Traction

Provided resistance is felt at the wheels all will drive as required, however, if all the diffs are unlocked and one wheel loses traction, all the power takes the path of least resistance and goes to the spinning wheel so the rest all stop. When you lock the centre diff, the drive is split 50/50 so both axles get their fair share. The only problem then, is that if a wheel on either axle spins the opposite number stops. So, for example, if both left wheels are spinning on a slippery grass verge, the two right hand wheels, possibly on tarmac, will do nothing. Classic situation. The truth of the matter is that a three-differential permanent four wheel drive system which is completely unlocked is actually only a one wheel drive under some circumstances. Hence the need for a centre diff-lock at the very least. And all that does is make your full-time system as effective, mechanically, as a part-time system with four wheel drive engaged.

So, the centre diff must be locked to guarantee drive to both axles and axle diffs must be locked to guarantee drive to all four wheels. It is all the fault of the differentials which are essential on high traction surfaces to enable all the wheels to rotate at different speeds when the vehicle manoeuvres, i.e. the inside wheels on a corner rotate slower than the outside wheels and the front pair have a higher mean speed than the rear pair. So, we can't do without differentials on the road, but they're the weak link when driving on low traction surfaces.

Land Rover get away without having diff-locks in the axles as standard fit because their suspension is extremely flexible and, in most situations, wheels stay on the ground and driving. The G's suspension is stiffer in roll, so it tends to lift wheels more readily when off-road. Consequently, it actually needs its axle diff-locks to tackle the more serious conditions.

Leeu
Hmmm........Yes if the centre diffis locked the front wheels should be rotating at the same nett rate as the rear ones either in the same direction or contra-rotaing depending on whether the front diff is engaged.
Hmmm
Is your centre diff not working/engaging then?
M

Leeu, from what you have described in your first post, I don't think anything is wrong. It is just the way differentials behave, but it is dependent on how much traction or resistance each tyre is "feeling". If none of the diff-locks are engaged, you only need one wheel to lose traction and start spinning, for whatever reason, and the other three will stop turning. If only the centre diff-lock is engaged, then you have 50/50 drive to each axle, but the individual axle diffs will cause trouble if one wheel on either or both axles starts to lose traction and spin. Next stage is to lock them; rear first then front, which is the why the switches are sequenced that way. You will also find that if you have all the locks engaged and you unlock the centre diff, it will automatically unlock both axle diffs too. Locking centre diff will also automatically disable the ABS, as a matter of interest.

All differentials have one input and two outputs. Torque is only delivered to both outputs if they offer the same resistance. So if one output has no resistance, because of a spinning wheel for example, all the torque goes to that output and the other gets nothing. In reality, torque is proportioned between them according to their resistance. On a full-time 4x4 system all wheels generally offer some resistance so all wheels tend to drive all the time, at least until one wheel starts to spin. On the 460's part-time system, in 2wd, there is no drive to the front axle; it just idles, so in 2wd it will get bogged far more easily that the 463 with its diffs unlocked.

I'm beginning to get bogged down myself here! :lol:

If a unlocked 463 looses traction to one or both front wheels, it becomes a 0wd. A 460 in the same situation is still a 2wd.

Not actualy true, for it to be a 0 wheel drive none of the wheels would be driven. In the scenario you describe drive is still going to the wheel without traction. It might not be going anywhere fast but it is still one wheel drive. :)

If you placed a 463 on 4 slabs of concrete, one under each wheel it would drive off... as would a 460.

If you remove a slab from one of the rear wheels and it was free to rotate in mid air, both a 460 and 463 would fail to drive off as the power goes to the wheel with no traction.

However if you removed a slab from under one of the front wheels and it did not contact the ground it would spin freely in a463 and no other wheel would turn, but a 460 would still drive off...

In real life when off roading one out of four wheels in a 463 needs to loose traction where only one in two in a 460.

Simple? Correct?

Ian,

I like your explanation, simple yes correct? Sort of.

I’ve demonstrated exactly that by putting rollers under 1 wheel and defying numerous “experienced” off roaders to drive off. I would say 80% of these “experienced, knowledgeable” people were baffled by what’s going on (or not! :lol: )

IMHO, having said all that in “real life off roading” it’s pretty rare to have zero or virtually zero traction to a single wheel in addition to trying to start from stationary (no momentum to carry you past this situation) and therefore find yourself in this position.

In other words I’m happy with full time 4WD vs part time but it is a miss understood and often over rated thing.

Russ

Imagine my surprise when I became completely stuck in a small snow filled drainage ditch after pulling over to allow an oncoming car to pass on a single track road.

The angle of the ditch had left the rear offside wheel slightly in the air and because I was unable to lock the centre diff I was only getting drive to this wheel.

The final humiliation was being pulled out by a local farmer in his ancient LR pickup who was only to happy to tell me how bollox all this foreign 4x4 stuff was. I don't think my explanation about vacum pipes and difflocks really made much of an impression on him. :oops:

I would not blame the G for getting stuck because it was not working properly. However it does show the weakness of permanent 4x4 with an open centre diff.

I’ve never thought to try this with a 4wd but you can create your own primitive traction control which may have got you out of that situation.

If it’s one of the fronts without grip just left foot brake, if it’s one of the rears judicious use of the handbrake will transfer torque away from the wheel with loss of traction.
Fly by wire throttle cars don’t generally like left foot braking though.

Russ