inside the differential case, a set of gears (called “spider gears”) allows power to be distributed to both axles. The crucial characteristic of an open differential is that it always sends power to the wheel with the least resistance.
- The “One-Tire Fire” Syndrome: This is its fatal flaw for performance. If one wheel loses traction (e.g., it’s on a patch of ice, a puddle, or simply lifts slightly during aggressive cornering, or has less grip during a burnout attempt), all the engine’s power instantly diverts to that spinning wheel. The wheel with actual traction gets no power, and you’re left with a single tire spinning uselessly, while your car goes nowhere fast.
When a car turns, the outside wheel travels a longer distance than the inside wheel. Without a differential, your tires would chirrup, hop, and drag through every corner. The differential allows the axles to spin at different speeds so you can turn smoothly.
| Feature | Open Differential | Posi-Trac (LSD) |
| Power Delivery | Path of least resistance | Distributed to both wheels |
| Burnout Style | One-Tire Fire | Two Black Streaks |
| Driving Feel | Smooth, invisible | Can have slight “chatter” in tight turns |
| Internal Parts | Basic Spider Gears | Gears + Clutches + Springs |