Club 80-90 Forums

There is a lot more to it than just filling with the said weight of silicone.
Rememder slightly out and u will have one that slips or one that eats ure gearbox.

Nice one Aidan. Sounds too simple you know. Can anyone explain why these things can get SO stiff as to shear off the input shaft like the old one in my shed?????

I would imagine that if you cook the silicone oil long enough and hard enough and let the "steam" out through a cracked seal, that you would end up with a solidified mass.

That should do it ...

The liquid does n't seem to get solid enough to do that, seems to slip under extreme loads...

well then ..other theoretical scenario;

You manage to "beach" the Syncro midriff on a big boulder, rear wheels just scraping the surface, fronts suspended high in the air. With lots of brute force and ignorance you spin all fours on full revs, the van inches slighly forward, front wheels smack down onto a hard, tractable surface, come to an aprubt stop because the front overhang has just buried itself while rears still spin at full throttle and van is still stuck on boulder...almighty crack ...vc sheared ...

yes, no ...maybe?

Under normal circumstances viscosity drops with temperature, doesn't get thicker...

If the oil level in the diff as a whole drops due to leakage, then all sorts of things can happen, including sheared drive splines.

All the syncro's that I have owned that have not been looked after very well have all had low oil levels in the front diff. The front box gets quite hot quite quickly on longer trips, but remains cool when I have driven without prop shaft. I think that the heat is coming from the VC. If the oil level is low then it's gonna cook its self as the heat cannot transfer out of the vc and escape. I still dont understand why this makes the VC stiff, as hot oil is thinner. I do remember reading that the VC works by a shearing action??? Also would not the tolerances between the plates reduce when hot. My syncro has a tight vc which is ideal for off roading ice and snow etc, best soloution is the de-coupler as far as I can see. I do agree with peasant though that 4 wheel drive is generally safer, but taking the strain out of the system, even if it were just for motorway trips has to be a good thing. Looks like there is some good money to be made in VC rebuilding. It is unlikley that the silicon fluid is gonna cost hundreds of pounds for such a small quantity.

These 'de-coupler ' things are they available, can the be added, is it a difficult job?

syncrosimon wrote:It is unlikley that the silicon fluid is gonna cost hundreds of pounds for such a small quantity.

It wouldn't suprise me if it did cost alot! Why does no-one in Germany appear to be offering this? Perhaps the famous TUV? Or not as easy as we'd like to think...?

Tiny percentages in the amount of liquid effect the performance of the VC... and surely the only way to test it properly is either in a vehicle, or on a test stand (such as Dorfbrunnen have with their original SDP test stand - which allows them to produce the infamous VC graph).

I guess on the plus side, many new vehicles, such as the XC90, Fiat Panda, Grand Cherokkee... work on a VC system, could well be the same liquid...

http://www.v70xc.com/forums/showthread.php?p=55676

...sound familiar:?

So the market for VC refurbs could well extend beyond Syncros...

The VC works on a principal that when the plates are a a different speed(wheel spin)the fluid will heat up causing expansion and thickening causing drive to the front.A VC is also stiff when cold so only sliping when at normal running temp.
IF it whos that simple i would be refurbing them.
U can get the fluid but u need to purchase a 50gallon drum at a time and this costs a furtune.Then u also come accross damage to different parts in the VC that need replacing.

I still dont understand why this makes the VC stiff, as hot oil is thinner.

Exactly!

HarryMann wrote: It took quite a few years to find the trick to getting these things to act as an auto-lock clutch in a controlled and predictable way. Without the slots and holes and other clever tricks, you just get a steadily decreasing torque Vs speed curve as fluid viscosity, even silicone fluid, decreases in the normal way with temperature - quite the opposite of what you really need.

Natural fluid shearing action betwen alternate plates, stimulated by the holes, slots and burrs raises the temperature...
As the temp goes up, so does the pressure, the air-gap diminishes as it diffuses into the oil, probably as minute bubbles, though other forms of diffusion are possible.
Plates deform fractionally, not in straight pairing modes, but in 'sets' and as the gaps between those decrease, shear forces increase rapidly (despite the viscosity reducing).
Transmitted torque then increases rapidly slowing relative motion...

This happens extremely quickly and as the temp and pressure then drop again, the whole action cycles.

There are obviously much more complex ramifications at each of those stages and esepcially the part that non-homegenous air diffusion plays within the various fluid areas.

Something to note: When the VC is locked and trasmitting high torque, the internal temp and of course pressure doesn't keep increasing... as a cycling action sets in (stability)

So what is probably the worst condition for a VC of this type, is a relative motion betwen the shafts just insufficient to trigger pessure-rise and auto-locking, where temps are high for long periods, but not quite high enough.

This can be caused by too high differential tyre sizes, as well as previous internal wear, fluid damage or loss or incorrect filling, preventing or delaying proper and timely auto-locking.

VC locking characteristics as well as power transmission rating can be tuned by a wide variety of parameters, including number of plates, their diameter, gaps, hole arrangements and surface finish, overall size and volume, air gap and fluid viscosity as well as heat transfer characteristcs of the housing.

To contain the extreme pressures generated, they are heavily built as well as having to have very tight seals - which are responsible for a good deal of the initial static and low-speed torque transmitted (30~50 N-m)

They are often used in combination with other clutching devices, both mechanical and electro-mechanical, across axles as LSD's as well as between axles. Some quite complex mechanical servo and electro-mechanical control devices have been incorporated to garner more control but the basic auto-locking behaviour from modifying the friction plates and plate group design acts as a quite remarkable free-wheeling clutch with speed sensitive auto-locking.

As Russel says, refurbing a VC for reliable use over an equivalent mileage to the original product is not a facile or cheap task - despite the apparent simplicity of the device.

On silicone fluids...

They do have a lower viscosity ~ temperature change than many other fluids, but it's still a negative slope (i.e. its a Newtonian fluid)

They have high resistance to temperature degradation

They are particularly poor lubricants (low lubricity)

Also, although the plates in a VC get extremely close at times, they don't actually touch under normal operation (until things go wrong!)

some links I've found

http://autospeed.drive.com.au/cms/A_108266/article.html

http://www.vanagon.com/syncros/technica ... index.html

If you refill a VC so that it is full it will burst its seals. Never refill so it is full. (The proper percent air is 84% to 86% according to an estimate from another expert familiar with Viscous Couplings but this percent cannot be confirmed yet.)

there is a patent for a dynamic VC valve that works on magnetised particles with an electromagnet that "plugs" a resevoir
http://www.patentstorm.us/patents/62699 ... ption.html
I take it the outer VC casing spins? which would make attatching anything difficult

an interesting quote from the subaru manual that implies plate to plate contact for the transfer of drive
http://www.clubwrx.net/forums/showthread.php?t=27819

Silicone oil is heated and expanded as differential action continues. This causes the pressure of
air inside the viscous coupling to increase and the pressure of oil between plates to decrease. As
a result, the inner and outer plates are pushed together. This direct plate-to-plate contact causes
a non-viscous operation to occur, and this phenomenon is called “hump"|.

In reply to karlT's question about the de-coupler.. they are available from Cj at syncrospares, or many european syncrohouses. I dont think they are very difficult to fit, once the box is out it's more or less a bolt off of the old gearbox nose, the bit where the prop shaft exits to go forwards. And a bolt on of the new one. Most kits come with all the piping to take the vacuum control to the dash where it fits in the missing center location. You can see the green plastic lens of the engaged light which is already there. Then you can have a row of three little green lights when you are off-roading. VW designed this decoupler to be used before they perfected the viscous coupling, so it's effectively an original equipment fit.
I am gonna fit one to mine, then the stiff VC will be a bonus, no more worries about damaged CV joints, props and diffs. Of course a properly working VC would also be fine, but I like the thought of that extra button.

PS work on my van is finally about to start. Full respray, camping interior including petrol night heater and compressor fridge, rebuilt 3rd gear syncromesh and decoupler. CJ has already fitted a brand new VW 2.1 dj engine as I bought it without an engine. It probably wont be ready till next year. I cant wait, it has sat on the drive for too long.

http://autospeed.drive.com.au/cms/A_108266/article.html

http://www.vanagon.com/syncros/technica ... index.html

If you refill a VC so that it is full it will burst its seals. Never refill so it is full. (The proper percent air is 84% to 86% according to an estimate from another expert familiar with Viscous Couplings but this percent cannot be confirmed yet.)

A lot of that vanagon.com and syncro.org stuff is wrong

The fill-factor for our VCs is not about 85%, AFAIK, but 100% fill or too close to it fill would of course cause it to burst.

Also, there are constant references on the web about VC's as working due to thickness and viscosity rise with temperature - this is not their mode of operation. Polyorganosiloxanes exhibit a continuing decrease in viscosity with temperature..

Thickness/viscosity does rise though with age as the fluid gets damaged by metallic particles and oxidation, although later developments in VC fluids include additives to counter this. I don't know whether the original SDP fluid was this advanced, but I think not.

Direct plate-to-plate metallic contact is not crucial to torque transfer and torque 'hump' onset, from my understanding

and from that same forum a later quote agrees, that this is pure speculation...

I understand what you are saying about the center unit, however in a traditional viscous diff the plates do not touch. If ours do it is news to me.

Subaru is the only organization that really knows what's in there and how it is supposed to work.