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Old 12-07-2006, 03:05 PM   #2
Butt Dyno
Street's closed, pizza boy
Member#: 17301
Join Date: Apr 2002
Chapter/Region: MAIC
Location: NoVA
2016 FoST ST3
2006 Evo #17 STU


What size swaybar should I get?

1. Consider your other modifications.

If you already have a very stiff car (for instance, you're running coilovers with very high springrates) you will have less body roll than someone with a softly sprung car. As such you will not need as big a swaybar for the same effect. Suspension tuning is very much a black art. Some like to go with softer springs and stiffer swaybars, some like to go with stiffer springs and relatively soft swaybars, and some like to make everything massively stiff and see what happens.

Also, if you are running a fairly aggressive alignment (for instance, lots of negative camber up front) you might not need as big a swaybar for the same reasons.

The point is - your suspension is a SYSTEM. You cannot make decisions in a vacuum - the whole system must be considered when you are changing things. This also means you should think about upgrading both of your swaybars rather than just the rear or just the front.

2. Consider your starting point.

Most of the turbo Imprezas come with a 20mm front swaybar and a 13-20 (??) mm rear swaybar. Wagons typically come with smaller rears than sedans.

Here's a list of known stock swaybar sizes:

3. Consider your application.

First, here's an excellent post by Turn In Concepts on the subject:
As for what size bars to get, and this is my opinion based on experience, and is in consideration of solid bars (that's what I have the most experience with, and it makes for an easier comparison to stock).

For the rear bar - 22mm is good. You can pull off 24mm but the inside tire gets too light, and you lose traction in hard turns if you are on the stock springs. If you are considering going to 24mm or larger then you really really need to consider upgrading your springs first. See, the springs will stiffen not only the ride, but will also increase roll resistance. Now, stiffer springs does not have to mean a crappy ride. If you do your homework, and get a spring that a) doesn't have a stupidly low drop and b) are good for the operating range of your struts then the ride will feel just fine.

For the front - traditional thinking has held that if you want to fix understeer you want the rear to step out more. While this does work to some extent, I feel this is the wrong way to approach it. Sure CONTROLLABLE oversteer with the back stepping out is fun, and a good thing, but the front is still going wide. As part of this traditional thinking folks scream get a bigger rear bar, and leave the front alone or go smaller. I have to disagree on this. Here's why -

First, let's look at the technical reason. The dynamic camber curve for the mcpherson strut setup sucks. It just does. That's a tradeoff of running this type of setup, and all setups have their good and bad points. What happens is this - you enter a turn at speed. Your ourside front corner gets loaded with weight. That weight compresses the suspension. When the suspension compresses the camber of that wheel, where that tire is loaded, rolls toward positive camber. If you're running very little static negative camber this will actually start to make the inside edge of the tire light, reducing the width of your effective contact patch that resists lateral acceleration. This lose of traction results in that "push" or understeer condition. This is why folks who are more into performance run a decent amount of negative front camber. Sure, they give up some straight line accel and decel grip doing this, but that's more than offset by the fact that they can maximize the width of the contact patch in turns to help resist higher slip angles due to lateral acceleration. If you lower the car any with springs or coilovers that camber curve gets worse. Oh, and the added bumpsteer sucks too. Fortunately the fine fellows at 6Gun have figured out a way to help that, but that's for a different long post I'll most like use to answer somebodies questions in the next few days.

So, what they heck the the point in my saying all that? Well, let's get back to the front swaybar. The front swaybar will increase the effective spring rate of the wheel without effecting the ride spring rate. In other words, it will help keep the suspension from compressing so much at the loaded corner. Less compression means less loss of camber. Less loss of camber means that you can tune to, and maximize the contact patch of the tire to resist that push. Now, one of the things you need to consider - just like you can't make something from nothing, you can't make a good contact patch if you have nothing to give it to begin. Because the suspension will still compress some you need some negative camber in the front to start with. See the bajillion threads on alignment specs for where to start.

Now, for the second item I'd like to bring up something I've thought long and hard about for a long time. For ease I'll call it the philosphical side of the front suspension. Here's what we've got - a car that understeers or pushes quite a bit. Sure, you could throw a big bar in the rear and get the rear end to step out to help eliminate that understeer, but I feel that this is the wrong approach. The front of the car is still pushing, but the rear just happens to be stepping out at a faster rate than the front. The net result is a wider than what I feel is necessary lateral travel of the car. In other words, the rear is whipping around to point the front of the car where you want it to go, but the front of the car is still sliding sideways and taking the whole thing sideways making for a wider path than really is needed.

See, this stems from my philosophy that move oversteer does not necessarily mean less understeer. The front is still pushing. I feel the fix for this is to setup the front end of the car to truly reduce understeer as much as possible. This will make the front tuck in more, and while your tires will be resisting more lateral acceleration taking more grip from straight line acceleration (think traction circle), you'll be able to unwind from the turn faster thus getting you back toward the acceleration portion sooner than your competition. In other words, you'll be able to drive a tighter, and more controlled line.

Additionally, by reducing understeer as much as possible you won't have to set up the car for so much oversteer. This will also lead to more control, a tighter line, and the ability to accelerate sooner.

Now, that's great an all, but if you're looking for a simple answer as to what size bar to get then here it is (I'm giving these in terms of whiteline bars as I am most familiar with them):

Rear - 22mm to 24mm, but if you go to 24mm then you really want to think hard about upgrading your springs. If you go higher than 24mm then you are taking the wrong approach and trying to fix shortcomings of your springs with swaybars, and this is not the right way to do it. Get a proper spring for what you do, and then fine tune with a swaybar.

Front - for the street and mild autocross, and mild track work look at 24mm up front. If you start to get into it more then 26mm is good (WL makes a 24-26 adjustable front). These are also very good sizes if you are not a smooth driver as they won't make things too "darty" with super quick tuck of the front end into the apex. For autocross junkies or track whores who are smooth overall and can anticipate the "dartiness" then look at the 27 to 29mm front bar.

Now, two last thing on these considerations: You need a brand and size, and you need a vendor.

For the brand and size please do some research in the car parts review area. After receiving the parts you choose, and installing them please leave your feedback on them in the car parts review area so you can help others in the future.

The vendor part works the same way. Please research the vendors you are considering in the vendor review area. After the completion of the transaction please leave your feedback for that vendor in the vendor review area so that you can again help others in the future.

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I tried but I really can't say it any better than that. Thanks TiC!

Clint has another writeup about this on IWSTI:

One additional note - the bigger you go, the more likely it is that you'll hear random clunking/popping noises from the front of your car. I think this generalization is true, but feel free to correct me

I've heard that a bigger front swaybar will make the car understeer. Is this true?

It is, and it isn't. Basically, for our cars with their Macpherson strut configuration, the benefit that a front swaybar gives you in terms of managing camber under load outweighs the putting-too-much-weight-on-the-tire drawback.

DrBiggly has practiced this speech many times and has gotten pretty good at it:
Originally Posted by DrBiggly
To answer your question directly, you get a +3 or +4 addition in front grip from the camber curve improvement but a -1 in front grip from the "additional weight on the tire" part of the equation that comes from having a larger bar, so overall traction in the front of the vehicle is increased. Of course body roll is decreased, but as a bonus turn-in is quicker.

The car feels like it is understeering less because the limit of grip is a lot higher, while in reality at the limit the car is actually understeering a bit more. Hence why some folks say the car understeers less on a bigger front bar, and then others who haven't tried it bust out the "book and GT4" knowledge and proclaim that a bigger front bar will make the car understeer more.
I *REALLY* recommend reading that thread. It's an excellent discussion of the topic.

For a much longer post by DrBiggly about the front swaybar (i.e. if you don't believe him and you think a bigger front swaybar is a bad idea for our cars), here is a longer version of that post:

Will this swaybar fit my car?

From the failed "Will this fit my car" thread:
Originally Posted by Scooby921
WRX sedan and wagon RSB's are interchangable. WRX's can use STi RSB's, but they will mount to the outside of the endlinks. WRX and STi FSB's are interchangable. Wagons use different FSB's than sedans and can't swap.
There are some exceptions. This is a weak part of this FAQ, so feel free to suggest additions.

The big-ass 27mm Whiteline has been made to fit on a 2002 WRX Wagon:

How do adjustable swaybars work?

A swaybar will come with multiple sets of holes at the end, where the bar attaches to the endlinks. Here's an old picture of my Cusco bar, which is said to be the equivalent of a 22, 23 or 24mm bar depending on which hole is selected:

Basically - the longer you make the bar, the softer it is. Short but sweet thread:

The nice people from Whiteline chimed in with their explanation:
Originally Posted by WL Flatout
"A swaybar is actually a torsion spring not unlike a coil spring. Imagine you could uncoil your coil springs, and then hold each end and twist. This is pretty much what happens with the coil when it does its work as the shape forces the material to twist through out its length as it's compressed.

Going back to our straightened piece of spring steel, with your arms holding each end, imagine that each of your arms represent the swaybars "arms" and the spring steel rod represents the "back" or centre portion of the bar between the arms. As you try to twist the ends, a certain amount of flex happens in your arms but most of the action happens across the back of the bar. Hence, the formula calculating swaybar rates requires a value for the length of the back of the bar as well as the length of the arms.

The formula does not allow for a left or right arm value, just a total that acknowledges that the total arm length vs. the back length is the key calculation affecting the rate. We know that shortening the effective length of the arm by choosing a hole closer to the back of the bar will reduce the leverage ratio hence increase the rate but this affects the total arm length so can be done on one side only. In this way, a 2 hole per arm Blade adjustable bar does actually have 3 different settings; a 3-hole bar has 5 different settings. Additional mounting holes on the chassis end multiply the options further.

There are some assumptions and exceptions to these examples, specifically the issue of ultra short arm lengths and its effect on suspension preload. That is, asymmetric adjustment can preload the suspension if the arms are very short. The other issue is that the bars arms also deflect so that is taken into account by the formula but the amount of deflection is governed by the size and shape. Friction and deflection in mounting bushes will also affect the bars outright behaviour.

You may also notice that some of our adjustable use a longer Blade (flattened area) than others. We use this to fine-tune the adjustment range as the Bladed portion deflects a lot less than the simple round bar. (Think of a structural steel I-beam). Even the height and width of the Blade is used to tune the final outcome when designing new bars.

It's therefore important to understand that a 22mm physical diameter swaybar can be made to behave like something totally different just by changing the shape of the bar and its ends. Adjustable bars are a very useful suspension balance tuning tool and a better understanding of how they work should help you get a better result".
So, a bigger rear bar makes the rear end step out earlier but doesn't help the front, right?

I got this PM and felt it was worth posting:
This is incorect. Increasing the rear bar size does not just make the rear step out earlier, with no benefit to the front. When you are in a hard left hand turn, most of the cars' weight is on the right hand wheels,...if you had a big front sway bar and no rear bar,...the front right tire would be overloaded, and the front left would have almost no weight on it at all, might even be airborne.

The rear tires would both have weight on them however, the right more than the left of course. Adding a rear bar would take weight off the left rear and add it to the right rear. IT WOULD ALSO REDUCE THE TOTAL BODY ROLL OF THE CAR. This would in turn take some weight off the overloaded right front tire (allowing it to grip slightly better), and add weight to the left front tire,...which until now, was not able to help the car around the corner much, if at all.

The net effect is that the rear bar will certainly cause the rear to step out sooner that it would have with no rear bar (or with a smaller rear bar),...but at the same time, the front WILL have MORE grip.

You need to increase the rear bar size until you almost achieve a neutral car,...and if you still have too much body roll, increase both bars from there. (This might only involve buying an aftermarket front bar and moving the endlinks inboard on the aftermarket rear bar you already have).
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Last edited by Butt Dyno; 03-03-2009 at 07:34 PM.
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