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The Design of Permanent 4WD Vehicle

Remember when 4WD is engage all the wheels are being forced to turn at the same time and revolution. Driving dead straight on a dry, firm surface will cause no problem. The moment the steering wheel is turned, say to left, the wheels on the right-hand side of the vehicle are having to travel a greater distance around the arc than those on the left.

This will tighten up the transmission considerably as both axles and all the wheels are forced to rotate at differing speeds. That is transmission wind-up. Long term, this will lead to severe tyre wear and eventually damage to the gearbox. Changing gear is difficult with transmission wind-up. This situation does not arise off road because of the type terrain. Muddy, wet or stony surfaces will allow wheel slip without the driver knowing and this will cancel out any potential wind-up.

The design of permanent 4WD vehicles therefore needed careful consideration of transmission wind-up. The problem was delat with installing a third differential. Whilst the other two are located, respectively, on front and rear axles this additional one is inside the gearbox. It allow both axle to rotate at different speeds.

However, if one axle's wheel revolves faster than the other while driving off-road in slippery conditions this center differential will permit the drive to go the easiest route - the power will go to the axle that can spin its wheels most freely. This is the last thing you want because if you have one wheel on a slippery surface and the other three wheels on firmer ground everything transfer to that one wheel.

Thus, all permanent 4WD vehicles are equipped with a centre differential lock on that central differential. This has the effect of stopping the centre differential working so that drive will be equalised between the front and rear axles. This lock should be in the 'lock' position when on slippery surfaces and off-road but unlocked whenever driving on dry, hard surfaces.

4WD Routine Maintenance

One of the first jobs that should be done after a 4WD trip is to thoroughly clean and inspect your vehicle. I f the last trip was a through any muddy terrain there will be lumps of hardened mud around the wheels arches and underneath most other part of the vehicle. Unless this mud is removed, corrosion will eventually occur at these places.

To get rid of this mud you need plenty of water under a good hose pressure. Alternatively, if you have a local garage with one of those "Do It Yourself" pressure car wash units, you can clean the underside at very little cost.

One the underside is clean, it is very easy to check and make sure that everything else is O.K. Items that need special checking are:

1. Brake hoses - for cracks or cuts in the rubber.

2. Brake lines - check to make sure that no wiring have been crushed.

3. Wiring - check to see that no wiring has been broken (ot torn out) particularly around tail lights and front lights.

4. Tyre - inspect the wall of the tyres for any sign of cracks in the casings.

5. Oil leaks - check around the engine, gearbox, transfer case and differentials for any signs of oil leaks. Check all oil levels and top up if necessary.

6. General - do a very careful inspection underneath to make sure that sticks have not wedged in around the chassis and body or up in around the gearbox area.

7. Tie rods - make sure that the tie rod are not bent.

8. Bolts - check all bolts to make sure that they are done up tightly.

9. Rubber bushes - check and see whether all the rubber bushes are still intact on the spring hangers.

10. Grease vehicle - whilst you are still underneath it wouldn't hurt to give it a grease.

11. Brakes - clean out the brake drums and around brake shoes. Check level of brake fluid. Adjust brakes if necessary.

12. Radiator - remove bugs and other matter from the radiator core.

13. Battery - maintain clean, tighten connections to battery. Check electrolyte level.

14. Air cleaner - remove excess dust and grit from element. Replace element if necessary.

Any item that needs fixing should be attended to as possible so that your 4WD is quickly back in a ready condition to be able to undertake the next trip away.