Transmission wind-up is the stress or torque created in the components of a vehicle when it is placed in four wheel drive and made to turn corners.
Transmission wind-up is also called axle wind up or drive-line binding. They are all the same phenomenon and can all create expensive damage to tyres, transmissions, transfer cases, axles, drive shafts and CV Joints.
Why Transmission Wind-Up Occurs
The illustration on the right demonstrates what occurs when a vehicle corners. All four wheels travel different distances and therefore must rotate at different speeds during the turn. It is why track and field athletes receive a head start when they run on the outside lanes of a circular track.
In a two wheel drive car with the rear wheels driven, the differential allows each wheel to rotate at it's required speed. The front wheels are free-wheeling and can rotate of their own accord.
Part Time 4WD
A part time 4WD operates in exactly the same manner until 4WD is selected and the front locking hubs have been actuated. At this point the vehicle becomes a four wheel drive with equal power being delivered to the front and rear differentials and finally to the wheels. Part time four wheel drives do not usually have centre differentials.
Constant four wheel drives have a centre differential that supplies power to both front and rear wheels at the same time. This centre diff allows the front and rear wheels some 'slippage' so the vehicle can make turns. If a Constant 4WD has the facility to 'lock' the centre differential then power is delivered equally to the front and rear diffs - much like a part-time four wheel drive.
Transmission wind-up occurs when true four wheel drive has been selected and the vehicle is driven on a non-slip surface.
Dirt, gravel, snow and most loose off-road surfaces allow the tyres to break traction to compensate for the constantly powered drive-line.
Concrete, bitumen and other unforgiving surfaces don't easily allow this wheel spin. Turning a car in 4WD on hard surfaces creates torsional stress back through the drive-line. Transmission wind-up begins to manifest itself as tyres begin to 'skip' over the surface. Steering becomes heavy and the vehicle begins to shudder. The transfer case eventually becomes 'locked' in four wheel drive and cannot be de-selected. The vehicle may also refuse to advance any further. Keep driving and something has to give. Tyres begin to tear up and the weakest link must break. Transmission components, diffs and axles are all extremely expensive to replace.
How to Remedy Transmission Wind-Up
If your 4WD shows any symptoms of transmission wind-up then stop driving immediately. If you cannot de-select four wheel drive then you can slowly reverse the vehicle until the wind-up has been released. If you are in Low 4x4 attempt to shift into High.
Another remedy is to jack up a single wheel and shift into High 2WD. The release of the torsional stresses in the drive-line can cause the raised wheel to spin quite viciously.
The article Free Wheeling Hubs contains information regarding part-time four wheel drives and locking hubs.
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