VCUs - The sweet spot

Anyone tried swapping the freebee vcu for a vw one?

[GALLERY=][/GALLERY]

Lots of interesting stuff above about how the VCU works and how it interacts with ABS etc. Wow!! – BUT


Focussing to my original subject matter – “The sweet spot”


I have now managed to obtain and fit a couple of hall effect sensors and matching rev counters to the front and back propshafts of my 2005 Td4 Auto Freelander.

See the attached pictures


My tires are a matched set of Michelins with 1mm more tread on the front – all running at 30psi.

As stated on Hippo’s “Measure yer wheel fred” my wheel measurements were:- . (Full tank of diesel - TD4 Auto 2005 HSE)
Front 346mm Rear 351mm difference 5mm


My VCU can do a “One Wheel Up Test” of 12ses for an 8kg weight at 1.2M and 28sec for a 5kg weight at 1.2M – so is is pretty good nick and well able to slip as required. Also pretty similar to Hippo’s “One Wheel Up Test” Graph


After about 16 miles at varying speeds up to 75 my temperatures were Rear diff 40c VCU 25c (for comparison damper weight thing next to it 20c) IRD 60c



Now let me state at the outset that this vehicle – a 2005 Td4 Auto Facelift - gives me 3.3” lag at a rear wheel for 20 revs of a front wheel (My Two Wheel Up Test). Which is consistent with a 0.2% difference front to back from the IRD (39/22 x 38/21 x 14/45 = 0.997979797 1-0.007070707 = 0.0020203))


I have also looked through the IRD level plug and counted 22 teeth on one of the pair next to the front diff. Assuming it meshes with a 39 tooth gear that’s consistent with a 0.2% difference front to back from the IRD (39/22 x 38/21 x 14/45 = 0.997979797 1-0.007070707 = 0.0020203))


I have also taken off the front pinion from the IRD and counted 24 teeth on the gear opposite the crown wheel which assuming it meshes with a 29 tooth gear is consistent with standard Td4 gearing (29/24)


So it’s a “later” standard Td4 IRD consistent with that described in the 2004 Freelander Spec Sheet which quotes the IRD as having a ratio of 1.467 (39/22 x 24/29)


(I know there is difference of opinion about whether this brochure is accurate and whether/when /if the IRD ratio changed but I do not wish to get into that argument –

I can only speak for My Vehicle and a 2003 Td4 Manual on which I also did my Two Wheel up Test with the same result)



So what did I find?


On a flat road straight road at speeds of up to 75MPH the propshaft revcounters are indicating the same speed.


See pictures where I have the rev-counters shown alongside a laptop with OBD11 showing Engine rpm and road speed – the speed showing by OBD co-related exactly with my Tomtom – the vehicle’s speedo was hopelessly fast and has been dis-regarded


Sometimes on a bend the rear prop does one rev less


Trying to maintain a constant speed up a steepish straight hill at around 50MPH I sometimes see the rear 2 revs behind the front prop.


The spec for the instruments claims accuracy to +/- 2 rpm up to 5000 but I find them absolutely consistent to each other in similar conditions so in terms of difference one to the other I suspect the accuracy to be better than within 2 revs.


When connected to the same battery and therefore switched on at the same time they maintain a completely simultaneous refresh time of about 1 second (if I connect them to different batteries I can get them out of step - they then maintain that consistently but it makes it difficult to get comparative data as we are looking at only 1 rev in circa 3000 at speed and very small speed changes have a greater effect on the readings)


Now what does that tell us?????????


For me :-


as the 0.2% difference in IRD ratio front to back only allows the front wheels to go about 6 propshaft RPM faster than the rear wheels at circa 75MPH (circa 3000 prop revs)


– and the theoretical loaded radius from the STATIC tire measurement is 5mm less on a 330mm radius front tire or 1.5% which equates to circa 45 propshaft RPM at circa 75mph (1.5% x 3000)

• and as the revcounters are showing only up to 1 Prop RPM slippage in straight flat running at speed –( Let’s assume that to be accurate for a moment)
  
  
  
  -and 1 prop rev per minute = 1 / Diff ratio 3.21 Wheel revs/minute = 0.31rpm or 3.21 x 60 = 192.6secs/rev - and a “One Wheel Up Test” is only 45 degrees – therefore a 1 rpm Prop rev = a “OWUT” time of 192.6 /8 = 24seconds.
  
  From Hippo’s graph that’s a load of around 6 kg at 1.2 M
  
  
  

• lets talk ft lbs ‘cos I’m 73 y’know
  
  
• 
  
  
• that’s a torque of 53ft lbs
  
  
• at the prop that’s a torque of 53/3.21 = 15.ft lbs
  
  
• at 3000 revs that’s only 8.5hp
  
  
  
  No more power than that can be transmitted through to the back wheels without generating more slip at the vcu and stiffening it up further
  
  
  That’s all virtually fact rather than assumption - except for possibly the accuracy of my revcounters
  
  
  
  Now for the guesswork
  
  
  
  My guess is – as per my hypothesis at the beginning of this thread – that tire flexibility is well able to soak up the theoretical differences in wheel speed –caused by the loaded rolling radius differences (only measured STATICALLY – must vary at speed) which is partially offset by the gearing difference.
  
  We are all familiar with slip angle in tires which manifests itself in understeer at the front or oversteer at the back – there will be a similar component for forward drive – and then there is the torque reaction from the wheels which lift the front of the car when you put your foot down thus unloading the front wheels and reducing the loaded rolling radius difference front to back – that’s only two factors – tire maths are scary!
  
  
  
  After all this is not two steel pinions per side on a steel rack between front and back – it’s four wheels (disks wrapped in a flexible rubber bag full of wind) on a tarmac road.
  
  
  
  In short I believe my prop slips around 1rpm or less at circa 75 and the transmission is quite lightly loaded at around 8.5hp – when required like slipping offroad my clever hippo stiffens up and does the business.
  
  
  
  1 prop rpm slippage x 15ft lbs = 15ft lbs/min of work done lost presumably in heat = 0.35watts which is a tiny amount of heat so I would not expect my VCU to get very hot – and it doesn’t
  
  
  
  So I think with the Minimal 0.2% IRD difference front to back Land Rover have hit “The Sweet Spot”
  
  
  Discuss?
  
  
  I’ll get me coat
  
  
  (Don’t you just love the smell of Napalm & EP90 Hypoid first thing in the morning)
  

interesting reading .. however ..
i got sort of lost after ..

Now what does that tell us?????????

Click to expand...

but it be after midnight .. maybe after morning coffee i'll find my way ..
until such a time .. i'll aim to keep below 75 mph .. or 3000 rpm .. whichever comes first ..

well done on the propshaft revcounter readout project

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Just scanned over this (about 3 times!). All interesting stuff and very impressed.

I do find it strange though that the props would spin at 'exactly' the same rate because (as you say) there is an inbuilt 0.2% geared difference and known tyre diameter differences - ie the likelyhood of everything matching up perfectly - even with 'torque reaction' and ambient VCU torque transfer - just doesn't seam right. I'm not questioning your findings - just musing why/how.

At 75MPH the props were spinning at an identical rate - I was wondering if this was because they were trying to spin at a 45RPM difference and the VCU was in hump mode - but I'm sure this can't be the case as the car wouldn't 'feel right' - and there would still be slippage due to the 'equlibrium'.

Just more confused now.

Will come back and have another read later!

Nice work dfossil.
Now for a challenge, wot happens when one tyre is soft, say 20 psi or even less. General consensus is the two shafts will rotate at significantly different speeds, but is this fact or fiction?

There may be a significant difference in prop speeds - but not as much as the tyre difference. When my IRD went, it also demolished a tyre. The tyre was completely trashed across its whole width, melted away and the belt showing in places. So I can only deduce that it was low on pressure which created a difference across the VCU which went into hump mode and therefore put extreme pressure on the IRD & tyre through the rear diff.

I suppose running an under inflated tyre in a 'normal' car will also trash the tyre - but I'm sure this was much worse/quicker. The tyre wasn't visibly 'flat'.

A short run on the tyre shouldn't trash anything, but I'd still do it with your spare.

GrumpyGel said:

Just scanned over this (about 3 times!). All interesting stuff and very impressed.

I do find it strange though that the props would spin at 'exactly' the same rate because (as you say) there is an inbuilt 0.2% geared difference and known tyre diameter differences - ie the likelyhood of everything matching up perfectly - even with 'torque reaction' and ambient VCU torque transfer - just doesn't seam right. I'm not questioning your findings - just musing why/how.

At 75MPH the props were spinning at an identical rate - I was wondering if this was because they were trying to spin at a 45RPM difference and the VCU was in hump mode - but I'm sure this can't be the case as the car wouldn't 'feel right' - and there would still be slippage due to the 'equlibrium'.

Just more confused now.

Will come back and have another read later!

Click to expand...

props spinning at the same rate on a straight road is the point of a vcu surely ,but with front wheel drive feel as lr engineers told us back in the 90s

jamesmartin said:

props spinning at the same rate on a straight road is the point of a vcu surely ,but with front wheel drive feel as lr engineers told us back in the 90s

Click to expand...

Props spinning at the same rate on a straight road has nothing (or very little) to do with the VCU. The VCU is there to allow small amounts of slip - so at small amounts or no slippage, the VCU should be doing (next to) nothing.

The OWUT takes longer with reducing weight - a 5KG weight will take longer than an 8KG weight. Therefore if there is any force (or difference in prop speeds) even if that force is just 0.5KG (or equivalent in prop difference) there will be slippage - just less.

@dfossil also says "Trying to maintain a constant speed up a steepish straight hill at around 50MPH I sometimes see the rear 2 revs behind the front prop." - so that shows a straight line can give difference. That difference is greater than the "1 rev" difference he saw going round a corner - although maybe cornering was at slower speeds.

GrumpyGel said:

Props spinning at the same rate on a straight road has nothing (or very little) to do with the VCU. The VCU is there to allow small amounts of slip - so at small amounts or no slippage, the VCU should be doing (next to) nothing.

The OWUT takes longer with reducing weight - a 5KG weight will take longer than an 8KG weight. Therefore if there is any force (or difference in prop speeds) even if that force is just 0.5KG (or equivalent in prop difference) there will be slippage - just less.

@dfossil also says "Trying to maintain a constant speed up a steepish straight hill at around 50MPH I sometimes see the rear 2 revs behind the front prop." - so that shows a straight line can give difference. That difference is greater than the "1 rev" difference he saw going round a corner - although maybe cornering was at slower speeds.

Click to expand...

it has all to do with vcu otherwise they would be forced to drive as ratio

I know what you're saying @jamesmartin - that's why I'm confused with the results.

The above tests were done with a 5mm tyre size difference - equating to 1.4% - less the IRD's 0.2% under gearing, so should give a difference in prop speeds of 1.2%. The figures are slightly different - but at a 75mph 3,000rpm prop speed, that equates to a difference across the VCU of 36rpm - easily within the equipment's 2rpm accuracy - so the kit should be able to show what's going on.

As I said above, I'd be amazed if torque reaction etc would equal out that 1.2% 36 rpm difference 'exactly'. So yes I agree, its probably the VCU equaling up the props - but I do find it difficult to imagine there would not be some slippage in the VCU - maybe it takes it down to an amount that is not within the 2rpm accuracy of the test equipment.

This also means that the tyre tread has to be able to flex by the 1.2% difference, less any slippage in the VCU. If a block is 40mm wide - it would need to flex by about 0.4mm without wearing them down - I suppose this can be distributed front to back - so 0.2mm.

@dfossil , have you tried on a level road straight but at lower speeds (10mph, 30mph) where any ratio difference would result in a lower slip rate across the VCU?

Running at 7.5mph that would give a prop speed on 300rpm and a slip rate of 3.6rpm. Still within the test equipment's accuracy - and probably at a speed where the VCU will allow slippage.

It's not just the tread that flexes in a tire - push your vehicle sideways and see how much the sidewalls allow the vehicle to move for small forces applied - they are stiffer front to back but by no means rigid - and at speed gawd knows whats happeningI
Also when you do a OWUT you feel a fair bit of "stickshin" before the VCU moves at all - I felt about 40ft lbs on my bendy bar torque wrench before I got movement after then it was nice and well within normal numbers. If the transmission is reasonably happy in a straight line and normally loaded at a comfortable speed the tires may give enough more easily than the VCU.
I have not done much at slow speed - one thing that is difficult is to maintain very steady speeds - I can play around with tire pressures, loads in the back etc, find an empty car park for slow steady turns etc. Experience so far is limited to fairly fast running, long easy bends, and trying to load it up more on the hill. I think when going slowly and not steadily the readings jumping around seem to unsettle the instruments slightly and possibly sort of flutter the refresh rate making it difficult to capture comparative readings. - You really can only get them with a camera - first impression on slow speeds straight is when I get a steady reading it's still virtually same front to back.
I need to find a better straighter hill, a decent carpark etc and maybe a co-driver - it gets pretty busy with all the instruments.
Early days - looking forwards to others input however -thanks for comments so far folks

The link below I think gives us a clue to what is going on here - it discusses amongst many other things two tire radii - the loaded rolling radius which we have been measuring on Hippo's "measure yer wheel fred" and the effective rolling radius- and the penultimate paragraph on page one reads:-
"The effective rolling radius turns out to increase with increasing speed and increasing inflation pressure"
In other words as I read page one the "effective rolling radius" which will be the one which sets our front to back relationship, will always be less than the natural diameter of the tire and greater than the static loaded radius measurement and will increase with speed.
After page one this paper goes on to a point where I glaze over but is mostly talking about cornering etc and seems to have dealt with the bit we are examining here.
But it explains why we don't see full wind up based on static loaded rolling radius measurement - and helps me to understand why the faster we go the nearer to one to one front to back we will see - but with always that tiny 0.2% giving us a bit to the rear.

https://www.tut.fi/ms/muo/vert/11_tyre_as_car_component/handling_input_rolling_radius.htm

I only read page 1, but the last para begins "A radial-ply tyre rolling radius appears to be almost constant for varying speed" - are we not all running radials these days?

As an update on my Prop Shaft Revcounters experiment in post #122 of this thread:-

Having found an empty pub carpark with no kerbs half a mile from home I took the opportunity to do some slow speed prop revcounter trials.

Same setup as before. ie 30psi all round etc

For info it’s about 1mph/40 revs when looking at these.
(over 30 mph they always read =/= on a straight flat road at steady speed)
Straight
Forwards Front Rear
1000 999
56 55
Backwards 36 34

Full lock
Forwards 129 112
120 106
102 90
Backwards 120 108
101 86
Half lock
Forwards 145 142
102 98

Went straight home and got my infrared thermometer –
IRD 40C VCU 27C Damper 16C Rear diff 23C


Dropped rear tires to 20psi – that made my STATIC loaded tire radius
the same front to back - Did 1 mile round the block:-


Straight Front Rear
988 990
-so the rear axle was pushing the rear prop! Circa 0.2% !
Rechecked temperature VCU 28C

then Corrected tire pressure (;-))
I will be visiting shortly where I know there is a very steep straight quiet hill so will examine the behaviour on that then.
(Sorry about the formatting of my tables - didn't carry over the same from the draft -but I think you can still follow them)

Interesting stuff. I think all the above figures are what you would expect? The last one being "exactly what it says on the tin"!

It shows your testing setup is spot on.

My mind is a mess though - just done a day/nighter - need some sleep!

OK - last bit of info on my prop revcounter setup (posts #122 & #134 - this thread) - going up a steep hill at slow speeds got 2 or 3 revs faster at the front prop as the extra power to climb gets shared more with the back - coming down was more difficult to get a stable reading as the balance between brakes and auto-box engine braking and trying to maintain a steady speed was difficult. - seemed to get spells of both leading and lagging at the front prop though - not too reliable but might be happening - it was a roughish little road.
Hey @4Bee4Bee - did you get your idea going? - it would be great to compare results.
I think so far my findings are very much in line with @jamesmartin - his description of "a front wheel drive vehicle with a four wheel drive feel" as per Land Rover's intention (very light power transmission to the rear normally)- coupled with a very ready 4x4 effect immediately when required - which makes them a bit more nimble than the tratters in light off-roading - to their horror (;-))

Excellent work. I haven’t had a chance to read through it fully to digest the information yet, but it looks interesting.

I’ve been working away too much over the past few months, so haven’t had much of a chance to even drive the Freelander, and it seems the times I have been back there has always been a mod or niggle to sort out. It seems a bit like whack-a-mole at the moment, in that I sort one thing out, and something else shows up. The most recent being the LP fuel pump making a nasty loud noise, so having to sort a replacement to avoid a full breakdown.

The upshot is that even though I picked up a reconditioned VCU and bearings from Bell just before Christmas, I haven’t got around to getting the new tyres on the Freelander sorted, and the propshaft back on. So my original plans to make a propshaft monitor have been put on hold for a while. And actually, reading through the excellent feedback on here, I might go for making a multi-temperature monitor first, putting sensors on the rear diff, IRD, and VCU, (and ambient, as my pre-facelift doesn’t have that), and putting the numbers up onto a little display on the dashboard. Maybe replacing longer clock module. It should be straightforward to do. And not too timeconsuming, and give me an indication of if anything is going wrong with the drivetrain. The prop monitor can be added to this after.

I will post progress / results as this moves forwards.

Looking forward to getting the four-wheel drive going again, as you can look a bit of a berk wheelspinning a hippo!

Jim

firstly thanks to all for such an informative thread.
secondly let me introduce myself, my name is Andrew and I am an Aerospace Engineer. Like to think I am an experianced DIY mechanic.
thirdly and reason for the post is I just bought a 2004 TD4 HSE Freelander 76k on thr clock.
I read most of the thread before picking the car up and the drive home from Newcastle to Burnley.
I had already decided the replace the VCU as 270 quid seemed cheap as a preventative measure.
Drining home I would decribe the car as strained, bit like the brakes were sticking but I stopped and they were fine. I
I think it was also lucky it was lashing down.
After passing Harrogate there are some large downhill gradients so I decided to lift off the accelerator at 60 mph tonsee what happened, the car slowed and reduced to 50 by the bottom of the hill. nothing scientific there but would not expect on such a steep gradient the car to slow. normally on that road I have to brake several times to keep the car at 60.
when i got home I decided to try the reverse on full lock test but on a gravel road. The car stopped almost immediately and felt as best as I can describe fighting itself. I checked the gravel and the two wheels which would be travelling the greater arc had made marks that suggested there were not turning and had skidded as the car stopped reversing.
Drove the car to my garage and did the one wheel up test with a 1.2 metre bar and the wheel didnt want to move, so rang Bell who advised not to move it any further and have supplied a recon VCU. on removing the old unit it was obvious that it had been off recently. some had marked the VCU and the marks were still lined up perfectly suggesting the VCU was not moving at all. New VCU not yet fitted but will be very soon. I will have a drive to Harragate amd repeat the downhill test to see if its just normal freelander behavior. certainly the car drove dramatically different with the VCU off, but I did only drive it up and down our lane.
So thankyou to one and all as I would have just carried on driving.my new car and fear I would have suffered a major drivetrain failure very quickly
Andy

Hopefully I can also add some info to the thread that might help some people. I have a couple things that maybe of interest or at least add light onto some things that have been said.
In the manufacturing industry there is some thing called the bathtub curve and the "mean time between failure".

you can google them for detailed explanation.
This is used in reliability engineering and puts Data to what people see. You will get early VCU failures and you will get ones that last 120,000 or more. So when some have said 70,000 miles is the service life that could argued as the "mean time between failure".
I have know idea if LR used these principles but they are statistically proven. Some manufacturers design it for a mtbf.
So personally when I saw a couple of specialists mention that figure, it probably gut feel and not based on data, but I thought why gamble. its going to fail and it will take more expensive items with it.
hopefully that puts some data around the debate of times of failures and perhaps some enlightenment.

Lowbank said:

firstly thanks to all for such an informative thread.
secondly let me introduce myself, my name is Andrew and I am an Aerospace Engineer. Like to think I am an experianced DIY mechanic.
thirdly and reason for the post is I just bought a 2004 TD4 HSE Freelander 76k on thr clock.
I read most of the thread before picking the car up and the drive home from Newcastle to Burnley.
I had already decided the replace the VCU as 270 quid seemed cheap as a preventative measure.
Drining home I would decribe the car as strained, bit like the brakes were sticking but I stopped and they were fine. I
I think it was also lucky it was lashing down.
After passing Harrogate there are some large downhill gradients so I decided to lift off the accelerator at 60 mph tonsee what happened, the car slowed and reduced to 50 by the bottom of the hill. nothing scientific there but would not expect on such a steep gradient the car to slow. normally on that road I have to brake several times to keep the car at 60.
when i got home I decided to try the reverse on full lock test but on a gravel road. The car stopped almost immediately and felt as best as I can describe fighting itself. I checked the gravel and the two wheels which would be travelling the greater arc had made marks that suggested there were not turning and had skidded as the car stopped reversing.
Drove the car to my garage and did the one wheel up test with a 1.2 metre bar and the wheel didnt want to move, so rang Bell who advised not to move it any further and have supplied a recon VCU. on removing the old unit it was obvious that it had been off recently. some had marked the VCU and the marks were still lined up perfectly suggesting the VCU was not moving at all. New VCU not yet fitted but will be very soon. I will have a drive to Harragate amd repeat the downhill test to see if its just normal freelander behavior. certainly the car drove dramatically different with the VCU off, but I did only drive it up and down our lane.
So thankyou to one and all as I would have just carried on driving.my new car and fear I would have suffered a major drivetrain failure very quickly
Andy

Click to expand...

76k seems a very low mileage for a VCU to be so shot but if the previous owner neglected the tyres or had different makes/sizes of tyres it would account for it. My first Freelander had 104k on it and the VCU was totally shot whereas my latest one has 120k on it and the VCU was tight but still turning, so mileage will only be an indicator of VCU condition and shouldn't be relied on.