4WD and Tread Depth

[AlfaJack]

Is it ok on a 4x4 to run different tread depths like 8mm Front or Rear and say 3mm tread at the other end?

Got it in my mind that it could ruin the diffs or something?

I asked the same question on alfa155.org and some said it would be fine as its got a centre diff - others said they had experienced cars where all 4 need changing regardless of a centre diff.

Any ideas?

[cuore_sportivo_155]

it depends on the kind of diff.... for a Q4 it's fine....

and a bit of a moot point really.... what about the different weights and differences in pressure on the front and back axles? this will make the running circumference different anyway....

[AlfaJack]

True, guess there are a lot of variances.

Cheers!

[Steve Webb]

The center diff will take up any differences between front and rear rolling circumferences, but if they are too far out, say brand new tyres at the front and almost bald at the rear then there is going to be alot for the center diff to even out and could I suppose cause damage or excess wear.

If you look at tyre websites you will also see that the same 'sized' tyres from different manufacturers will have different rolling circumferences.

[AlfaJack]

Thanks I will be going to a couple of new fronts - rears have about 4mm left I think - I will get the same type (Uniroyal Rainsports) if I can 9heard they have stopped making them

[Evodelta]

It is not the centre diff which deals with front to rear 'slippage' and any differences between rolling diameters, it is the viscous coupler (VC) which does this.
This can cope with the minor differences between worn and new tyres. It is only massive differences such as our (in the UK) standard in-the workshop-roller MOT brake test and towing a car with two wheels in the air which will see immediate damage to the centre diff as the CV locks up and transfers the torque loading to it.

[wrinx]

...standard in-the workshop-roller MOT brake test and towing a car with two wheels in the air which will see immediate damage to the centre diff as the CV locks up and transfers the torque loading to it.

My last MOT was done, very gently, on the rollers. When I asked the guy about it he said it's fine, they do lots of permanent 4wd cars (they are Jap specialists too) and have not had a problem???

How much damage are we talking about?

wrinx

[Evodelta]

If there was any damage you would probably know it by now. As the VC gets older the silicone liquid in it loses its viscosity and it slips easier than when it would have done when it was new or if it has been uprated or renewed, it is also down to the operator, some are very gentle and sense when the car is trying to pull itself out and immediately stop, it is still not a wise move to put a 4wd car on the rollers though and the brake test must be done on the road.
I have a friend whose local MOT station wrecked his transmission doing the roller test, it cost them thousands ??? to put it right. To quote directly from the MOT testers manual:

Manual section 3.7 page18:

Vehicles NOT to be tested on a roller brake tester are,
1/ More than 1 driving axle permantly engaged
2/ limited slip diff
3/ belt driven transmission
these vehicles should be tested using the decelerometer.

I have written an article/discussion on VC removal from an integrale with pictures etc for our Lanciasport site, we even opened up the VC itself and photographed it so the reader can see what goes on in there, you can have your VC refilled with thicker uprated and new fluid for about ?300. If the owner/Mod of this site (Steve?) would like the article you can have it to publish on here.

[Steve Webb]

It is not the centre diff which deals with front to rear 'slippage' and any differences between rolling diameters, it is the viscous coupler (VC) which does this.

Bit of a mix up with termanology here, when I refered to the centre diff I was refering to the ferguson diff as a whole (open diff + VC) but you are correct, any large difference in rolling diameter front to rear will cause premature wear on the VC part of it.

Surely, if I remember correctly, when they do the rear wheel brake tests on the rollers for the MOT, they brake the rear wheels seperately, which will cause far more problems with the Torsen rear diff as this operates and distributes torque far more quickly.

Oh and yes, EvoDelta, would be great to have that VC article on the site. Could you send me through a copy / link to it.

Steve

[Steve Webb]

Oh and when they do 4wd mot's over here in France, they have two extra roller plates which they slip under the wheels not being tested, so all wheels rotate when the brakes are tested. Saying that though the car isn't tied down at all!

[Evodelta]

Bit of a mix up with terminology here, when I refered to the centre diff I was refering to the ferguson diff as a whole (open diff + VC) but you are correct, any large difference in rolling diameter front to rear will cause premature wear on the VC part of it.

Surely, if I remember correctly, when they do the rear wheel brake tests on the rollers for the MOT, they brake the rear wheels seperately, which will cause far more problems with the Torsen rear diff as this operates and distributes torque far more quickly.

Oh and yes, EvoDelta, would be great to have that VC article on the site. Could you send me through a copy / link to it.

Steve

The VC does not suffer from any increased wear, it either overheats and gets damaged (the plates buckle) it or it simply locks up, transfers the load and winds up the diff damaging this and/or the gearbox instead. It was certainly an eduction taking it all apart!



Can you PM me your email address Steve? I have the article in a 2600k Email format already with the pics attached. It would be an honour to donate it, I find your site very useful and informative. Please feel free to edit or add where neccesary as it is written with the integrale owner in mind.

[Steve Webb]

The VC does not suffer from any increased wear, it either overheats and gets damaged (the plates buckle) it or it simply locks up.

Oooooookay, so from your quote above, a VC either works or doesn't. And from an earlier post

As the VC gets older the silicone liquid in it loses its viscosity and it slips easier than when it would have done when it was new or if it has been uprated or renewed,

Now as an example someone runs tyres that result in a different front/rear rolling circumference, which in turn means a rotational difference that needs to be taken up by the VC. Now the work that the VC has to do results in the liquid in the VC heating up, but not to the point where it will overheat and physically damage the internal plates. Will this not cause the silicone liquid to lose its viscosity at a faster rate than if the rolling circumferences front/rear were the same and the VC were sat there essentially doing nothing.

I've never had to think about the ins and outs of the VC so am genuinely interested.

Cheers

[Evodelta]

Oooooookay, so from your quote above, a VC either works or doesn't. And from an earlier post:

Yes/No! This quote of mine is a retort as to what happens when the transmission is abused and forced (MOT rollers) Not under normal circumstances, where it slips as to how much is needed of it.

Now as an example someone runs tyres that result in a different front/rear rolling circumference, which in turn means a rotational difference that needs to be taken up by the VC. Now the work that the VC has to do results in the liquid in the VC heating up, but not to the point where it will overheat and physically damage the internal plates. Will this not cause the silicone liquid to lose its viscosity at a faster rate than if the rolling circumferences front/rear were the same and the VC were sat there essentially doing nothing?

I've never had to think about the ins and outs of the VC so am genuinely interested.

Cheers

Well almost, the silicone fluid gains viscosity due to heat not loses it, in simple terms - the friction caused by the plates slipping immediately heats up the fluid which thickens and causes the plates to lock directing the torque to the wheels which are gripping. I think that in this instance of only the most gentle of slippage there will be no heating of the fluid.
The fluid is not only rated as torque (see below) but at temperature too, I have noted the location of the VC; It is outboard and away from the rest of the transmission so it can run cooler and be less affected by the engine and gearbox temps.

The original fluid was rated at 120Nm, (but will by now have gone even less viscous) which allows for the difference in tyre circumference, but not extreme abuse, I have 300Nm fluid in my track car, I would expect this to be less forgiving to tyre diameters, it's certainly less forgiving to drive! Although this is also due to many other modifications, certainly when you jack the car up on one side you cannot spin the f+r wheels easily by hand like you could before, it's a struggle with both hands.

<<I've never had to think about the ins and outs of the VC so am genuinely interested.>>

Well, discussing VCs is far more interesting than watching Saturday night TV, X Factor anyone....? (You must have some similar nonsense in France too)

[Steve Webb]

Well, discussing VCs is far more interesting than watching Saturday night TV, X Factor anyone....? (You must have some similar nonsense in France too)

We've got a sky dish over here, so there is no getting away from X-factor or Strictly come dancing or any of those bloody programs. Don't help that the missus is a fan. Mind you British TV is far better than French, apart from the stuff late at night obviously.

Anyway whats the reasoning behind the size/shape/number of plates in a VC, the one on the Range Rover is far larger in diameter, but only about 6" long?

[Evodelta]

Hopefully you got my email?

Oh dear, I see you are treated to the same rubbish on a Saturday night as I am! No Topgear tonight i'm afraid either due to the snooker, I shall instead seek solace in one of Leeds' many fine restaraunts.

I would initially say that the bigger plates will be there to spread the load of the large amounts of torque created by a (hopefully*) large engine trying to drive those huge wheels. But then again.....

You could have a longer VC with more plates, or a shorter one with less plates and still have the same surface area to work with. It could also have been sized that way simply to fit in where it does, or taken from another vehicle.



* On peering under the bonnet of a newish Landrover recently I was rather dismayed to find a Rover K series engine in there, it was akin to being taken to the engine room of HMS Invincible and finding a Hyundai 1.3 coupled up to the propeller....

[Steve Webb]

Heres a pic of the RR VC to show you how different they are in shape,

Must be down to the torque requirements and how they react to slippage.

Anyway, I know what you mean about LR putting silly little engines in the freelander, seems abit daft to me. You'll be reassured that the engine room on the RR is filled nicely with a nice big lump of Solihul V8.