Overview of AWD Systems

[Thermo1223]

Originally Posted By: edwardh1

how does the toyota highlander awd work? - says it (2 rear wheel drive) comes on when needed???????

Sounds like a haldex system...which is rather poor indeeed. The older Highlanders had a 3 diff system with a center planetary diff. I'll agree to most of what he wrote. HangFire, The Subaru's act the same way, never fully disengaging the clutch to the rear and based on the incline sensor, throttle etc...send more power to the rear as needed. You can feel it when driving a little harder, adding more throttle pushes the car through the corner. I guess his beef is just the clutches in general. He would rather have center diffs on everything and done mechanically I bet. That is how my old Forester was and you could tell. It drove like a mostly RWD vehicle unless you pushed it.

 

[440Magnum]

Originally Posted By: VNTS

Originally Posted By: OneEyeJack

I can still remember in the 80's a Jeep Grand Cherokee unable to move because both wheels on one side were spinning. I pulled him out. He was really upset at getting stuck because the salesman told him this was a Jeep and the four wheel drive system was do effective that this would never, ever happen. It's my experience over the years that the better the system, the tires and all that then as you went exploring in the desert all you did was guarantee that when you did get stuck it was much worse. In other words a better system just meant more digging.

Ahh, ZJ didnt start until 1993, so your 80's Jeep must of been a Cherokee.

There was a "Wagoneer" trim level of the XJ Cherokee in the 80s. Or maybe the OP was thinking Grand Wagoneer (although a Grand Wagoneer would probably have had QuadraTrac, and would be less likely to get stuck.) Any system with open diffs front and rear (even with a locker in the center) can get in a situation where both wheels on one side spin. A limited slip in the rear usually is good enough to deal with that issue.

 

[440Magnum]

Originally Posted By: OneEyeJack

It's my experience over the years that the better the system, the tires and all that then as you went exploring in the desert all you did was guarantee that when you did get stuck it was much worse. In other words a better system just meant more digging.

Which is exactly the school of thought behind "leave it in 2WD until you need to get un-stuck."

 

[k24a4]

I could wrap my pedestrian brain around about half of that, but boy, did he have some funny lines: " Ferdinand Piech...in his mind he's the Leader of the Universe.", "unless your AWD dreams are fulfilled by the ability to jump over the Dept of Trans snow mound left by their plows at the end of your driveway.", "...if all these companies' engineering reps ever got around to chatting to each other at SAE conferences or just wandered around in a cloud of their own private thoughts." Good stuff. Wonder what he would have to say about the Forester AWD with the manual gearbox. Salesman said it's split 50/50 all the time, but what do I know...

 

[Thermo1223]

It tires to keep it that way but it will vary and it has no center diff locker so unless you have LSD in the rear you could technically get it stuck.

 

[MikeHigg]

I really enjoyed reading this!

 

[Ammofirst]

Wanted to bring this back to life. It was a Great write up, thanks to the OP. I also detest the weak mass market awd systems. I thought Cadillac dropped the 3 Diff system. My moms Escalade has the full time awd but it's a 2012. I'm also interested in the GKN system that's replacing some haldex units. XT-5 / Buick LaCrosse & 2018 Regal / Focus RS. Etc. I don't believe it features a center differential but has a clutch back for each rear wheel. Suppose to produce real torque vectoring at the rear. Anyone care to chime in ?

 

[meep]

cute, and I agree in general with the OP. It's hard to follow. I bought a 97 legacy some years ago. (this is a detail he didn't get) - in 1997 Subaru had two different AWD systems in their north American offerings. Manuals got a center differential that also had a limited slip component. That car was glued to the ground. It was the highly desirable way to go. Automatics were FWD with a limited slip clutch (not differential) to the rear. I was recently corrected in my understanding of the automatic's operation -based on what I was told, the automatic was more aggressive however, than today's clutch-to-the-rear systems in that when unpowered, it was fully engaged. During turns, the clutch was electrified to release in variable fashion. these systems were known to chatter during turns as they got older, and people would install a service fuse under the hood to electrify and "release" the clutch. So I have to give them some credit, they biased in the direction of real AWD with part-time release rather than FWD with part-time assist. As the prior owner of a gen 1 MDX, I must say its SH-AWD is rather limited in what it can do, purely because of the limitations of the dual-clutch spool in the back. The service spec for lockup is about 150ft/lbs of torque per axle, or 300 ft/lbs total for the entire rear drivetrain. Assuming a 30" tire, that's something around 260-270 pounds of thrust from the back axle. It is certainly effective in the snow, and adds a lot of confidence, but will reach its limits quickly in harder conditions. If I'm not mistaken, this is the same architecture used in the ridgeline. I do not know if the ridgeline gets stronger clutches, being that they claim "lockable" 4wd. -Meep

 

[UG_Passat]

Originally Posted By: edwardh1

how does the toyota highlander awd work? - says it (2 rear wheel drive) comes on when needed???????

The current one is FWD until AWD is needed (up to 50% torque to the rear). the older Highlander (1st 2 generations) were a fixed 50:50 split. in other thoughts.... if Haldex coupling is good enough to be used on a Bugatti Veyron and Chiron, it's probably acceptable for use on VAG Transverse AWD vehicles. The Haldex Gen 4 coupling on my Tiguan does qualify for AWD, as the default 90:10 split. The cream of the Haldex Gen 4 crop was the XWD on the Saabs, where it had another haldex coupling in the rear for torque vectoring. Audi's Quattro Ultra Technology is a FWD car until slippage is anticpated and goes AWD. Subaru manuals (excluding STI) had a center differential with a viscous coupling attached to it, and depending on trim level/model a viscous coupling rear LSD.