So I'm new here and a new owner of a Freelander and I've seen plenty of posts making reference to the need to have evenly matched tyres on all four corners to prevent drivetrain problems....

Mind if I ask why? I've only seen references to having different tread depths or tyre pressures causing problems but nothing to support the theory, not challenging this as such but am interested to know what the impact tread depth / pressures have and how this can effect a vehicle that bumps along at varying speeds on varying terrain?

Cheers in advance oh and Happy Christmas!

It has been discussed on here many times. eg...

https://www.landyzone.co.uk/land-rover/vibration.270721/#post-3286682

Have a read of this thread as well to show how 4 seemingly identical tyres, but that slightly different, can screw up the transmission...

https://www.landyzone.co.uk/land-rover/feels-like-driving-with-brakes-on.260939/

not sure but think it is something to do with winding up prop shafts, difference between front and back tyres and then side to side is a winding up of differentials, ie if there to great a difference in rotation of opposite shafts they eventually either bust the shaft or wreck the diff. In actual fact the bumpier the ride the better cause that would then release the tension on the shaft. I am sure there is a more scientific way of putting it but I'll leave that to the experts.

FreddieA said:

So I'm new here and a new owner of a Freelander and I've seen plenty of posts making reference to the need to have evenly matched tyres on all four corners to prevent drivetrain problems....

Mind if I ask why? I've only seen references to having different tread depths or tyre pressures causing problems but nothing to support the theory, not challenging this as such but am interested to know what the impact tread depth / pressures have and how this can effect a vehicle that bumps along at varying speeds on varying terrain?

Cheers in advance oh and Happy Christmas!

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The freelander problems are well documented and are typically suffered due to prolonged on road driving when the permanent 4 wheel drive train is under greater stresses, the freelander drive train is subject to the increased grip offered by tarmac which can be made worse by mismatched tyres etc.

FreddieA said:

but nothing to support the theory

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Thanks for the replies, I've read the linked posts and prob all that mention this phenomenon yet I'm still confused (or just stupid).

If its winding up of the transmission from mismatched tyre size that causes the problems, wouldn't this happen regardless of tyre size unless the hubs were always turning at the same rate which can't happen during tight cornering, when the traction control is active or hill descent is in use?

Thanks again and sorry for the stupid questions!

The 4X4 system contains a viscous coupling (VCU). This is the device that transfers torque from from to rear, when the front looses grip. However for the VCU to react quickly, it is designed to allow only a small amount of slip,before locking. If the tyres aren't all the same, the VCU can potentially try to transfer torque when not needed. If torque transfer happens on concrete, huge amounts of load is put on the drive train. This will make short work of the IRD and rear diff. Even a low tyre can cause catastrophic IRD failure. So for reliability the Freelander needs 4 identical tyres with the least worn on the rear.

The rear diff is a slightly different ratio to the front. Basically the rear diff is a slightly lower ratio than the front. This means the rear is turning the rear shaft of the VCU faster than the front VCU shaft. The VCU is designed to compensate for this, to a point. When cornering, the slightly lower diff ratio equalises the VCU shafts rotation speeds. When manoeuvring the VCU will transfer torque, but the slower slip rate keeps torque to a minimum and doesn't cause damage.
When HDC is in operation, the wheels don't lock, so the VCU isn't transferring excessive torque.

The IRD and diff are damaged with sustained overloading with incorrect tyres, not brief torque spikes like when manoeuvring or turning corners.

You should bare in mind that a VCU operates on a rate of slip basis. At low rates of slip it will not transfer much torque (assuming it is in good condition). When the rate of slip increases it changes to transferring a lot of torque.... so that the rear wheels will provide traction when a front slips.

TBH I'm not sure anyone on here knows exactly how a VCU operates - even though some may tell you they do.

It operates in 2 modes, viscous mode where the viscocity in the fluid drags plates around and once the slip gets to great it goes into hump mode where the VCU is essentially locked up. Unlike the popular thought that the fluid is non-Newtonian, or Newtonian I forget which, dyno tests shows that as slippage increases the rate of torque transfer lowers - however, if the rate drops by 10% but the engine is pumping out 50% more power - the net transfer is greater. Similarly a VCU can not permanently be in hump mode - as this would imply no slippage and no hump mode - however as soon as hump mode is lost, the rate of slip is back up to hump mode levels - so you are immediately back into hump mode - so essentially it is permanent hump mode.

As you can see, it is all very simple stuff!

Lord @Hippo has done lots of 1 wheel up tests. On a good working VCU it shows that a 5kg weight on a 1.2m pole should give you a time of under 60 seconds - as its a 1/4 turn, that's 0.25 rpm. It also shows that if you double the weight, the time is reduced by only 32%

Obviously, if you are doing a tight turn you will be going very slow, so even if there is a "large" rotational difference between front and rear axles the actual rate of slip should not be too large.

However, at 70mph the prop shafts and VCU are spinning at about 3000 rpm. So a small difference in tyre circumference of 1% equates to a slip rate of 30 rpm. If my calcs are correct, this is the equivalent of a 1 wheel up test with 3,000,000 kg on the bar!

Obviously something isn't right in those calcs - but it does show you how much stress a tyre size difference can put on the transmission..... I think.

GrumpyGel said:

You should bare in mind that a VCU operates on a rate of slip basis. At low rates of slip it will not transfer much torque (assuming it is in good condition). When the rate of slip increases it changes to transferring a lot of torque.... so that the rear wheels will provide traction when a front slips.

TBH I'm not sure anyone on here knows exactly how a VCU operates - even though some may tell you they do.

It operates in 2 modes, viscous mode where the viscocity in the fluid drags plates around and once the slip gets to great it goes into hump mode where the VCU is essentially locked up. Unlike the popular thought that the fluid is non-Newtonian, or Newtonian I forget which, dyno tests shows that as slippage increases the rate of torque transfer lowers - however, if the rate drops by 10% but the engine is pumping out 50% more power - the net transfer is greater. Similarly a VCU can not permanently be in hump mode - as this would imply no slippage and no hump mode - however as soon as hump mode is lost, the rate of slip is back up to hump mode levels - so you are immediately back into hump mode - so essentially it is permanent hump mode.

As you can see, it is all very simple stuff!

Lord @Hippo has done lots of 1 wheel up tests. On a good working VCU it shows that a 5kg weight on a 1.2m pole should give you a time of under 60 seconds - as its a 1/4 turn, that's 0.25 rpm. It also shows that if you double the weight, the time is reduced by only 32%

Obviously, if you are doing a tight turn you will be going very slow, so even if there is a "large" rotational difference between front and rear axles the actual rate of slip should not be too large.

However, at 70mph the prop shafts and VCU are spinning at about 3000 rpm. So a small difference in tyre circumference of 1% equates to a slip rate of 30 rpm. If my calcs are correct, this is the equivalent of a 1 wheel up test with 3,000,000 kg on the bar!

Obviously something isn't right in those calcs - but it does show you how much stress a tyre size difference can put on the transmission..... I think.

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A correctly working VCU will transfer ~ 440Nm of torque at 75 Rpm speed differential between shafts. So I think your 3,000,000 is a bit off GG.

Another thread for insomniacs !!

It is a surprise that none of this has ever been discussed on here before

Thanks for all the replies... think I'm getting my head round it a bit better now.

I know it's been discussed on here but not concisely or supported by with the theory (at least as I could see!) behind it as such so thank you

FreddieA said:

Thanks for all the replies... think I'm getting my head round it a bit better now.

I know it's been discussed on here but not concisely or supported by with the theory (at least as I could see!) behind it as such so thank you

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The Freelander isn't alone in this. All 4wd cars should be treated the same.

Alibro said:

The Freelander isn't alone in this. All 4wd cars should be treated the same.

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The plot thickens, never had any tyre size/pressure related drivetrain issues with and of my previous 4x4's which include a couple of Impreza's, merc ML, Navara, Series II and Disco along with the numerous 4x4' that my old man's had over the years...?? This is realy why I was confused in the first place..,

FreddieA said:

The plot thickens, never had any tyre size/pressure related drivetrain issues with and of my previous 4x4's which include a couple of Impreza's, merc ML, Navara, Series II and Disco along with the numerous 4x4' that my old man's had over the years...?? This is realy why I was confused in the first place..,

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If you have one wheel rotating at a significantly faster or slower speed than the other wheels (because it is soft or wrong size) then it will cause transmission windup. This is a known issue with Freelanders and I've been told a potential issue for other 4WD cars. How much it effects other cars I don't know but since no tyre depot in the country and only a handful of owners will know anything about it then I suspect 99% of Freelanders out there have mismatched tyres.
This is one of the reasons Freelanders have bad reputation for blowing up transmissions.

FreddieA said:

The plot thickens, never had any tyre size/pressure related drivetrain issues with and of my previous 4x4's which include a couple of Impreza's, merc ML, Navara, Series II and Disco along with the numerous 4x4' that my old man's had over the years...?? This is realy why I was confused in the first place..,

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The Navara and SII are "switchable 4WD" and are 2WD most of the time. If you put them in 4WD there is no center diff and wind up will occur - but you only put them in 4WD on terrain where wheels are slipping - so not a (big) issue.

The Disco is "permanent 4WD", but has a center diff. So the center diff will 'absorb' any differences in axles speed - but if tyres are too different in size, it may not distribute torque as well. Once in diff lock, same as above applies.

I'm not sure on the Imprezza, but the Legacy is "AWD" - like the Freelander. It needs matching tyres or else problems do occur through wind up. With the Legacy though, there is an electronically actuated clutch and when problems occur, you can pull the fuse on the clutch and it won't wind up.

The term AWD All Wheel Drive seems to have changed over the years from what it originally meant. Int UK about 20 years ago Volvo started making vehicles which could drive all 4 wheels as opposed to just 2. The market already had 4x4's like the tratter and RR (the Freelander was still int development at the time). These were vehicles built to drive all 4 wheels and go oft road, as opposed to what Volvo were about to start making: road cars with ability to drive all 4 wheels. Volvo wanted to make 4 wheel drive vehicles but had to disconnect them from the eggsisting 4x4 market. The term AWD was created to allow them to brag the vehicle could power all 4 wheels, but not state it was an oft roader as per the traditional 4x4 meaning of an oft roader.

Over the years the 4x4 and AWD meaning seem to have changed. Both can power all 4 wheels but the 4x4 branded product is built to go oft road as it's physically stronger with increased ground clearance. An eggsample would be the structural strength to cope ok balancing on 3 of 4 wheels oft road. The AWD isn't built to do this regularly (or at all) and dun't have the ground clearance or increased torsional strength. Hence the AWD name given to cars without the additional ground clearance of a 4x4.

Peeps and manufacturers have gone a step further with this by differentiating different types of drive (and control of) to the wheels as being 4x4 or AWD. This adds a bit of confusion as it goes against the original meaning. Some of this is down to the "eco marketing" hype of whatever theory they're trying to push to make a sale. For eggsample both the FL1 and FL2 are 4x4's. Both have increased ground clearance and additional strength for oft road use. The FL1's 4x4 set-up is there and available all the time but only triggered when yer need it. The FL2 has 3 options: automatically engaged when needed, if the owner selects it permanently via optional terrain response setting or when it pulls away from stationary (momentary selects 4x4 to reduce wheel spin when pulling away).

The AWD drive systems provided in normal cars give the same desired effect of driving all 4 wheels when required, but they don't have the 4x4 physical strength of our trusty go anywhere ard as nails Freelander's.