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#3: 08-11-2011, 02:46 PM
 
 broknindarkagain
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Swaybar FAQ

This FAQ has been written by Jamal

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.

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 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 a huge advantage, or a big problem, depending on the suspension geometry. In our case, it's a big problem. A tire creates the most grip when the entire tread is in contact with the road. 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 sidewall. That's not good for grip. Additionally, when the suspension compresses past a certain point the tire will camber out, 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. More negative camber in the front means more cornering grip in the front and less understeer. I daily drive with over -2 degrees of camber and have not noticed any additional tire wear. However, it is important to have the toe set to 0 or slightly in, rotate tires frequently, and make sure all of your suspension and steering components are in good condition.

The other part is to not let the car roll as much. Reduce roll, and you reduce how far the tire leans over, and you improve grip. That's where stiffer suspension and swaybars come in. Since this is a thread about swaybars, I'll start talking about swaybars now.
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. On a stock Subaru, it's been calculated that the swaybars do about twice as much to resist roll as the springs.
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 the roll center (that's a hint at why roll center height is important). Both the springs and the swaybars resist this rolling force.
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.

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.
So then I should buy a rear swaybar?
Yes, probably. To reduce understeer, you need more rear roll stiffness. But to improve grip, you 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.

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. Just a big rear swaybar takes away rear grip, which will improve the balance, but it doesn't help with the severe lack of grip that exists in the front. Just a big front bar will help with the grip lost by the front due to body roll, but you end up with a car with a very front-biased roll stiffness.

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, or rather with the weight and suspension geometry of our cars, 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 450 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 whitelines to go with my sti struts and springs. I'm pretty happy with the setup, although I intend to buy stiffer springs and stickier tires.
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.


Fitment:

We've all heard the term that Subarus are like lego and you can interchange many parts between models and years. This mostly holds true for swaybars but there are some exceptions:

Generation differences
In the front, all Legacy swaybars are the same from 90-04, with the only difference being turbo vs. non turbo. There are also two different bushing and mount types, but bars are the same.
The FRONT 05-09 swaybars are different and do not fit on other legacies (although they interchange with 08+ imprezas).
Rear swaybars for an 00-04 Legacy must be specific for that model.
Rear swaybars for an 05-09 Legacy must be specific for that model.
Front swaybars for a '10+ must be for that model.
Rear swaybars for a '10 Legacy are the same as 08+ imprezas and foresters.

Turbo and non-turbo front bars are different
Because of the crossmember and exhaust differences, turbo and non-turbo bars do not interchange.
Impreza rear bars don't fit any Legacies
Our Legacies have a little bump that sticks out of the spare tire well and interferes with Impreza rear swaybars. So while a Legacy bar will fit perfectly on an Impreza, things don't work the other way around. This also probably applies to Forester swaybars.
The new wider-tracked WRX and STi front bars don't fit
The 02-07 WRX sedan has a wider track and therefore different control arms and lateral links. This means that the front swaybar has to be wider. WRX Wagon and non-turbo front bars DO fit on any 90-04 Legacy, following that you are not trying to mix turbo and non-turbo bars. Sti rear swaybars are also wider.
90-91 and 92-99 rear swaybars are different.
The 90-91 uses a droplink setup that goes into a hole in the rear trailing arm. 92-94 uses the C-links that are found on most newer Subarus. Swaybars might be interchangeable. More details of the differences can be found here:
http://bbs.legacycentral.org/viewtopic.php?t=34005
Some legacies do not have rear swaybars
None of the 92-94 L models came with a rear swaybar. I don't think Brightons or some L models did either after 95. Adding one is a really good idea. To do so you need mounting brackets, lateral links, and endlinks. More details here:
http://bbs.legacycentral.org/viewtopic.php?t=26464
Sizes

Okay finally we're here. Regardless of the car you have, there are different swaybars out there. Here's a list of sizes that came on different models of Subarus that will fit 1st and 2nd gen Legacies.
These may not all be correct and it would be great if people could confirm the stock sizes

Turbo Front
Legacy
91-94: 18mm
GC WRX/STi: ?
02-07 WRX wagon: 20mm
Non-turbo Front
Legacy
90-94: 18mm (19mm air suspension?)
95-99 2.2: 19mm
96-99 2.5: 20mm
Impreza
93-01 L sedan: 18mm
93-97 Wagon: 19mm
93-96 LS: 19mm
OBS: 19mm
98-01 RS: 19mm
02-07 RS/TS/i: 20mm
Tribeca: 25mm
Rear
Legacy
90-92 non-turbo sedan: 16mm
90-92 non-turbo wagon: 17mm?
92-94 L: n/a
92-94 LS, LSi sedan: 16mm
92-94 LS, LSi wagon: 17mm?
94 Ti/GT/Mi: 17mm?
91-94 turbo: 18mm
95 (all): 15mm
96-99 2.2: 15mm
96-99 2.5: 16mm
95-99 outback: 18mm
Impreza and 00+ Legacy bars do not fit.
Whiteline Aftermarket

Turbo front
BSF18 - 20mm
BSF18X - 22mm
BSF20Z - 22mm adjustable (22-21)
BSF20X - 24mm
BSF20XZ - 24mm adjustable (24-23)
BSF20XXZ - 27mm adjustable (27-26)
Non-turbo front
BSF19 - 20mm
BSF19X - 22mm
BSF19XZ - 22mm adjustable
rear 90-91
BSR17 - 18mm
BSR17X - 20mm
BSR17XZ - 20mm adjustable
rear 92-99
BSR19 - 18mm
BSR19X - 20mm
BSR19XZ - 20mm adjustable
BSR19XXZ- 22mm adjustable
Other Aftermarket
Rallitek - 22mm non-turbo front bar
Addco - 1" F, 7/8" R
others?
Bushings, mounts, and endlinks

The bushings in the endlinks and on the swaybars are made of rubber and they flex. That reduces the effectiveness of the swaybar. Replacing them with urethane or spherical endlinks is generally a good idea and really improves responsiveness because the bushings and endlinks aren't flexing as much before the swaybar loads, and will cause the swaybar to load more quickly. The only thing you need to keep in mind is that urethane bushings need to be well greased or they will bind and pop and possible tear apart. Whiteline makes urethane bushings in most sizes. Spherical bearings will also wear and eventually start to make noise.

The rear swaybar mounts have been known to break with larger swaybars. Whiteline, Perrin, and Cobb all have stronger mounting brackets available. The cheapest replacement option, however, are the 04-07 Impreza mounts, which are reinforced. They use a new bushing, and that Whiteline part number is W0406-[mm].

As far as endlinks go I'm partial to Kartboy and Whiteline but pretty much anything will work.


^Courtesy of Jamal for all his hard work documenting his experiments ^

Last edited by SBT; 09-27-2011 at 04:47 PM..