the vc has a limited control as once it heats performance drops off rapidly. I think steep hills do kill it rather quickly particularly if using oversize tyres. Im also considering that the fuel pump tweaks affect it also. Standard setup eould hardly pull off down a hill!
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Towpack
Well-Known Member
If you have a VC (which is working ok) you will never have a fully open diff as you have in a transfer box without one. What you have really is a limited slip CD, hence the difference between limited slip and fully locked will always be less than fully locked and fully open. IMO the ‘response’ of the VC will be instant as it’s operation is based on fluid dynamics and it doesn’t have to wait for anything to engage mechanically. A bit like a shock absorber maybe?
Towpack
Well-Known Member
Why would hills and oversize tyres kill it? It’s only working to any degree when traction is lost at either end, irrespective of the vehicles attitude or what size tyres it has. The difference in speed (relative to each other) between the F & R diffs will be unaffected by tyre size as long as they’re all the same of course. All that non standard tyre size affects is overall gearing and performance.
What I was saying that it heats up quicker when the load is greater. Bigger tyres increase the gearing/drag and hills do what hills do. I can only talk from experience but disconnect your front shaft and go for a normal drive. you will see how speed, acceleration and terrain affect its performance. Once it is working it produces heat and lowers the viscosity causing it to slip until it no longer provides drive. once it heats up and drive is lost then you need it to cool before getting drive again.
Having just penned that and wikipedia'd it.......my theory is upside down. when it heats up the viscosity increases locking the drive.
'Rotary viscous couplings with interleaved, perforated plates and filled with viscous fluids are used in automotive systems to transmit torque.[1] The device consists 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, to about 80% by volume. 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
SO why does it loose drive after a period of constant use? Might just grab a new can to collect the worms in....
Having just penned that and wikipedia'd it.......my theory is upside down. when it heats up the viscosity increases locking the drive.
'Rotary viscous couplings with interleaved, perforated plates and filled with viscous fluids are used in automotive systems to transmit torque.[1] The device consists 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, to about 80% by volume. 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
SO why does it loose drive after a period of constant use? Might just grab a new can to collect the worms in....
http://www.tlocuk.co.uk/forums/viewtopic.php?t=6175
A quote stolen from that other forum...
The VC has a set of plates and heat sensitive solution (silicone based I think?) so as there is slip between the front and rear axle (i.e. differential speeds) the VC turns more, the silicone heats up and stops separating the plates and they bind, causing a locking action. Obviously this happens fairly quickly to provide some "limited slip" function. Obviously if it gets to the point of fully locking the centre diff, then the lack of differential speed/action between front and rear drive shafts results in the fluid cooling down, the plates separate and the VC "opens" again.
A quote stolen from that other forum...
The VC has a set of plates and heat sensitive solution (silicone based I think?) so as there is slip between the front and rear axle (i.e. differential speeds) the VC turns more, the silicone heats up and stops separating the plates and they bind, causing a locking action. Obviously this happens fairly quickly to provide some "limited slip" function. Obviously if it gets to the point of fully locking the centre diff, then the lack of differential speed/action between front and rear drive shafts results in the fluid cooling down, the plates separate and the VC "opens" again.
Towpack
Well-Known Member
The heat is generated by the amount of slip, not the load on the transmission which will depend on what gear you’re in.
That’s because it wasn’t designed or intended to cope with total loss of drive to the front axle.
Does it? It is in use (almost) constantly due to the difference in rotational speed between F & R axles on anything but a straight road. On snow and ice, more so. Running with a prop removed for anything more than a short test is a sue way of damaging it permanently IMO.
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I guess the question of how much drive/Usefulness can we expect from the vc has evolved from the handling characteristics of an 80 on slippery surfaces and whether or not ones VC is functioning correctly. Having tried to drive with a broken drive flange (which I didn't know had broken) only to loose drive a few miles later left me quite puzzled at that time until I subsequently worked out what was happening. So having realised the amount of drive that can be produced by the VC left me wondering why the truck stepped out rather aggressively, as surely the VC should have kept more power going to the front if it was loosing grip at the back. As it is I would rather not have it than be under any false sense of security.
From the above experience, without a front drive flange the truck would go a reasonable distance on the flat but going up hill soon lost drive. Therefore I deduce that the amount of torque transferable is limited and therefore increased loading, RPM, Tyres, gradient do affect its performance based on the driving I have done with it.
If the only apparent failure mode of a VC is total lock-up, Having done several thousand miles with a small radius tyre on one corner, doesn't seem to have caused a failure in my case so I can only assume that it is working. I have already acknowledged the fact that my problem may be unrelated to the VC.
SO all that said am I expecting too much from a VC to give improved handling similar to a CDL rather than just to keep the truck moving forward when surfaces change??
From the above experience, without a front drive flange the truck would go a reasonable distance on the flat but going up hill soon lost drive. Therefore I deduce that the amount of torque transferable is limited and therefore increased loading, RPM, Tyres, gradient do affect its performance based on the driving I have done with it.
If the only apparent failure mode of a VC is total lock-up, Having done several thousand miles with a small radius tyre on one corner, doesn't seem to have caused a failure in my case so I can only assume that it is working. I have already acknowledged the fact that my problem may be unrelated to the VC.
SO all that said am I expecting too much from a VC to give improved handling similar to a CDL rather than just to keep the truck moving forward when surfaces change??
Towpack
Well-Known Member
VC’s can fail either way giving full lock up or fully open although the former may be more common as I believe there are owners on here who have had this and removed it. If it goes the other way then you might as well leave it in there assuming you’re not bothered about having it working.
A simple test is to jack up one front wheel, handbrake on and transfer box in neutral. If the front wheel is locked solid the VC is seized, if it spins freely it has lost function and is fully open. If it turns with resistance then the VC is working. How much resistance is debatable. There is probably a figure in LB/FT but I've no idea what it is. Mine ‘feels’ quite stiff for what it’s worth but it does slip.
A simple test is to jack up one front wheel, handbrake on and transfer box in neutral. If the front wheel is locked solid the VC is seized, if it spins freely it has lost function and is fully open. If it turns with resistance then the VC is working. How much resistance is debatable. There is probably a figure in LB/FT but I've no idea what it is. Mine ‘feels’ quite stiff for what it’s worth but it does slip.
Although I’m effectively banned from this thread 
someone has to get it back on topic...
Today, at last (well tonight actually) lousy photos, but it is dark...
someone has to get it back on topic...Today, at last (well tonight actually) lousy photos, but it is dark...
Our balcony view slightly later in the week. Home now though, the flight on Thursday was cancelled due to air traffic control strike so we had 6 extra days in the end!
Looking just fabulous uHu 
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A bit late in posting..
This is from the carnage of last week.
This is from the carnage of last week.
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the rain falls a little different up here in winter nick..
was a awesome few days.
cars and landrovers abandoned all over the place. a couple of local lads spent the night in there 80 completely snowed in. they loved it..
was a awesome few days.
cars and landrovers abandoned all over the place. a couple of local lads spent the night in there 80 completely snowed in. they loved it..
Towpack
Well-Known Member
This was our back garden last week. 14" of level snow.....
.....and the LC, waiting expectantly
By Monday most of it had gone although I heard on the radio that the Snake Pass, Cat and Fiddle and Axe Edge were still closed. We live on top of one of the "Seven hills of Sheffield" so we're quite high up at just over 600ft ASL although you can stick another 1000ft on that with the aforementioned routes.
.....and the LC, waiting expectantly
By Monday most of it had gone although I heard on the radio that the Snake Pass, Cat and Fiddle and Axe Edge were still closed. We live on top of one of the "Seven hills of Sheffield" so we're quite high up at just over 600ft ASL although you can stick another 1000ft on that with the aforementioned routes.
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