Transmission Wind Up in Viscous Coupling AWD

[rache]

Hi, I was wondering just what is going on in the video below?

The driver is saying this that the wheel spin that can be seen is caused by transmission wind up and not his use of the throttle as I believe.

I accept that the Sportrak with a mechanical 4wd and no centre differential can suffer wind up if used on the road in 4wd, but didn't think it was possible on a slippery surface like in the video. But I don't think that Viscous coupling systems could have wind up in any circumstances.

The driver has said this: "The tight turning circle I did was to try and show there is some wind-up occurring between the axles on the Sportrak (as would be expected because of no central differential), however even though there is a viscous differential on the Freelander there appears to be some seen on them, more so on the benchmark vehicle. I suspect there is less resistance between the tyre and the ground than the resistance in the viscous differential. The release of wind up can be seen as a momentary increase in speed of the left-hand rear wheel because it is taking the tightest radius."

Is he trying to pull the wool over someone's eye?

Here the video he is explaining:

 

 

[garrycol]

I have got no idea what the video is trying to show - to me it shows the Freelander AWD system doing exactly what it is supposed to do locking up and driving the rear wheels when there is slippage in the front wheels.

To over simplify, the Freelander is a front wheel drive car and when the VC detects its input speed as driven by the engine etc is different to its output speed which is driven by the rear wheels it locks up.  On solid surfaces the VC input and output are the same so it is unlocked - if the front wheels spin or the front wheels are turned sharply the input to the VC is faster than the output driven by the rear wheels so the VC locks and drives the rear wheels.

The only time you will get full on windup is when the VC has failed as it completely locks up driving the rear wheels all the time - if on a solid surface the IRD rear PTO will fail.

 

 

[tobac]

This is an interesting topic. I had a Freelander 1 for a number of years and I noticed when I backed out of the garage and did a reverse turn I could hear the rear wheel slip. I further investigated the Freelander system and found that the viscous coupling were a source of trouble. I replaced the coupling with a new one and it resolved the problem. A friend had a diesel Freelander 1 and he had the misfortune to have similar problems. He wrecked his transfer box and decided to replace the viscous joint. Very soon after he did his rear differential. This guy had different tyres with different amounts of wear on his car. The tyre reseller who happened to be my brother worked out if you replaced the tyres on your car they all needed to be exactly the same. If you only replace 2 tyres they must be put 1 on the front and 1 on the rear or else the viscous coupling will be constantly working and it will eventually tighten up and fail. Always replace the tyres in full sets. Many people are not aware of this point. Hope this helps.

[rache]

Thanx for the replies, but from the quote in my opening post, you will see that the driver is saying that the wheel spin is caused by a perfect working VC.

Perhaps if we take this one at a time, can you get wind up on the slippery surface as seen in the video in any vehicle?

It is my belief that the wheel spin see (accompanied by exhaust and engine revs) is caused by the driver using the throttle and not the build up of wind-up?

I accept that to make the rear wheels spin the front ones must be spinning although they are out of camera view and this shows the VC transferring drive to the rear wheels, but the question is; Is that wheel spin caused by transmission wind-up as claimed in the quote in my opening post?

Its just that when I repeated the test, the wheels didn't spin as seen here and so wondered why the same car is not doing it immediately after the first video was taken? Also, if transmission wind-up is a constant thing caused by turning in a tight circle, why is the wheel spin only intermittent? I really think the first video is a hoax - am I right?

 

[Tanis8472]

1 on front and 1 on rear will surely make it fail and also cause dangerous driving conditions.  If replacing only 2 tyres they must go on the back "axle".  This is in the manual. 

[tobac]

Hi Tanis8472,

 

This may be true in a perfect world. All tyres SHOULD be changed at the same time. But if for some reason you only replace 2 tyres you are risking ruining the viscous coupler if you don't do as I suggested with 1 on front and 1 on the back. This has been checked and confirmed with tyre and transmission engineers. As for the questions from Rache. My 4wd garden tractor which has no centre diff. will cause chirping on one wheel every so often on tarmac surfaces. On grass you don't notice it as it slips easily. On the Freeland the viscous coupling has a certain amount of resistance in it to turn. If you have a new unit on the bench you may have difficulty slipping it by hand. ie there is some friction. In use on grass the traction on the slipping wheel is obviously less than the friction in the coupler hence the wheel spins as it is trying to keep up with the other wheels.

[rache]

57 minutes ago, Tanis8472 said:

1 on front and 1 on rear will surely make it fail and also cause dangerous driving conditions.  If replacing only 2 tyres they must go on the back "axle".  This is in the manual. 

The rear differential is geared so that the rear half of the propeller shaft turns 8% faster than the front half to ensure that no drive goes to the rear wheels in normal road driving when the only the front wheels drive and the rear ones get dragged and rotate the rear propshaft/VC just slightly faster. The VC is not activated by this small speed difference, for if it was, it would try to slow the rear wheels, which wud be a braking effect on the car.

Technical References:

Landrover Workshop Manual (Description and Operation) 47.6, states:

“The rear wheels are 0.8% under driven, so in most conditions the vehicle is effectively front wheel drive, with the rear wheels turning the rear propeller shaft slightly faster than the IRD drives the front propeller shaft. Since the speed differential is low, the increase in viscosity of the silicon jelly is marginal and there is little resistance to relative rotation of the slotted discs."

 

However, the rolling diameter of worn tyres on the rear compared to new one on the front could adjust the 0.8%, hence the advice to fit new tyres to the rear in preference to the front. Wrong tyre pressures and conflicting rolling diameters can create a braking effect in the VC and have an impact on fuel consumption.

 

One on front and one on rear would seem to be theoretically the best solution for VC issues but there may be a conflict with braking balance across axles although I imagine that ABS would counter such a problem?

 

This is a shame, because since the front tyres do more braking than the rears, it would normally be a preference to have the best tyres where they are needed more.

[rache]

29 minutes ago, tobac said:

Hi Tanis8472,

 

This may be true in a perfect world. All tyres SHOULD be changed at the same time. But if for some reason you only replace 2 tyres you are risking ruining the viscous coupler if you don't do as I suggested with 1 on front and 1 on the back. This has been checked and confirmed with tyre and transmission engineers. As for the questions from Rache. My 4wd garden tractor which has no centre diff. will cause chirping on one wheel every so often on tarmac surfaces. On grass you don't notice it as it slips easily. On the Freeland the viscous coupling has a certain amount of resistance in it to turn. If you have a new unit on the bench you may have difficulty slipping it by hand. ie there is some friction. In use on grass the traction on the slipping wheel is obviously less than the friction in the coupler hence the wheel spins as it is trying to keep up with the other wheels.

So in a nut shell, you are saying that there is no wind-up on a slippery surface like the one in the opening post video?

I have had a VC on a bench, cut them open etc (see pic) and the reason why you can't turn them by hand (when working) is because the seals are so tight to cope with the high pressure that can occur within them I believe that the seals offer somewhere like 40Nm resistance (please don't quote me on that figure but I do have an SAE paper with the figure). You can undo the grub screw so that no pressure can build up or cut on in half, but either way, you will find that it is the seal that prevents it being turned. Some people 'test' a VC with a bar an watch how easily if falls/rotates but this is only a test of the seal which will loosen over prolonged use.

[Tanis8472]

On 22/10/2016 at 1:51 PM, rache said:

The rear differential is geared so that the rear half of the propeller shaft turns 8% faster than the front half to ensure that no drive goes to the rear wheels in normal road driving when the only the front wheels drive and the rear ones get dragged and rotate the rear propshaft/VC just slightly faster. The VC is not activated by this small speed difference, for if it was, it would try to slow the rear wheels, which wud be a braking effect on the car.

Technical References:

Landrover Workshop Manual (Description and Operation) 47.6, states:

“The rear wheels are 0.8% under driven, so in most conditions the vehicle is effectively front wheel drive, with the rear wheels turning the rear propeller shaft slightly faster than the IRD drives the front propeller shaft. Since the speed differential is low, the increase in viscosity of the silicon jelly is marginal and there is little resistance to relative rotation of the slotted discs."

 

However, the rolling diameter of worn tyres on the rear compared to new one on the front could adjust the 0.8%, hence the advice to fit new tyres to the rear in preference to the front. Wrong tyre pressures and conflicting rolling diameters can create a braking effect in the VC and have an impact on fuel consumption.

 

One on front and one on rear would seem to be theoretically the best solution for VC issues but there may be a conflict with braking balance across axles although I imagine that ABS would counter such a problem?

 

This is a shame, because since the front tyres do more braking than the rears, it would normally be a preference to have the best tyres where they are needed more.

 

 

[FridgeFreezer]

The Freelander viscous locks up (due to heat generated internally) after a certain amount of slip, could explain the video. When I drove mine to Bosnia in summer (40degC+ in Mostar) it was noticeably stiffer on the very hot days.