VCUs - The sweet spot

B34R · Oct 30, 2015

Hippo said:
Sorry but yer theory is wrong. ABS brake pulses wheels to assist directional stability which is it's first point of use, but does help brake sharper due to the on/oft repeating pulse as a secondary element when brake force applied changes from on to pulsing. All 4 wheels are braked when the foot brake is applied. Only skidding wheels are brake pulsed by the abs, on an if and when basis of it happening. ABS is quick to activate if within the correct speed range. This reaction is so quick the prop wouldn't have enough time to slow to increase the differing speed with the other prop.

I think you missed my point entirely - I'm not talking about the point past the point of lockup where the ABS kicks in. (I'm entirely aware of how ABS works!)

My point is that with no centre diff the drivetrain prevents a single wheel or an entire axle getting to the point of locking while the vehicle is at speed.

If you take a model of the drive train, the VCU tries to force the front axle to rotate at the same speed as the rear axle.

This means if you stand on the brakes really hard in a situation that would normally lock both front wheels - you're in a situation where the brakes have stopped the front diff, but the rear wheels still have traction and are turning. In this situation you have Rear (200 RPM) = VCU LOCK = Front (0 RPM)

Which means you end up in a torque fight through the VCU where the rear axle is trying to force the front axle to not keep turning and the locked front axle is trying to bring the rear axle to a complete stop. Which means that when you REALLY stand on the brakes to the point of lockup the VCU is redistributing the brake force through all four wheels.

With me so far?

When you apply this one step further...

Remember a differential is a basic equation of input RPM x 2 = left rpm + right rpm

In a normal FWD car this equals gearbox output RPM x 2 = left wheel + right wheel so when you stand on the brakes and lock up one front wheel there's an escape route for the extra rpms. IE Your engine RPMS will suddenly drop because a wheel is locked so theres a torque escape route.

The freelander VCU drivetrain is a special case though - you cannot lock just one wheel, you'd HAVE to lock one front and one back at the same time to compensate. If you break it down to a mathematical model (and assume that the front and rear axles are the same ratio for the time being) you're in a situation where:

Front Left Wheel RPM + Front Right Wheel RPM = Rear Left Wheel RPM + Rear Right Wheel RPM

So if you're spinning at 200 RPM:

200 + 200 = 200 + 200

So if a front wheel attempts to lock you end up in a situation:

200 + 0 != 200 + 200

And if you go back to the example above trying to lock a single axle you get:

0 + 0 != 200 + 200

In reality it'll allow a certain amount of slip, BUT, it will do it's damndest to prevent a lock OR transmit a sympathetic lock to another axle.
The overall effect of this will be that the drive train tries to eliminate an axle starting to move towards not spinning at the same speed as the others - effectively redistributing any excess brake force from the front axle to the back axle. (I'm betting that's another reason they altered the ratios front to back, to stop the front axle transmitting a complete lock to the back axle in an emergency.)

The freelander is a special case in this - the tratter boys won't see this effect because they have a centre diff which means if you lock an axle you get the same effect as a purely FWD car - the escaping torque goes back to the engine.

GrumpyGel · Oct 30, 2015

jamesmartin said:
yes immediately before failure in the case of bearings

Once again, I disagree. Bearings are chosen with tolerances built in. So if it is designed to operate at (say) 95c there will be a tolerance that allows it to run at up to (say) 200c before rapid deterioration occurs. If it is under stress overload and the oil film between contact surfaces is lost, there will still be oil around the components providing a cooling effect - and in the case of the IRD there is a cooler for the oil connected to the engine's cooling system.

So heat will rise for a period of time before failure occurs - it will not be an "immediate" heat and fail situation.

jamesmartin · Oct 30, 2015

GrumpyGel said:
Once again, I disagree. Bearings are chosen with tolerances built in. So if it is designed to operate at (say) 95c there will be a tolerance that allows it to run at up to (say) 200c before rapid deterioration occurs. If it is under stress overload and the oil film between contact surfaces is lost, there will still be oil around the components providing a cooling effect - and in the case of the IRD there is a cooler for the oil connected to the engine's cooling system.

So heat will rise for a period of time before failure occurs - it will not be an "immediate" heat and fail situation.

checking bearing temps isnt an indication of vcu ,only the speed of box and or having a bearing about to fail, when a bearings failed enough to produce heat it will fail very quickly after and not long before it melts a bearing next to it can be completely unharmed though teeth on any supported gears will be destroyed,one of the ways you can tell if a failure happens while box still has oil is the carbonised surfaces of failed parts

Nodge68 · Oct 30, 2015

@ B34R.
On a Freelander, when a wheel locks, the VCU doesn't tie the ends together like a solid link. It allows some slip, giving a semi solid connection between front and rear. This actually helps reduce locking of the wheels to a degree. However if you lock the front solid, the VCU allows enough slip to prevent the rear locking up too.

Nodge68 · Oct 30, 2015

jamesmartin said:
checking bearing temps isnt an indication of vcu ,only the speed of box and or having a bearing about to fail, when a bearings failed enough to produce heat it will fail very quickly after and not long before it melts a bearing next to it can be completely unharmed though teeth on any supported gears will be destroyed,one of the ways you can tell if a failure happens while box still has oil is the carbonised surfaces of failed parts

When a unit like a diff or transfer box is taking power, an amount of heat is created. However I'm sure you know this already

The thought of temperature checking the rear diff was to see if it's taking excessive torque, like then the VCU starts to stiffen. The theory being, more drag on the VCU = higher torque transfer. Higher torque = higher heat loadings absorbed by the diff. If we can get a large enough sample of diff temps, we might have a guide by which we can say. Diff at X temp after 10 miles at 60, VCU is ok. However if the diff exceeds X temp then the VCU is goosed. It's difficult to use the IRD for this temperature measurement because the cooler will also act as a heater, which I suspect is more often than not.

Alibro · Oct 30, 2015

GrumpyGel said:
Although the GKN doc makes it look like the plates touch to achieve hump mode, I've just read over quite a bit of that SAE doc again and it looks like the plates do not actually need to touch to ramp up the torque. The plates only need to come within 0.1mm distance apart for this to happen.

Are you sure they are perfectly flat? It apears the distances that make a difference are less than 0.1mm - is the colouring on the plates in your pic shaping that was manufactured in?

Once again GKN's doc shows the plates touching in hump mode - but only at the edges of the "inner plates" (I presume these are the ones driven from the central shaft). It states that these inner plates are "axially movable" so presumably they can shift position on the shaft to move closer to the "outer plates" (presumably those driven by the casing) that it describes as "axially fixed".

Interesting stuff!

The disks are perfectly flat, what you can see is shading where there has been some wear. They are free to slide forward and backward in the VCU with the fluid in between as there is around 1cm of free space.

B34R · Oct 30, 2015

Nodge68 said:
@ B34R.
On a Freelander, when a wheel locks, the VCU doesn't tie the ends together like a solid link. It allows some slip, giving a semi solid connection between front and rear. This actually helps reduce locking of the wheels to a degree. However if you lock the front solid, the VCU allows enough slip to prevent the rear locking up too.

Oh yeah, I'm quite happy to admit that it's an in proportion thing, but don't underestimate it.

Theoretically speaking, taking ABS out of the picture for a second.

If you put both feet in to the brake pedal at 20mph in the wet, without ABS, the front wheels will lock first (and as you apply more pressure the back ones will eventually come in.) We've all been there at one point where something unexpected got us... (I'm sure someone will come along in a minute with a holier-than-thou attitude about that statement.)

But, consider this scenario in a freelander, if you go both feet in at 20 mph and the front wheels lock, you're suddenly in a situation where the VCU is slipping at greater than 800 rpm. This will transmit a MASSIVE braking force back to the rear diff and due to newtons laws about equal and opposite reactions will transmit an accelerating force to the front wheels trying to unlock them.

The prop turns at very high rpms even at low speeds and can you imagine it trying to tolerate an 800 RPM slip? It'll transmit the excess brake power into the rear axle.

You might say youll not notice the difference if you're braking that hard anyway, but, its the lesser effect - when you attempt to lock up a single wheel - the slip speeds are immense and the vcu will put massive effort into stopping it.

Hippo · Oct 30, 2015

B34R said:
I think you missed my point entirely - I'm not talking about the point past the point of lockup where the ABS kicks in. (I'm entirely aware of how ABS works!)
Your making what is a very simple design far too complex.

My point is that with no centre diff the drivetrain prevents a single wheel or an entire axle getting to the point of locking while the vehicle is at speed.

If you take a model of the drive train, the VCU tries to force the front axle to rotate at the same speed as the rear axle.
Not strictly true. The vcu has has 2 options: resistive or activated. When resistive it tries to resist both props turning at differeing speeds, but only up to the point of activation when it does make both props turn at the same speed, minus the very ver yquick/small release period before locking up again. the resistive period has a varying resistance linked to the differing speed of the props.

This means if you stand on the brakes really hard in a situation that would normally lock both front wheels - you're in a situation where the brakes have stopped the front diff, but the rear wheels still have traction and are turning. In this situation you have Rear (200 RPM) = VCU LOCK = Front (0 RPM)
No you don't. The FL1 has 4 wheels, each with it's own brake. If you put yer foot on the brake really hard the FL1 will stop, assuming the brakes are in good working order. The time to do this from 65mph demonstrated below. Under this condition you've braked so hard the max force is applied to all brakes. The front wheels dip due to the additional weight on them from braking. They're still turning, as are the rear wheels which have a slightly different brake force applied as there a reduction value in the system to reduce pressure to the rear. The rear wheels will still brake sharply and turn just like the fronts. If on ice then the rear wheels would soon be on the same surface as the front wheel is (or at least was) at speed as you pass over the ice.

Which means you end up in a torque fight through the VCU where the rear axle is trying to force the front axle to not keep turning and the locked front axle is trying to bring the rear axle to a complete stop. Which means that when you REALLY stand on the brakes to the point of lockup the VCU is redistributing the brake force through all four wheels.
Locked front wheels won't happen so this is void. You can only lock wheels like that if on loose or a slippery surface like ice when braking, which the rear wheels would be on too within a few meters of the FL continuing to move forward.

With me so far?
I understand what yer saying but yer looking too far into how you think the concept works. Eggample below:

When you apply this one step further...
You don't need to apply anything other than the brake. I'm sorry but yer looking into this far too much and complicating things. What you haven't taken into account is that abs pulse braking isn't constant pulses. It's enough pulses to bring the wheel back into control over a small period of time which is dependent on a number of factors like brake fade and heat as well as the wheel slowing to allow grip if there is some. That's control of speed for correction of directional stability. The period of pulsing is small. The speed of which the props need to differ to get the vcu to activate is well beyond the time the abs pulses many times to control the wheel so the vcu won't have time to activate.

Remember a differential is a basic equation of input RPM x 2 = left rpm + right rpm

In a normal FWD car this equals gearbox output RPM x 2 = left wheel + right wheel so when you stand on the brakes and lock up one front wheel there's an escape route for the extra rpms. IE Your engine RPMS will suddenly drop because a wheel is locked so theres a torque escape route.

The freelander VCU drivetrain is a special case though - you cannot lock just one wheel, you'd HAVE to lock one front and one back at the same time to compensate. If you break it down to a mathematical model (and assume that the front and rear axles are the same ratio for the time being) you're in a situation where:
This is just theoretical crap and nothing more. Take yer Freelander oft road and film the wheels to see what they do. Me comments stop ere as the rest is a concept based on a theory that is wrong. Sorry.

Front Left Wheel RPM + Front Right Wheel RPM = Rear Left Wheel RPM + Rear Right Wheel RPM

So if you're spinning at 200 RPM:

200 + 200 = 200 + 200

So if a front wheel attempts to lock you end up in a situation:

200 + 0 != 200 + 200

And if you go back to the example above trying to lock a single axle you get:

0 + 0 != 200 + 200

In reality it'll allow a certain amount of slip, BUT, it will do it's damndest to prevent a lock OR transmit a sympathetic lock to another axle.
The overall effect of this will be that the drive train tries to eliminate an axle starting to move towards not spinning at the same speed as the others - effectively redistributing any excess brake force from the front axle to the back axle. (I'm betting that's another reason they altered the ratios front to back, to stop the front axle transmitting a complete lock to the back axle in an emergency.)

The freelander is a special case in this - the tratter boys won't see this effect because they have a centre diff which means if you lock an axle you get the same effect as a purely FWD car - the escaping torque goes back to the engine.

Comments above in red. sorry but yer theory is wrong.
Click the vid to play then click the you tube button to open a new window where they will then play.

B34R · Oct 30, 2015

Hippo said:
Comments above in red. sorry but yer theory is wrong.
Click the vid to play then click the you tube button to open a new window where they will then play.

I'm sorry but the theory is sound.

All cars are biased to lock the front brakes before the rears after all of the safety systems have failed as a last resort. This means that when you jump on the brake pedal hard in low traction the fronts will attempt to lock first. The vehicle does not get type approval without this feature.

In this situation if at 20mph the front propshaft attempts to reach 0 when the rear is still spinning at 800 rpm.

We may have to agree to disagree on this point as the math is sound.

Hippo · Oct 30, 2015

GrumpyGel said:
I think your are right @Hippo but I wouldn't be so categorical in that statement. One thing MHM talks about is that if a front wheel loses traction, the VCU is turning the back wheels (ie the VCU has achieved hump mode) within 1/4 (or was it 1/8) of a wheel rotation - this is a small fraction of a second and there are not many ABS pickups in 1/4 of a wheel turn. As the VCU works of axle differences, the same will/may be true if a wheel locks - ie within 1/4 turn of another wheel it is back spinning before there have been enough ABS pulses for the ABS system to detect a lockup.

I think in fact though you are right because there is slack in drive train that may be taken up (ie allowing more ABS pulses) and VCUs works on axles not wheels - so the ABS would probably handle all (most?) wheel lockups. It may be though that the VCU would bring a locked then ABS pulsed wheel back up to speed more quickly than road grip alone and/or without impairing the handling of the car in that situation. Conversely it may be that the 2 systems working concurrently may confuse/impair to full benefit of either!

There was an interesting discussion a while back about a fella who has a house down in the Alps and spends a lot of time down there. He was asking advice on whether a Freelander would be a good choice to switch to from his Defender. One point that came up was on (sharp) turns where there may be ice on the road. When turning the rear wheels would want to turn more slowly, but the VCU would want to keep them turning at the same speed as the fronts - if the car's rear wheel came across low grip (ie icy) would this result in the rear wheels spinning and inducing oversteer?

The vcu would try to keep both props at the same speed when in resistive mode but fail (and therefore slip a bit) unless the differing speed was fast enough to activate it. At that point it would make both props turn at the same speed but at this point you would have lost grip anyway, spun 1 or more wheels and traction control would have kicked in to try to control wheel spin as well as the vcu characteristic changing from resistive to activated. There's a lot going on in a very small amount of time so it's too complex to image one action is an event triggered by another. It is sort of but it's a combined event of a mixture oif things happening. In simple words it's just the brillance of the FL's clever tc system, coupled with it being light weight and it's ability to select 4x4 automatically when needed. Freelander rules

Hippo · Oct 30, 2015

Nodge68 said:
When a unit like a diff or transfer box is taking power, an amount of heat is created. However I'm sure you know this already
The thought of temperature checking the rear diff was to see if it's taking excessive torque, like then the VCU starts to stiffen. The theory being, more drag on the VCU = higher torque transfer. Higher torque = higher heat loadings absorbed by the diff. If we can get a large enough sample of diff temps, we might have a guide by which we can say. Diff at X temp after 10 miles at 60, VCU is ok. However if the diff exceeds X temp then the VCU is goosed. It's difficult to use the IRD for this temperature measurement because the cooler will also act as a heater, which I suspect is more often than not.

My diff runs hotter with the vcu fitted than when it's not fitted. We did some tests on this some time ago but can't find the fred now.

Hippo · Oct 30, 2015

B34R said:
I'm sorry but the theory is sound.

All cars are biased to lock the front brakes before the rears after all of the safety systems have failed as a last resort. This means that when you jump on the brake pedal hard in low traction the fronts will attempt to lock first. The vehicle does not get type approval without this feature.

In this situation if at 20mph the front propshaft attempts to reach 0 when the rear is still spinning at 800 rpm.

We may have to agree to disagree on this point as the math is sound.

Can you film such an event where you brake very hard and the fronts lock up and the rears continue to turn? The FL applies brake force to all wheels when the foot brake is used. If you lock the front wheels then yer skidding forward. Within 2 meters the rear wheels will be on the same surface. they will skid as well.

But who cares. Under 4 seconds from 65mph int that bad when you think about it.

Nodge68 · Oct 30, 2015

My first Freelander was an early basic 1.8. This didn't have TC or ABS. I did lock the front wheels on a couple of occasions. As far as I could tell, the rear wheels didn't lock. This shows that the VCU slips to a degree, under extreme loads. The front brakes are able to apply much more braking force than the engine can apply acceleration forces. I suspect that the VCU has a maximum torque transfer capability. Under engine drive, this isn't exceeded. Under heavy braking, I must be. If this wasn't the case, all wheels would lock under heavy braking.

Nodge68 · Oct 30, 2015

Double post

Hippo · Oct 30, 2015

Nodge68 said:
My first Freelander was an early basic 1.8. This didn't have TC or ABS. I did lock the front wheels on a couple of occasions. As far as I could tell, the rear wheels didn't lock. This shows that the VCU slips to a degree, under extreme loads. The front brakes are able to apply much more braking force than the engine can apply acceleration forces. I suspect that the VCU has a maximum torque transfer capability. Under engine drive, this isn't exceeded. Under heavy braking, I must be. If this wasn't the case, all wheels would lock under heavy braking.

It wouldn't surprise me if the rears locked at the same time as the fronts when on tarmac. The rear being lighter so less footprint on tyres. Especially when the fronts dip and take more of the weight and no tc.

Nodge68 · Oct 30, 2015

Hippo said:
It wouldn't surprise me if the rears locked at the same time as the fronts when on tarmac. The rear being lighter so less footprint on tyres. Especially when the fronts dip and take more of the weight and no tc.

I'm sure only the fronts locked. It's almost worth trying this while filming the VCU to see for sure. I suspect that LR designed in a maximum torque transfer through the VCU.
The Freelander wasn't the only vehicle to use the VCU. The earlier Scooby used them too. They are, or were available with differing torque abilities to fine tune the 4WD system for different uses.

Hippo · Oct 30, 2015

Nodge68 said:
I'm sure only the fronts locked. It's almost worth trying this while filming the VCU to see for sure. I suspect that LR designed in a maximum torque transfer through the VCU.
The Freelander wasn't the only vehicle to use the VCU. The earlier Scooby used them too. They are, or were available with differing torque abilities to fine tune the 4WD system for different uses.

I tried some time ago but stopping within camera shot from speed was difficult. Especially close enough to see what happens. I wanted to see the abs pulses. You need to manually set the focus on the car where you think it will stop, or it will auto focus on the background and the car is a blur.

GrumpyGel · Oct 30, 2015

Nodge68 said:
When a unit like a diff or transfer box is taking power, an amount of heat is created. However I'm sure you know this already
The thought of temperature checking the rear diff was to see if it's taking excessive torque, like then the VCU starts to stiffen. The theory being, more drag on the VCU = higher torque transfer. Higher torque = higher heat loadings absorbed by the diff. If we can get a large enough sample of diff temps, we might have a guide by which we can say. Diff at X temp after 10 miles at 60, VCU is ok. However if the diff exceeds X temp then the VCU is goosed. It's difficult to use the IRD for this temperature measurement because the cooler will also act as a heater, which I suspect is more often than not.

I don't think you are going to get much or a response in a request for diff temps - the OWUT is easy to do and there's not a huge response - probably just about enough to get an answer as in @B34R's post.

Another factor we do not know about is what the worst type of wind up, is or what type is the best to try and detect - by this I mean is it a sharp rise in wind up when turning and the wheels turn at different rates (ie spike in temp) or a continuous amount of wind up all-be-it at a possibly lower force (ie gradual increase in temp over time). If we detect the latter, is it to late because wear from the first has already caused to much of a problem.

If we can detect wind up through temp change - how would be the best way of detecting it? The oil obviously absorbs the heat and dissipates it through
mass of oil in the box - then through disipation into the casing and to atmosphere and with the IRD through the cooler - assuming that it runs at a hotter temp than then engine's coolant. We can therefore test how much above ambient temp to check for a general increase over time.

To say that heat is not generated before bearings fail, is obviously not true. Hippo's observation that the rear diff operates at a higher temp with the props installed proves this - everything is turning at exactly the same rate regardless of whether the props are installed - the only difference is that there is more stress involved with the props as it has to turn it and also combat resistance from the VCU. The level of temperature rise must surely be proportional to the amount of stress.

Anyway, all this talk isn't getting my Freelander back to 4WD - I'm out-a here and down the inspection pit.

Hippo · Oct 30, 2015

We did have a fred where there were several post's of peeps diff temp. 10 or more and there was a pattern starting.
In my view there's 2 things a vcu will do: resistive before becoming active.
I think with age the resistance is greater for the same differing speed across it.
Therefore putting more stress in the transmission than it would if it were newer, as it's resistance increases with age or by some factor.
Hence diff temp would be higher as the effect of transmission wind up would put more pressure on the transmission.
If we know the typical OWUT result then we can compare vcu's under test to see how they're doing.
If as I think we have proved vcu's have a higher resistance when older, then temp result of rear diff could be a wdatum for stress in the transmission.
This additional stress could be down to tyres as well as the vcu.
For me I think it's the increased resistance with age which is the worse type of stress in the transmission.
This is because the vcu is resistive the vast majority of the time from measuring prop speeds.
With age a vcu would apply additional resistance which puts more stress on the transmission.
Hence why when something brakes the vcu has a higher resistance when compared to others..

jamesmartin · Oct 30, 2015

jamesmartin said:
checking bearing temps isnt an indication of vcu ,only the speed of box and or having a bearing about to fail, when a bearings failed enough to produce heat it will fail very quickly after and not long before it melts

Hippo said:
We did have a fred where there were several post's of peeps diff temp. 10 or more and there was a pattern starting.
In my view there's 2 things a vcu will do: resistive before becoming active.
I think with age the resistance is greater for the same differing speed across it.
Therefore putting more stress in the transmission than it would if it were newer, as it's resistance increases with age or by some factor.
Hence diff temp would be higher as the effect of transmission wind up would put more pressure on the transmission.
If we know the typical OWUT result then we can compare vcu's under test to see how they're doing.
If as I think we have proved vcu's have a higher resistance when older, then temp result of rear diff could be a wdatum for stress in the transmission.
This additional stress could be down to tyres as well as the vcu.
For me I think it's the increased resistance with age which is the worse type of stress in the transmission.
This is because the vcu is resistive the vast majority of the time from measuring prop speeds.
With age a vcu would apply additional resistance which puts more stress on the transmission.
Hence why when something brakes the vcu has a higher resistance when compared to others..

rear diff temp will largely vary depending on speed and length of journey plus weather, and it stands to reason that any differential in speed between back and front will give stress the greater the difference he greater the stress and the shorter life of vcu

VCUs - The sweet spot

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