AWD Systems / Technologies Differences between Subaru and Toyota and Honda

[murphysf]

Hello,

I have often heard that the Subaru AWD system is superior to what comes is say a Toyota RAV4 or a Honda CR-V.

I am looking for information or a good article that explains and compares the Subaru AWD system to the Toyota and Honda.

I've googled the topic and had no luck.

Does anyone know of any sources they can share or could accurately explain these systems?

 

[scott43]

I would caution on the use of the word superior. They're different with different intents. It's probably better to define what you want out of the vehicle.

 

[murphysf]

I would caution on the use of the word superior. They're different with different intents. It's probably better to define what you want out of the vehicle.

Thanks for the post.

I am looking to learn about Automotive Technology on its own, so do not want to cloud the topic with a discussion about application, etc.

 

[François Pugh]

I want it to last forever, not wear out my tires faster than a one-wheel drive car, and never get stuck. I want part time with selectable lockable centre, front and rear diffs, but I see @murphysf just posted that he doesn't want to cloud the issue.
BTW

 

[scott43]

They're all relatively the same. Front diff, rear diff with some type of coupling unit in the middle. It's more the implementation where they differ. The knock against the lighter weight versions is that they're not true four wheel drive. However they're also much lighter and get better mileage.

 

[Ogg]

Subaru systems always have some power going to both the front and rear wheels. The RAV4 and CRV only power the rear wheels if the front ones slip. In the RAV4 the AWD only operates up to 25mph, I'm not sure about the CRV but it's probably effectively the same. Either will get you going in slippery conditions but the Subaru will provide better overall stability, IME.

 

[scott43]

The centre diff or transfer unit can be clutch, full locking gear or viscous coupling. Most of the lighter weight units are viscous coupling with some kind of electronic mechanism to proportion torque to the rear diff based on sensor input and software. So for many of these vehicles they're basically front wheel drive until the computer decides conditions warrant some rear diff bias. This gives you better mileage but it also means the system is a black box to you and you don't really know when AWD is kicking in. So it can be unpredictable for the driver.

 

[murphysf]

Subaru systems always have some power going to both the front and rear wheels. The RAV4 and CRV only power the rear wheels if the front ones slip. In the RAV4 the AWD only operates up to 25mph, I'm not sure about the CRV but it's probably effectively the same. Either will get you going in slippery conditions but the Subaru will provide better overall stability in dodgy conditions, IME.

I understood that the Toyota system was 85% front 15% rear and then would increase the rear as necessary based on the slippage.

 

[scott43]

Some Subaru systems have a more full time system so it's much more predictable but also more gas thirsty. Axle diffs are much like centre diffs. They proportion torque to each wheel. Some are open (power goes to either wheel, no control) or they have some type of locking mechanism, usually clutch pack. "Posi" is basically this. Both wheels are driven 50/50 with the clutch pack to allow different wheel speeds based on conditions in a purely mechanical resistance method.

 

[François Pugh]

This site explains a little bit for Subaru, even if it is a somewhat of a Subaru hype marketing site.

Symmetrical AWD Explained | Quality Subaru - Wallingford, CT

If you've shopped for a Subaru vehicle, seen or heard a commercial, you've likely heard of Symmetrical AWD. Click here to learn why its so much different from the competition!

www.qualitysubaru.com

 

[scott43]

Fully locking systems allow you to lock the centre diff at 50/50 for very balanced torque application. You can also lock the axle diffs as well. Thus is what most people can true four wheel drive. It has some good applications as well. It's really all about what you're trying to accomplish as each is a compromise.

 

[Ogg]

I understood that the Toyota system was 85% front 15% rear and then would increase the rear as necessary based on the slippage.

You're probably right, I lost interest in them when I found out about the no AWD over 25mph thing.

 

[murphysf]

This is in conflict with my above comment.

I just found this about Toyota / Lexus AWD

The transmission features an integrated transfer case and a viscous center differential that provides a 50/50 front-to-rear power split. The system requires no activation by the driver. If slippage occurs at the front or the rear, the viscous center differential automatically and transparently directs power to the wheels with the most traction.

The traction control provides further "fine tuning" by sensing individual wheel rotation speeds and braking the appropriate wheel when necessary.

The fact that the AWD system is supplying torque to both axels all the time gives it superior handling characteristics to other so-called AWD systems where only one axel has torque until slippage occurs. These are "reactive" systems, compared to a "proactive" system.

 

[scott43]

Not calling you a liar but I believe the Toyota system can be manually locked but it automatically disengages above 40kmh or something. In normal use it is either 100/0 or 90/10 I believe. Maybe the bigger trucks like the Sequoia are different, probably the four runner, but the Rav 4 and other smaller vehicles are as I've stated.

Edit: you're quoting an article so it's really the article I'm questioning. No disrespect intended

 

[scott43]

The software for some of the earlier AWD systems in Honda's was not very predictable or intuitive. There would be front wheel spin, a significant delay (0.5 - 1.5s) and then the rears would start pushing. By that time you're well into an understeer condition which is then turned into an oversteer condition pretty quickly. Very scary if you're trying to be delicate and actually drive at the limit. They subsequently modified the software to start rear wheel drive more quickly and in a more stepped manner so its much better than it used to be. Coupled with the now-standard vehicle stability control system and really it's very hard to tell what wheel is spinning when and where you're at in the slide phase. Really, in a modern SUV like a Rav4, you have to REALLY mess up to get into a bad spot.

 

[François Pugh]

This site explains how a Torsen diff (use in rear of some Subarus) works as compared to a plated LSD (also contains a link if you want the details s on the plated LSD too).

Torsen vs. plated limited slip differential. – KMP Drivetrain Solutions

In this blog, we would like to explain how a Torsen limited slip differential works and how it differs from a plated LSD, like the Drexler LSD.

www.kmpdrivetrain.com

 

[François Pugh]

 

[James]

I used to have a 95 Range Rover County. It had some sort of silicone filled front/rear power transfer unit in the center differential. One time I had the front driveshaft out to replace the u joints. I drove it and parked it in a steep short driveway. In park, handbrake on. Someone came to the house and said our car was in the middle of the street!
It had crept backwards without the front driveshaft in. The center drum handbrake wasn’t tight enough to hold it.

 

[Bill Miles]

Back when more Subarus had manual transmissions, the manual and automatic systems were different. I once asked a Subaru salesman what the difference was and why, and he didn't know.

OTOH, it doesn't much matter what system you have when the snow gets deep enough that the tires don't get a good grip on the ground.

 

[Snowfan]