Advice: Bogie-drive basics

All bogie or tandem-drive trucks require an inter-axle differential, also referred to as the 'power divider', which allows the forward and rear axle to turn independently of each other, thereby relieving stress on the axles, reducing tyre wear, and allowing the vehicle to corner more effectively.

Most trucks have a switch located on the accessory switch panel that enables the inter-axle differential to be locked or engaged. When the inter-axle is in the 'engaged' position, it causes the differential action of the power divider to act as a 'through drive', transmitting power equally between both the forward and rear axles, which provides better traction on slippery surfaces. On some trucks, this switch has a guard to protect it from being activated accidentally.

Knowing when to engage the power divider will enable the truck to move easily through certain low-traction situations. For example, often when driving over steep driveways one wheel may be left freewheeling in the depression. By engaging the power divider, 50 per cent of the drive is sent to both axles, which enables the axle with both wheels on the ground to move the vehicle.

Once the 'freewheeling' wheel is back on firm ground the power divider can be unlocked.

However, a word of caution: it is extremely important to ensure that none of the wheels are spinning. This means that in some instances you will have to bring the vehicle to a complete stop before engaging the power divider lock. Failure to do so could destroy the rear axle.

Typically, you would anticipate when you might need increased traction, and slow the truck to a steady speed under 40km/h or stop it entirely. As stated earlier, it is extremely important not to lock the differential while wheels are spinning or traction is minimal, or when descending steep grades or travelling at speeds over 40km/h. Lock the differential before you encounter these conditions.

When it is safe to do so, put the inter-axle differential lock switch in the LOCK position. A light in the warning cluster or on the switch will illuminate to indicate the power divider is locked (engaged). If you LOCK or UNLOCK the differential while moving, let up momentarily on the accelerator pedal to relieve torque on the gearing and allow full engagement of the clutch (in this case 'clutch' refers to the mechanism that locks the wheels. 
Then drive the vehicle through the poor traction area, keeping your speed under 40 km, failure to do so can damage the power divider unit.

Conversely, if your vehicle is fitted with an automatic transmission, it may be necessary to momentarily shift the transmission to the neutral position to allow the main differential lock splines to fully engage or disengage.

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When you reach normal road surfaces or better road conditions where the differential lock is not required, switch the differential lock to UNLOCK. Then let up momentarily on the accelerator pedal to relieve torque and allow the clutch to disengage. Once you unlock the differential, normal vehicle handling will resume and the light on the warning cluster or on the switch will turn off.

It is advisable to periodically activate the power divider switch, in the depot yard for instance, to make certain the valve operates freely; under normal conditions the switch should be in kept in the UNLOCK position.

If you happen operate a tri-drive unit, then you will typically find that the front axle in the tri-group will have the power divider permanently in the lock position and the middle axle's power divider will operate in the same manner as the tandem-axle unit as described above.

Rear axle wheel differential locks, or cross locks as they are sometimes referred to, are also driver-controlled and operated by the carrier-mounted air-actuated shift unit. When the diff lock is engaged, it positively locks the wheel differential to provide improved traction under adverse road conditions.

The differential lock is controlled through an electric switch or air valve mounted in the cab. The locking mechanism is air operated to engage a mechanical clutch that locks the wheel differential. The lock's disengage action is spring operated and permits the wheel differential to return to normal operation.

When the Driver Controlled Diff Lock (DCDL) switch is placed in the unlock position, air pressure supply is shut off and air pressure is released from the shift unit allowing the spring to release the clutch – the axle once again returns to act independently of the opposite axle.

Look ahead for wet, muddy or icy patches on the road. Stop your vehicle and make sure none of the wheels are spinning then move the dash-mounted switch into the LOCK position, ahead of time.

Some vehicle manufacturers will allow the differential locks to be engaged while moving but only if the vehicle does not exceed 40km/h and there is no wheel spin. Let up momentarily on the accelerator pedal to relieve torque on the gearing and allow the sliding clutch to fully engage. When the differential lock is engaged, an indicator light will be on, or in some cases an audible signal will sound.

It's important to note that when the differential lock is engaged, the vehicle will under-steer, requiring a longer turning radius for a given turn.

To disengage the wheel differential lock, shift the dash-mounted switch into the UNLOCK position. Then momentarily let up on the accelerator pedal to relieve torque and allow the sliding clutch to disengage. 
Once the differential lock has disengaged, the indicator light and/or audible sound will deactivate.

Take note that occasionally the differential lock may not disengage immediately due to torque 'wrap-up'. When this occurs, drive normally for a short distance with the control lever still in the UNLOCK position until normal road forces rectify this situation.

When engaged, the wheel differential lock will cause the vehicle to under-steer, meaning that the vehicle will not turn as quickly, and more steering effort will be required.

Engagement of the wheel differential lock while the wheels are rotating at different speeds may cause shift fork scoring and bending, clutch teeth failure and/or axle shaft shock failures.

This system should only be used when poor traction is encountered and it must always be disengaged when traveling on solid road surfaces.

If you are in any way unsure about the operation of the power divider or diff lock in your specific vehicle, refer to the driver's handbook or seek advice from an authorised dealership.