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Topic # 114527 22-Feb-2013 14:07 Send private message

Hi have been reading lots but can't quite figure out which system has the best traction in say when there's an odd snow shower in Dunedin or half icy roads in south island winter

Let's just compare Subaru Forester and Nissan X Trail

The problem is not all 4wds are created equal in fact far from it. In the most basic of four wheel drive there is a centre differential that transfers torque from front to back (i think). In the simplest system it doesnt transfer enough torque to the rear wheels and you might as well not call it a 4wd. sometimes the centre differential gets too hot and it shuts down altogether, and you're back to 2wd.

Between each axle front or back, in the simplest system if the right front wheel spins because it has no traction, progressively more torque is sent there depriving torque to the front left (the path of least resistance??) so you have no drive at all!


So how does the Nissan and the Subaru compare? (I'm comparing arguably the top two but i want to know which is better ...)

Nissan X Trail 4x4i is front wheel drive and sometimes sends power to the rear when needed AND when it thinks its needed. It can transfer between 100/0 % up to max 50/50 %. So if you really need the torque at the back - bad luck. And it can only do 50/50 up to 40km/h, so no good on the open road. What if there is right -left imbalance? If the right front is slipping the computer brakes the right front, increasing the resistance to torque to all the torque is diverted to the left front where there is friction. same if happens in the rear. Problem is it doesn't brake the wheel all the time, only in pulses so it may or may not get you out of trouble.

Subaru: i have no idea .... but people (AUS X Trail forums) rave about how it is superior ...
For one it has full time four wheel drive. In fact the centre differential doesn't actually transfer torque from front to back. It is connected directly to the engine so it distributes torque between front and back (subtle but important difference perhaps?).

But can it go 90/10 to 10/90? and what happens in left - right imbalance? Thanks if you know




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  Reply # 767690 22-Feb-2013 14:59 Send private message

There is All wheel drive (AWD), Four wheel drive (FWD) full time, 4WD Part time, 4WD with high and low ratio, 4WD with High and Ratio and locking Diffs, and 4WD Hi and low ratio the locking hubs. Al these are designed to do certain things.

If you are planning on just going on back country roads etc AWD is sufficient, if you plan to do beaches, river beds etc you should go to 4WD, it is best to have part time selectable 4wd ( Refer Suzuki Grand Vitara which is actually very good on and  off road and has locking diffs) this saves a lot of fuel when just doing normal day to day driving.

If you intend to go into rougher stuff, mud, snow( deep that is ) etc then 4wd with locking diffs and Hi Low ration is something you should consider.

Which ever you decide take time to learn how to use your vehicle, driving a 4WD is a LOT different to driving a car.






Mike

 Interesting. You're afraid of insects and women. Ladybugs must render you catatonic.

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  Reply # 767817 22-Feb-2013 18:23 Send private message

Generally 99% it won't make a difference if you have 4WD drive or not. For most it's a waste of fuel. 4WD does nothing unless you need to accelerate out of something or a climbing a slow slippery grade (IE skifield).

If you hit black ice mid corner (I have half a dozen times in Central Otago) then you are simply a passenger until you get to tarseal again. I've had the rather unnerving experience of driving a Forester rather sedately round an unmarked lefthand corner (IE one you would take at 100k) at 70k and slipping across into the other lane, where the car gripped again and carried on my way.

Nothing I could do but pooh myself and praise a deity.

Of course if you're going proper 4WDing then you'll need something different. Non turbo Subaru's are inefficient, complex to maintain and more sluggish than their 2WD inline rivals.

The only other time I could think 4WD might save you is if you happen to in a position of rather dramatic oversteer, it may you let drive out of it.



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  Reply # 767828 22-Feb-2013 18:33 Send private message

no i'm not talking about hitting black ice at 100kph - the reference to open road driving was for say heavy rain - where 4wd WOULD make a difference in car stability where it won't go into oversteer so easily like you mentioned

if you live in hilly parts of the south island with non sealed road, soft roaders DO make a difference in many conditions and yes you would be driving slowly of course

- snow
- rain
- i wasn't going to say icy morning but yes, icy mornings - they do randomly appear in Dunners when you want to go to work in winter mornings

what i'm interested in is how Subaru deals with those 2 conditions - transfer of torque front-back and left-right




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  Reply # 767829 22-Feb-2013 18:36 Send private message

if anyone knows, that is

the internet suggests 90/10 10/90 but not sure ...

and i'm wondering how can subaru (non lockable diff) be superior to soft roaders with lockable diffs

yes i understand that proper off roaders will do the job but that's not the point of this thread :)




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  Reply # 767844 22-Feb-2013 18:54 Send private message

As you know the center diff is the diff that controls the torque between the front and read axles, this can be either locked, LSD or open, If you have an open diff on the front or rear axels ( as the vast majority of 4wd road vehicles do) having one wheel in the air will cost you ALL of your drive.
Subaru use the term full time 4wd, but this does not mean all 4 wheels have drive under all circumstances, if this was the case you would never be able to turn the car without scrubbing the wheels.
Several manufacturers of late use the practice of braking a wheel on one side of the axle to control the torque distribution .
In all reality the system you choose should depend on the terrain you are trying to traverse. choose a light vehicle with suitable tires and you don't need as much traction.
4wd on tarmac , especially with tight diffs will kill your tires.
Hard cornering on gravel for example can lift or lighten a front or rear wheel , this can effectively turn a 4wd car into a rear wheel drive car for a brief time until traction is regained ( due to the action of the center diff)
recovering from oversteer is easier and faster if the pulling force is primarily from the front of the car, and torque from the rear will tend to push the front tires forward rather than let them turn naturally.
What sort of ground are you actually planning on traversing?

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  Reply # 767856 22-Feb-2013 19:10 Send private message

joker97: no i'm not talking about hitting black ice at 100kph - the reference to open road driving was for say heavy rain - where 4wd WOULD make a difference in car stability where it won't go into oversteer so easily like you mentioned

if you live in hilly parts of the south island with non sealed road, soft roaders DO make a difference in many conditions and yes you would be driving slowly of course

- snow
- rain
- i wasn't going to say icy morning but yes, icy mornings - they do randomly appear in Dunners when you want to go to work in winter mornings

what i'm interested in is how Subaru deals with those 2 conditions - transfer of torque front-back and left-right


I have a Subaru and find it is less likely to lose traction than a 2wd when things get slippery.  If you completely lose traction then it slips just like any other car with rubber tyres.

You still have to be very gentle when it gets slippery, 4wd helps but it's not a magic bullet.



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  Reply # 768028 23-Feb-2013 08:43 Send private message

turnin:
What sort of ground are you actually planning on traversing?


well there are terrains of 45 degree gradient (no kidding!) and about half of those are unsealed soil or unsealed gravel on the hills obviously, it gets very nice in the winter - you see all the otago students with subarus "challenging" themselves up to see the white blankets

but unsealed soil terrains of 45 degree gradient in the best of days a 2WD will struggle - there is a guy down the road whose driveway is a 100m stretch of unsealed soil 45 deg - he has a grand cherokee and a 2wd toyota van and the neighbour has a 2wd nissan pulsar - they speed up like mad, hit the driveway and wheelspin from start to finish :) fun! (i have never seen them going up in the wet of the frost! probably just stay out i think!)




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  Reply # 768029 23-Feb-2013 08:45 Send private message

anyway as you may have gathered i have an x trail ...

i take it no one knows about how subaru's well advertised full time "symmetrical AWD" work in poor traction ... it just does!

i'm just curious as to how it compares with another well aclaimed part time system like mine works.

btw its known suzuki grand vitara's one is better than the xtrail's but these three are the top. don't get a honda/toyota/hyundai (maybe even audi not sure) and hope to do any off roading :)




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  Reply # 768046 23-Feb-2013 09:41 Send private message

You've probably seen the marketing blurb http://www.subaru.com/engineering/all-wheel-drive.html
But actually when it comes down to it, they are quite different depending on what sort of gearbox you have in them.
http://drive2.subaru.com/Win09/Win09_ItsWhatMakes.htm .

In essence they use the abs sensors to put a value on wheel rotation speed and compare them to each other to ascertain which wheels are slipping, if they all are, then an accelerometer and throttle position sensor can determine the rest .
Whilst in all cases the wheel speeds are "monitored" by computer it's quite surprisingly that it's only the 5spd auto and the six spd manual from the wrx sti that use a computer to "alter" the mechanical function of the diffs, the rest rely on viscous units. Viscous units are not usually thought of as being able to transfer much torque though, they tend to slip especially as they heat up, think of a spoon in golden syrup, if you get that diff too hot  it will slip quite badly, hence less drive. either way it probably makes for an effective system.
Not too sure about the whole balance thing being too relevant. Lots of Manufacturers rave on about having perfect 50/50 weight distribution, which yes is a good starting point but it makes no compensation for the relatively massive 70 kilo's of fuel sloshing around in your tank or the 50-140kg weight of your passenger or even if you have one.

so if you're going up 45 degree hills I'd go for good tyres( European tyres tend to have less silicon content, hence better grip on snow / ice - not too sure about 4wd tyres though), less weight (probably half the reason Suzukis are popular amongst serious off-roaders)  Really the tighter the diffs the better, keep the car as flat as possible and possibly try driving up backwards , 2 reasons , reverse is usually the highest ratio gear in the gearbox which will save your clutch and the weight of the motor being over the wheels with the most weight on them might help too.

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  Reply # 768089 23-Feb-2013 10:55 Send private message

1500kg for the forester, 2100kg for an xtrail,
I think that's probably the biggest difference, Both seem to use braking to assist torque distribution l/r whilst it seems the xtrail can totally lock the center diff at low speeds. The Subaru might have better software for certain situations, hard to know, manufacturers are getting better at hiding specs behind acronyms. On paper a mechanical locking diff should be superior to a viscous. At least you're putting yours to good use, mostly they get used to go up and down the Auckland motorway :) 

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  Reply # 768098 23-Feb-2013 11:17 Send private message

Here's a link to a reasonable write up on Subaru's.
http://legacygt.com/forums/showthread.php/explanation-continuous-active-and-vtd-awd-systems-honest-attempt-48112.html?s=34775ac99e17c1c15195de9c0753d3c8&

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  Reply # 768131 23-Feb-2013 12:06 Send private message

Thanks for that forum link mudguard, interesting read.

Cheers
Cyril

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  Reply # 768256 23-Feb-2013 19:00 Send private message

Just remember that 4wd does not give you any more grip on ice than a 2wd.
It might give you more traction to gain some speed, but you still only have 4 tyres to steer and brake with.

Personally I'd rather have a switchable 4wd than constant AWD/4WD.
4wd is a massive drag on the engine hence much higher fuel useage.



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  Reply # 768364 24-Feb-2013 00:20 Send private message

TheUngeek: Just remember that 4wd does not give you any more grip on ice than a 2wd.
It might give you more traction to gain some speed, but you still only have 4 tyres to steer and brake with.

Personally I'd rather have a switchable 4wd than constant AWD/4WD.
4wd is a massive drag on the engine hence much higher fuel useage.



The actual 4 wheel drive isn't much more of a drag, but the weight of the vehicle certainly is so the heavier vehicles tend to have big engines as a result. I ended up getting an old RAV4: all-wheel drive with a manually lockable centre diff (button on the dash), but no locking on the front/back diffs at all or low ratios. Fuel efficiency is great, so is ground clearance, and the extra traction delays the point where one wheel might start spinning. You forget about the centre diff unless you can predict some problem in advance so it tends to be used only in first gear when you realise the diff lock might be useful, but it did smell a bit like a hot clutch when I took it up Muriwai once.

On open road, I find dodgy corners can be managed using a quick gear change into the corner as you would in a rear-wheel drive, as opposed to some of the 2+2 vehicles that are FWD until you hit the button. Obviously you try to avoid brakes completely if you hit water or ice, since you try to keep all wheels rolling at any cost! So 4WD would give you better traction at slow speeds to let you attempt any steering you can with only engine braking.




Qualified in business, certified in fibre, stuck in copper, have to keep going  ^_^

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  Reply # 768365 24-Feb-2013 00:32 Send private message

Interstingly you are the first rav owner that's said they are good on gas. Usually comments are more along the lines of gas guzzler

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