GrumpyGel
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and surely the greater the stress, the higher the temp at which the diff operates.
Lawn cut - still no nearer to 4WD!
VCUs - The sweet spot
- Thread starter dfossil
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Lawn cut - still no nearer to 4WD!
jamesmartin
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which is widely effected by the other factors so not a good diagnostic tool, best test is feeling how stiff vcu is
Hippo
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Don't have the results to hand at the moment but in general I found there was an upper temp with and without the vcu being fitted. Lower of the 2 being when the vcu wasn't fitted. Ambient temp makes a difference at the start of the trip but after several miles it didn't make that much of a change. I took measurements daily over a period of time on the same trip to work. Also a few longer trips of 50+ miles.
jamesmartin
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depends on oil viscosity too
Diff temperature is relative to work being done. The higher work done (torque transfer) the higher the temperature. LR new this, this is why the cooled the IRD. The IRD is taking huge amounts of torque on small diameter gears and bearings. This pushes oil temperature up unless controlled by the cooler. The rear diff, having a larger case and lower torque transfer, dissipates it's heat without the need for extra cooling. However it's heat will rise above ambient as it's taking torque. I, like Hippo have measured this myself.
Here is a (poor) scan of a page in my F.A.G. bearing catalogue - general case I know but suggests normally bearings are good up to 120C after which they have several grades of heat treatment up to 300C
Attachments
For my Discovery, I have a full systems diagnostic doodad that allows my to log and graph the ABS sensor feedback, straight to my smartphone or tablet. This would be handy for providing information on wheel speeds front and back, under heavy braking. Sadly this very clever doodad won't work on the Freelander 1. It doesn't even recognise it's in the data port, so I can't test it myself.
B34R
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If I remember correctly, all of the Scooby AWD systems use a visco coupling in the gearbox centre diff (and are just as prone to exploding the bearings as the freelander is!)
Everything upto the WRX imprezas use viscous and it was only the heavier cars and the STi models that got 4wd and a true centre diff.
Some of the VW group 4wd systems also use it, as most of the reading about visco fluids I've been doing are about rebuilding the VW VCU. (Theirs was serviceable
)The viscous coupling is actually a great idea, as long as it's serviced properly, exploding transmission components are not confined to freelanders - in fact, the Subaru boys tend to get it a bit worse than we do - their centre coupling dies and takes most of the gearbox bearings with it. Most of them need an entire gearbox rebuild by 70k as you can't get at the VCU to change it/service it without pulling the box.
B34R
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Yeah, to be honest I don't think it's something that you'll ever really see in 99.99% of cases though - which I think is what hippo might have been getting at.
The brakes on a freelander are actually pretty good and most of us drive them fairly respectfully (sometimes out of lack of choice, we don't like being seasick.)
To get the effect I was describing you need special circumstances - like mud on the road, a patch of ice or snow under low braking force.
On snow it'd be a pretty cool thing to try and see the effect of - if we get any this year I may attempt it in the works car park and see what happens.
My example mainly centres around the idea of when you're driving on snow for example? You tend to trundle about slowly at 20 mph and only ever apply the brakes at what, 20%?
In a normal FWD car it'll quite frequently lock up one or both of the front wheels as you hit slippery/compacted snow even though you're only braking lightly.
(If we assume that the freelander has a standard 60/40 bias on the brakes, if you apply 20% brake force to the pedal you get 12% to the front wheels and 8% to the rear wheels - a 50% difference which is enough on loose surfaces to easily break the front wheels away in low traction scenarios without locking the back up. I accept this is a rare scenario, it's purely as an example. It's also massively more common if you're offroading on wet mud.)
In a freelander it's my theory (and the PDF sae paper alludes to it) that something special happens at this point - the VCU will try to stop differences in wheel rotations which will either redistribute the low braking force from that wheel to the other three wheels and attempt to keep it spinning. The whole point of the VCU was to keep both axles spinning at the same (or similar) speed under acceleration, but a knock on effect of that is that it also applies under deceleration.
In real world daily driving, you won't notice any effect as the point of sticktion on the tyres is VERY high on tarmac, so the braking forces involved generally mean when you go to lock up the front wheels you REALLY mean it - and the rear wheels are milliseconds behind in locking up (unless you have ABS etc etc.) I'd love to see the scenario involved where you mentioned that you've had the front wheels locked - the paper linked previously talked about "sausages" of viscous fluid forming and rolling between the plates. I wonder if this happens in the VCU?
B34R
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The '03 WRX engine has a peak torque just shy of 300NM
1st gear is 3.454
Final drive is 4.111
Which means at the centre diff they're popping about 4,259 NM of torque at the diff (I think.) Sort of similar to a freelander in first gear but remember that the 1st gear on a freelander is FAR shorter and the car is far heavier (and I bet the rolling radius of the wheels are smaller amplifying the torque laid down on the road.)
Our VCUs end up with far less torque going through them iirc because the propshaft is stepped up? [EDIT: Not quite I think, looking at this diagram the VCU sits on the back of the output shaft, so it's spinning at a slightly lower torque, probably similar to freelander kind of specs, or not far off.]
And yeah, their VCU is right inside the case at the back:
B34R
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Yeah, they have very non linear power curves on the higher performance models too - they'll jump 200hp in a matter of 250 rpm which must be pretty traumatic on the drive train.
Interestingly looking at the diagram above they're set up identically to a freelander which is that they're FWD with a viscous coupling off to the back...
GrumpyGel
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That's an interesting cut away - it shows it has a VCU (9) AND a Center Diff (10)!
Does that mean it is actually a Limited Slip Diff rather than just a VCU?
I think they must use the VCUs differently in different models because I wasn't aware that they had a conventional diff to the rear - just the coupling, and that the VCU could be disengaged by pulling a fuse - ie there must be an electrically actuated clutch between it and the gearbox/front diff.
Does that mean it is actually a Limited Slip Diff rather than just a VCU?
I think they must use the VCUs differently in different models because I wasn't aware that they had a conventional diff to the rear - just the coupling, and that the VCU could be disengaged by pulling a fuse - ie there must be an electrically actuated clutch between it and the gearbox/front diff.
jamesmartin
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in a rr is an automatic locking diff, but because of the drag in a vcu imparts similar stress to a limited diff till it locks
B34R
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Ugh, really need to get the 4wd back together on mine, with all this crap weather it's behaving like a dog on a polished floor.
*scrabblescrabblescrabble*
I'm amazed at how easily the front tyres break away from the lights - though k series and budget tyres combined make it a bit frisky.
*scrabblescrabblescrabble*
I'm amazed at how easily the front tyres break away from the lights - though k series and budget tyres combined make it a bit frisky.
Hippo
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We've been told in the past by so called eggspurts 70 degrees is a marker point where the vcu has been over heated. I assume it's far hotter inside and this is the temp we feel outside. This would be the dissipated temp created by the sheering process. We can't prove this. Looking at the scooby setup their vcu is located in the gearbox which is a hotter environment, so presumably operating in a location which has a higher ambient temp.
The VCU in the Scooby works much like the VCU controlled centre diff in the RR P38. It's basic function is to limit the amount of slip in the diff. This gives enough slip so stop axle wind up, while giving drive to each axle. These VCU controlled diffs don't take and don't need to take as much torque as the Freelander type. I only know of a few vehicles with a VCU in line with the prop shafts. The Freelander, obviously. Then there's the VW Synchro system used in some earlier VW camper based vehicles. These VW VCUs were servicable units. The Renault Scenic RX4 uses an almost identical system to the Freelander except it's VCU is bolted to the front of its rear diff. The VCU can also be found inside some rear diffs, acting as a LSD.
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