Originally Posted by beyond
219, are you saying that as far as you know, all AWD's except for Quattro and Subaru are Haldex derived? Any sense of what "very few" comes out to? Will be in the market eventually (my beloved 4 Runner has 149,000 miles) and curious to know.
Well, look for any vehicles that have a longitudinal engine/trans and AWD, and that will tell you who has a true AWD platform. Subaru and Audi (Quattro models) are the two that come to mind. I'm having a hard time thinking of others, but I know there are a few. This kind of layout is only used when AWD or RWD are the main drive configurations for the platform. Note that AWD is not the same as 4WD, so most trucks and traditional SUVs (the big body on frame ones) don't count (a few of the big SUVS do).
Most manufacturers are doing FWD platforms these days. Basically, if the vehicle has a transverse engine and other FWD models in the lineup, then the AWD system is going to be FWD-based (like the Haldex, but that is just one type). Examples are the Honda Element, CR-V, Pilot, Ridgeline; Acura MDX, RDX; Toyota RAV4, Highlander, Ford Escape, Volvo wagons/SUVs, and so on...
There's nothing really wrong with these FWD systems, but they tend to be heavier, more mechanically complex, and less balanced than the true AWD systems. The transverse engine layout makes a lot of sense for FWD, but seriously loses it's benefits when it is adapted for AWD. It's kind of a kludge.
I say all this from an engineering standpoint; most consumers don't really make the distinction or care. But, it does matter. For instance, my Acura RDX is about 400 lbs heavier than the similarly sized/equipped Outback XT it replaced. Instead of balanced and neutral/oversteer handling like the Outback had, the RDX understeers (plows) until you really get on it and the SH-AWD kicks in. Acura has used electronic trickery to minimize the understeer, whereas it was never a problem on the Subaru. This is one example, but there are others.
The main knock against the Haldex system, besides that it is FWD based, is that it is a part time system. It only sends power to the rear wheels when the fronts slip or a computer decides it should send power to the rear wheels. In contrast, all of the Subaru systems are full time with varying default front/rear power distributions (my old XT wagon was nominally 45% front / 55% rear and shifted power around from there). In addition, Subaru has proactive AWD systems whereas Haldex is reactive. For years, the Subaru systems were capable of proactively shifting power fore and aft under acceleration and deceleration. In other words, they shifted power in order to prevent wheel slippage, not just in reaction to it.
Over time, the other manufacturers have caught up in certain ways. The SH-AWD in my RDX shifts power fore and aft, and side to side in the rear, in order to augment handling. It will actually overdrive the outside rear wheel in corners to offset the vehicle's inherent understeer. It's quite cool, but I like to remind people that the simpler Subaru AWD systems often don't suffer from understeer in the first place! One thing I dislike about SH-AWD is that it only kicks in when I apply healthy throttle in a corner, which may not be appropriate. My XT wagon had neutral to oversteer behavior with very light throttle inputs, and was better suited for real world driving. And in snow, the SH-AWD is purely reactive. I'm pretty much driving a FWD vehicle until the front wheels slip and power is transferred to the rears. In my XT, the fronts and rears would have all been driving as I headed into the snow, which I consider an advantage. It will be less likely to slip in the first place.
Finally, you should look at what kinds of differentials they all use. My XT wagon had a true limited slip differential in the rear. This means that, whatever portion of power going to the rear axle (55% nominal), it could deliver all of it to one or both wheels. Whichever wheel slipped, power would automatically head to the opposite wheel with no loss. A lot of "AWD" vehicles have simpler differentials and rely on traction control, via braking, to reign in wheel spin. The problem is that a maximum of half the axle's power can be applied to any one wheel. So when it clamps down on a spinning wheel, you lose 50% of the power to the axle, and the other wheel only gets at most the remaining 50%.
I have had limited experience with the Haldex system in a relative's Ford Freestyle (now called the Taurus X). The Taurus and Freestyle share a platform with a couple Volvo models, and all share the same basic powertrain. I found the system to be decent in snow, about like my RDX. It would keep you out of trouble if you were a sensible driver, which is all most consumers want. It's on par with most of the AWD systems on the market. But it was a far cry from the tenacious, clawing feel I have gotten out of my Subarus. I drove my Outback XT wagon into a parking lot with 16-20" of unplowed snow a few years back, in order to get front row parking at a nearby ski hill on a powder day. With the nearly 9" of ground clearance and superb AWD, I had no reservations at all. I would not try the same with my RDX; it only has about 6" of clearance and the AWD system is just not as aggressive. I think Subaru owners/fans will know what I am talking about here!