How does AWD work in slippery conditions?

[jeansain]

I know that in a conventional 2-wheel-drive vehicle, most of the traction is transferred to the spinning wheel, in slippery conditions, which is why those are dangerous in snow/ice. Those with traction control balance traction between the 2 wheels when that happens, thereby making driving safer.

 

In a MT AWD sube is traction 25% on all wheels at all times, including slippery conditions? If not, how does it work?

 

Understanding how it works would let me know how confident I should be when driving my MT Impreza OBS in inclement weather.

 

Thanks

[AK-Brando]

My understanding is the Auto tranny Subaru's are electronically controlled AWD, where the Manual tranny Subaru's are viscous coupled AWD. What this means, and the way it was explained to me, is the Auto detects a difference is wheel speed and sends power to the wheel that is turning slower. Normally the Auto is 90% front, 10% rear (I think). Upon hard acceleration, the Auto will change to 50% front, 50% rear. The Manual tranny has to have some slippage before the power is transfered. I think the Manual is normally 60% front and 40% rear, but I am not sure on those numbers.

 

Maybe somebody has more detailed information, or can correct mistakes I have made in my assumptions.

[canajun2eh]

I can't answer your question about "how does it work". All I can say is that it's great!! You shouldn't have any serious problems, except when all 4 wheels are on ice. Under that condition, you need to go easy on the gas pedal.

 

If you deliberately try to make the car slide, it will behave as if it's rear wheel drive. Thus, you correct a slide the same way you do on a rear-wheel drive car. Turn the front wheels in the same direction the rear is sliding, and apply power.

 

I suggest that you find an empty parking lot and practice (watch out for the lamp posts). This might be hard to do in Toronto -- you don't get nearly the same bad driving conditions we do here in Ottawa.

[coloradosubarules]

I know in my 90 legacy the front wheels get 90% and the rear gets 10% of the power. This all changes when the vehicle senses slipping. It transfers power from the wheels that slip to the wheels that grip.

 

 

I think most Subarus are like that except for the STi which has a user adjustable center differential.

 

Hope this shed some light..

[jamal]

MT (except STi) has a full time 50-50 split.

 

Some Subarus (91-92 Turbo Legacy, 98-01 RS, and WRX I know (think) for sure) have a rear LSD. Without the LSD, it's possible to just spin one front and one back tire. On cars with a rear LSD and an open front diff, you can spin both back tires and one front tire. AFIK, the STi is the only car with limited slip in the front.

 

Yes, ATs have a 90-10 split until the fronts start to slip or if the gear selector is in 1 or 2 (in my 93 Legacy, anyway), in which case it goes to 50-50. I think some cars have a viscous coupling, where speed differences cause the fluid to shear, heat up, thicken, and send power the back wheels, kind of like a torque convertor, and some have a locking clutch-type center diff where a sensor detects a speed difference and engages the diff.

 

I don't know how VDC works, but I assume it just brakes a wheel when it starts to slip, so the power will go to the side that has traction.

 

The new Honda AWD-SH system looks pretty impressive. it uses clutch pack in the rear diff to transfer additional power to a wheel instead of braking the other. Check out this sweet video.

 

You can't really say that an AWD car handles like a RWD car on a slippery surface. You can sit there and plow the front wheels on-throttle, and other times you can kick the back end out and do awesome donuts. Just depends on weight transfer.

 

I don't know if some of this is wrong, so don't quote me on it. I should probably just not post when I'm not sure I know what I'm talking about.

[rweddy]

.

 

The new Honda AWD-SH system looks pretty impressive. it uses clutch pack in the rear diff to transfer additional power to a wheel instead of braking the other. Check out this sweet video.

 

 

I don't know if some of this is wrong, so don't quote me on it. I should probably just not post when I'm not sure I know what I'm talking about.

Kind of.

First off, the AWD systems used in Subarus differ, depending on what transmission you have.

Automatic-equipped Subarus use a computer-controlled hydraulic clutch pack, similar to those found in an automatic transmission, to transfer torque to the rear axle when the sensors detect a 20% difference in axle rotation speeds between the front and rear axles. Normally torque is split 90 front/10 rear. Not really all that different from most of the other AWD systems.

Manual-equipped Subarus use a viscous coupling, similar to a limited slip differential (and certain full-time 4WD transfer cases) where under normal, dry circumstances the torque is split 50/50 until the physical difference of the rotational speeds of the axles causes the hydraulic fluid in the coupling to heat up to the point where it acts as a clutch, forcing the two axles to turn at the same speed. It's basically the sameas full-time 4WD without the extra low gear. It can transfer torque front and back almost 100/0 or 0/100 if necessary. Better than most.

 

BTW Honda's system is not very good. Go test a CRV, they are front wheel drive until the real slips then the rear kicks in but by then it is too late.

[coloradosubarules]

Is that true of my 90 legacy auto AWD? Does it go 50/50 in first and second? I would really love to get a straight answer on this.

[Setright]

I can't answer that, but I would like to share this view on the Honda SH-AWD:

 

Looks the business, is the logical progression for AWD, but I expect that it is

BORING to drive.

 

The system cancels out the drivers influence on cornering line via the throttle. If I want the back wheels to slide out, I will step off the throttle sharply, and no Honda ECU is to deny me that!!!

 

Besides, all those electronics make me uneasy. I prefer the mechanical AWD in my MT Sube. It won't pull any weird stunts because some programmer didn't find a bug in the software...

 

Once you have had one tail-out skid moment in an ESP equipped Mercedes, you lose faith in all these modern electronic systems.

[jeansain]

Manual-equipped Subarus use a viscous coupling, similar to a limited slip differential (and certain full-time 4WD transfer cases) where under normal, dry circumstances the torque is split 50/50 ...

I have read that before.

 

What I'd like to know is the following: when a wheel starts to spin, is traction transferred to the other wheel on the same axle, and if so, how is that done?

 

For instance, in my 2-wheel-drive Mercedes with traction control, when one of the rear wheels starts spinning, the system can apply the brakes and/or back-off the throttle opening until both wheels turn at the same speed.

[rweddy]

I have read that before.

 

What I'd like to know is the following: when a wheel starts to spin, is traction transferred to the other wheel on the same axle, and if so, how is that done?

Check this page out.

http://auto.howstuffworks.com/differential9.htm

[blitz]

Is that true of my 90 legacy auto AWD? Does it go 50/50 in first and second? I would really love to get a straight answer on this.

 

On my '02, only the 1 (low) postion locks-in the 50/50 split, but I think that certain earlier years allowed both the 1 & 2 positions to do it, but I can't confirm it.

[Legacy777]

If you physically move the gear selector to the 1 postion it will force 50/50 torque split on the automatics. I'm not positive about the 2 position however. It may, or may not.

 

As for the info in the above threads, it's pretty much correct.

 

The 4EAT system is actually pretty good, and will in most cases react faster then the MT system will. This is simply because the AT system is electrical based. Yes it does have its short-comings, but it's not bad.

 

The MT's system. The fluid in the center diff is actually a silcone based fluid. There are clutch plates in there. Inner, and outer plates. When a speed difference occurs, this fluid heats up and causes the plates to lock up. The most torque split that can occur is 50/50. I believe it's a common misconception with most AWD systems that power can be transfered all the way front or back. It can't, the best you can hope for is both front and rear output shafts spin at the same speed, which is 50/50 torque split. You can not take away power from the front or rear output shafts.

 

The new VTD is actually pretty cool. It uses a center diff with a planetary gear system that controls default torque split via the gearing. So under normal conditions the default torque split is 45/55. There is a clutch pack that when engaged locks the inner and outer planetary gears together and forces 50/50 torque split. The system is for the most part, pretty much on or off, so you have the default torque split or 50/50.

 

The only draw back to the subaru system compared to fancy high priced systems is that power can only be transferred forward or backward, and not side to side.

 

The VDC system is subaru's answer to that though. It's a traction control system that can brake individual wheels and keep the car on the intended path and limit slip if you have one wheel spinning in the front or rear.

[northguy]

In a word - great.

[jeansain]

The MT's system. The fluid in the center diff is actually a silcone based fluid. There are clutch plates in there. Inner, and outer plates. When a speed difference occurs, this fluid heats up and causes the plates to lock up. The most torque split that can occur is 50/50...

Thanks everyone for your posts!

 

I am not worried about front to rear torque distribution as this has most to do with torque than safety, but about left-right split on the same axle. Indeed, if both wheels on the same axle are not turning at the same speed (in a straight line) you have a safety prob.

 

When a wheel starts spinning, how does the other one - on the same axle - react? For the silcone based fluid to work here, it would have to be IMHO, not only in the center diff but also in each individual diff.

 

If, as most say here, the system works great, there has to be something that makes it work!

 

Josh: Does each diff have this silcone based fluid? If so, how quickly does it react (heat up and thicken) to transfer torque to the non-spinning wheel?

 

TIA

[coloradosubarules]

Thanks for the info. Now I will just keep it in 1st when I go playing in the sand.

[americanyouth]

Don't think its been corrected yet, but '98 RS' did not have LSD.

[WoodsWagon]

If you don't have a limited slip in the axel differentials, then it will do the normal taking away power from the wheel with better traction and giving it to the spinning one manuever. Say your rear left wheel looses traction, it starts spinning. this caused the driveshaft speed to increase, but the front driveshaft speed stats the same because the front whells still have traction. this makes the center differential lock up and force the front driveshaft to turn at the same speed. this limits the amount one wheel can spin, and the car moves forward. now if a wheel on the front is slipping at the same time as one on the rear, both will spin at equal speed, but the car will go nowhere. Unless you have a limited slip in your axel differential. The best AWD would have three limited slips. One in the front, one in the center, and one in the rear differentials.

[jeansain]

OOPS!

[jeansain]

If you don't have a limited slip in the axel differentials, then it will do the normal taking away power from the wheel with better traction and giving it to the spinning one manuever. Say your rear left wheel looses traction, it starts spinning. this caused the driveshaft speed to increase, but the front driveshaft speed stats the same because the front whells still have traction. this makes the center differential lock up and force the front driveshaft to turn at the same speed. this limits the amount one wheel can spin, and the car moves forward. now if a wheel on the front is slipping at the same time as one on the rear, both will spin at equal speed, but the car will go nowhere. Unless you have a limited slip in your axel differential. The best AWD would have three limited slips. One in the front, one in the center, and one in the rear differentials.

The way you describe it (assuming I don't have LSD), if only one side of the vehicle hits some icy road while driving, while the other side stays on dry pavement, I could lose control of the vehicle! Whereas my traction control equipped, 2-wheel drive Mercedes would stand a better chance?!

Please note that this scenario (called "black ice") is not at all far fetched. It happens quite often here in Canada during the winter.

[Setright]

No. Your Sube would have to spin two wheels, that takes more effort than the Merc to spin just one.

[subarufanatic]

well it all depends on what tranny you have... I think everyon here is correct about the manual tranny. I havent seen anyone post anything about a 4eat with manual/power mode on them. If your subaru has a manual button on it then that is the way you help lock your 4wd in. Under snowy or icy conditions without the manual button pushed in the subaru I have doesnt grip as well. When i put it in manual 2nd, i can go through anything.

[Legacy777]

The manual button does not lock the car in 4wd mode.

 

this is from my site on the manual button's function

 

Manual Button

The manual button has some controversy as two what its function is. But I'm here to disavow any of those rumors. Firstly, the automatic transmission normally runs a power split of 90% front 10% rear. This is under normal driving conditions. Secondly, the manual button ONLY works when the shift lever indicator is in the positions "3" or "2".

Ok, so you're ready to hear what this thingy does. It is a very primitive version of traction control. When the shift lever is either 3 or 2, and the manual button is depressed, it will keep the transmission from going into first gear. By doing this, you have less torque, more evenly distributed power, and a less chance to slip and slide around.

So to recap, when the shift lever is in the 2 position the transmission stays in 2nd gear. When the shift lever is in the 3 position the transmission chooses between 2nd and 3rd gear. If the shift lever is any other position then the two above, the manual button does absolutely positively nothing.

[jamal]

The way you describe it (assuming I don't have LSD), if only one side of the vehicle hits some icy road while driving, while the other side stays on dry pavement, I could lose control of the vehicle! Whereas my traction control equipped, 2-wheel drive Mercedes would stand a better chance?!

Please note that this scenario (called "black ice") is not at all far fetched. It happens quite often here in Canada during the winter.

 

 

Well, say you're driving with one side of the car on ice and one side on pavement. If you floor it, both the tires on ice will start to spin. Since pretty much no power is going to the wheels on dry pavement, and the wheels on ice aren't getting any traction, the traction from the wheels on pavement will probably keep you going straight.

 

The traction control in the Benz does help, but the Subaru will still be better. The case of spinning two wheels is pretty extreme, and in most cases there will probably be pretty even traction from left to right, and enough so that you don't just spin two wheels, so power is going to all four.

 

The AWD-SH system I mentioned is different from what they have in the CRV.

[jeansain]

Well, say you're driving with one side of the car on ice and one side on pavement. If you floor it, both the tires on ice will start to spin. Since pretty much no power is going to the wheels on dry pavement, and the wheels on ice aren't getting any traction, the traction from the wheels on pavement will probably keep you going straight...

There is no question that I feel safer driving the Sube in the snow than the Merc (we've already got enough of the white stuff here for me to have experimented with both). However I am not sure I agree with you on what you said. If I floor it on black ice with the Sube, I think I'd probably end up doing a 180...

 

Hopefully I'll never have to find it out the hard way!

[blitz]

Well, say you're driving with one side of the car on ice and one side on pavement. If you floor it, both the tires on ice will start to spin. Since pretty much no power is going to the wheels on dry pavement, and the wheels on ice aren't getting any traction, the traction from the wheels on pavement will probably keep you going straight.

SNIP

 

The solution would be to merely apply a little brake along with throttle and away you go...

 

I've always preffered open diffs in combination with good tires for slippery driving for reasons of better directional control. Limited slips tend to promote a lot of lateral movement whenever traction gets sparse.