Tight viscous coupling

[silverbullet]

The net result is the same. I find this tyre pressure/rolling radius/vc preload matter intriguing.
You say tomato, I say "it's a citrus fruit, not a vegetable"

[Ralf85]

Have you noticed how many hits this has had compared to everything else on the site. Could it be the raunchy subject title?

Patrick

[syncropaddy]

Have you noticed how many hits this has had compared to everything else on the site. Could it be the raunchy subject title?

Patrick

Well when you think about it, its words like 'tight', 'coupling' and my avatar that does it. Nobody gives a monkey's about de couplers or VCs

[HarryMann]

1) I think my glass half-full comment might have been taken out of context, in fact, was so wishy washy as to be meaningless - i.e. I think I knew what I meant at the time (but not now)

2) I seem to remember discovering when doing a TRC chart years ago that tyre pressure does not in fact make a busting lot of difference to TRC (within normal limits), as it doesn't in fact stretch the carcass peripherally, it just means that it deforms into a rolling 'S' shape less, retains its circularity better, and therefore produces less rolling friction (but Revs per mile doesn't change)

[syncropaddy]

No Andrew, it can't lock in hump mode really, and of course the beauty is the natural feedback system, as soon as it locks, it cools, and comes out of lock, so the mechanism is one of rapid cycling, holding the device at a nominal torque to lock sufficiently - almost self-regulating

'Lock up' is the wrong term here Clive. My mistake.

Ideally, a working VC should not, under proper working conditions, transmit anything to the front axle but this isnt going to be the case too often. Typically there will be a very small % of torque transmitted to the front axle and, on a long trip, on a motorway for example, varying degrees of 'service station wind up' will be experienced (a better description than lock up). This will be due to thermal expansion of the fluid, the rate of which will depend on how loose or tight the VC is and the distance/time travelled. On my green Syncro this was very noticeable but on my Multivan Syncro it is hardly noticeable at all. The amount and ferocity of this wind up will depend on a few things. Firstly the speed difference between the plates when driving along. This should be minimal if you are on a motorway but if there is a difference then this is what causes the friction and friction causes heat and heat causes expansion of the fluid and if the fluid is allowed to expand so much as to allow the vacant cavity to be filled 100% then we get into the 'Hump mode' eventually.
Hump mode is interesting because it is caused by hydraulic pressure acting on the plates. The hotter the fluid the more it wants to expand and the more it wants to expand, the greater the hydraulicing effect. Now it must be said that at this point there is no mechanical locking effect inside the VC, only a heat dependant hydraulic lock. This hump mode is 'tuneable' by the amount of fluid in the VC - more fluid = Tighter VC. The difference between a loose VC and a tight VC is the conditions inside the VC and the time it takes to get to Hump Mode.

[silverbullet]

1) I think my glass half-full comment might have been taken out of context, in fact, was so wishy washy as to be meaningless - i.e. I think I knew what I meant at the time (but not now)

2) I seem to remember discovering when doing a TRC chart years ago that tyre pressure does not in fact make a busting lot of difference to TRC (within normal limits), as it doesn't in fact stretch the carcass peripherally, it just means that it deforms into a rolling 'S' shape less, retains its circularity better, and therefore produces less rolling friction (but Revs per mile doesn't change)

1. Well I took it as an oblique "engineers see the world differently" reference. Fine by me.
2. Does that mean that our friend with the tyre pressures/vc post is mistaken or is a walking pace distance covered test between axles not representative of what goes on at "normal" traffic speeds?

[Mudlark]

So coming at this from another tack, could the same VC behave differently in different vans in your opinion ?

The reason i ask is that in my first syncro (Autosleeper hightop) I got an awful lot of scrubbing on the wheels after a bit of a journey; made turning into your tight camping spot ever so interesting and ever so bad for the turf. Found and bought a new VC and the problem was gone; i kept hold of the old VC.

A year or two later we discovered that the Dodoka's VC was failed open iirc, so we fitted my aggressive VC to that but no-one to my knowledge, nor I the odd time I drove it between Bristol and Coney farm ever had a sense of it being a problem VC or experienced the same kind of scuffing i had had in the autosleeper. in fact the dodoka behaved and performed admirably with it!

Never been able to get my head round why this might be ?

[syncropaddy]

So coming at this from another tack, could the same VC behave differently in different vans in your opinion ?

Yes I would think so but Im going to have to sit down and think about that one before I make a definitive statement. I have a feeling that every Syncro that came off the production line probably drove differently to some little degree under certain circumstances.

[syncrosimon]

This is better

Just took the syncro for her MOT this morning, and with the de-coupler was able to test the brakes on the rollers. Picked up a slight imbalance on the rears which a few turns of the adjuster resolved. I like this safety aspect of the de-coupler although it is a pain if your MOT tester is not friendly. I had not picked up the imbalance whilst driving, but could have caused an instability, especially at speed in an emergency stop situation. With all the motorway work I do with the family on board this is reassuring.

[Mudlark]

Yes I would think so but Im going to have to sit down and think about that one before I make a definitive statement.

It was a headscratcher for me; the only 'variable' i could come up with was one of the operating temperature of the front diff; the Autosleepr always had a slight whine on it which in terms of wear could have meant it ran hotter ?

Other than that i drew a blank. But it could have meant that if i had spent some money on having the front diff overhauled i could have saved on the VC !

[silverbullet]

It was a headscratcher for me; the only 'variable' i could come up with was one of the operating temperature of the front diff; the Autosleepr always had a slight whine on it which in terms of wear could have meant it ran hotter ?

I reckon you've nailed it there. If it was whining then it would have run hotter, the bearing loads would have increased a fair bit and that in turn would have pushed the VC up into it's operating range.
Add on the extra load of the Autosleeper fittings, which would have meant a higher duty cycle on the whole drivetrain anyway, so operating temps would be elevated.
Seems reasonable?

[HarryMann]

Well I took it as an oblique "engineers see the world differently" reference. Fine by me.

Ian, perzaxtly as intended

[syncropaddy]

This is better

Just took the syncro for her MOT this morning, and with the de-coupler was able to test the brakes on the rollers. Picked up a slight imbalance on the rears which a few turns of the adjuster resolved. I like this safety aspect of the de-coupler although it is a pain if your MOT tester is not friendly. I had not picked up the imbalance whilst driving, but could have caused an instability, especially at speed in an emergency stop situation. With all the motorway work I do with the family on board this is reassuring.

Speed in a Syncro .......!!

[MoonlightCustoms]

So coming at this from another tack, could the same VC behave differently in different vans in your opinion ?

The reason i ask is that in my first syncro (Autosleeper hightop) I got an awful lot of scrubbing on the wheels after a bit of a journey; made turning into your tight camping spot ever so interesting and ever so bad for the turf. Found and bought a new VC and the problem was gone; i kept hold of the old VC.

A year or two later we discovered that the Dodoka's VC was failed open iirc, so we fitted my aggressive VC to that but no-one to my knowledge, nor I the odd time I drove it between Bristol and Coney farm ever had a sense of it being a problem VC or experienced the same kind of scuffing i had had in the autosleeper. in fact the dodoka behaved and performed admirably with it!

Never been able to get my head round why this might be ?

Would it have any thing to do with weight distrubition? if there is more weight on one end the other will lose grip quicker and bring the vc in sooner?

Just a thought - Ill get my coat

[Mudlark]

I reckon you've nailed it there.

The whine was not excessive; just noticeable at higher cruising speeds. But if this was a significant factor you might have expected the new VC i fitted to have exhibited the same problem?

Would it have any thing to do with weight distrubition?

I have never seen any discussion about how any syncro should be loaded relative to the VC's operation; if that were significant then surely towing would be another factor to consider ?