Centre viscous couplings

[Chris]

Following on from the manly chest beating topic on then centre difflock dashboard switch recently I was working on SP yesterday when it had one wheel off the ground. I thought I'd give it a turn to see what sort of resistance there was. Now this is going to be hard to quantify I know and is genuinely a question, not the basis for a punch up :angry-cussingargument:

OK, it turns; it certainly didn't spin that's for sure. But with both hands on the wheel, I could turn it fairly easily. I suspect one of my Kids might have been able to turn it with some effort. It was a bit like a typical 80 series handbrake if that gives you an idea

I appreciate that the more it slips, the warmer it becomes etc and fluid dynamics comes in and it all gets a bit scientific, but I am still curious as to just how much drive the VC actually gives. Could it be than mine is stuffed? It's great off road and no different to any of the other 4 cruisers that I have owned in that respect. Sticking CD in when it's slippery makes a HUGE difference. If like me you have stupidly grippy tyres that cannot slip (seriously very hard to spin them) so the VC doesn't really come into play, what actually happens? Does it act as a locked diff and provide equal drive fore and aft? Because I'm not getting that feeling really.

I am genuinely struggling to understand how this works - not the VCc coupling unit, several of you have explained that well and I get it, but I don't understand necessarily what's happening in each situation. My feeling when I rotated the wheel yesterday was that if this was all the drive (in reverse - does that make sense?) that the wheel was getting, then it didn't feel like much for a great big truck.


Thoughts? No joined up writing please.

Chris

 

[adrianr]

I have only heard of them failing by locking up from overuse and they stay locked which is a bit like having your CDL switch on all the time, rather than failing by not working. To be fair I've not gone hunting for that answer. Does it actually take out some of the "looseness" in the drivetrain which is probably less noticeable in an auto than a manual in any case?

If they fail from overuse then I've never intentionally let mine do owt, and press the button at the first indication traction is less than normal road surface. So snow, mud, grass, any kind of loose track and CDL punched in and she's ready to rhumba.

Taking the viscous coupling out completely appeals to me, being able to drift is something i miss from the Pajeros and would be useful (OK maybe more fun than useful but thats my story and i'm sticking to it) in the snow. Doughnuts and figure of eights in a cruiser, gosh getting all dreamy there, Sorry.

 

[pugwash]

i think mine has failed though- and not locked up!

had the front prop off a couple of weeks ago, and had to reverse on and off the driveway to check the gearbox- forgot to egage teh CDL and the car would move on ly very veyr slowly- and not at all if there was a small slope. this suggests that the VCD wasn't doing what is't supposed= ie detect speed differential between the front and rear props and lock up!

Sure there must be away of draining out the old fluid and putting new in!

 

[Chris]

They look pretty solid to me. Small, very heavy and sort of stamped together.
I have a spare one, perhaps I could have a play with it.

Chris

 

[Cptsideways]

They should work on the Non Newtonian Fluid principle & there are effectively thousands of little friction plates seperated by goo.

This one was seized solid pretty much, the goo inside had solidified

 

[Chris]

That's proper handy that is. But the question still remains as to just how much drive they actually provide.

Chris

 

[Dave Docwra]

only having one wheel off the ground & turning that one wheel only will not turn the prop thats connected to the [strike:38klg2mq]knee[/strike:38klg2mq] CD...but good news your diff is ok..

 

[Cptsideways]

Generally they provide an amount of torque per revolution over rotational speed. I'd say a fair amount going by the size of it compared to viscous lsd's I've seen.

As an example an Mx5 viscous diff (1/4 the size)

Measured at the wheel hub bolt 17ft 1b over 90deg in 10 seconds, so your torque wrench should click if you try & do it faster than that.

It would not surprise me if this required 80ft lb to do the same, I know they wont do a brake roller test at mot time if working.

 

[Chris]

Dave, It most definitely turned the prop shaft. I watched it go round. Can't remember if it was in neutral on the transfer box. Certainly was in N not P on the auto box.

Chris

 

[Dave Docwra]

Ignore me if it was not in P.. Bob

 

[Chris]

Okey dokey. Gotcha.

C