As you know the Legacy has been given a much more advanced rear multilink suspension compared to its Impreza brethren. This offers many advantages over its Chapman strut counterpart used in the WRX/STi.
Advantages:
1) Better camber control. This allows you to use much less static camber as the multilink system adds camber under compression and body roll.
2) Better control over suspension movement, scrub, and toe-in/ toe-out. This is why road going race cars including F1 use them.
These advantages allows it to outperform an equally setup chapman system by virtue of better tire control.
Disadvantages:
1) Complexity. It is more complex than a chapman strut.
2) Limited suspension movement. A chapman strut allows the wheel to move through a greater range of motion. This is why WRC cars still use them.
3) Difficult to tune if you dont know what you are doing.
As to which is better. Given equally capable setups a multilink/ wishbone system will almost always be better on the tarmac. A strut system will be better/ more durable offroad.
Part 2: Breaking the Multilink into its component parts.
The legacy's multilink is broken down into 4 links.
1) Trailing Link.
2) Upper Lateral Link.
3) Lower Lateral Link.
4) Toe Link.
Trailing Link: This link's sole purpose is to localize fore and aft movement in the suspension. It is mounted parallell to the long axis of the car and moves in an arc. The mounting of the fulcrum point relative to the spring provides anti-dive and anti-lift geometry. Because the pivot point isolates movement to one vector it does a better job of controlling fore/ aft movement of the car.
Upper Lateral Link/ Lower Lateral link: This is what controls/isolates side to side movement of the wheel and provides camber control. These for the most part are "double wishbones" but are not in a wishbone shape because it does not have to control fore and aft movement like a double wishbone system. The fore/aft isolation of the wishbone has been seperated and given to the trailing link as mentioned above.
The upper and lower lateral links are what we call an unequal length arm system. This effects camber control because the longer lower link moves in a larger arc than the shorter upper link. What this means is that the lower arm arcs out more than the upper arm under compression. Tilting the top of the wheel into the car. This is EXCELLENT for handling as it adds camber when it is needed the most for cornering. This allows you to run a lower amount of static negative camber and have camber gain when diving into a corner. This is also why you gain static negative camber when you lower your car.
The unequal length concept is so effective that F1 and Champ cars still use it in thier suspensions today.
Toe Link: This link effects the toe in/ toe out of the car. Intrestingly it is also the longest arm in our multilink suspension. This means it moves in the widest arc. The toe link is mounted the most rearward in the suspension meaning under compression it will toe the wheel inward.
Toe-in is responsible for greater stability at the expense of turn in response and tossability.
Toe-out gives you better turn in response but can be very twitchy at speed.
It seems like Subaru designed the multilink with saftey in mind and set the toe arc to add toe-in under compression. This makes it very stable and predicatable in the corners but sacrifices the ass end out opposite lock behavior.
Thats all for now folks. This is what I gathered from staring at my suspension for a bit.
