What makes Subaru's AWD system 'better' than a Honda or Toyota implementation?

[fuzzybabybunny]

From videos on YouTube you see tests where CRVs or RAV4s are unable to get unstuck or make it up a hill while a Subaru has no difficulty in the exact same scenario. Is it all a matter of the Honda/Toyota AWD systems not being able to send enough torque to the traction wheels? Say that a car is on an incline and only the rear left wheel has traction. Subaru can send 100% of the engine's torque to just that one wheel while the Honda/Toyota can't send that much?

If this is the case, wouldn't the "fix" be as simple as allowing the center diff to lock (in the case of a clutch-activated center diff, to just lock the clutch) and applying brakes to the three wheels that don't have traction?

I don't see this example as having anything to do with the normal driving 60:40/50:50/40:60/whatever torque split or asymmetrical/symmetrical AWD design, but rather the work that the differentials do (or don't do) to transfer torque.

 

[The Boston Dangler]

crv and rav4 are just tall station wagons with token awd. the only thing they're good for is obstructing my view at intersections.

 

[Raizinman]

The Toyota RAV4’s biggest advantage against Subaru?

The RAV4 all wheel drive system knows when you need power to all four wheels and when it should save fuel economy.

How it Works: During normal driving, the RAV4 defaults to front-wheel-drive for improved fuel economy. Then, for improved cornering and driving performance, it sends power to rear wheels during turns and ascending steep hills. Think about this for a moment: when you don’t need all wheel drive, the RAV4 operates in front wheel. This means better fuel economy for you. When the vehicle doesn’t have to send power to all four wheels you save a ton of fuel.

These RAV4s were redesigned in 2013; after nearly two years the AWD RAV4 have now seen Spring, Summer, and Fall have reported BETTER than the 29 mpg highway rating.

How it Compares: Subaru boasts a symmetrical all wheel drive system. Translation: Subaru will always be in AWD so traction is constant. Great if you live in a place that never stops snowing. Comparing the Subaru Outback or Subaru Forester to the Toyota RAV4 and considering gas mileage to be important, there’s truly no competition.

The Specifics: The best AWD systems leverage software and wheel sensors to detect wheel slip as fast as possible. They then react by activating traction control to reduce or eliminate wheel slip while re-routing engine torque to the wheel with the best grip on the road. AWD with dynamic torque control are a riff on this theme and utilize an electro-magnetic coupler or (ECU). During normal driving, the RAV4 defaults to front-wheel-drive for improved fuel economy while still sending power to rear wheels during turns for improved cornering and driving performance (up to a maximum of 45% rear and 55% front torque distribution.)

Lock mode, on the other hand, essentially acts like Full-Time AWD on the RAV4 at speeds below 25 MPH by directing 50% of engine power to the rear wheels. Sport Mode provides smoother torque transfers between the front and rear wheels to improve steering by maximizing the traction of each wheel.

Modern production automobiles with AWD are not designed for EXTREME conditions. They are designed to assist in snow, rain, mud and the such all while obtaining the best possible fuel mileage. Just because a Subaru or Toyota can scale a 50 degree mud wall means very little to the average consumer.

 

[Kaido]

What makes Subaru's AWD system 'better' than a Honda or Toyota implementation?

Love. Its what makes a Subaru a Subaru :awe:

 

[Jumpem]

Subaru is the only one I would buy if having a capable AWD system is important.

 

[HitAnyKey]

It also depends on the model Subaru but in essence the full time aspect of it is great if you need full time traction. Live in a warmer zone, then only if you get alot of rain too would it make sense. That traction equates to a stickiness you don't get in alot of the other SUVs.

LOL @ The Toyota RAV4 = Tall Station Wagon. Yup pretty much. It actually is based off a Toyota Corolla platform. I think getting rid of the V6 and reducing the road clearance was a huge mistake for lineup that was supposedly an all conditions vehicle.

Wish Toyota would license Subaru's AWD system for the upcoming Hybrid RAV4.

 

[thescreensavers]

Subaru is full time Honda/yota are reactive systems so they are FWD until a slip is detected but it appears on some roller tests it doesn't send much power to the rear.

https://www.youtube.com/watch?v=jkiv-bWbLIo

 

[NutBucket]

The main reason Subaru fuel economy suffered for so long was the ancient 4 speed automatic. With the CVT the fuel economy is nearly the same as the FWD models of CR-V/RAV. I've found the epa numbers for our Forester are pretty spot on.

 

[MagickMan]

The 2 Subes I've owned fell apart after they hit 100k miles, YMMV however.

 

[fuzzybabybunny]

Subaru is full time Honda/yota are reactive systems so they are FWD until a slip is detected but it appears on some roller tests it doesn't send much power to the rear.

https://www.youtube.com/watch?v=jkiv-bWbLIo

Yeah, that's what I mean for the purposes of this thread. Automatic Subarus use a clutch-based center differential between the front and rear axles in addition to wheel braking to send various amounts of torque to each wheel.

For example, if the Subaru needed to send a large amount of torque to the left rear wheel only, it would lock the center differential's clutch and lock the brakes on the front and right rear wheels (they would be spinning freely otherwise since they have no traction). A good amount of torque from the engine will then transfer to the left rear wheel, enough to make the Subaru climb in the roller test.

That's nothing magical about this since applying brakes to the no-traction wheels is the standard way of transferring torque to traction wheels when using open differentials.

I'm just wondering why the Honda and Toyota systems seem to be worse at doing this.

I also believe that if Subaru wanted to, they could easily change the car to be 100% front-wheel-drive by simply disengaging the clutch on the center differential. But they won't because their whole thing is full-time AWD that is proactive (rather than reactive) in emergency situations.

 

[Zivic]

I'm just wondering why the Honda and Toyota systems seem to be worse at doing this.

.

because they aren't actually AWD... you need to think of them as FWD with rear wheel assist. it's more about marketing than it is actually functional/usable.

I know when I asked about some of these systems there are a lot of limitations... for example rear wheel assist only engages below 10mph, and won't stay engaged above 25. they aren't meant to be comparable to what subie offers; except for when john Q public is comparison shopping he sees, oh, this one is AWD as well....

Why are they worse? Because, again, they weren't ever meant to be as good

 

[michal1980]

The Toyota RAV4’s biggest advantage against Subaru?

The RAV4 all wheel drive system knows when you need power to all four wheels and when it should save fuel economy.

How it Works: During normal driving, the RAV4 defaults to front-wheel-drive for improved fuel economy. Then, for improved cornering and driving performance, it sends power to rear wheels during turns and ascending steep hills. Think about this for a moment: when you don’t need all wheel drive, the RAV4 operates in front wheel. This means better fuel economy for you. When the vehicle doesn’t have to send power to all four wheels you save a ton of fuel.

These RAV4s were redesigned in 2013; after nearly two years the AWD RAV4 have now seen Spring, Summer, and Fall have reported BETTER than the 29 mpg highway rating.

How it Compares: Subaru boasts a symmetrical all wheel drive system. Translation: Subaru will always be in AWD so traction is constant. Great if you live in a place that never stops snowing. Comparing the Subaru Outback or Subaru Forester to the Toyota RAV4 and considering gas mileage to be important, there’s truly no competition.

The Specifics: The best AWD systems leverage software and wheel sensors to detect wheel slip as fast as possible. They then react by activating traction control to reduce or eliminate wheel slip while re-routing engine torque to the wheel with the best grip on the road. AWD with dynamic torque control are a riff on this theme and utilize an electro-magnetic coupler or (ECU). During normal driving, the RAV4 defaults to front-wheel-drive for improved fuel economy while still sending power to rear wheels during turns for improved cornering and driving performance (up to a maximum of 45% rear and 55% front torque distribution.)

Lock mode, on the other hand, essentially acts like Full-Time AWD on the RAV4 at speeds below 25 MPH by directing 50% of engine power to the rear wheels. Sport Mode provides smoother torque transfers between the front and rear wheels to improve steering by maximizing the traction of each wheel.

Modern production automobiles with AWD are not designed for EXTREME conditions. They are designed to assist in snow, rain, mud and the such all while obtaining the best possible fuel mileage. Just because a Subaru or Toyota can scale a 50 degree mud wall means very little to the average consumer.

since your post reads like an ad

https://www.youtube.com/watch?v=wLkvqsQEV2Q

If you need to get up a slippery slope, then there's no point in getting the fake AWD of the RAV4.

 

[fuzzybabybunny]

because they aren't actually AWD... you need to think of them as FWD with rear wheel assist. it's more about marketing than it is actually functional/usable.

I know when I asked about some of these systems there are a lot of limitations... for example rear wheel assist only engages below 10mph, and won't stay engaged above 25. they aren't meant to be comparable to what subie offers; except for when john Q public is comparison shopping he sees, oh, this one is AWD as well....

Why are they worse? Because, again, they weren't ever meant to be as good

I mean mechanically, why specifically aren't they as good? "Because they aren't meant to" isn't a mechanical answer. Like I said, Subaru can make their cars FWD with "rear wheel assist" by simply telling the computer to disengage the center differential clutch. Then you instantly have 100% FWD on a Subie. And then when the computer engages the clutch (say, 20%) it transforms to the exact same rear wheel assist as a Honda or Toyota.

I can't imagine a mechanical reason for Honda or Toyota other than having a really shitty clutch-type center diff that can't lock...

 

[Accord99]

For example, if the Subaru needed to send a large amount of torque to the left rear wheel only, it would lock the center differential's clutch and lock the brakes on the front and right rear wheels (they would be spinning freely otherwise since they have no traction). A good amount of torque from the engine will then transfer to the left rear wheel, enough to make the Subaru climb in the roller test.

Subaru and Honda (and Toyota) all have several different AWD implementations that make different compromises in terms of weight and space, fuel mileage, cost, compatibility with drivetrain and market segmentation..

This is what Honda's best implementation, SH-AWD, can do:

https://www.youtube.com/watch?v=FJZxVefta68&t=1m24s

https://www.youtube.com/watch?v=KqzTsuPiTVA

 

[michal1980]

I mean mechanically, why specifically aren't they as good? "Because they aren't meant to" isn't a mechanical answer. Like I said, Subaru can make their cars FWD with "rear wheel assist" by simply telling the computer to disengage the center differential clutch. Then you instantly have 100% FWD on a Subie. And then when the computer engages the clutch (say, 20%) it transforms to the exact same rear wheel assist as a Honda or Toyota.

I can't imagine a mechanical reason for Honda or Toyota other than having a really shitty clutch-type center diff that can't lock...

on one car review comment section for the crv, a poster claims that Honda limits torque going to the back wheels by have a pressure relief valve set low to protect the rear diff

video of test failure
https://www.youtube.com/watch?v=jkiv-bWbLIo

post and comments

http://teknikensvarld.se/honda-cr-v-4wd-system-is-not-working-again-163708/

 

[thomsbrain]

What makes Subaru's AWD system 'better' than a Honda or Toyota implementation?

Marketing!

Subarus use torsen limited slip differentials which means that if a single wheel goes airborne or looses all traction (ice), the entire system takes a dump and sends all the power to the wheel with no traction, which is the exact opposite of what you would want it to do. They are good in the snow with all four wheels grabbing something, but you can get stuck just trying to turn up a steep driveway in the dry.

https://www.youtube.com/watch?v=w4oAKOCRy9g

While older versions of Honda and Toyota's AWD systems have had varying levels of ability to react to front-wheel slippage in a timely manner and/or had limitations in torque distribution, Honda's current CR-V system is very advanced and will even send torque to the rear under normal, non-slip conditions to for the sole purpose of affecting suspension squat under acceleration. Pretty clever stuff. And that's not even getting into the SH-AWD torque vectoring magic stuff. Honda pioneered street-car active torque vectoring with the Prelude SH, years before Ferrari tried it.

The video of the CR-V not able to move up the incline with both front wheels on "ice" is a good case for real, lockable 4WD, but at least it takes two wheels on the same axle with no traction to strand a CR-V, instead of just one wheel anywhere for a Subaru.

 

[HybridSquirrel]

This is a good video to explain:

https://youtu.be/ooQRxlChvMw

 

[michal1980]

Marketing!

Subarus use torsen limited slip differentials which means that if a single wheel goes airborne or looses all traction (ice), the entire system takes a dump and sends all the power to the wheel with no traction, which is the exact opposite of what you would want it to do. They are good in the snow with all four wheels grabbing something, but you can get stuck just trying to turn up a steep driveway in the dry.

https://www.youtube.com/watch?v=w4oAKOCRy9g

While older versions of Honda and Toyota's AWD systems have had varying levels of ability to react to front-wheel slippage in a timely manner and/or had limitations in torque distribution, Honda's current CR-V system is very advanced and will even send torque to the rear under normal, non-slip conditions to for the sole purpose of affecting suspension squat under acceleration. Pretty clever stuff. And that's not even getting into the SH-AWD torque vectoring magic stuff. Honda pioneered street-car active torque vectoring with the Prelude SH, years before Ferrari tried it.

The video of the CR-V not able to move up the incline with both front wheels on "ice" is a good case for real, lockable 4WD, but at least it takes two wheels on the same axle with no traction to strand a CR-V, instead of just one wheel anywhere for a Subaru.

Torsen is supplemented with stability/traction control which can use the brakes to lock one wheel to simulate a limited slip diff.

 

[Pulsar]

The Toyota RAV4’s biggest advantage against Subaru?

The RAV4 all wheel drive system knows when you need power to all four wheels and when it should save fuel economy.

How it Works: During normal driving, the RAV4 defaults to front-wheel-drive for improved fuel economy. Then, for improved cornering and driving performance, it sends power to rear wheels during turns and ascending steep hills. Think about this for a moment: when you don’t need all wheel drive, the RAV4 operates in front wheel. This means better fuel economy for you. When the vehicle doesn’t have to send power to all four wheels you save a ton of fuel.

These RAV4s were redesigned in 2013; after nearly two years the AWD RAV4 have now seen Spring, Summer, and Fall have reported BETTER than the 29 mpg highway rating.

How it Compares: Subaru boasts a symmetrical all wheel drive system. Translation: Subaru will always be in AWD so traction is constant. Great if you live in a place that never stops snowing. Comparing the Subaru Outback or Subaru Forester to the Toyota RAV4 and considering gas mileage to be important, there’s truly no competition.

The Specifics: The best AWD systems leverage software and wheel sensors to detect wheel slip as fast as possible. They then react by activating traction control to reduce or eliminate wheel slip while re-routing engine torque to the wheel with the best grip on the road. AWD with dynamic torque control are a riff on this theme and utilize an electro-magnetic coupler or (ECU). During normal driving, the RAV4 defaults to front-wheel-drive for improved fuel economy while still sending power to rear wheels during turns for improved cornering and driving performance (up to a maximum of 45% rear and 55% front torque distribution.)

Lock mode, on the other hand, essentially acts like Full-Time AWD on the RAV4 at speeds below 25 MPH by directing 50% of engine power to the rear wheels. Sport Mode provides smoother torque transfers between the front and rear wheels to improve steering by maximizing the traction of each wheel.

Modern production automobiles with AWD are not designed for EXTREME conditions. They are designed to assist in snow, rain, mud and the such all while obtaining the best possible fuel mileage. Just because a Subaru or Toyota can scale a 50 degree mud wall means very little to the average consumer.

https://www.youtube.com/watch?v=Uwr5_Sd47Us

3 years old.... but I really wonder where you're getting your information from about AWD systems.

 

[fuzzybabybunny]

Marketing!

Subarus use torsen limited slip differentials which means that if a single wheel goes airborne or looses all traction (ice), the entire system takes a dump and sends all the power to the wheel with no traction, which is the exact opposite of what you would want it to do. They are good in the snow with all four wheels grabbing something, but you can get stuck just trying to turn up a steep driveway in the dry.

https://www.youtube.com/watch?v=w4oAKOCRy9g

While older versions of Honda and Toyota's AWD systems have had varying levels of ability to react to front-wheel slippage in a timely manner and/or had limitations in torque distribution, Honda's current CR-V system is very advanced and will even send torque to the rear under normal, non-slip conditions to for the sole purpose of affecting suspension squat under acceleration. Pretty clever stuff. And that's not even getting into the SH-AWD torque vectoring magic stuff. Honda pioneered street-car active torque vectoring with the Prelude SH, years before Ferrari tried it.

The video of the CR-V not able to move up the incline with both front wheels on "ice" is a good case for real, lockable 4WD, but at least it takes two wheels on the same axle with no traction to strand a CR-V, instead of just one wheel anywhere for a Subaru.

I think you're getting some things incorrect.

The video you posted is of a Subaru converted to RWD only (and by the looks of it, an open differential in the rear axle as well judging by the wheel spin). A stock Subaru will climb a hill just fine when three wheels have zero traction but one wheel has traction.

I repeat, having up to three wheels with no traction is not a problem for a Subaru. The system will just lock the center diff, apply brakes to those three wheels, thus sending torque to the single wheel with traction. You will never see a stock Subaru with a wheel madly spinning in the air for any amount of time. The system will detect that that wheel obviously has no grip, apply the brakes to that wheel, thus sending torque to any wheels that it senses has grip.

SH-AWD Torque Vectoring from Honda is admittedly better than the Active Torque Vectoring from Subaru. Torque vectoring between the front and rear axles is old news - any AWD system does it by default. TV is simply the transfer of torque from one side to another. The devil is in the details though - how MUCH torque does a TV system transfer? As we can see in Honda's failed hill climb when the front wheels have no traction, the TV system doesn't transfer enough torque to the rear wheels.

TV on a single axle between the left and right wheels is much more interesting. Honda's TV can actually overdrive a wheel so that the output shaft on that wheel spins FASTER than the input shaft. So for example, in a cornering situation the outside rear wheel will actually create more torque and spin faster than the inside rear wheel.

This is different from Subaru's ATV solution which would apply slight braking pressure to the inside rear wheel in order to transfer more torque to the outside wheel. Subie's implementation is excellent at keeping a car's course true and is good for traction control scenarios, but you'll slow down when it does it because it's applying the brakes ie. I'm not going to win any races with it. To be honest though, from a safety standpoint I prefer this implementation. When I'm in the middle of a turn and hit a patch of black ice, my Subaru immediately slows the car down and the system does its magic to keep my original course. It's a weird feeling. The system backs off on the throttle and does all independent braking work for you - all you have to concern yourself with is maintaining your original steering position.

I was once in a narrow tunnel in the Rockies in a curve when I hit a patch of ice and I didn't even come close to sliding into oncoming traffic. The Toyota Landcruiser that my buddy was driving went full-on into the oncoming lane but luckily there weren't any cars.

 

[Accord99]

I repeat, having up to three wheels with no traction is not a problem for a Subaru. The system will just lock the center diff, apply brakes to those three wheels, thus sending torque to the single wheel with traction. You will never see a stock Subaru with a wheel madly spinning in the air for any amount of time. The system will detect that that wheel obviously has no grip, apply the brakes to that wheel, thus sending torque to any wheels that it senses has grip.

Not every Subaru AWD system does this. And even for those that do, some of the torque is wasted in the brakes so it's not as effective as an active vectoring system.

 

[fuzzybabybunny]

Not every Subaru AWD system does this. And even for those that do, some of the torque is wasted in the brakes so it's not as effective as an active vectoring system.

I'm sure that older Subarus without electronic traction control won't have brake-based torque vectoring.

BTW, the torque isn't lost through braking, at least directly. If anything, it's normalized between the two sides of the open differential:

http://oppositelock.**********/your-differential-and-how-traction-works-1661277563

(this example is for a FWD car capable of supplying the axle with 1200 ft-lb of torque)

If, with our example, you apply 300 ft-lbs to the tire on ice, and it slips, the sensors detect the slip and brake applying a resistance to that side of the axle and fooling the differential in to thinking that your Mu has changed to 1 or greater in which case the differential then distributes the torque based on a system that’s no longer split Mu resulting in a 50/50 torque split. With our 1200 ft-lbs of Trg , 600 ft-lbs can be applied to the high traction wheel and 600 ft-lbs of resistance is being met by the brakes being applied on the low traction side. 500 ft-lbs, if you recall, was our made up limit for overcoming inertia, thus…forward progress. I will say it again, you aren’t locking the diff when you brake a wheel, even though it looks like it visually, what you are doing is tricking the open differential into normalizing the torque equation, not biasing torque./quote]

If 1200 ft-lb can be applied to the rear axle, but one wheel is in midair, that one wheel may only take 2 ft-lb of torque to spin. Open diffs always balance the torque between the two wheels, so the wheel with traction will only have 2 ft-lb of torque. Now if you apply 600 ft-lb of resistance to the midair wheel via a brake, the wheel with traction will have 600 ft-lb of torque.

With open diffs + braking, you lose 50% of the available torque coming to the axle. If the axle has 1200 ft-lb of torque available, the max torque that any wheel can get is 600 ft-lb. With an active vectoring system one wheel could theoretically get 100% of the torque, so all 1200 ft-lb with the single wheel with traction.

So if we have a Subaru with three traction-less tires and only open diffs + braking:

Assume that after the transmission and after any additional gearing, the Subaru can supply a total of 1200 ft-lb of torque to four wheels. We lock the center diff and apply brakes to the two front traction-less wheels. That leaves 600 ft-lb for the rear axle. Then we apply the brakes to the rear wheel with no traction. That remaining wheel that actually has traction now only has 300 ft-lb of torque left to get the car unstuck or up the hill or whatever.

 

[andylawcc]

Subaru is full time Honda/yota are reactive systems so they are FWD until a slip is detected but it appears on some roller tests it doesn't send much power to the rear.

https://www.youtube.com/watch?v=jkiv-bWbLIo

i like the video, and also Honda's explanation.
http://teknikensvarld.se/honda-cr-v-4wd-system-is-not-working-again-163708/

 

[andylawcc]

Subaru and Honda (and Toyota) all have several different AWD implementations that make different compromises in terms of weight and space, fuel mileage, cost, compatibility with drivetrain and market segmentation..

This is what Honda's best implementation, SH-AWD, can do:

https://www.youtube.com/watch?v=FJZxVefta68&t=1m24s

the Audi looks like it could have made it though, the driver looks like he was hitting the brakes when it was gaining traction.

 

[fuzzybabybunny]

i like the video, and also Honda's explanation.
http://teknikensvarld.se/honda-cr-v-4wd-system-is-not-working-again-163708/

Very cool. So the technical reason looks to be a combination of a light - duty center differential that is programmed to not lock (just continually slips) and software to limit torque ON PURPOSE, which IMO is retarded. If the car is going below 15 mph and wheels are slipping, just send max power to traction wheels FFS. Screw gas mileage because it doesn't matter in this situation. Making sure the car doesn't get stuck is way more important.

I've seen quite a few Hondas and one Mitsubishi where the back wheels had solid traction and the front tires were just spinning while the back wasn't doing anything. Looked pretty sad. The Mitsubishi was on a concrete incline and had parked the front wheels on a big patch of wet leaves. It went absolutely nowhere despite the rear wheels being on solid dry concrete. The front just spun and spun.