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Permanent 4WD and tyres


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2 hours ago, London Best said:

Joust my opinion:

Any diff which is not strong enough to withstand a few fractions of an inch difference in rolling diameter is not strong enough/has no place on/will not last long on a 4x4.

I had never thought of the difference in diameter until Scotslad mentioned it. 

 

But I am with you, again a difference of say 12mm tread depth is less than an inch difference in diameter. Surely it's not that significant?  

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1 hour ago, ClemFandango said:

I had never thought of the difference in diameter until Scotslad mentioned it. 

 

But I am with you, again a difference of say 12mm tread depth is less than an inch difference in diameter. Surely it's not that significant?  

Multiplied by 3.1417 for the difference in rolling circumference it starts to look more significant, but are we not talking about supposedly ‘tough trucks’?

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I must mention this to my independant VERY experienced tyre man next time I visit. I will have my ear plugs in because I do not like rude language:lol:

I have owned three Range Rover Classics, and a Land Rover 90 and have a friend who was one of the design engineers on those vehicles at LR and this subject has never been raised. 

 

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30 minutes ago, London Best said:

Multiplied by 3.1417 for the difference in rolling circumference it starts to look more significant, but are we not talking about supposedly ‘tough trucks’?

It's a fair point when you put it like that. 

I wonder how much difference it actually makes?

Edited by ClemFandango
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6 minutes ago, ClemFandango said:

It's a fair point when you put it like that. 

I wonder how much difference it actually makes?

I know if you run a Land Rover axle with a 7.50x16 on one side and a 205x16 on the other side it warms the diff up a lot! Needs must situation at the time.

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3 minutes ago, London Best said:

I know if you run a Land Rover axle with a 7.50x16 on one side and a 205x16 on the other side it warms the diff up a lot! Needs must situation at the time.

I'll take your word for that. I don't have the balls to try it myself. 

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8 hours ago, Walker570 said:

I must mention this to my independant VERY experienced tyre man next time I visit. I will have my ear plugs in because I do not like rude language

I have owned three Range Rover Classics, and a Land Rover 90 and have a friend who was one of the design engineers on those vehicles at LR and this subject has never been raised. 

 

Its not an issue on either of those vehicles transmissions!

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  • 1 month later...

Mk1 Kia Sorento has this  problem with 4wd on demand system iirc. The Kia was not meant to be a permanent 4wd vehicle only when it detected slip. Having different diameter tyres front and rear caused some sort of clutch to engage all of the time and it would wesr out. No real effect on the vehicle except it made it 2wd. Below is a summary from owners website. Maybe your vehicle has a similar system?

TOD system, these are well made transfer boxes but kia made a mess of the control system, these are keeping the dealers "very" busy, they are not 4x4 they are only 2wd but the transfer box controller monitors the prop shafts and switches in 4x4 as and when it thinks it is required, the problem is these have not to be in 4x4 on hard ground because they do not have a centre diff to prevent wind up, if the tyres are not the same make model ply rating etc then due to the difference in rolling radius the 4x4 starts to cut in and this burns out the mag clutch in the transfer box, 

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11 minutes ago, matone said:

Seems difficult to grasp for many but 2 axles turning at different speeds without a centre differential = something has to give!!!

What gives is down to the integrity of the drive train.

As the prevoius poster says this same can manifest itself with some AWD systems ,depending on the management system controlling 2nd axle engagement.

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On 20/08/2020 at 08:51, B725 said:

The Freelander 1 was the same if you were only changing two tyres, on my Freelander 2 I have the wheels changed around every service so the tyre wear is more even. 

Was told, when I bought a Freelander 1 years ago that it was set up to feel as though it was front wheel drive and the gearing was running quicker to the front, than the rear. Advised to put new tyres on the rear if only changing two so not to put extra stress on the IRD and rear diff. Didn't mention anything about the useless viscous coupling!

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I ran mud tyres on the rear and all terrain tyres on the front last winter and nothing bad happened, there must be a small difference in size between the two given the more aggressive tread pattern on the muds.  All my diffs are still intact for now.

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Iirc, it is vehicles with the Haldex system that usually require 4 tyres changing at the same time,  apparently it doesn't like to be out of alignment by more than 10mm or so. 1st used on the Audi Quattro rally cars in the 80s, overly complicated system in my view. 

S

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Nobody's really managed to put their finger on the problem yet.

When you turn a corner, each of your four wheels all end up travelling a different distance and therefore running at different speeds.

OR

If running in a straight line, if any of your four wheels are a different diameter they will be wanting to turn at different speeds

If you're in 4WD and your prop shaft doesn't have a centre diff splitting the front/rear axles, and the surface you're driving on isn't slippery, you're going to be suffering drivetrain wind-up - in other words, forces building up due to the wheels wanting to travel at different speeds.

This is what makes diffs and driveshafts go pop.

Just a random google search on the subject, well worth a read...

https://www.offroadaussie.com/transmission-wind-up-what-is-it-why-you-should-avoid-it/

Only stick it in 4WD when you have to!

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As I understand this it does not affect true open differentials.

It does however knacker viscous couplings like the one on the freelander (uneven tyres is a big killer of freelander viscous couplings) and certain other vehicles, and electronic systems that use differences in wheel rpm to do fancy stuff, commonly referred to as differentials because people then understand roughly what they do.

 

 

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On 20/08/2020 at 10:59, martinj said:

I wanted to change from General Grabber road tyres to General Grabber All Terrain on my Freelander 2 and was told to change all four at the same time. They said I would see warning messages due to the difference in diameter, if I only changed two.

Note:- General Grabber All Terrain are a bit noisy

Strange that, as I have G.Grabber AT3's on the four wheels on my FL2. Depending on the road surface, they are 90% quiet.

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Differentials - there is a clue in the name.  When driving in a straight line the differential enables both wheels on the same axle to rotate at the same speed.  When you get to a corner the inner wheel has less distance to travel than the outer wheel and the differential compensates for this without scrubbing either tyre.

There is another kind of differential, known as an LSD or limited slip differential - where that above compensation is limited to provide sportier handling characteristics. 

There are several different kinds of all wheel drive (AWD)systems, some are permanent and some part-time.  The latest top end systems can dynamically move traction between axles and even individual wheels to maximise grip through the use of differentials that have automatic clutches on each output drive shaft (to the wheel).

The part-time systems (i.e. on an AWD Freelander or Audi Q3) tend to use something like a Haldex.   The Haldex is an electro/hydraulic clutch unit that sits in front of of the rear differential.  A so equipped vehicle will be predominately front-wheel drive unless the ABS brake sensors detect that the front wheels are starting to rotate (can be as little as a 1/4 of a rotation) faster than the rears; when this happens then an electrical signal is sent to the Haldex which 'wakes up' engages it's clutch and the rear wheels are driven as required.   The advantage is that fuel and tyre wear savings are achieved.  The Haldex clutch can compensate for the front and rear axles rotating at slightly different speeds.  Tire changes in axle pairs are recommended.

The next rung up is permanent four wheel drive and this typically, on modern vehicles will find a clever centre coupling to distribute drive to each axle - some are simple 50/50 splits, others are much more sophisticated and will shift drive (up to 85% on some systems) to whichever axle has got the most grip. These systems typically need all four tyres replacing together - they will all wear at roughly the same rate anyway.

 

 

 

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