November 10, 200421 yr Does AWD work ALL the time, or this there some sort of sensor?? sounds like something that would go bad..
November 10, 200421 yr Does AWD work ALL the time, or this there some sort of sensor?? sounds like something that would go bad.. yes, yes, no.
November 10, 200421 yr Author yes, yes, no. All four wheels at the same time driving aLL the time? WOW... I love my car!!!
November 10, 200421 yr Author What actually I thought that only the front wheels drove, unless slippage was detected...
November 10, 200421 yr The automatic transmissions have a torque split of 90/10 under normal straight dry driving conditions. So the front wheels are going to be most of the work. WHen slip occurs, the torque split can increase up to 50/50 Manual transmission cars have 50/50 torque split all the time. Manual tranny cars use a viscous limited slip differential to control torque transfer, while the autos use electronics.
November 10, 200421 yr Author The automatic transmissions have a torque split of 90/10 under normal straight dry driving conditions. So the front wheels are going to be most of the work. WHen slip occurs, the torque split can increase up to 50/50 Manual transmission cars have 50/50 torque split all the time. Manual tranny cars use a viscous limited slip differential to control torque transfer, while the autos use electronics. thanks legacy!!! RIGHT ON! Great board, btw.
November 10, 200421 yr Does AWD work ALL the time, or this there some sort of sensor?? sounds like something that would go bad.. 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. 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.
January 17, 200521 yr 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. I understand that TCU controlls the clutch pack via solenoid C. I undenstand that 50/50 split meand closing the clutch pack but cannot understand how the 90/10 torque split is generated. The clutch is either open or closed. Would 10% torque mean closed only 10% of the time with rapid cycling? That would generate a lot wear and tear on the clutch. Or maybe partial closing of the clutch (90% open)? But that would potentially produce slipping and even more wear of the clutch pack. Anyone please?
January 17, 200521 yr The AWD puts more emphasis on the rear wheels to accelerate, because the weight of the vehicle shifts backward. More emphasis goes on the front wheels for engine braking, because the weight shifts to the front. During normal cruising, the wheels are evenly distributed. It also senses the individual speed of the wheels and compare it to the relative speed of the car. If there is some discrepancy, the power will shift according to where the power is needed. I've had occasions where I'll take a hard turn on a slick street and the car will start fishtailing. It senses that after about a half a second, then the back wheels drop out and the front wheels kick in, allowing the car to pull itself out of the skid.
January 17, 200521 yr Yes, all the wheels, all the time. Very few true PERMANENT four wheel drive saloon cars out there. Most are just pretenders.
January 18, 200521 yr the power will shift according to where the power is needed Yes, I remember that from the comercials, But cannot comprehend how that sophisticated transfer of power is ecomplished by the solenoid C and clutch pack in 4EAT. Again, is the clutch pack working in on/off mode with rapid cycling ("digital") or in "analog" mode with the amount of torque transfered is proportional to the ATF pressure at the clutch pack?
January 18, 200521 yr Mine's a standard so it's probably different. Although I just had an AWD experience tonight. I came out of a car wash, then proceeded to make a U-turn at a light. I made the U, but the wheels were still wet so I ended up fishtailing and drifting, my Forester going diagonal down the three eastbound lanes of Mockingbird Lane. All I did was let off the gas a bit, and the front wheels, being cut all the way over to the right, kicked on and my car jerked itself straight. AWD is effin' awesome.
January 18, 200521 yr Shuttle2Moon, chances are you already know this. But just in case... All the discussion so far has been front to rear power transfer. But if your car has the usual open differentials (not limited slip), then you can still loose traction side-to-side from one wheel on EACH axle. Under the right (or wrong) conditions, your AWD could become 2-wheel drive with power to only one wheel up front and one wheel at the rear. Still, it beats the heck out of the 1-wheel drive that a 2-wheel drive car becomes when it starts slipping!
January 18, 200521 yr Never mind, I found the answer on the NASIOC forum. Apparently, the amount of torque varies with the pressure of ATF at the clutch pack, so it is an "analog" system in my description and the clutch engagement is only partial with the normal 90/10 split driving and more complete with about 50/50 split ("1" gear); the setting is in between (60/40) when slipping is detected in "D". FWD fuse should relieve ATF pressure and disengage the clutch pack completely.
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