When both propshafts are turning at virtually the same speed, the VCU is running free and not transmitting drive.
It will only start to lock up when the two props try to turn at different speeds, when the front wheels lose traction, or you jack a back wheel up an try to turn the rear prop and not the front.
Land Rovers description on how it works is clear enough.
A working vcu always has drive across it, be it drive from the engine end pushing or from the rear wheels dragging, in straight line driving with perfect tyres.
Iin normal driving the rear wheels are turning the rear propshaft and rear of the VCU slightly faster than the IRD is turning the front propshaft and front of the VCU, the silicon gel in the VCU remains fluid.
Doing the one wheel up test makes the rear of the VCU turn and not the front, the silcon gel resists the shearing effect by trying to lock the VCU up.
It does not prove what forces are being appled through the VCU during normal driving.
Land Rovers own description of how it works will do for me.
The fluid seems to be able to stay fluid all the time but change it's "stickiness" properties whilst doing so.
I agree the one wheel up test is a stationary comparison and not what's happening during driving (you would need to film the speed of the props with and without the vcu to do this, with perfectly matched tyres on level straight ground. But make sure you do it at slow speeds as the camera won't be fast enough to capture the spinning at speed - even with flags on the props
). the one wheel test is the best comparison we have but it's only a guide. It does pick out vcu's which have a higher resistance across them than others.
There is always a resistance across a working vcu. In the case of straight line driving with perfect matching tyres, the rear prop will turn faster than the front prop, which casus a ratio difference across the vcu. This has an effect on the Freelander, be it to slow it down due to transmission wind up with the drive from front or rear dragging winning, to overcome what would happen with front wheel drive only.
It would be nice if LR went further with their explanation. A vcu doesn't just switch on and off. It has resistance across it all the time if it's working correctly. Try turning it by hand yourself. It's the varying resistance v temperature of the fluid and it's reaction which produces the magic.
As the ratio of speed of front/rear prop increases, the sheering effect in the vcu become greater. The differing plates rotate at differing speeds, and this speed increases as the ratio increases. The sheering effect creates heat which has an effect on the fluid in the vcu. The greater the heat the more the fluid inside the vcu wants to grab the plates and reduce the differing speed at which they turn at, in comparison to each other. Hence the vcu starts to seize, or betterer put, the resistance across the vcu becomes greater and the vcu seizes momentarily if the ratio is great enough to cause it too. When the vcu is in this state the lack of differing speed of the differing plates means the sheering effect reduces and the fluid cools. This cooling effect allows the vcu to slip more as the resistance across itself reduces. This causes the speed difference between the props (and hence the inner vcu plates) to increase again. The vcu's plate don't fully release their grip as the fuild in the vcu will cool until the ratio of differing speed between the props (and hence the plate) increases to an extent where the sheering effect causes the fluid to heat up again, and the resistance across the vcu increases.
It's this varying resistance across the vcu which is a reaction of the fluids ability to rapidly warm up and cool down inside the vcu, which is why the vcu works so well. If you can mentally visualise the rapid alternating from "some resistance" to "more resistance" to near or seized, and back again due to the expected drop in resistance thereafter (due to cooling when the plates get closer to a 1:1 ratio), then you can appreciate why the vcu has the ability to vary the resistance across itself. It's the rapid heating/cooling which is proportunal to the changing ratio applied to the vcu, which varies the resistance (and therefore drive) across it. It's this ability of varying resistance which is key to the vcu reacting to conditions (ratio of drive across it) to achieve a state of equilibrium resistance, with respect to the ratio across it. This equilibrium state varies as the ratio across it varies. Hence the vcu varies the resistance across itself, whilst monitoring/reacting to change of ratio across itself. On an "as and when required" basis. It's when this goes wrong the vcu feks up.
