Viscous Couplings...in WORDS!

[Zak33]

I love a challenge and this is my challenge to myself.... to explain in WORDS with no piccies,how a visocus coupling works...

it can be in an auto box or a limited slip diff, or the centre diff in a 4x4 car.

Lets take a fluid....lets make it from a silicon compound. Lets get the dudes in the Laboratory to make it thin enough to flow, BUT lets give it a special property.....a SHEARING PROPERTY

Shearing.....(pronounced SHEERING)..the ability for a fluid thicken if its pulled in two direction rapidly, in close proximity. So either end of a bowl of it aint interested, BUT if you were to put two metal rulers into a bowl of it, holding them really close together but not touching , (like holding your hands together,but a millimetre apart )and then SLIDE THEM in different diections while maintaining a small gap between their surfaces.....(like when you rub your hands together to keep them warm, but not quite touching).......the silicon fluid thickens and slows them down....

Right....imagine a baked bean tin.......imagine the lid of the baked bean tin...cut it off, take the beans out and wash that tin. Now the lid of that tin....cut grooves in it, nice and neat....lots of holes and grooves....all equal and symetrical.

Now make 15 of those lids just like that one....1 bean tin, 15 lids....all grooved....and now drill a hole down the centre of them all.....right down the middle.

Now take 8 of the lids...and stick them inside the tin.....at perfect equal distance....from bottom to top.....8 lids, all perfectly fitted.....with grooves in......and each fitted in perfect ditances apart....

Now.....cut that tin perfectly in HALF....down its length so you have 2 cross sections....and you can see all the levels of these lids, plates......dont forget there are holes perfectly down the centre of these..

Now...take the other 7 lids and put them onto a metal rod, (a drive shaft) Put them onto the end of it and stick them to that drive shaft so that they are spaced pefectly to fit between the plates inside those half bean cans....

Place that shaft and its plates into one half of that bean can....and now put the other half of the bean can on top, seal it up and now drill a little hole, fill it with Silicon fluid, and put a bung in.

We now have....a bean can with plates in it, and in between those plates are plates connected to a drive shaft that sticks out of the end of the bean can. In between all those plates, with their holesand grooves, is silicon fluid....that doesn't like to be "sliced" or sheared.

Lets weld a drive shaft to the bottom of the bean can....the drive shaft out of the TOP is connected to the other 7 plates, and this new one is conected to 8 inside the can.

Lets put it in the middle of a 4 wheel drive car......as the centre differential.

When we pull away SLOWLY the front wheels pull away and the back follow....the 2 drive shafts turn at the same speed and the silicon fluid does sod all.

When we HAMMER the gas in 1st on a wet road...the front wheels slip...they spin up...

the back doesn't..no power to it....YET...but cos the front shaft is spinning and the back one ISNT..the 15 plates are having a party

7 of them are spinning their arse off, and 8 aint...and the silicon is being SHEARED...and it thickens...FAST....

and the gaps in the plates is full of thickening silicon...and it sets like treacle and the disks connected to the rear shaft start to turn inside cos they are nearly GLUED to the front ones now..its turned like CONCRETE in there..all those grooves and holes and slots are bunged up with thick jelly and its getting thicker...

and the back wheels get power!

and to themore the two ends vary in wheel spin the quicker it thicken...and as soon as they equalise the 7 plates spin the same speed as the 8 and the silicon goes all thin all of a sudden...and we're back to 2 wheel drive....



Hows that?

[TiG]

Okay I understand it now, took several re-reads - the thing that I find most shocking is the fact that a 4wd is infact just 2wd while driving in a straight line.

Is this the norm?, or is this just one of the ways of doing diffs?

TiG

[Knoxville]

wow, nice post dude

thats some damn complicated stuff took a few re-reads but i think i've got it now, always wondered how they worked

[Tumble]

Mine's 4x4 all the time. Clunky old mechanical centre diff in there. Viscous couplings appear in things like Range Rovers & Freelanders. Fancy technical cars like that

[sdp]

That's a good explanation, done well

A fluid clutch works in a similar way. I only know a little about the old types but you have an impeller and a turbine with a tiny gap inbetween. There's oil (might be something more advanced now) in the casing that holds the impeller and turbine and as the engine turns the impeller oil gets flung across to the turbine. At low engine speeds it's not enough force to turn the turbine/driveshaft but as engine speed increases the force from the oil being flung increases and the turbine starts spinning slowly, and thus drives the driveshaft. At high revs the 'slip' between the impeller and turbine (i.e. difference in rotation speed) is around 1 - 2%.

Probably wrong about a few details, and I'm not sure if it'd be called a 'fluid coupling' but that's roughly it as I recall.

[3Dfx]

...

wtf is he on about?

[Zak33]

Originally posted by TiG
Okay I understand it now, took several re-reads - the thing that I find most shocking is the fact that a 4wd is infact just 2wd while driving in a straight line.

Is this the norm?, or is this just one of the ways of doing diffs?

TiG

Well in SOME cars, their is a set of mechanical gears/cogs to set a rate of torque from front to back.....so you can get a rear wheel drive or a front wheel drive bias Good examples are Escort Cosworths that were rear wheel drive bias. But cars like Vauxhall Calibra 4x4 use what I described above...and as Tumble says, so do some newer 4x4's like Discos. When on a motorway at 80mph you dont need 4x4...so why turn a rear diff?

BUT for most modern performance family hatchbacks, it uses that setup I have described....but not a Bean tin

However....let me allay your fears.....

Imagine a GEARING that allowed the PROP SHAFT, running front to back, with the Viscous diff IN IT, to be TURNING 3 times as fast as the wheels, and then the rear diff knocks the road speed back to normal 1:1 ratio..its more sensitive


Now....when the front wheel spins only 1 third of a turn of slip, the centre diff has turned once already....

by the time your front wheels have even THOUGHT about spinning up you are getting torque TRANSFERED away to the back wheels...


SO.,....while hooning itaround a roundabout and you have too much juice, the front starts to drift wide....and in doing so is travelling a lot further than the rear.....because the viscous diff is spinning at multiple times the wheel speed it's already reacting...is LIVE...and it works a treat.

HOWEVER...for stability under braking, when ABS may allow wheels to be freed off and a car is better as a 2wd in most cases, an electronic clutch dissengages the rear wheels from the action.

ALSO.....imagine if you could effect the bias to the rear wheels.....maybe put silicon in that only sheared to a vertain percentage....you can then restrict the bias that ever goes to the rear wheels.

BAD NEWS THOUGH.....what happens when your half worn tyres (all 4 are worn nealry the same) geta puncture in ONE and its not fixable....

NEW TYRE......*drum roll*

THIS TYRE has a larger rolling circumferance than the other three

The diff is always working.....it wears faster
Thats why 4x4 performance cars are SUPPOSED to have ALL 4 tyres in one go

Scary huh?

[Knoxville]

Originally posted by Zak33
BAD NEWS THOUGH.....what happens when your half worn tyres (all 4 are worn nealry the same) geta puncture in ONE and its not fixable....

NEW TYRE......*drum roll*

THIS TYRE has a larger rolling circumferance than the other three

The diff is always working.....it wears faster
Thats why 4x4 performance cars are SUPPOSED to have ALL 4 tyres in one go

Scary huh?

Skylines, evo's and escorts cossies all suffer from this :/

The tires on em also wear from the inside of the wheel outwards for some reason, not sure if its anything to do with this.

[Zak33]

they do suffer from that BUT....

the reason the buggers allwear on theinside is summit else.

To get those cars to do what they do so well (GRIP) they have lots of CAMBER on them....ie the tops lean in and the bottom leans out...

and it wears the tyres more.

You're good Knoxx..really good

[TiG]

The type described above sounds like its a lesser solution?, cheaper some way by the way its described - is this the case is a cheap mans 4wd.

I understand that its a bit more sensitive than i had assumed, the 3:1 ratio makes a lot more sense, but it seems odd to me.

This to me would be indicated a permenant 4wd by that measure?.

Anyway of finding out what the 4wd Octavia I was looking at had in it?.

TiG

[Zak33]

Octa via has exactly what I described.....ie 4x4 only in demand.

Its most usefull for pulling caravans, and it wasn't right for you at all....it most definately wouldn't have helped in recent situations.

Permanent 4x4 involves both a mechanical midle diff AND a viscous coupling built within it (or other form of limited slip device)

The Escort (and Sierra) Cosworth 4x4 had mechanical difs, as did Integrales and Evos and Subarus.....they DO have a viscous coupling in there too, to help keep the balance right.

Skyline is a whole nother world....there are more elctronics on that car thanb anyone could even guess at.,....BUT in essense the Skyline has all its diffs, (front rear and centre) controlled by a computer linked to sensors for yaw, weight transfer, power, abs, traction control etc.

Its less pure but so well sorted its a tool

With a mechanical diff in the centre you set an actuall torque bias to from or rear or 50/50

[Richie]

I wrote a large article on the 4x4 system of the Vauxhall Cavalier Turbo about 2 years ago and its still on my website if anyones interested -

http://www.cav-turbo.co.uk/4wd_transfer_box.html (viscous coupling bit near the bottom)

Photos, pics, diagrams, explanations etc....


Good explanation from Zak but pics and diagrams can sometimes help

[TiG]

Thats great Richie, thanks for that very interesting reading and helped a lot.

I dunno what i'd do without a great team of people here - i wouldn't know half as much about cars as i now do....

Thanks
TiG