DRIVING A PART-TIME 4WD
Around half of the new 4WDs sold come with part-time 4WD drivelines and our bet is that 99 percent of them will be improperly operated. The part-time driveline is poorly understood by 4WD owners and 4WD sales people.
Driving a Japanese 4WD ute, a Troopy, a 76 Series LandCruiser wagon, a Patrol, a Suzuki, a Jeep Wrangler or old-shape Cherokee, a Challenger, a Jackaroo, a Frontera, a Terracan, a Sorrento, a Musso or a Rexton? Or one of the older Japanese 4WD wagons in the market? You’d better understand how a part-time 4WD system works, or you could be in for expensive transmission repairs – just through driving it incorrectly.
Despite many articles on the different types of 4WD systems the traditional and most common part-time 4WD driveline is still little understood, while accidental abuse of the part-time driveline continues to break components. For example, typical damage in Jeeps is broken front diffs and in Toyotas it’s transmission bits.
Part-time 4WD drivelines have no central differential or wet clutch pack, which means that the front and rear propellor shafts must turn at the same speed when 4WD is engaged.
Driving a part-time 4WD vehicle on high-friction surfaces in 4WD mode causes transmission damage and tyre wear, because the driveline has no mechanical way of providing the necessary differential action between the front and rear axles. The only ‘out’ for the torque build up is in wheel slip and that can’t happen if there’s too much tyre grip.
When operating a part-time 4WD, the machine should be driven on high-grip surfaces in 2WD and then put into 4WD mode when conditions warrant it, such as when a bush track becomes steep and loose or slippery.
Even more importantly, the driveline should be put back to 2WD mode when the track is no longer steep, loose or slippery.
It’s also vital for transmission life that the vehicle be in 2WD mode when turning tightly on surfaces that don’t need 4WD for traction.
Even with power steering you can feel the extra loading in the steering when you’re using 4WD unnecessarily. That load is ‘wind up’ torque in the gear train and it can break things.
Although part-time 4WDs dominate working vehicle numbers there’s very little training given to their operators in the correct use of the part-time system – the most glaring omission being careful instruction on when to use 4WD and when to disengage it.
There is also great confusion about the function of free-wheeling hubs. A manual or automatic free-wheeling hub contains a spring-loaded sliding collar, which couples and uncouples the front axle shaft to the front hub.
By isolating the front drive assembly using free-wheeling hubs, 4WD makers save some fuel consumption, vibration and noise, but the main reason it's done is to prevent premature wear in the front differential, which is designed to ‘drive’ the front wheels, not to be ‘driven’ by them.
Some owners fear they'll ‘wind up’ their driveline and increase
tyre wear if they drive on high-friction surfaces with the hubs locked while in 2WD mode - they won’t. Locking the hubs doesn’t engage 4WD: it just readies the front axle for 4WD engagement in the transfer case.
Once the hubs are in ‘lock’ mode and with the vehicle running straight ahead, four-wheel-drive can be selected via the transfer case lever or dashboard switch, without stopping the vehicle, because the front and rear propellor shafts are turning at synchronous speed.
Automatic free-wheeling hubs are a popular way for 4WD makers to get around the inconvenience and misunderstanding of manual hubs. Auto hubs function in much the same way as manual hubs, but are locked by stopping the vehicle and selecting 4WD. Thereafter, the hubs remain locked, even if the transfer case lever is moved back into the 2WD position. They're unlocked by reversing for a few metres and so they’re unreliable in ‘vehicle rocking’ manoeuvres, where you’re trying to extricate the machine from a hollow by building up forward and backward momentum. Auto hubs are likely to disengage in these circumstances, which is why Nissan put a manual override on its Patrol automatic front hubs.
‘Shift on the fly’ 4WD engagement is a further refinement of the auto hub principle in which 4WD can be selected without the need to stop the vehicle for initial selection. However, the basic driveline of these systems is still part-time, so they can’t be operated on high-friction surfaces in 4WD mode without the risk of transmission damage.
« Go Back