XC60 D4 AWD rear wheel torque

[Quacker]

Originally Posted by Philip Fisher

You mean that they don't have a "locking" centre and cross axle diff I assume?

They would have to have a cross axle and centre diff or they would be undriveable.....

No, I actually mean that they physically do not have differentials at the centre or rear. The back consists of a crown wheel and pinion shaft to turn the drive through 90 degrees to the half shafts, plus a clutch pack either side which are independently variably engaged as drive is required to either wheel.
Obviously the main drive to the rear needs to be at least partially engaged to deliver drive to the rear, and the clutch pack for that is all there is also.
As I understand it, the only differential on these cars [Evoque and Disco Sport] is the one for the front drive axle, which is part of the transmission/gearbox. This system enables electronically controlled torque vectoring for every wheel for all conditions.

Ive just done a little search to see whether this is explained with illustrations somewhere and found this..
https://www.gkn.com/en/our-divisions...2016/twinster/

Note that it doesn't specifically mention the lack of differentials in that explanation but I think you'll agree that it is self evident from the illustration of the back axle cutaway.
I was unaware that it is also used in the Ford Focus RS but it now makes sense as to how they were able to provide the now notorious 'drift mode' to that model.

Maybe I should add that there is no physical centre diff in either the Honda system or indeed the Volvo's Haldex system or any other Haldex application. The necessary differential speed between front and back axles is taken care of by disengaging or variably slipping wet clutches. This is why these systems are not sensitive to variations in tyre wear front to back, where machines with an actual centre diff are. In a physical diff, difference in rotational speeds of back and front result in the pinion gears** spinning rapidly, where if the two shafts [front and back in this case] are rotating at the same speed, as in driving a straight line, the little pinion gears at the centre of the diff do not themselves rotate at all. They only rotate inside the diff when there is an output speed difference.

** The pinion gears and pinion shaft are two quite different parts. The pinion shaft is the input shaft with the tapered helical gear that drives the crown wheel to which the differential cage is attached.

[ukden]

Originally Posted by Quacker

Ive just done a little search to see whether this is explained with illustrations somewhere and found this..
https://www.gkn.com/en/our-divisions...2016/twinster/

Are you saying that the MY18 XC60 uses the GKN system and not the Haldex one?

[Haldex]

No, all non-T8 SPAs have the Borg Warner (Haldex) unit.

Here's a simplified scheme of the SPA AWD drivetrain:



T8s (not true AWD- as there's no propeller shaft..) use one of GKNs solutions:

https://www.gkn.com/en/our-divisions...ies/2016/xc90/

[familymangreg]

OK, so go easy hear, I don't understand an awful lot about differentials etc....

When we were in the lakes a few weeks ago, we parked at the side of the road, on a gritty and slightly lumpy bank. There was plenty of space and I didn't think the car was going to a have any issue getting off the road. However, as we drove forwards up the bank, the car ground to a halt and wouldn't get beyond a certain point, we had several attempts, gently building the power each time. The front nearside wheel was over a dip in the ground and just about not touching, but the car seemed to be putting all the power to that wheel, which was clearly just spinning.

I couldn't understand why the car was not putting power to the rear wheels which appeared to be doing nothing. If it had I'm sure it would very easily have moved forwards.

Having read the above, I suspect that putting the car in off-road mode might have put additional torque to the rear wheels and dealt with the situation. Would you expect this to be the case?

:wq

[ukden]

Originally Posted by Haldex

No, all non-T8 SPAs have the Borg Warner (Haldex) unit.

Thanks, I thought I had lost the plot.

Situation normal, just mildly confused.

[Quacker]

Originally Posted by ukden

Are you saying that the MY18 XC60 uses the GKN system and not the Haldex one?

Volvo use Haldex on all but the hybrid I think. The hybrid uses a GKN rear axle, but I'm not sure yet as to its design and whether it is a Twinster variant or one unique to Volvo/Geely

[Quacker]

Originally Posted by Haldex

No, all non-T8 SPAs have the Borg Warner (Haldex) unit.

Here's a simplified scheme of the SPA AWD drivetrain:



T8s (not true AWD- as there's no propeller shaft..) use one of GKNs solutions:

https://www.gkn.com/en/our-divisions...ies/2016/xc90/

That illustrates what I said about Haldex [and the GKN and Honda systems] in that it clearly shows that there is no centre differential between the front axle and back axle. Just a clutch pack. There are only two differentials, one across the front axle and one across the rear. In the GKN system used by LR, Ford in the RS, and Volvo in the hybrid, there is no rear differential either, only the one across the front axle, which is built inside the transmission final drive.

The Volvo's [non-hybrid] 95% to 5% drive balance front to back in normal driving isn't actually any drive at all to the back. It is just the parasitic drag within the clutch pack at the back end, at the front of the rear differential, due to the viscous quality of fluid between the discs and separator plates and their very close proximity. It just happens to transmit about 5% of the torque through to the back wheels even when the pack is fully disengaged.

[Haldex]

Originally Posted by Quacker

The Volvo's [non-hybrid] 95% to 5% drive balance front to back in normal driving isn't actually any drive at all to the back. It is just the parasitic drag within the clutch pack at the back end, at the front of the rear differential, due to the viscous quality of fluid between the discs and separator plates and their very close proximity. It just happens to transmit about 5% of the torque through to the back wheels even when the pack is fully disengaged.

Yes, I'm sure this is how it ends up with the 5% sent to the rear..

[Haldex]

Originally Posted by familymangreg

Having read the above, I suspect that putting the car in off-road mode might have put additional torque to the rear wheels and dealt with the situation. Would you expect this to be the case?

Well, offroad mode would ensure 50% of torque is sent to the rear wheels- but in any mode, with one front wheel spinning the system should automatically divert the torque to the non-spinning wheels.. See the demonstration vid:

https://youtu.be/c2ScHaitFSY

Could be an electrical problem of some sort? (haldex pump?) I'd have it looked at by the dealer in any case..