AWD a lie?
#22
12-22-2005, 07:44 PM
RE: AWD a lie?
ORIGINAL: acvega
a similar situation happened to me oncewhen i was getting off a highway in the snow. i was at the stop sign at the bottom of the ramp and it was snow covered, well i went to pull out, and the car wasn't moving (this was before my abs pump died). tires were spinning all over the place, well at least one in the fron and one in the back. EDL is a weak system and a poor excuse for controling wheel spin. in really mucky situations or in icy conditions it is almost completely ineffective, at least in my experience with it. Heinzanova mst have had poor traction in the rear as well and the EDL system just wasn't effective in that situation. I didn't pull that off of multiple sites, just aquired knowledge since owning this car. audi's all wheel drive is functional but dated compared to many others. TORSEN is a bragging point for audi because they make great diffs for alot of great vehicles(military hummer's, s2000's, lot's of others that are big names. Having front and rear open diffs is what hurts it. heck the old all wheel drive DSM's (eclipses, talons, lasers) came with factory viscous rear LSD's, most newer suby's have rear lsd's, all brand new evo's, sti's have three LSD's. sorry, did alot of reading about all wheel drive systems when i got the audi and all the differences between them.
ORIGINAL: CheckMyVitals
...i researched.... he's right... for what i could tell until i stopped looking around.
if you didn't pull that off multiple sites... then kudos my man...
but can you explain this specific situation? it's definitely piqued my interest
ORIGINAL: acvega
Audi's use a mechanical or helical center diff based on worm and pinion gears, not a viscous. It has a 2:1 bias ratio, meaning it's 50:50 power split with a 60:40 transfer in either direction. And it is a limited slip diff (LSD). the front and rear diffs are open diffs that rely on the brakes to limit spin(EDL, electronic differential lock). those of us with bad abs pumps no longer have EDL cause it uses the abs pump to regulate spinning. A vicsous center diff is based on clutch packs and viscous fluid. most of the other AWD cars use viscous diff. (subaru, mitsu). power transfer in either direction can be almost infinite. the more wheel spin, the tighter the differential locks up in a viscous unit. In a mech unit like the audis, it can only go what the gears are ratio'd for, so if the fronts are spinning but the reas have grip, the torsen can only send up to 60% power to the rear. a viscous could send 100% if it was setup that way. Stasis engineering sells an upgraded torsen center for the audi's with a 4:1 bias or 80:20 for like $500, it can send 80% of the power to the gripping wheels as opposed to the stock 60%.
Audi's use a mechanical or helical center diff based on worm and pinion gears, not a viscous. It has a 2:1 bias ratio, meaning it's 50:50 power split with a 60:40 transfer in either direction. And it is a limited slip diff (LSD). the front and rear diffs are open diffs that rely on the brakes to limit spin(EDL, electronic differential lock). those of us with bad abs pumps no longer have EDL cause it uses the abs pump to regulate spinning. A vicsous center diff is based on clutch packs and viscous fluid. most of the other AWD cars use viscous diff. (subaru, mitsu). power transfer in either direction can be almost infinite. the more wheel spin, the tighter the differential locks up in a viscous unit. In a mech unit like the audis, it can only go what the gears are ratio'd for, so if the fronts are spinning but the reas have grip, the torsen can only send up to 60% power to the rear. a viscous could send 100% if it was setup that way. Stasis engineering sells an upgraded torsen center for the audi's with a 4:1 bias or 80:20 for like $500, it can send 80% of the power to the gripping wheels as opposed to the stock 60%.
if you didn't pull that off multiple sites... then kudos my man...
but can you explain this specific situation? it's definitely piqued my interest
definatly good to know man. thanks a lot
#23
12-22-2005, 08:19 PM
RE: AWD a lie?
Excellent link to differences in between differentials. http://www.houseofthud.com/differentials.htm
Here are a couple of excerpts I found interesting. I added the notes to indicate what section I took the text from.
So what's wrong with the open diff?
The problem is an open diff always tries to balance the torque. That's a hard statement to get a grasp on, but it means that if the spider gears are pushing on both drive gears and one of them offers lots of resistance (tire sitting on pavement) and the other side offers no resistance (up in the air, or sitting on a patch of ice), then it will find a happy balance where both sides are receiving almost no torque at all. All the rotational energy is guided to the side with the least resistance. In the end, that side spins very fast and the pressure on each drive gear is the same.. Almost no torque is needed to spin one wheel, and therefore almost no torque is going to the other side as well. Anyone who's driven on snow or ice knows this trick.
Note: Taken from EDL section:
Also note, it does not try to prevent wheelspin! If both sides lose traction at the same time and spin at the same speed then EDL has no idea anything is wrong. A more advanced system measures the difference between the speeds of all 4 wheels during acceleration and assumes that anything spinning faster than the slowest rotating wheel is slipping. A cheap system will apply the brakes. An expensive system will reduce the throttle until the problem goes away. BMW's and new VW's use this method to prevent wheelspin.
So why use EDL if it's so bad?
Well, the torque transfer is only really bad if the input torque is high and the resistance at the side where it's being transferred to is high. Such as.. When you're spinning one tire on the pavement and the engine is cranking out 175 ft/lbs of torque. When it kicks that torque across from one side to the other, it finds that the other side (also on pavement) initially has lots of traction and therefore a lot of resistance which means that the drivetrain takes the brunt of the stress all at once. Worse case, is when the side that is slowed, slows enough to regain full traction, while the other side starts spinning. Then the EDL compensates and brakes that side.. and the whole process repeats back and forth many times.. Solution.. take your foot off the gas.
On the other hand, if one tire is on snow, and the other is spinning on ice and the EDL has you accelerating slowly, the engine is producing little torque and if you tried to gas it hard, the torque transfer from the EDL meets up with little resistance on the snow so the torque "escapes" as wheelspin on the snow side. If it's ice and pavement then yes, you want to avoid mashing on the gas, but pulling away smoothly shouldn't present a problem as each pulse of the ABS transfers into a small bit more acceleration of the car.
Note: Entire section on Quattro/Torsen is good, here is a good piece of it...
It is capable of going from an open differential to say 60% locked differential condition absolutely instantly (zero lag), so many would argue that it's about as close to perfect as it gets for performance driving. There are no clutches to wear out. Several AWD systems like the Quattro system put a torsen diff in the center of the car to control slip between front and rear wheels. This system does not have the problem the VC does with the ABS. A torsen diff only distributes torque when it's under load. When it's freewheeling all the wheels can turn at different speeds as the ABS may desire.
(Note: It looks like the current Audi Quattro system only uses the torsen diff in the center, and some other LSD or EDL at the axles. )
The disadvantages are that the mechanism is a bit heavier, more mechanically complicated thus expensive, and can't be tuned or adjusted dynamically. Plus, it reacts so fast and is so even handed that it literally makes a torsen diff equipped AWD car boring when you'd really like to hang it all out. If you want to go 100mph in the snow nothing beats Quattro. If you want to slide sideways or spin and recover, forget it. I've heard that VW's new 4motion system is much more fun because of its electronic control.
Here are a couple of excerpts I found interesting. I added the notes to indicate what section I took the text from.
So what's wrong with the open diff?
The problem is an open diff always tries to balance the torque. That's a hard statement to get a grasp on, but it means that if the spider gears are pushing on both drive gears and one of them offers lots of resistance (tire sitting on pavement) and the other side offers no resistance (up in the air, or sitting on a patch of ice), then it will find a happy balance where both sides are receiving almost no torque at all. All the rotational energy is guided to the side with the least resistance. In the end, that side spins very fast and the pressure on each drive gear is the same.. Almost no torque is needed to spin one wheel, and therefore almost no torque is going to the other side as well. Anyone who's driven on snow or ice knows this trick.
Note: Taken from EDL section:
Also note, it does not try to prevent wheelspin! If both sides lose traction at the same time and spin at the same speed then EDL has no idea anything is wrong. A more advanced system measures the difference between the speeds of all 4 wheels during acceleration and assumes that anything spinning faster than the slowest rotating wheel is slipping. A cheap system will apply the brakes. An expensive system will reduce the throttle until the problem goes away. BMW's and new VW's use this method to prevent wheelspin.
So why use EDL if it's so bad?
Well, the torque transfer is only really bad if the input torque is high and the resistance at the side where it's being transferred to is high. Such as.. When you're spinning one tire on the pavement and the engine is cranking out 175 ft/lbs of torque. When it kicks that torque across from one side to the other, it finds that the other side (also on pavement) initially has lots of traction and therefore a lot of resistance which means that the drivetrain takes the brunt of the stress all at once. Worse case, is when the side that is slowed, slows enough to regain full traction, while the other side starts spinning. Then the EDL compensates and brakes that side.. and the whole process repeats back and forth many times.. Solution.. take your foot off the gas.
On the other hand, if one tire is on snow, and the other is spinning on ice and the EDL has you accelerating slowly, the engine is producing little torque and if you tried to gas it hard, the torque transfer from the EDL meets up with little resistance on the snow so the torque "escapes" as wheelspin on the snow side. If it's ice and pavement then yes, you want to avoid mashing on the gas, but pulling away smoothly shouldn't present a problem as each pulse of the ABS transfers into a small bit more acceleration of the car.
Note: Entire section on Quattro/Torsen is good, here is a good piece of it...
It is capable of going from an open differential to say 60% locked differential condition absolutely instantly (zero lag), so many would argue that it's about as close to perfect as it gets for performance driving. There are no clutches to wear out. Several AWD systems like the Quattro system put a torsen diff in the center of the car to control slip between front and rear wheels. This system does not have the problem the VC does with the ABS. A torsen diff only distributes torque when it's under load. When it's freewheeling all the wheels can turn at different speeds as the ABS may desire.
(Note: It looks like the current Audi Quattro system only uses the torsen diff in the center, and some other LSD or EDL at the axles. )
The disadvantages are that the mechanism is a bit heavier, more mechanically complicated thus expensive, and can't be tuned or adjusted dynamically. Plus, it reacts so fast and is so even handed that it literally makes a torsen diff equipped AWD car boring when you'd really like to hang it all out. If you want to go 100mph in the snow nothing beats Quattro. If you want to slide sideways or spin and recover, forget it. I've heard that VW's new 4motion system is much more fun because of its electronic control.
#26
12-23-2005, 02:35 AM
RE: AWD a lie?
nah the button says "esp off". But a benz is not an Audi so ur prob right crazyhawk.
#27
12-23-2005, 01:27 PM
RE: AWD a lie?
ORIGINAL: apg96
did u ever get the car out? If so how?
did u ever get the car out? If so how?
had to back up and keep twisting the car around until I got enough traction somewhere and a little momention to break thru each thing holding me back are by area.
AKA L33T DRiving SK1LLz
Or how ever you kids spell it
