Foz AWD systems / Sand capabilities / SF thru to SJ

[Tweaksta]

My recent experiences in deep, soft sand at Coongie Lakes has got me thinking about the ways that Subaru have refined and (hopefully) improved their AWD systems throughout the years.

Let me give you some background about my findings:

Up until my most recent trip, I have always tried to maintain a low speed and maintain traction when driving on mud, clay and sand - eg: when driving along a sloshy muddy track in the Capertee Valley in pouring rain I would hold it at 10-15 km/h and I could feel all four wheels pushing us along until a slippery bit - then I'd feel the LSDs & AWD system do some easy work and we'd continue onwards.

At Coongie on sand the above method only got me so far. Once I had reached the steeper part of the dune, in order to maintain momentum and keep afloat on the sand, I found that higher RPMs were necessary resulting in more of a rally style where all four wheels where rotating faster than they would be in a higher-traction situation. I'm by no means an expert on sand dune driving - but I'm thinking this style is just part and parcel with the conditions.

Then, when I did a sharp turn too slowly I sunk right in. The only way out was to apply a great deal of power at high RPM and get all four wheels spinning (saved by the LSD).

Now, I am quite happy to employ these methods in my current SF as they are pretty fun and easy to use.....and it did get me there and back again!

But I guess the question I am trying to ask is about the newer Subies with VDC and X-Mode. It is inevitable that I'll end up with a new Foz one day. I think I want X-Mode. I just wonder how it performs or copes with soft sand dunes which need to allow some wheelspin to occur.

1. Would the VDC just be going bonkers with lots of small slippages being detected at all four corners every couple of milliseconds?

2. Would X-Mode kill off the throttle response needed to maintain momentum and 'float'.

3. Have any Offroadsubarus members with newer models done any tests or have any tales to tell?

4. On your average two week trek across central Australia (or USA) how much wear would the system place on your brake pads?

Would love to hear everyone's thoughts.

 

[pezimm]

Hey Twesksta,

Interesting topic and I would love to hear back from the techies out there.

I would've thought that the system is designed in a way to detect slippage differential, so that if 1 of the 4 wheels starts to slip a lot more that the other 3, it gets braked a little until it complies.

If we look at more developed traction control systems, like Land Rover's terrain response (I think that's what it's called), they have different settings to suit different terrains. So the traction control or vehicle dynamics control, would behave differently, allowing more or less slippage...

I think there's where the future lies for 4WDs:
1) Great torque curve on engines
2) CVT transmissions to put the optimum torque out to the wheels
3) Fine tuned and well developed VDC or TC systems to suit

As I said, very interesting topic...

Pedro.

 

[Beigewagon]

At least one of the members of the Vic Subaru 4WD club has the last model Forester before X-Mode. The member with the auto XT has to switch off the VDC/TC in sand so as to not get stuck. The VDC/TC applies the brakes to the spinning or slipping wheels, and when most wheels are slipping, it tries to stall the car.

Beigewagon.

 

[Kevin]

My Triton has ESC stability control, and traction control. When offroad I turn ESC off as it gets very confused in slippery conditions. Traction control stays on unless I remove the TC modules as there is no switch. While I did encounter a few deep soft patches in the Simpson mid-year TC did not seem to interfere; however, I think for deep soft beach sand dunes I will need to remove the TC modules. It would be a similar situation for the Subies I believe.

BTW - I have had the Triton stall in circumstances mentioned by BW above. Very unusual for an auto to stall!

 

[Shiv]

1. Would the VDC just be going bonkers with lots of small slippages being detected at all four corners every couple of milliseconds?

2. Would X-Mode kill off the throttle response needed to maintain momentum and 'float'.

3. Have any Offroadsubarus members with newer models done any tests or have any tales to tell?

4. On your average two week trek across central Australia (or USA) how much wear would the system place on your brake pads?

Would love to hear everyone's thoughts.

1. Yes VDC will practically bring the car to a stop on soft sand. X-Mode would allow for more wheel slippage but again, it will slow progress and be more of a hindrance than an aid in soft sand.

2. Depends how soft the sand is. But yes it will 'throttle back' the car once it detects excessive slippage of the wheels. I would suggest knocking off all electronics when driving in sand. One thing I like to do when trying to crawl over obstacles is switch the vehicle into "Intelligent" mode on the SI-Drive. Gives me more throttle travel therefore letting me be a bit more precise.

3. My only comment would be that the first thing I noticed with my '08 Outback VDC when I went off road was how superior the standard AWD system was to the one in my '02 Outback VDC. Its just sorts itself out and I only call on the VDC for more demanding situations.

4. Can't comment.


Here's an interesting video of the brand new Outback doing a hill climb:
[ame="https://www.youtube.com/watch?v=hrdb_UVTa20"]2015 Subaru Outback takes on the TFL Cliffhanger: Ultimate Off-Road Review - YouTube[/ame]

They turned X-Mode off but forgot to turn VDC off. So unfortunately this car didn't get given a fair chance getting up that hill. I'm yet to make a judgement as to how well CVT works off road...

 

[MiddleAgeSubie]

Hmm, I cannot comment about deep soft sand. In soft sand of a few inches, my VDC Subaru is flawless, if it comes to a halt, it will restart.

On more or less firm if loose ground, the point is to keep the traction control off AND THE FOOT ON THE GAS if you get stuck.

The traction control off button does ONE THING: turn off the function that cuts power to the engine when the computer senses slippage. It is not as bad as it sounds and I have done my toughest hill to date with the traction control ON. In fact, I have not found much of a difference as far as hill climbs go, no matter how slippery. Where it must be definitely OFF is in mud, sand, and snow precisely in order to prevent the car from coming to a stop.

The guys on the video did a TERRIBLE job on the hill.

If some of you are into posting comments on youtube, please explain to the self-proclaimed testing "experts" there that the car would have climbed if they had kept the foot on the gas once stuck, instead of letting go, and perhaps turned traction control off as well. Maybe AT tires would have made it easier?

The hill on that video does probably not even require traction control off, so long as the right foot remains planted on the gas. It may take several seconds, even 10+ for the car to regain traction with a little turning of the wheel left and right, if necessary. I have done MUCH WORSE than that. What a joke.

As for mud, I have driven through a lot of Utah mud which is as bad as it gets, clay, usually a few inches but with sections of up to a foot. There is no such thing as VDC cars coming to a stop because of a crazy brake application. So long as the traction button is OFF and you keep your momentum, the car just keeps going.

Though I might not have as extensive experience as many others here, I have seen varied terrain (except for deep sand) and I have not been in a situation that has made me either regret VDC or wish I had rear LSD.

Except for the wheelbase and overall footprint, you will likely LOVE the new XT Forester.

HOWEVER, the 2.5 naturally aspirated Foz in the US comes either with a CVT with NO PADDLE SHIFTERS or MT with no dual range. I would not consider either suitable for climbing tough hills. When I did a classic 100 mile road in Utah a ranger told me about a CVT no-shifters Subaru (i.e. a 2.5 n/a Foz) that failed to climb--who knows if it was the CVT or the driver. Either way, I would not buy a car that has just D and a low mode of D that is nothing like a true low gear, let alone low range.

Finally, BRAKES.

I have 25,000 miles on my OB brakes with about 300 on challenging terrain that often involves crawling for miles and a thousand more on easier off-pavement. The brakes are on target for 75,000 miles. The brakes take slow descents with frequent tapping to prevent lock and sliding well enough (minor noise). However, after descending quickly on what was the far smoother side of a 12,600 foot pass (close to 4,000 m), after a slow climb from the other side, the brakes were screaming and shouting in pain.

EDIT: x-mode is only an enhancement of VDC, presumably specifically meant for CVT transmissions. All it does that matters is to make the wheel-braking faster and to allow for a sort of hill descent control.

 

[Tweaksta]

Interesting replies.....

Got me thinking more...

It sounds like the current software in these systems isn't calibrated for sand. With some programming and calibration by Subaru they could eventually include a 'mode' for sand - just like Land Rover. This would, as Pedro said, allow for some slippage and kind of slacken the response of the VDC. It's primary function would be to brake individual wheels to mimic an LSD or (potentially one day) a diff lock.

With some newer model drivers currently switching off the electronics in slippery situations it seems that the old LSD is still a very good way to keep all four wheels powering.

As Shiv mentioned, there are also CVTs to include in the equation......I am super suspicious of these. They are good at keeping the engine at peak torque but what would happen if the load on the wheels became huge - like in soft sand? Once I had stalled and become stationary, I needed a massive surge of torque to spin the wheels and 'break' me out. Direct mechanical linkage did the job well - I kind of feel like a CVT is a sensitive and breakable item and shouldn't be stressed. I guess time will tell.

 

[Tweaksta]

Thanks for your awesomely relevant input MiddleAgeSubie.....you have a very modern system so it's great to hear your experiences.

So X-Mode is mainly for slow crawls and rocky hill descents - that would make sense. I reckon X-Mode is what they should now refine and recalibrate into several modes with varying levels of response.

 

[MiddleAgeSubie]

There is a Subie offroad group in Texas that posts pics and impressions on the Foz forum. So far, I have seen no difference in actual performance of x mode vs Vdc in the sense of the former being more than a convenience feature. What they said about XT with CVT and xmode vs XT with 4Eat and my system is that the latter had easier time restarting on a steep hill.

 

[NachaLuva]

When it comes to a basic SF or SG Foz with no fancy electronic driver aids lol, I've been on a very steep learning curve. The much maligned 4kg viscous centre diff works adequately if in good condition. I suspect those who say it doesnt work have a centre that needs replacing. 4 tyres of equal size & type and NO handbrake turns people

Two open diffs is a recipe for losing all momentum. A rear LSD esp if plated makes a big difference but the most critical time that momentum is lost is while doing a steep climb with diagonal ruts. All the weight is on the back wheels so they usually stay on the ground but as soon as a front wheel is lifted, the engine races & you lift off losing momentum. To counter this you can do 3 things. Install 1) a front LSD, 2) a locking centre diff or 3) learn how to drive through the brakes.

1 and 2 means splitting the box and sourcing the parts required, not cheap but very effective.

3 is free, just not easy. I've used it a lot to keep up with others with locking centres on tracks no way would I have made it without. You keep your left on the brakes which adds resistance to each wheel esp the fronts. You also apply a lot more throttle to keep the revs up. As a wheel is lifted and the engine starts to race, dont back off the throttle, instead apply more brake. As the wheels grip more & the revs drop, increase throttle & reduce braking. Its a constant balance between the two. Sounds harder than it is and it really works . I've used it on sand, mud, clay, gravel & rocks & it works on all of them. I would prefer XMode though... :rotfl:

I haven't personally driven either XMode or VDC but I have followed a previous thread on VDC with much interest, incl official Subaru videos on how to use it offroad.

You don't switch off VDC. It applies braking to whichever wheel is slipping sending more torque to the wheels with grip. You turn off TC so the ECU doesnt limit power from the engine. This puts the VDC into offroad mode. The Subaru vid shows an OB easily climbing a steep muddy bank doing this.

MAS's comments seem to back this up. VDC on, TC off & keep the power on

 

[Tweaksta]

Soon enough we can all look forward to torque vectoring diffs....maybe then, and only then will we be getting close to finding a better technology to a true locking diff. Until that time I am happy to work with slip - limited slip, but slip all the same - lol.:lol:

 

[jf1sf5]

When it comes to a basic SF or SG Foz with no fancy electronic driver aids lol, I've been on a very steep learning curve. The much maligned 4kg viscous centre diff works adequately if in good condition. I suspect those who say it doesnt work have a centre that needs replacing. 4 tyres of equal size & type and NO handbrake turns people

Hmmm, not sure about the centre that needs replacing because when its killed, it stays "locked"…not good for asphalt driving but it should be better offroad !

 

[MiddleAgeSubie]

Much of what NachaLuva said is very specific to his type of AWD, which is not at all what you would be getting.

Unlike him, I would not worry about any front wheels being lifted, but my system, too, is not what you would be getting.

Either way, traction-wise, there is little to worry about.

 

[NachaLuva]

Hmmm, not sure about the centre that needs replacing because when its killed, it stays "locked"…not good for asphalt driving but it should be better offroad !

I've heard both, some that lock up or bind, others that become open.

MAS, yeah the bit about driving through the brakes was very SF/SG related. Its basically a crude manual version of VDC :biggrin::lol:

 

[MiddleAgeSubie]

It is not an option anymore, unfortunately, because the brake, at least in US-spec ones, now overrides the gas. This is all because of the Toyota recall story...

 

[NachaLuva]

Here's a good example of the early Subaru AWD system at work & how useful a rear plated LSD is:

[ame="https://www.youtube.com/watch?v=odDwsvCsqRI"]'97 Subaru Forester AWD System - YouTube[/ame]

 

[Ratbag]

^ Actually, even the much maligned rear vLSD helps in my experience.

My old Impreza didn't have one (but was still very capable on very slippery surfaces - think the Old Coach Road at Skenes Creek after 4" of rain ... ).

Roo2 does have one, and it is quite noticeably better than the open rear diff in the Impreza.

BTW, when the vLSD in the centre fails, it will (mostly) turn your car into a FWD. If the rear vLSD fails, it will become an open diff.

Other than damage caused by abuse, mostly these items are extremely reliable.

The benefits of vLSDs over conventional LSDs are very considerable. They require no adjustment, and also require no driver intervention of any description to work very well. This latter is quite unlike AWD on demand systems, which seem to react just after one needs them to be there ...

The downside of the vLSD vs the more conventional forms is that they are not quite as effective. However, they are far smoother in operation - basically undetectable in almost all circumstances.

There are always costs and benefits no matter what is being considered. One must always reach some sort of compromise that suits one's purpose.

 

[ateday]

I am quite impressed with the LSD on my MY03.
Soft sand, rough tracks I can feel one rear wheel start to spin and then the diff starts to work very quickly with traction restored.
Makes it a lot easier.

 

[simxs]

BTW, when the vLSD in the centre fails, it will (mostly) turn your car into a FWD.

This is not the case with a centre viscous LSD. It is important to distinguish between a viscous LSD and stand-alone viscous coupling.

The latter has been used in some Volvo and Volkswagen AWD vehicles among others and is an arrangement whereby there is fixed drive to the front differential (as per a FWD car) and the drive to the rear goes via a viscous coupling unit. If the viscous coupling fails such that it cannot bind and transfer torque then, yes, you are left with FWD only.

A centre viscous LSD on the other hand (as fitted to many manual Subarus) is primarily a geared differential and most of the time acts accordlingly whereby it sends the output from the gearbox to the front and rear differentials with the torque split equally between the two. This is neatly arranged in Subarus by the centre differential at the back of the gearbox sending one of its outputs forward to the front differential via a shaft residing concentrically within the hollow lower shaft of the gearbox. The rear output simply goes to the rear propellor shaft and on to the rear differential. The vLSD's 'limited slip' function is achieved by way of a viscous coupling unit between the two outputs which results in the front and rear outputs being partially bound together as the difference in speed between them increases (for example if the front wheels are on ice and start spinning faster than the rear wheels). If the viscous coupling unit in the differential fails such that it cannot bind, you are left with an open differential and therefore 50:50 front:rear torque split regardless of conditions.

For either arrangement however a more common type of failure of the viscous coupling unit is for it to actually bind too readily, rather than not bind at all. (In fact mine has been doing this for some time.)

 

[Ratbag]

^ Thanks for that clarification and correction, Sim.

I have seen what the centre diff in a Subie looks like after it's failed - it's ugly ... .