[LjasonL]

Swaybars

To deal with sway bars, you need to understand that everything is compromise, nothing you change on your car is wholly good, there will be a compromise of good effects and bad effects. If you know what the compromise is, you can make a more intelligent decision on what parts you want.

First, what is understeer and oversteer? It simply refers to the tendency of the front end to lose traction before the rear end (understeer) or vice versa. The technical term for this is TLLTD. Taken from another site:

Originally Posted by http://www.teamscr.com/sway.html

TLLTD stands for Tire Lateral Load Transfer Distribution. While this term may sound complex, it simply measures the front-to-rear balance of how lateral load is transferred in a cornering maneuver and is commonly used to compare the rate of lateral traction loss between the front and rear tires. You probably understand this already as the concept of "understeer" and "oversteer."

If you're reducing oversteer, then by definition you're increasing understeer. I see people say they did something to decrease oversteer, then immediately add that they did this without increasing understeer. This is impossible, as they are just words for opposite values of TLLTD. You can visualize this by a number line with oversteer at the right and understeer at the left. Understeer is just negative oversteer. When this person 'reduced oversteer' they indeed added understeer, it's just that in this case that added understeer canceled out too much oversteer and brought them closer to neutral. I think the confusion here is a result of the 'understeer is a bad word and we don't want anything to do with it' mentality. Again, it's all a compromise.

Okay let's get to the point. Sway bars do three things:
1. Increase weight on outside tires
2. Reduce body roll which reduces camber change when cornering
3. Change front to rear weight distribution (changing oversteer and understeer)

I'll copy this from another site since they do a better job of explaining these effects clearly than I do:

Originally Posted by http://www.nsxprime.com/FAQ/Performance/swaystrutbars.htm

Effect 1) is not a good thing for handling. The coefficient of friction for your tires decreases as more weight is applied. Read that twice to make sure you got it. If your car weighs 2000 lbs and 1000 is in front and 1000 in back (500 each corner), your cf might be .8 for illustration purposes. Your total grip in front is 500 * .8 + 500 * .8 which is 800. As you go into a turn, some of the weight in front transfers to the side so you may have 600 on one front corner and 400 on the other. However, at 600lbs, the cf may be only .7 and at 400, the cf may increase to .85. (Each tire has different values here.) Your total grip now is 600 * .7 + 400 * .85 which is 760. This is a decrease in total front grip. When you add stiffer sway bars, you increase the weight transfer to the outside which further decreases the cf for that outer tire and, therefore, for that end of the car.

Effect 2) is usually the primary reason for sway bars. When the body of the car rolls over, it takes the suspension with it. This causes the tires to ride on the outside edge of the tire and not get a flat contact patch. NSXs are very good in this area, but can still be improved (I suppose).

Effect 3) is neither a positive or negative absolutely. It depends on your car. If your car understeers, it may be due to effect 1) on your front suspension. Too much weight is transferred to the front outside tire which reduces its cf and may be causing it to break loose from grip. A way to prevent this is to increase roll resistance in the rear with a stiffer rear sway bar. The stiffer rear sway bar would resist body roll and apply more force to the outside rear tire instead of applying so much to the outside front tire. Obviously, you can go too far (too stiff in rear) which would transfer too much weight to the rear outside tire and cause it to decrease cf too much and eventually break loose.

So to make it short, the positive effect of sway bars are that they can reduce camber change due to body roll. The negative is a stiffer sway bar will decrease traction on the wheels it's applied to. And an effect that can be either positive or negative is they change the handling balance of the car. This is probably going to be your biggest concern when picking a bar. To put it simple, if you want to reduce understeer (change to more rear % TLLTD) then you can either add a bigger rear bar or a smaller front bar. Usually the bigger rear bar is preferable to a smaller front. The opposite is also true, if you want to reduce oversteer (more % front TLLTD) then you either add a bigger front bar or a smaller rear bar. Again, adding the bigger front bar is usually preferable.
For some reason there's a myth running around the Subaru community that this isn't the case - that adding a bigger front bar will reduce understeer some how. This of course does not agree with the physics of how sway bars work, but you don't have to take my word on it. Whiteline, one of the most respected manufacturers of sway bars for Subarus, had this to say:

Originally Posted by http://www.whiteline.com.au/default.asp?page=/faqswaybars.htm

As most factory vehicles are biased towards understeer, fitting of the larger rear swaybar will help in providing a more neutral characteristic in the handling at the limit. This is due to the increase in roll stiffness at the rear, which loads the rear wheels more unevenly and provides slightly less grip at the rear than previous.

At first this may sound sacrificial, however, as the rear end is resisting more of the roll, the front end resists less in proportion, leaving the front wheels more evenly loaded, therefore more available front end grip. In the end an increase in overall grip can be achieved by balancing the vehicle. A WRX or other front torque biased all wheel drive vehicle will benefit even more due to combined front end steering/traction demand.
...
The balance (and grip increase) of the car could also be achieved by reducing the front swaybar stiffness, however its roll stiffness would be reduced and roll camber would suffer. This would lead to large amounts of positive camber being gained on the outside wheels/tyres when cornering. This would result in a wheel/tyre that would not be at its optimal camber setting at the limit of handling.

A tidbit often pointed out is that the serious auto-x Subarus often use big front bars. Many misinterpret this as evidence that the bigger front bar really does reduce oversteer. The real reason this is done on serious race cars is because of something called motive traction, which we haven't discussed yet. When you put that big rear bar on your car to reduce understeer, the way it does this is by linking the two rear wheels together; as the outside corner is compressed in a turn, the bar rotates and compresses the inside tire too. If the effect is extreme enough, the inside rear tire will actually lift off the ground, and when you go to accelerate out of the corner, you'll just get wheelspin due to the lack of motive traction at the rear end. So what do you do? Add a bigger front bar to increase motive traction at the rear, and then you can accelerate out of the corner sooner. As you probably know by now, this comes at the price of more understeer bias. For the race cars, it's worth a little more understeer to be able to get on the power sooner coming out of a corner. Again, compromise...



So now that you know how it works, the next question is probably 'What will work best for me?' Well, that's not a simple question to answer. This has a great deal to do with the rest of your suspension setup (other factors that increase stiffness such as stiffer springs), your driving style, what you intend to do with the car, and just what feels best to you. You're going to be faster with a setup you feel more comfortable/confident in than a setup that has more overall grip but makes you nervous. Most of the time you actually want a bit of understeer bias, and not true 'neutral' handling that's often claimed to be the holy grail.
What do I use? Adjustable rear sway (22-24mm), stock front sway, STi front strut tops, front strut bar, and a set of springs meant for a sedan which, due to the extra 80 pounds in the back of a wagon, gives me a slightly stiffer rear spring rate than similar wagon-specific springs. This part wasn't really by design but just because nobody was making wagon-specific springs at the time I bought them. How does it feel? A bit of understeer on throttle with smooth lift-throttle oversteer. I do have that rear motive traction issue we discussed, but it's not an issue for me because I don't competitively race my car. This will only be an issue at the absolute limit, which you will almost never (or should almost never) reach on the street. I only know of one road where I can actually get that back inside tire spinning coming out of corners, and the car is AWD so I still have the front end pulling anyways. The balance is more important to me than the motive traction issue so I leave it be. If I were to try and competitively race the car I would add a bigger front sway to combat it; in fact, I've been thinking about adding a *slightly* bigger front bar to get quicker turn in at the expense of a little more understeer. Compromise...

I encourage you to read the articles I quoted, especially the first one which includes a controlled real-world test of sway bar effects on TLLTD.

http://www.teamscr.com/sway.html
http://www.nsxprime.com/FAQ/Performa...ystrutbars.htm
http://www.whiteline.com.au/default....aqswaybars.htm