jamesmartin
Well-Known Member
snow etc is were it really comes in rather than having to engage or disengage a center difflock on patchy road surfaces etc
Chaser
New Member
yep that figures out of thin air........the is NO drive to the rear when driving in a straight line with no front wheel slip.
jamesmartin
Well-Known Member
yes there is
Theoretically there isnt, James.
If both halves of the VCU are being driven at the same speed, then there will be no shear between the two plates, no heat generated and the fluid will not change its viscosity, thereby giving no drive between the two halves. if there is a difference in drive between the front wheels and the rear IRD output, that will give a difference to the two halves, as the rear wheels will be turning at the same rotational speed as the front wheels, but the output shaft (ie front of VCU) will not.
As for how effective it is, theoretically, the VCU can (with a wheel spinning, causing differential rotational speeds on the VCU) react within 1/10th of a turn of the VCU. I defy anyone to engage difflock in the same space of time.
Unfortunately (and its a big point), there is no maintenence and no qualified test available to determine whether the VCU is working properly or not, so the only thing owners can do is test the VCu with the test available as often as they can , and at the slightest doubt, get it replaced.
If both halves of the VCU are being driven at the same speed, then there will be no shear between the two plates, no heat generated and the fluid will not change its viscosity, thereby giving no drive between the two halves. if there is a difference in drive between the front wheels and the rear IRD output, that will give a difference to the two halves, as the rear wheels will be turning at the same rotational speed as the front wheels, but the output shaft (ie front of VCU) will not.
As for how effective it is, theoretically, the VCU can (with a wheel spinning, causing differential rotational speeds on the VCU) react within 1/10th of a turn of the VCU. I defy anyone to engage difflock in the same space of time.
Unfortunately (and its a big point), there is no maintenence and no qualified test available to determine whether the VCU is working properly or not, so the only thing owners can do is test the VCu with the test available as often as they can , and at the slightest doubt, get it replaced.
jamesmartin
Well-Known Member
try one ,they will slip to allow differential ,lift a rear wheel
It aint a switch 
. It can give a progressive locking up. In fact it is designed(???) to. it is designed to react to the heat generated by the shearing of the liquid dure to differing rotational speeds. This causes it to lock up, causing an immediate reduction of temperature, which changes the viscosity, allowing it to slip, which causes it to heat up..........
. It can give a progressive locking up. In fact it is designed(???) to. it is designed to react to the heat generated by the shearing of the liquid dure to differing rotational speeds. This causes it to lock up, causing an immediate reduction of temperature, which changes the viscosity, allowing it to slip, which causes it to heat up..........
jamesmartin
Well-Known Member
try one ,i know how they work
So under normal road only driving the freeby is basically front wheel drive with no power going to the back wheels.
If the fronts slip then the VCU will shear a bit sending power towards the back until it no longer shears. Right?
Is there a upper limit then of how much shear/power will go to the rear?
Just trying to figure it out as I'm curious as I thought they always had power going to the rear and if not then I would like to know the correct story of it.
If the fronts slip then the VCU will shear a bit sending power towards the back until it no longer shears. Right?
Is there a upper limit then of how much shear/power will go to the rear?
Just trying to figure it out as I'm curious as I thought they always had power going to the rear and if not then I would like to know the correct story of it.
see in red.
jamesmartin
Well-Known Member
on a freelander the inputs from both ends of vc arent the same ,
i think we may be under-complicating this
Nope - Its a **** design, that can fail in a catastrophic manner, destroying the majority of the drive train.... If it just failed with no drive, it wouldnt matter too much, but often, the first thing an owner knows is when the IRD has been destroyed
.
Freelaner
Landy Driver
Sadly i've been thereNope - Its a **** design, that can fail in a catastrophic manner, destroying the majority of the drive train.... If it just failed with no drive, it wouldnt matter too much, but often, the first thing an owner knows is when the IRD has been destroyed
and you're absolutely bang-on with that comment
jamesmartin
Well-Known Member
theoretically it is how they work but try a new one mhm see if you can find point of no torque transfer at all
I am sure it will, James, purely because of the friction inherent within the system. We are talking semantics. Theoretically there is no transfer, but in practice there will be. All down to the plate and viscous fluid characteristics.
I refer you to the following....
A viscous coupling is a mechanical device which transfers torque and rotation by the medium of a viscous fluid. It is made of a number of circular plates with tabs or perforations, fitted very close to each other in a sealed drum. Alternate plates are connected to a driving shaft at one end of the assembly and a driven shaft at the other end. The drum is filled with a dilatant fluid, often silicone-based. When the two sets of plates are rotating in unison, the fluid stays cool and remains liquid. When the plates start rotating at different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat and become nearly solid because the viscosity of dilatant fluids rapidly increases with shear. The fluid in this state will effectively glue the plates together and transmit power from one set of plates to the other. The size of the tabs or perforations, the number of plates, and the fluid used will determine the strength and onset of this mechanical transfer.
What I do find confusing is the following graph....
Which implies that its minimum torque transferrence is just before it locks solid
jamesmartin
Well-Known Member
yes i know the theory ,borgwarner gave us a course on them in 90s when we took over build of rrc t/box builds ,im just saying due to reality of vc and speed difference of front back you allways have drive to rear wheels, equal drive as rr would have been better for transmission
Chaser
New Member
under normal straight line driving with no front wheel slip the rear half of the prop turns faster than the front half. there's no argument about this, it's defined by the difference in front and rear ratio.
so the VC slips in two directions, the front only "drives" the rear half when there is front wheel slip and the speed of the front half of the prop exceeds that of the rear half.
until that front slip happens the rear wheels and rear prop are being rotated/dragged around by the motion of the vehicle, in other words the rear diff is in an overrun condition.
so the VC slips in two directions, the front only "drives" the rear half when there is front wheel slip and the speed of the front half of the prop exceeds that of the rear half.
until that front slip happens the rear wheels and rear prop are being rotated/dragged around by the motion of the vehicle, in other words the rear diff is in an overrun condition.
Last edited:
Mainwaring
Active Member
it's odd isn't it.
It's like the VCU is generally a pretty good idea on theory but it's been almost too engineered.
It's like the VCU is generally a pretty good idea on theory but it's been almost too engineered.
jamesmartin
Well-Known Member
dont know if over engineered springs to mind ,they wanted small 4x4 with front wheel bias, as they new market wouldnt be the usual land rover buyers but mondeo man ,v/c is cheap way of getting that ,if theyd gone for equal drive v/c wouldnt have given so many problems
