Figures are running torque at the VCU, they come from a 2001 copy of L series rave. There's no indication of method for measuring so assume this to be a design specification.
Reading on the net about shear thickening fluids I'm not convinced the explanation of the VCU operation in the article NI posted is correct. I don't think that temperature plays such a primary role in the activation of the VCU at all. (Note that there is no temperature given with the torque figures).
This seems to be a more likely explanation, Dilatant - Wikipedia, the free encyclopedia
If VCU operation is temperature dependant how does it activate almost instantly even in winter when it's -5c outside and also do the same at say 30c. Once such a mass has heated and locked how would it cool down?
 
Chaser said:
Figures are running torque at the VCU, they come from a 2001 copy of L series rave. There's no indication of method for measuring so assume this to be a design specification.
Reading on the net about shear thickening fluids I'm not convinced the explanation of the VCU operation in the article NI posted is correct. I don't think that temperature plays such a primary role in the activation of the VCU at all. (Note that there is no temperature given with the torque figures).
This seems to be a more likely explanation, Dilatant - Wikipedia, the free encyclopedia
If VCU operation is temperature dependant how does it activate almost instantly even in winter when it's -5c outside and also do the same at say 30c. Once such a mass has heated and locked how would it cool down?
Click to expand...

Thanks for citing the source.

I always thought the silicone fluid was 'rheopectic', speaking of corn starch, did anyone see Chris Evans walking on flour mix (batter) in the One show?

Dow Corning produce the silicone DOW CORNING® 3-9161 VISCOUS COUPLING FLUID

I emailed them sometime ago, they have two distributors here in the UK. Not sure if they supply to trade only, or minimum order.... if so possibly could do a group buy in LZ??

Could even source from Europe or US driectly



Regarding the temp issue, you could have a valid point, the annotation on the graph (link above) is slightly misleading


We know the VCU is sealed (welded), so how do the re-con firms (Bells and Ashcroft) unseal them to replace the plates which have the microscopic burrs.

There is a lug in the VCU itself, drilling this will allow access.

I have a spare prop which I might tinker with......any ideas ;)
 
Interestingly, Ian Ashcroft (from Ashcroft Transmissions) has a slightly different take on the VCU:

The coupling is a multiplate clutch arrangement and works by transmitting the drag from one set of plates to the other. The drag is controlled by the viscosity of the fluid, the plate spacing and the number of plates.

As the fluid temp increases the drag reduces allowing more slip creating more heat and if allowed to continue leads to change of state of the coupling fluid, goes solid, this change cannot be reversed.

Contrary to popular myth the coupling does not 'lock up' after a certain amount of slipping, it transmits max torque when cold ie max fluid viscosity.
Click to expand...

Difflock :: View topic - Freelander VCU bench test?
 
Contrary to popular myth the coupling does not 'lock up' after a certain amount of slipping, it transmits max torque when cold ie max fluid viscosity
Click to expand...

Ashcrofts were one of my sources also, but only via telephone - I always enjoy a comment when the experts blow away the myths.

As an aside. I guess one of the many problems with trying to 'define' a test of any sort (re. torque test) is establishing exactly the same conditions for each test - Almost impossible - which makes any 'result' as such pretty meaningless.

If only the resident expert (rippedoff) where able to gift us all with more of his words of wisdom......:hysterically_laughi
 
Dont wish to set the arguement going again, but Rave states that it is the shearing action which invokes the thickening of the VCU and transmittion of torque, not heat.

"From Rave...

VCU
The VCU automatically controls the transfer of drive to the rear wheels by limiting the speed differential between the front and rear propeller shafts. The unit is supported in two propeller shaft bearings attached to the floor cross member.
The VCU comprises a short cylinder which contains an input shaft supported in a roller bearing race at the front and a ball bearing race at the rear. Within the cylinder, slotted discs are alternately attached to the outer surface of the input shaft and the inner surface of the cylinder. An output shaft is welded onto the rear of the cylinder. The input shaft is attached to the front propeller shaft and the output shaft is attached to the rear propeller shaft.
On models with 1.8 K Series engines, a torsional damper is bolted to a flange on the output shaft.
The cylinder is a sealed unit filled with a silicon jelly. The viscosity of the silicon jelly increases (it thickens) when subjected to shear. When there is a speed differen
 
Last edited:
as there is no sure answer to the VCU'S life span then the tippex test is still good but a rule off thumb needs to be made so i would say check more often after 70.000 miles
 
I think the answer, Ming, is to test it as regularly as you can justify doing it. You can never test it too often :D.
 
Re-reading Ashcroft's post, he does not suggest that temperature (heat) was the mechanism by which the fluid operates.

Rather he suggests that heat is a failure mode of the fluid.
 
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