If you’ve read the previous article and know your way around a 4WD, there’s a good chance one question will start forming: what about the differential, when winching off the vehicle wheels? This can be answered simply by the statement in the Bush Winch FAQ: “The Bush Winch can be used on both automatic and manual vehicles whether AWD, 2WD or 4WD, with or without Traction Control, and there is no requirement for a Differential Lock.” This article explains why that’s true.

What a Differential Actually Does
Every vehicle has a differential sitting in the middle of the axle between its drive wheels. When a vehicle goes around a corner, the outside wheel has to travel further than the inside wheel, so it needs to turn faster. The differential is what allows the two wheels on the same axle to spin at different speeds while both still receive power from the engine.

A differential will always direct power to whichever wheel offers the least resistance. On firm ground that’s not a problem, because both wheels have similar grip. However, lift one wheel off the ground, or put it on ice or loose sand, and the differential will happily direct all the engine’s power to that one freely spinning wheel, while the wheel still sitting on firm ground, which has grip and could move the vehicle, gets none. That’s the classic “one wheel spinning uselessly while the vehicle goes nowhere” scenario. It’s exactly why differential locks and traction control devices exist, to force both wheels to turn regardless of how much grip each one has.

The Natural and Reasonable Assumption
This being the case, it’s logical to assume a wheel-mounted winch would run straight into this problem. The thinking goes: if the winch is attached to one wheel and that wheel is under load, resisting rather than spinning freely, won’t the differential simply do what it always does and divert all the power to the other wheel, which is spinning freely?

What Is Done to Negate This Issue
The Bush Winch kit provides two winch drums, one for each wheel of the chosen axle. Each wheel has its own drum and its own winch rope running out to its own anchor point. Both wheels are doing the same job simultaneously. Both are under tension, both are winding in rope, and both are pulling the vehicle forward together.

The differential is effectively bypassed. When both wheels on the axle are pulling hard on their winch ropes connected to the anchor points at the same time, there is no side of least resistance for the differential to favor. It simply does what it does on a normal road, splits the engine’s power between the two wheels.

Traction Control
Traction Control works on a similar principle to a differential lock, but electronically rather than mechanically. It constantly monitors for a wheel that’s spinning faster than the others, as a sign that the vehicle has lost grip, and briefly applies the brake or cuts power to the spinning wheel to redirect engine power to the one that still has traction. It’s the modern, computer-controlled answer to the same “one wheel spinning free” problem a differential creates.

The reason it doesn’t interfere with the Bush Winch is the same reason the differential doesn’t cause a problem: there’s no spinning, ungripped wheel for it to react to. Both wheels on the winching axle are under tension and turning at matched, controlled speed as they wind in their ropes. Traction Control has nothing to correct.

The next article in this series looks at the physics of a real recovery, working through what actually happens in soft sand and how a wheel-mounted system compares to a conventional bullbar winch step by step.

This is the second in a seven-part series exploring the Bush Winch recovery system. Read “What Is a Bush Winch?” first if you haven’t already, as this article builds directly on it.