Originally Posted By: CapriRacer
I am of the opinion that on those vehicles with a transfer case, the 4X4 feature shouldn't be used except at slow speeds - I'm thinking less than 15 mph.
1) Do I have this right?
2) Is there a note in the owners manual to this effect?
The reason I am asking is that I am discussing 4X4 usage at 45 mph at 45 mph - and his position is that it's OK.
Hey CapriRacer, in addition to the above, the engineering reasons for not running locked are sound.
Consider the limit of friction for the tyres against the road...dry roads, you are looking at u=1 roughly...ice or sand, much, much less.
The tyres are all different sizes, and regardless, turning a corner, or different weight distribution giving effective radius differences...they are all different sizes.
They will wind up the axles, they will load up the gears in the transfer cases and diffs, until there is a torque (described by HokieFyd as "binding" - it is, you can't get the selector out of 4WD sometimes...but reversing a few feet will unbind it).
So you are travelling forward, and the axles, boxes, and gears are wound up through all of the tyres travelling at different rotational speeds.
They will wind up until one breaks, or nearly in every stocker, one or two wheels are at their limit of adhesion...it's the same torque as required for that grinding start to a burnout, and it's there, while ever in 4WD locked. Ice and snow, it's nothing, bitumen it's a lot.
Power, however, is torque times speed, so as you go faster, the torque is turned into "circulating power"...there's possibly 3-400hp, running through the drivetrain, the majority being regained at the other end of the vehicle.
The engine only has to make up for the frictional losses in the drivetrain, maybe 10% of the circulating power, 30-40 hp.
Imagine putting 5-6KW of heating into each compartment in the drivetrain, and imagine what that does to temperatures...
it's precisely how massive industrial gearbox manufacturers test the gearboxes and cooling systems, they couple a few together, and apply a static torque between them....tehy don't have to apply Megawatts, the gearboxes are transmitting MW, but only KW are required to keep them spinning.