Updated 2/13/18
I've noticed a lot of swaybar related threads in the short time I've been around here, and I also have noticed a lot of misinformation and contradictory advice. So I thought I'd polish up my old legacycentral FAQ and post it over here.
I'd like to start by talking about how our cars handle and what happens to our tires and suspension when we go around corners.
From a flat out, on-road performance standpoint, a subaru starts out at somewhat of a disadvantage. Off the showroom, these cars are set up to understeer, and understeer some more. And then keep on understeering. Understeer is when, in a corner, the front tires break traction and begin to slide so you are turning the wheel but the car is going straight.
One reason for this is the AWD. You have four wheels powering the car, and give the fronts the additional task of steering. That right off the bat means that while accelerating and turning, the front tires have more work to do and will lose traction first.
Another problem is the weight distribution. On top of asking the front tires to both accelerate the vehicle and steer, you've gone and put more weight on them. Guess where that leads.
Now we come to the alignment and suspension geometry. When the suspension moves, the alignment changes. This can be advantageous with proper suspension design. In our case, the suspension design was based more on packaging concerns, cost, and travel for rally. A tire creates the most grip when the entire tread is in contact with the road. So driving along in a straight line or braking we want the tire to be standing straight up with the tread flat on the ground for even tire wear and optimum braking traction. However, if we set the camber to 0, that ideal contact goes out the window as soon as the steering wheel is turned. When you go around a corner with the stock car, the car leans over, the tire leans over, and you end up doing most of the cornering with the outside edge of the tire and the sidewall. Obviously the sidewall is not supposed to be in contact with the road and that's not good for grip. Additionally, when the suspension compresses past a certain point the tire will actually start to lean outwards, which makes the problem even worse.
On top of these inherent handling disadvantages, the cars are setup to push at the limit. This is because it's generally safer to go off the road straight than it is backwards/sideways, and inexperienced drivers tend to have trouble dealing with oversteer. There's also something called lift-throttle oversteer, and it's a common cause for crashed wrxes.
Okay so what do I do about it, and what does this have to do with swaybars?
One part of the solution is the alignment. Start with the tire leaned in (negative camber), and when the car rolls the tire ends up with more tread contact. This is especially important in the front, where we have more weight and more load in a corner. More negative camber in the front means more grip while cornering and less understeer. How much camber you should have depends on the use of the car and the tires among other things. A pure racetrack or auto-x only car will often have at least -3 degrees (stock is 0 to -1) along with stiff suspension and low profile tires that flex less. On a street car that mostly gets driven in a straight line and never sees high cornering loads this would not be a good thing and you would see uneven tire wear. You can still get away with quite a bit though and I daily drive with -2.5 degrees of front camber and the tires look pretty good. However, it is important to have the toe set to 0 and make sure all of your suspension and steering components are in good condition. The alignment as you drive down the road, brake, acclerate, and corner will change compared to what it is on the rack because of flex in bushings and suspension components. This gets worse as the bushings age.
So we've established that alignment is important, and why it is important, so you can probably see where I'm going with this post about swaybars. If the car sees less body roll, we reduce how far the tire leans over and improve grip. That's where stiffer suspension and swaybars come in.
What is a swaybar?
A swaybar (aka anti-swaybar, anti-roll bar etc), is a bar that connects the left and right sides of the suspension. It twists to resist the suspension on one side of the car from being at a different height than the other side. Swaybars exist to add roll stiffness without adding ride stiffness so that a car will ride comfortably but still stay relatively flat in corners.
What is roll stiffness?
Roll stiffness is a car's resistance to body roll. Body roll is caused by cornering force. Cornering causes a lateral acceleration, and that force acts through the center of gravity about an imaginary point called a roll center (that's a hint at why roll center height is important). Both the springs and the swaybars resist this rolling force. A cool guy who goes by stretch actually went through all the work of measuring out the suspension geometry of an STi and found that even the stock swaybars contribute about twice as much resistance to roll as the springs.
Why is roll stiffness important?
A car will more roll stiffness will have less body roll and the suspension will load and react more quickly. That is good for two reasons:
1) the tires stay flatter on the road and create more grip
2) the car is more responsive and changes direction more quickly
Great... What does that mean for me?
Swaybars let you add roll stiffness efficiently and also adjust the front to rear handling balance of the car.
A very important rule of suspension tuning is that adding roll stiffness to one end reduces the grip.
The front and rear of the car each resist body roll. Given a constant cornering force, that resistance will be proportional to the roll stiffness on each end, so the end of the car with more roll stiffness will resist more roll.
Doing more to resist roll means that more weight is transferred and that tire does more work.
More weight transfer means less proportional grip.
So a car that understeers probably has too much front weight transfer, and not enough in the rear. Adding rear roll stiffness (or taking it away from the front), will increase weight transfer in the rear, and reduce rear grip relative to the front, making for a more neutral handling car. Another way to put it is that we are reducing weigh transfer to the outside front, keeping it flatter, lowering the load, and increasing grip. So even just putting on a rear swaybar will reduce the amount of roll at the front of the car.
So then I should buy a rear swaybar?
Yes, probably. To reduce understeer, you need more rear roll stiffness. But to improve front grip, you also need less body roll. Can you see the conflict here?
A big front swaybar drastically reduces roll, and keeps the tires in better contact with the road. That improves grip. It also increases the weight transfer on the front end. That reduces grip. The interesting thing is that our cars roll so much and gain so little camber under compression that a big front swaybar can, in some cases, reduce understeer but in my experience that only happens on an auto-x course for some reason.
What it comes down to is that the car really doesn't have even close to enough roll stiffness to keep the tires in good contact with the road. If you want to make the car handle well, it's most important to add more overall roll stiffness. While swaybars are most effective at doing this, you still can't just go out and buy giant swaybars meant for racing because we run into issues with damping and suspension independence. However, a modest upgrade is always a good thing, and 22mm front and rear bars are a very safe bet for almost any Subaru (new cars with different rear geometry are their own case and an off-road oriented car will not want much swaybar).
In some auto-x classes, only a front swaybar is allowed. If you're in this situation, get a massive front bar from addco or strano. Otherwise, it's ideal to add roll stiffness to both the front and rear ends of the car.
How big is too big?
Well, that depends on the intended use, the tires, and the rest of the suspension.
In theory you want the bars to be as small as possible to maintain suspension independence and keep damping properties as consistent as possible. In reality, we are driving average sedans (and wagons), that are fairly tall and heavy and have less than ideal suspension designs. So, you need A LOT of swaybar to handle well on road. Honestly, more than is available for our cars. A good track setup for an impreza with sticky street tires generally involves about 500 lb/in springs and 27mm bars. With R-compounds more spring rate is required.
So, if you're looking for great on-road performance, bigger swaybars are very important, and for a Legacy, there isn't a readily available swaybar that I would consider too big. I have the 22mm front and rear adjustable whiteline bars and could not be happier.
One thing to keep in mind is that bigger swaybars add spring rate that the dampers have to deal with. So if you have 100k mile stock struts, I'd be wary of slapping on some big bars without at least throwing on a set of fresh KYBs.
Okay, well, I think I have a good start. I need to expand on the last few parts a little and update the next post for the newer Legacies.
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Originally Posted by ScaryFatKidGT
