Freelander 1 Rear wheel 'bounces' in tight turns

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Joe_H said:
Hi Gemsdad - I would caution getting a recon vcu from Portugal (depending on the supplier - and I only know of one) - I have one and have just received a Bell engineering one as the Portuguese one is simply too loose !. It will only engage 4x4 with excessive wheel spin - I had the first replaced and the second is just the same !.
Go for a quality one ! (ie Bell Engineering !)
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Hi Joe,i have no need for a vcu,i was just passing on details of someone who has recently bought one from Portugal,and he seems happy with his vcu,thanks anyway.
 
gemsdad said:
But if you are going to get a superior product,then it is worth the extra carriage costs.Or the exchange item.
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Again Gemsdad, absolutely agreed ! :) - that is exactly what I did and no regrets at all. (From Bell)
The only other thing to contemplate is a brand new unit depending on cost as no core surcharge is payable and also the cost of sending the core back (which can be rather expensive) is also not needed. When buying from the UK to a Eurozone country, for me it would be Bell - OR a new unit if the price was right.
I went for a recon Bell unit with bearings - Excellent ! - the difference between the Portuguese recon and the BELL Engineering recon was night and day - the Bell unit is superb.
Joe
 
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Just read on Bell's website (I bet it has already been covered) that ' a faulty viscous coupling unit (VCU), which should be changed at the same time as the IRD.'
Is this true?
 
The Mad Hat Man said:
I think you need to read up on the VCU operation. It has nothing to do with electromagnets. Have you seen wires coming out of yours?
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no haha what I meant was that in some post someone had mentioned replacing the viscous with a materal that hardens when energised and softens when not
 
Zejmjien said:
Just read on Bell's website (I bet it has already been covered) that ' a faulty viscous coupling unit (VCU), which should be changed at the same time as the IRD.'
Is this true?
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Hi, If there is a problem with the IRD, then yes, the VCU should be replaced when the recon IRD is fitted. However, you have not reported any IRD issues, but rather VCU issues so you only need to change the vcu and bearings (from the symptoms you describe)
Joe
 
Zejmjien said:
no haha what I meant was that in some post someone had mentioned replacing the viscous with a materal that hardens when energised and softens when not
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The vcu contains - basically - a non Newtonian fluid (a silicon base in this case) that has a 'relatively' low viscosity when the shear forces are low - rapidly increasing to a very high viscosity when a rising shear force is exerted. The locking ability of the VCU is hence directly related to the delta of the shear force acting on the fluid between the plates.
Custard is a good example ;-) - fill a swimming pool with it and you can run or walk rapidly across it no problem, however, walk slowly and you will sink :) ;) (True !)
Joe
 
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Joe_H said:
The vcu containes - basically - a non Newtonian fluid (a silicon base in this case) that has a 'relatively' low viscosity when the shear forces are low - rapidly increasing to a very high viscosity when a shear force is exerted. The locking ability of the vcu is hence related directly to the delta of the shear force acting on the fluid between the plates.
Custard is a good example ;-) - fill a swimming pool with it and you can run or walk rapidly across it no problem, however, walk slowly and you will sink :) ;) (True !)
Joe
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now that is a good explanation haha
 
just checked out the Brainiac experiment for walking on custard!!! had to see it....and its true!!
that is a great explanation! - thanks!!...
 
Trailstar said:
just checked out the Brainiac experiment for walking on custard!!! had to see it....and its true!!
that is a great explanation! - thanks!!...
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lol ;) , yes, the Brainiac experiment with John Tickle it is one of the best demonstrations of a 'non Newtonian fluid' to really see what happens... I suppose we could all fill our vcu's with custard, cornstarch or even tomato ketchup :) :)
The VCU is such a relatively simple device in design that takes advantage of the remarkable property of these fluids.. all good stuff !

Also, as a guide to how a vcu fails, as the fluid breaks down over time. then the fluid in the unit increases in viscosity causing the unit to actually change to mechanically be similar to a Newtonian fluid (A fluid with a constant viscosity) hence the vcu effectively never 'slips' properly causing wind up and eventual overload of the ird or rear diff - or indeed - both !.

In a normal working vcu - when normal driving is occurring, you can consider the shear force is in proportion (but constant AND very low) in relation to the very slight difference in ratios between the front and rear of the vehicle. Here the vcu is always slipping slowly but ! - as it's 'delta' is effectively non existent - the viscosity (low) remains the same and the vehicle is effectively front wheel drive.
In the event of a difference in speed front to rear occurring , (during wheel spin or loss of traction) then the delta suddenly is huge causing the lock up via the increase in viscosity, this will also now remain constant until the shear delta reduces again and the vcu allows the normal limited slippage. Remember it is the delta of the shear forces (the RATE of change) that controls the viscosity. It is not a change caused by heat etc. It is a physical property of these kinds of fluids. The reason the viscosity remains high in the event of slippage is that the delta causes the lock up, the delta then drops and the vcu starts to free itself, but, in the event of a remaining difference of speed between front and rear of the vcu it's delta again ramps up causing a lockup - this is a constant cycle but happening so fast as to effectively remain locked until the delta drops off and remains off causing the viscosity to instantly reduce and normal action takes over. So, a constant cycling of viscosity occurs when slippage occurs but too rapidly to sense.
All clever stuff.
Joe
 
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All this talk of VCUs containing non-Newtonian fluid that thickens with shear would be good, if it were true.

The fact is that as the VCU shears it loses its viscosity so allows the axles to move independently of each other more easily.

It is only once the rate of shear is sufficient to heat the fluid to a level that the pressure inside the VCU reaches a tipping point does it increase its viscosity - and at this point the viscosity rises almost immediately to 'hump'/locked mode.

This is born out by dyno results of VCUs, for example...

VCU_Viscocity.jpg


Credit : @The Mad Hat Man https://www.landyzone.co.uk/land-rover/the-definitive-freelander-vcu-testing-thread.171198/

But also many other sources including....

GKN themselves http://www.gkn.com/driveline/our-so...oupling/Documents/datasheets/Viscous-engl.pdf

Society of Automotive Engineers / Volkswagen http://www.easy2design.de/stuff/visco_sae.pdf

As for what characteristics a failing VCU exhibits, I'm not sure if there has been any scientific evaluation of this, but my conclusion is that they get to bloody stiff :)
 
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