When a tractor unit has a 6×4 or 8×4 configuration it has two axles that are drive axles. Some units are fixed, while others have a power divider to enable the driver to decide where to send the engine’s power.
These are used when driving in snowy weather or off-road, or when one axle is likely to wheelspin.
With tandem drive axle sets, both axles are driven by the engine at all times. The individual axles can travel at different speeds, and this happens when cornering (a small difference) or if one of the axles loses traction (potentially a large difference).
If one axle is lifted off the ground, all power will go through that axle and the wheels will spin, meaning there’s no power going through the axle that remains on the ground. This means that it’s possible to beach a tractor unit with tandem drive.
Tandem drive with power divider
The power divider works like a differential lock between the two axles meaning both axles will run at the same speed, even during loss of traction. Each axle’s differential means that the wheels on either end can run at different speeds to help with cornering.
Power dividers should be engaged when going uphill and downhill to reduce the risk of engine damage as they can reduce inadvertent wheelspin when going uphill and engine brake compression lockup when going downhill.
The power divider should be engaged when conditions are slippery, such as when driving off-road. They can be used at any speed.
How to engage the power divider
Don’t engage the power divider when the wheels are spinning and traction is lost.
- Ensure that the truck is travelling in a straight line and below 60km/h
- Release the accelerator pedal
- Press the power divider switch
- Depress the accelerator pedal and continue driving normally
To disengage it, drive in a straight line, lift off the accelerator, press the power divider switch, then press the accelerator.
Tandem drive with power divider and diff lock (cross lock) activated
This can also be called an interaxle cross lock. This locks the centre diff between the axles, as well as the diffs on the axles, meaning all wheels turn at the same speed. If one axle loses traction, the other axle will continue to turn at the same speed. This mode should not be used for tight cornering as it stresses the axle.
Cross locks are used when extra traction is required off-road.
How to engage the cross lock
- Stop the truck completely
- Press the power divider and cross lock switches
- Move off slowly until you get to an area where there will be enough traction to not have to use the cross lock
How to disengage the cross lock
- Stop the truck completely
- Switch off the cross lock; you can leave the power divider engaged, if required
- Move off slowly