Re: Why does Haldex work like this? () (vwtoys)
Hey guys,
Theres some misinformation flying around in this thread. AWD systems are tricky things to figure out, so it's not a big surprise.
Okay, there are two main types of AWD systems, the first is full time AWD and the second is part time AWD.
Full time AWD cars include manual tranny Subaru's, the EVO, and longitudinally engined Audi's.
Other systems from Nissan, Honda, BMW, transverse engined Audi's, transverse engined VW's, Mercedes and Porsche are part time AWD. That is, these cars either run in FWD or RWD mode. Typically these cars will have a power takeoff shaft coming out of the transmission and they will have a clutch system attached to the shaft that controls the amount of power going to the take off shaft. This can be either a Torsen, a clutch type LSD, or a Haldex. It doesn't really matter, they all do the same thing with varying degrees of success.
So, basically the way the full time AWD systems work is that the power is taken from the transmission to the center differential. The center diff basically has 3 shafts coming into it. One that goes to the transmission, one that goes to the front wheels and one that goes to the rear wheels. The center diff is therefore constantly distributing power front/rear. Note that it can distribute 100% of the power front or 100% rear or anything in between.
Now, part time AWD systems are different. The power goes from the engine to the transmission. In the case of a FWD car, the transmission has a direct connection to the front differential, just like if the car was FWD. There is also a shaft coming from the transmission that goes to the rear differential. The Haldex/torsen/LSD sits right in front of the rear differential. The shaft is always turning, it cannot be stopped. It turns as fast as the output shaft of the transmission. All the center diff can do in this case is transfer anywhere from 0 to 100% of the power of the output shaft into the rear differential. NOW, notice that the center diff in this case has NO control over the power going to the front wheels. So, the very most this system can ever transfer to the rear wheels is 50%, even when the diff is locked up solid (like welded together).
The Porsche system is essentially a FWD part time system rotated 180 degrees.
The RWD/AWD systems such as those in Nissan cars like the GTR, G35x, FX35/45 and BMW's X-series all have a RWD layout. Except if you look underneath the car, there is a shaft coming out the side of the transmission that heads back to the front differential. The car always is driving the rear wheels, but it can send up to 50% of the power up to the front wheels, and no more.
The only thing a Haldex does is it allows a 100% lock up of the center differential. Note that with enough development, you can use a Haldex differential in the front diff or the rear diff of a car, it is not specific to an AWD system. In contrast, a Torsen can only lock up about 90%, so clearly the Haldex has an advantage. The BMW X-drive is a Haldex type system and so is Nissan's ATTESSA. They are all clutch type, computer controlled differentials.
Hope this helps.
