Video of AWD in action. Not impressed!

[Michae1]

I don't own a V70XC but I've owned a 1998 V70R in the past and am seriously considering buying my first XC soon.

While doing my homework I came across this youtube video of the MY08 that seems to indicate to me a shortcoming the the Haldex AWD system.

You will find the video here. Just for your info seems to have been filmed in Israel.
http://www.youtube.com/watch?v=7uf923rKHpA&NR=1

In particular look at 2:40 to 2:50.

It seems to me that the one wheel that has no traction (because it is clear off the ground) is receiving all the torque while the wheels on the ground have virtually none.

Can someone please explain to me what it happening in this video?
Would the previous generation of AWD system handle the situation in the same way?

Thanks for your help.

[vtie]

I'm no 4x4 expert, but it appears to me that this is a typical situation where two crossing wheels lost traction (right front + left back). In such a case, straight awd is not sufficient, since the differentials on both axles will simply make the wheels without traction spin. That is why a real offroader has a locking differential.
In the case of the XC70 (and several other mild softroaders), this is mimicked by an electronic trick, using the DSTC circuit: the wheel that is spinning too fast is braked. In that way, the electronics avoid that the differential sends all the torque to the fastest spinning wheel. It's not as robust, but second best to the real thing. And it looks like, in the video, it got the job done finally.

To be honest, I think that this video shows that the XC70 is really more than up to the task it was designed for. I always considered it a station wagon that has the capability and confidence to take any road. I know very few station wagons that could do what the XC70 did in this video. This should be more than enough for the vast majority buying an XC70.

[Michae1]

Thank you for the reply vtie.

Yes that was my understanding too, that the brakes are applied to the spinning wheel so that the torque is transferred to the opposite wheel that has the grip. What I could not understand is why it was taking so long. I'd have thought that there was no need for the system to "think" about it, it would just happen without delay (milliseconds ).

There is another scenario that I wondered how the Haldex system would handle and that is if all four wheels are slipping. For example if you are on a muddy incline. The front right wheel starts spinning, so the brakes are applied to that wheel and the torque is transferred to the front left wheel. When that too starts spinning, up to 50% of the torque is transferred to the rear axle. Now while the front wheels continue to spin, the rear right wheel starts spinning. Again the brakes are applied to that wheel so that the torque is applied to the left rear wheel.

Now my question is, what does the Haldex system do when that last wheel starts spinning. If it applied the brakes to that wheel, well then both wheels at the rear would have the brakes applied. Hmmmm. In fact the front wheels would be in the same situation too.

Also what do people think about the fact that the MY2008 is only available with a space saver spare tyre?(At least that is the situation in Australia) I have a real problem with that for three reasons.

One. I simply don't think it is appropriate to use a space saver tyre on anything but a tarred road, or at least a DRY, smooth, dirt road for obvious reasons.

Two. Where is the full size tyre supposed to go? It's too big to go where the "space saver" tyre was being kept, and if you have your car full of camping gear, there is no room left for it to go, never mind the safety issues associated with having a wheel in the back of the car. What happens if you then have a frontal collision at 60 km/h??

Three. Doesn't the space saver cause excessive stresses on the Haldex system since it is of a smaller diameter therefore spinning faster than the opposite wheel?

I dismissed the Audi Allroad because it had a space saver!

Thank you.

[Michae1]

P.S.
I dismissed the Audi Allroad because it had a space saver!

I decided against the Subaru Outback because it doesn't have a cargo net built into the back seat that can be used when the back seat is down. It doesn't have the refinement of the Volvo (you know what I mean! especially after owning a V70R which had the Alcantara heated seats and a Dolby Pro Logic sound system), and most importantly I simply can't believe it is as safe as a Volvo XC70, especially during a roll over accident.

[PJ810]

In Australia the XC70 can be ordered with a full-sized spare. However, the tray that is there (under the floor) will need to come out. Your dealer will be able to demonstrate this.

I am however of the opinion that ALL cars sold in Australia should feature a full-sized spare due to the vast unpopulated distances and harsh (lethal) climatic conditions should one become stranded.

I carry 2 full sized spares when I go on long trips. Furthermore, a space saver will wreck your AWD system. Not sure how it affects Haldex but the viscous coupling unit in the older Volvos won't last long.

[tgwillard]

There are times when you want the wheels to slip and not have the DSTC apply the braking action. When driving in snow and/or ice, wheel slippage is to be expected with the Haldex providing power to all four wheels. I believe there is a switch to turn off the DSTC in situations like this.

[kave]

I just ordered a XC 70 today, and I am really impressed with that video.
My Subaru would not handle the same situation, it has proved many times.
Yes yes yes :)
Now if anyone can find out how to disable DSCT completely so I can enjoy my 4WD Volvo in snow to do some drifting :)

[staffann]

Thank you for the reply vtie.

Yes that was my understanding too, that the brakes are applied to the spinning wheel so that the torque is transferred to the opposite wheel that has the grip. What I could not understand is why it was taking so long. I'd have thought that there was no need for the system to "think" about it, it would just happen without delay (milliseconds ).
The TRACS functionality of the brake system requires the wheel to spin with a certain speed in order to react. I do not know about the cars from the new platform, but I know that the performance on cars of the older P2 platform was quite bad in this respect. But even with cars where the manufacturer prioritized this functionality, the performance would be much, much worse than with a proper clutch-based solution.


There is another scenario that I wondered how the Haldex system would handle and that is if all four wheels are slipping. For example if you are on a muddy incline. The front right wheel starts spinning, so the brakes are applied to that wheel and the torque is transferred to the front left wheel. When that too starts spinning, up to 50% of the torque is transferred to the rear axle. Now while the front wheels continue to spin, the rear right wheel starts spinning. Again the brakes are applied to that wheel so that the torque is applied to the left rear wheel.

Now my question is, what does the Haldex system do when that last wheel starts spinning. If it applied the brakes to that wheel, well then both wheels at the rear would have the brakes applied. Hmmmm. In fact the front wheels would be in the same situation too.


When you start in a muddy incline, things would work something like this: As soon as you give gas, the Haldex will start to transfer a significant amount of torque to the rear axle. Because of the weight transfer to the rear (due to the incline) it is still probable that one of the front wheels will be the first one to spin. The Haldex will see this and put even more torque to the rear until the speed of the front axle and rear axle are about the same. If it was only one front wheel that started to spin, it is possible that the TRACS system will apply the brakes on that wheel as well (but it will react slower than the Haldex).

If the Haldex transfers enought torque to make a rear wheel spinn, the brake will be applied on that wheel as well. The brakes will not be applied to more than one wheel on the same axle since the distribution between the axles is handled by the Haldex.

The above assumes that the antispinn system is disengaged, otherwise the engine torque will most likely be reduced as well. In cases like this it may be an advantage to turn of the antispinn - at least if the driver is experienced.

[staffann]

Now I've watched the film clip as well. In the driving situation between 2.40 and 2.50 it is the case that one front and one rear wheel has lost grip (hanging in the air). The TRACS system therefore has to activate and brake the wheels without grip so that torque can be transfered to the other two wheels. One can see the TRACS system working. In this case, the driver doesn't give enough gas - when the brakes are applied half of the engine power just becomes heat in the brakes so it is necessary to give a little more gas. In this case I cannot say that the driver does anything wrong - he is just a little bit carefull.

[mortensv]

Staffan, are you saying that the DSTC (isn't that what it is called on the newer Volvos?) will brake a spinning wheel even if the antispinn is disengaged?

[staffann]

The TRACS functionality (the T and C in DSTC), which acts as a poor mans electronic diff brake on each axle as I described before, is active even with antispinn off in the P2 platform. I would be surprised if it was any other way in the new cars.

It is common and in my experience essential to switch if the antispinn in some offroad conditions like sand driving. Otherwise the engine torque can be cut to the point where the vehicle stops moving because the system detects some spin. In order surfaces than sand you'll have to try it for yourself to see if antispinn on or off is the best. The Volvos are optimised for bad road use and not for offroad. Also different surfaces require the systems to be optimised differently. That is why the LandRover LR2 (Freelander 2) has a knob allowing you to choose between 4 different terrain modes depending on the surface that you drive on.

Note that turning off antispinn does put some extra demands on the driver in order to avoid too much torque leading to the car digging itself in.