true 4 wheel drive question

[ST165OKE]

talking to mates in the office and they all saying bout the whole 4 wheel drive only ever drives on 3 wheels at any one time.
is this true for all 4 wheel drives inc st165
anyone care to explain?

[Gary ST165]

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

hth

[ST165OKE]

cool so basically it is permanant 4 wheel drive so they can all go stick it up there backsides cheers

[Coyoteboy]

OK I'll start again.

Scratch all of that, ive still not decided exactly.

[bazza2541]

Aye, the 3 wheel drive thing is generally true. This is cos most standard AWD cars have an open front diff and some sort of limited slip differential at the rear.

[ST165OKE]

right sooooooo
no one knows basically??  lol

[bazza2541]

AWD is really 3 wheel drive unless there is a viscous centre diff, then its 2 wheel drive until the other axle is needed.
Other types of centre diff mean the car runs in 3 wheel drive all the time, UNLESS there is a limited slip front diff, which IIRC I have never seen on a standard production car.

[rubberduck_10_4uk]

Surely it will only be 3 wheel drive if one of the front wheels loses traction??

On an open normal diff, when driving in a straight line, both wheels are driven at the same rate, when you go round a roundabout the inside wheel goes slower and the outer faster say for arguments sake left to right rotation speed ratio of 65% to 35%, but both wheels are being driven. If one of the wheels loses traction all the drive will go to that wheel. So in a 2wd car you would have 1 wheel drive but no actual drive. And on a 4x4 you would have 3 wheel drive for that moment but only 2 wheels(the rears) actually driving you forward.....

[Gary ST165]

:lol: the 165 has only open diffs

would it be correct to state sometimes it is only a 2wd? i.e. only 1wd front and 1wd rear?

[rubberduck_10_4uk]

Are you wheelspinning Gary    Knowing you probably 

[T600]

Surely it will only be 3 wheel drive if one of the front wheels loses traction?

Seconded - until a wheel loses traction the torque is spread through all the driven wheels even without any LSDs.

Therefore the system is less likely to lose traction in the first place because the power is being shared more equally.  Once a wheel loses traction then the affect of that will vary depending on the number and type of LSDs, and yes that could mean that you revert to three, two or one wheel getting the balance of the torque. 

Still better than any FWD though, without an LSD that's no wheel drive when one slips, and it seems to take next to no loud pedal to make it happen on most of them.

[rubberduck_10_4uk]

just to clear up the '3 wheel drive' myth, from wiki 

To illustrate how a differential can limit overall torque, imagine a simple rear-wheel-drive vehicle, with one rear wheel on asphalt with good grip, and the other on a patch of slippery ice. With the load, gradient, etc., the vehicle requires, say, 2000 Nm of torque to move forward (i.e. the threshold torque). Let us further assume that the non-spinning traction on the ice equates to 400 Nm, and the asphalt to 3000 Nm.

If the two wheels were driven without a differential, each wheel would push against the ground as hard as possible. The wheel on ice would quickly reach the limit of traction (400 Nm), but would be unable to spin because the other wheel has good traction. The traction of the asphalt plus the small extra traction from the ice exceeds the threshold requirement, so the vehicle will be propelled forward.

With a differential, however, as soon as the "ice wheel" reaches 400 Nm, it will start to spin, and then develop less traction~300Nm. The planetary gears inside the differential carrier will start to rotate because the "asphalt wheel" encounters greater resistance. Instead of driving the asphalt wheel with more force, the differential will allow the ice wheel to spin faster, and the asphalt wheel to remain stationary, compensating for extra speed of the spinning ice wheel. The torque on both wheels will be the same - limited to the lesser traction of 300 Nm each. Since 600 Nm is less than the required threshold of 2000 Nm, the vehicle will not be able to move.
Note that an observer will simply see one stationary wheel and one spinning wheel. It will not be obvious that both wheels are generating the same torque (i.e. both wheels are in fact pushing equally, despite the difference in rotational speed). This has led to a widely held misconception that a vehicle with a differential is really only "one-wheel-drive". In fact, a normal differential always provides equal torque to both driven wheels (unless it is a locking, torque-biasing, or limited slip type).

[ChrisW]

Very thorough as always Ducky

Cheers

[rubberduck_10_4uk]

    

[Coyoteboy]

Its factually correct but not really correct in terms of "drive". Although it IS allowing equal torque to both wheels, that torque is vastly reduced from a "gripping" tyre and is eqaul to the slip torque of the spinning tyre.