I'm planning on making a mechanical replacement for the VCU!

Carrington224 said:

This idea would work but you'd need to retain the VCU, if an electromagnetic clutch could be grafted onto the rear prop in front of the VCU you'd have selective 2/4wd

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Or just replace the VCU?

Spacemonkey... said:

Yes, you could butcher an X Trail system to fit. Could be fun...

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Not that easy - was looking into it 6 years ago - easier to replace the complete rear subframe, but then the diff ratios are all wrong, plus the fact that the X-Trail electronic drive control takes feed from the ABS system, yaw & pitch sensors and the accelerator. Be too complicated and hugely expensive.

be cheaper to replace the VCU every year

Yeah, funny that- I replaced my VCU (takes no time) and all is well! I would like to change the diff ratios if I could to give 50:50 drive front:rear but can't see anyone who could do that. But that's all I would do unless I could lower 1st and 2nd gears a bit more.

This issue with the front/rear drive ratio difference. If this leads to constant slipping of the vcu during normal driving this is surely keeping it effectively "warmed". My point is that surely this means the vcu will transfer drive to the rear wheels quicker I the event of momentary loss of traction on the fronts, perhaps on an icy patch on tbe road in the winter months. Maybe witb 50:50 ratio you would suffer a slight delay in this situation......and pay the price......was it bad design.....or that way for a reason?

Well the old Rangies had 50:50 and their VCUs coped well enough, and when they gave up the ghost, they didn't knacker everything else, probably due to the 50:50, or more likely the better design strength of the transfer case over the IRD. The pick up of the VCU is microseconds from what I read so not really an issue. Whereas it is the 'keeping it warm' from constant slippage bit that knackers it in the long term. Probably. Different size tyres makes it slip even more. Yes, a 'centre diff VCU' will constantly slip but not as much as one with different front/rear ratios which will still slip in a perfectly straight line. That's the way I see it anyway- the less force of slippage you put on the VCU the less chance of unwanted slippage.

Yeh I suppose its the damage it can do once its tightened thats the real issue. Hmmm

The VW syncro van guys managed to develop one. Of course they have the "advantage" of having the syncro located inside a secondary bell housing.

google "vanagon syncro decoupler"

The Mad Hat Man said:

Or just replace the VCU?

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I don't always agree with MHM but this time I do. It's far easier to simply replace the VCU at the first sign of it stiffening!!

any gnu's?

If anyone is still looking for a potentially good way to disengage drive to the rear, have a look at the Steyr Puch converted fiat Panda 4x4 - the old square ones.

My brother in law just bought one for reasons I dont really understand. Anyway, it has a normal 4cylinder transverse engine with the gearbox to one side and the front diff poking out behind as normal for any FWD car, but it also has a crown wheel running off the diff, with an output shaft facing towards the rear of the car. There is then a very small, compact unit to disengage/engage drive to the propshaft and rear axle.
With 4x4 engaged, it has no centre vcu or diff, so only for use on slippery surfaces, and when disengaged, it is FWD only and the rear axle and prop just freewheel.
I have not seen inside this 4x4 selector, but it is very compact, about 8 to 10 inches long and maybe 5 or 6 inches diameter. drive is engaged by a simple lever in the middle, which goes up to a hand lever by the handbrake in the cabin, but could probably be operated by a decent solenoid or servo motor. one end is a flange to fit to the Panda prop, the other end I dont know...I assume a drive gear from the front diff. It is only 4 bolts, so if one could machine a drive flange to fit this end too, it could be fitted inline on the freelanders propshaft before the VCU, allowing drive to the vcu to be disconnected giving fwd only. It would need a support bracket, but I foresee no real problems there - The only question is how much torque it will cope with. It has no clutch system, so the car needs to be stationary or moving very slowly to engage 4wd.

Not that i know anything about tbis lol, but i do remember a wheeler dealer with an old suzuki sj (i thjnk) and someone had redesigned part of that system that all the seroius of roaders fitted notsure if tbat helps lol, but for the ratios problem what about somesortof overdrive system like was on my old spitfire?

Surely the ratio problem can be overcome by different tyre sizes front to back? And how about using a p.t.o type unit as a lorry might have fitted to engage/ disengage propshaft? Either with a small compressor to operate it, or possibly just a lever to move the fork?

surely a better way would be to replace the internal gloop with something that did not get too thick after time so the vcu keeps working

latev8 said:

Surely the ratio problem can be overcome by different tyre sizes front to back? And how about using a p.t.o type unit as a lorry might have fitted to engage/ disengage propshaft? Either with a small compressor to operate it, or possibly just a lever to move the fork?

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Or smthn like power take off that tractors use to power trailers like the hay baler etc that can be engaged and disengaged, basically u need a gesrbox working from a solenoid that has 1 gear and neutral whatever ratio..

Rule1 it has to be cost effective. No point in designing a system that costs £2K to impliment - might as well buy lots of replacement VCUs,

Dispute all of the failings if VCU's they are actually pretty good at doin what they are designed to do. And for the price and ease of changing one like for like. I agree with MHM and I will be staying with landrovers original design.

ZDomZ said:

Dispute all of the failings if VCU's they are actually pretty good at doin what they are designed to do.

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What's that then? Grinding down bearings or stripping teeth off gears?

lol I believe that fiat pandas use exactly the same type of design as I want t do for the freelander so I could do to get hold of one of them and have a look. As for the ratio issue it shouldn't make any difference to the ratios as I am just trying to figure out a way of mechanically joining the front and rear props as and when required so no change to the gearing at all

There's a thread on here somewhere about the fluid which I think LR use in suspension or something.
It contains suspended magnetic particles which align in the presence of EMF, effectively locking the fluid.

I thought that maybe it might work in a VCU alternative, using the counter rotation of the propshafts to generate the EMF, but
1) I don't think the EMF would be strong enough, but an "on/off" switch might work
2) The fluid is very expensive
3) It wears out over time, just like the VCU fluid, except it stops locking (open circuit) rather than stiffens to the point of being effectively locked like the VCU (closed circuit)

GrumpyGel said:

What's that then? Grinding down bearings or stripping teeth off gears?

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No that's a symptom of what happens when people and garages don't look after their drive train properly. Although I have first hand experience of this happening........

When the vcu is working correctly it is perfect for the job, and as has been pointed out the cost of a recon vcu vs the cost and time engineering an alternative....

There is no comparison as far as I am concerned. I also prefer the way the 4wd kicks in when you need extra traction without you needing to do anything. ie the confidence that you can pull away from a wet and greasy junction without issue, or where that tractor has left mud all over the road.. Even the difference of grip levels when cornering or going around a roundabout is reassuring and improves the drivability.