Why is the XC not permanent 4x4?

[revolver]

Can somebody please clarify why Volvo chose a four wheel drive system which is not permanently engaged. There are many systems in other cars which seem to have no ill effects from this, (Subaru/Land Rover/ VWTouareg) and therefore do not need the complication of a Haldex or rear disengagement mechanism.
I remember seeing a road test of a FF conversion of an Opel Monza which was compared with the standard car. The FF was heavier but the performance and fuel consumption were the same, which the testers accounted for by the all wheel drive overcoming the rolling resistance. The system had two viscous couplings, one for the centre diff and one across the rear, splitting the torque 70% to the rear and 30% to the front giving a rearwheel drive feel to the car. But the point they made about the viscous units was that not only did they divide the torque evenly driving the wheels but they also divided the braking forces. Volvo on the other hand seem to say that they need to disconnect the drive to the rear wheels for the best braking.
I have no complaint about my car, a 2001 XC70 which is working fine after 125,000 miles, but it seems that a lot of the problems on this site concern the rear coupling. Another thought I had on that was since the word 'snow' turns up a lot with the lack of rear wheel drive, could it be that the viscous fluid is too cold to heat up to work properly, especially if it has not been changed for a while.

[BillAileo]

The later model Volvo's do not use viscous couplings for the AWD. I believe the Haldex electronic unit which replaced the viscous coupling are used because supposedly one gets better milage only using AWD when necessary. I have found the system to work very well on both my XC70 & AWD S80.

Bill

[budrichard]

You need to understand what a 4x4 is versas an AWD.
A true 4x4 has two drive shafts from the engine to each of the front and rear axles with no other connection, consequently the axles always are turning and if on solid pavement where there can be no tire slippage, the slightly different rotation of front versas rear will build up a torque differential. To counter this torque, most 4x4 systems now employ either a full or part time selectable third differential to allow for this torque to dissapate.
Todays AWD systems employ some sort of power direction to all four wheels in varying methods, none of which has proven superior to another but they are not true 4x4 systems in most cases but functionality is what counts.
The electronic Haldex is used in many vehicles and has proven acceptable over time and in fact is the only way to couple the Stability type of control to the AWD system.
So, I don't know of any true permanent 4x4 systems on the market today, the last was the IH Scout, where you both connected the front wheels at the axle at the hubs and then shifted into 4x4. Later systems did this connection as you shifted and this type of system is still used by Jeep called 'Command-Track'. Jeep's 'Select-Track' system is 4x4 with the addition of a permanant 4x4 mode which merely engages the third differential but it is not true 4x4 as axle slippage is allowed and will bog down in deep snow or mud.
Most systems today are AWD in some form or varient and not true permamant 4x4.
The difference in performance is only noticable in conditions that call for constant power application so for most vehicles that never go off road, no one cares and for those that do, most forms of AWD are just as capable and in some situations probabaly more capable.
I haven't done a survey for a couple of years of all the 4x4 and AWD systems but all 4x4 systems from Porsche on down employ some sort of system or systems to disengage the front from rear axles under difficult off road conditions and the distinction between what is really 4x4 versas AWD has become blured so you have to read real close between the lines to try to understand how these systems really operate and the truth is the buyers don't care, all they want to see is AWD on the badge which is fine with the manufacturers because all they have to do is make the vehicle high profile, call it an SUV and now 'Crossover', install some sort of AWD system and the buying public thinks that they drive a true off road vehicle. Since these vehicles are never driven off road, they never find out what they really purchased.
Frankly, I've owned all the major systems and the latest E-Haldex employed by Volvo is the best for most on and off road driving which of course is why I drive one among other reasons.-Dick

[revolver]

Thankyou for that detailed reply Dick, and I think that you have answered my question by bringing the stability control into the equation. Also thinking about my own question I suppose that it is very awkward and expensive to engineer a differential into the front wheel drive transmission and thus a simple bevel to take the drive back is the solution with a decoupling device at the rear.
I am sure the Range Rovers and Land Rovers are all now permanent AWD with a centre locking diff if needed. I agree that ultimately these vehicles are only two wheel drive in extreme conditions unless fitted with a diff lock across each axle. I believe the Mercedes G Wagon could do this. The FF Ferguson system would end up with three wheel drive when the viscous couplings locked, one for the centre diff and one for the rear. I think that it was fitted to a few AMG Pacers.
As I said , like you I am very pleased with the XC and the way it has performed, (and I have tested it up a steep wet grassy bank with no problem at all) but looking at the amount of posts concerning loss of drive to the rear wheels of the XCs, I was trying to think of an answer!

[vtie]

Since these vehicles are never driven off road, they never find out what they really purchased.

In my experience, the AWD/DSTC coupled system of the XC70 is very useful, even if you stay on the road. In wet conditions, it makes a huge difference in traction and stability. My previous car was a pure front-wheel driven Saab 9-5, and that car was horrible when it comes to departing on a wet road, particularly when turning (e.g. entering a road from a crossection). In the XC70, it is very easy to notice that the real wheels are often being engaged in such conditions, and it's a good feeling.

[budrichard]

In my experience, the AWD/DSTC coupled system of the XC70 is very useful, even if you stay on the road. In wet conditions, it makes a huge difference in traction and stability. My previous car was a pure front-wheel driven Saab 9-5, and that car was horrible when it comes to departing on a wet road, particularly when turning (e.g. entering a road from a crossection). In the XC70, it is very easy to notice that the real wheels are often being engaged in such conditions, and it's a good feeling.

I never said that the Volvo wasn't very useful off road or on road, what I said/meant to say is that individuals purchase a high profile vehicle with an AWD system and think that they have an off-road vehicle which they don't have for a number of reasons, among them ground clearance, ability to lock out differentials and wheel articulation.
I believe the Volvo E-Haldex with DSTC IS very useful both off and on road!
As I said previously, that's why I drive one.-Dick