Overview of AWD Systems

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2,105
Location
Easton, PA
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.
 
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8,859
Location
Texas
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.
 
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8,859
Location
Texas
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."
 
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753
Location
MA
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...
 
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2,105
Location
Easton, PA
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.
 
Messages
478
Location
Western usa
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 ?
 
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5,008
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Southeast
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
 
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Parts Unknown
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.
 
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Many AWD systems today are computer controlled and electromagnetic activated. Predictive AWD is the way to go as the system applies the proper amount of AWD for the situation at hand.

A lot has changed in the six years since this topic started.
 

Pew

Messages
1,363
Location
Illinois
The S-AWC in the Evo has a pretty complex AWD system and is still probably one of the most advanced AWDs you can get in a road legal car now. It's super fun to read about and when I left the scene in 2015, the only ones who were able to adjust [tune] the S-AWC was Bosch with their own unit, requiring you renting out an entire track for the day and paying for the services which if I recall correctly, started no lower than $20,000. I don't think that has changed.

  • 3 hydraulic diffs: front, center, rear. All functionally open diffs. The center and rear diffs are active.
  • System is always 50:50 front/rear power split
  • 3 known sensor inputs: wheel speed, yaw sensor, and steering angle sensor
  • When the power starts to bias towards front/rear more than the other, the center diff will start locking
  • The rear diff can send 100% of it's given power to either side - so essentially up to 50% of the car's total power can be sent to either rear wheels (torque vectoring, also known as AYC-active yaw control for Mitsu.)
  • There are 3 AWD settings, tarmac, gravel, snow.
    • Tarmac allows the center diff to stay open the longest but will clamp the hardest once limits are reached
    • Snow will lock the center diff the earliest but the clamping force is much more gradual than tarmac, essentially allowing smoother locking operations. This was a favorite with drag racers.
    • Gravel is in the middle. Usually this was also a favorite when racing in wet conditions.
  • The traction control and stability had 3 settings.
    • Completely on - TC will disable engine power (horrific feeling I might add.) SC will operate the rear brakes as a way to help the rear rotate.
    • TC off, SC on
    • TC and SC off. Recommended for the track as leaving SC on will burn up the rear brakes

- Fun fact: There were high failure rates for the pump due to corrosion. The pump for the system was located behind the passenger rear wheel covered by nothing more than the body panel and wheel liner. If you lived in the salt belt, it wasn't a matter of if your pump failed but when and often before the car reaches 100K.
 
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5,790
Location
North Texas
Nothing beats a good driver. AWD or not. Don’t drive like a donkey. I’m no professional but common sense goes a long way. My wife’s 19 Pilot EXL is AWD and did awesome so did my Tacoma 2WD with weight in the bed, lol.
 
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199
Location
Central FL
I used to daily beat a '95 Legacy Scooby wagon around town a few years back. The only thing that stunk about it was I was the only guy that would show up on the engineering campus on time for work. Nothing much is more spooky than being the sole guy in an empty engineering hall and silent machines.

I have a brutal, sharp as a bowling ball AWD chevy sport truck now. She's an oldie. If I'm not completely off my game, she's got a limited slip rear, plain old diff in the front, and a viscous clutch center diff embedded in the transfer case. Nominal operation sees a rear biased 37:63 front:rear torque split in non- slip conditions. She can go 50:50, the manual says to just keep the throttle even as it transitions, but it will get there in a fraction of a second. No bump, groans, etc. Just sudden traction if the rear slips as the fronts start to pull harder.

I cannot seem to find any data on what sort of AWD system it is. The manual isn't a clue. It seems that the '03 Silverado SS has fallen off the internet radar except for old motor-trend/week reviews.

What sort of AWD system is she packing? She lacks traction control, Stabilitrack, has ABS... but I'm not aware of any time she used it to brake a slipping wheel. She just digs in and goes.
 
Messages
3,842
Location
Chicagoland
I used to daily beat a '95 Legacy Scooby wagon around town a few years back. The only thing that stunk about it was I was the only guy that would show up on the engineering campus on time for work. Nothing much is more spooky than being the sole guy in an empty engineering hall and silent machines.

I have a brutal, sharp as a bowling ball AWD chevy sport truck now. She's an oldie. If I'm not completely off my game, she's got a limited slip rear, plain old diff in the front, and a viscous clutch center diff embedded in the transfer case. Nominal operation sees a rear biased 37:63 front:rear torque split in non- slip conditions. She can go 50:50, the manual says to just keep the throttle even as it transitions, but it will get there in a fraction of a second. No bump, groans, etc. Just sudden traction if the rear slips as the fronts start to pull harder.

I cannot seem to find any data on what sort of AWD system it is. The manual isn't a clue. It seems that the '03 Silverado SS has fallen off the internet radar except for old motor-trend/week reviews.

What sort of AWD system is she packing? She lacks traction control, Stabilitrack, has ABS... but I'm not aware of any time she used it to brake a slipping wheel. She just digs in and goes.
From what I can find, it uses a NP/NVG149 transfer case which uses a viscous coupling to give you AWD.
 
Messages
478
Location
Western usa
A lot of snake oil and very little ability. Same issue has driven me nuts for years. A really big offender are the new Ram Trucks. What a load of B.S. they are pushing. Unless you get a base model with the old transfer case or the Rebel you get stuck with a 2WD machine that just sends power to the front when and if the rears start to slip. They even use the word lock on the dash to deceive the customers. Pathetic
 
Messages
1,023
Location
Upper midwest
Nice write up. I will add what I know about my "simpleton" Hyundai's FWD bias AWD. I want AWD mainly for high performance sport driving/track days. As we have seen with Audi AWD was so successful in road racing it was banned within a year in SCCA Trans-AM Series and after late coming to the IMSA Series and not running late in the season, Daytona and Sebring ( which they would of won both, if no mechanical problems foreseeable happen) Audi chose to stop racing the series after showing dominance. A small video starts at a point to watch for 8 mins. Yes, I know FWD bias AWD is no match for Torson diff'ed Audi's, and Subaru's AWD system, but the same advantageous characteristics are there never the less.



As with my wife's Rav4 and my lowered Hyundai Kona AWD 1.6T, when you put into sport mode you get a small form of full time AWD. Toyota documents 95/5 split in Sport Mode and Hyundai is somewhere in that ball park as when enable Sport Mode you feel a "push" of the rear diff engaging just like the Rav4 I also tow a small 1,200 total weight boat and if I don't enable the AWD I can feel and hear a straining load on the front diff. Engage the AWD Sport Mode button and no noise and feel of strain. Same thing happens with the Rav4 with a lightweight pop up trailer of the same weight. My Kona is rated to tow 2,800 lbs in Europe and Australia and the Kona is a world car, so it is the exact same except country specific suspension tuning. The Hyundai's rear diff's electromagnetic unit can hold 1,000 ft lbs of torque, a far cry from my 240-250 ft bs of torque. It would be nice to have a front LSD or even better both diff's with LSD. Even with open diffs I can out handle FWD in the rain by a far margin. Also AWD adds weight in the rear and makes a nose heavy FWD car a better handling AWD car. When I added a hitch and a full sized spare to my car I jumped up by a large margin in over all handling as my weight distribution is far better. I lost some acceleration by the extra total weight all three of those components, but the joy of a better balance is worth it as a drivers car. You can come 90 mph into an exit ramp and throw it into a corner and stick into a balanced car without a lot of understeer and no FWD weight transfer oversteer that sucks in the "curbed forest."

A agree, Subaru has a nice AWD system. The OP knows how nice AWD is in a street racer same with the Mitshu EVO AWD system. I don't want to go back to 2WD as I had to make that choice as I was going to buy a Hyundai Kona N and Hyundai went cheapskate, and made The Kona N a "LIFTED" 2WD hot hatch. A high amount of people are passing on the Kona N since they only wanted it in AWD. It would of costed Hyundai $750 to put it in. They charge $1,400 for AWD in the Kona's. Everyone who were itching for a Kona N AWD said they would paid that in a heartbeat. Hyundai left a lot of sales on the table with that stupid move. They would stole 1/4 to 1/3 of the Golf R AWD market and that is after they "lost" existing Hyundai owners who would of bought the Kona N in AWD.

.
 
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