Sway Bars and Endlinks on a Subaru WRX and STI

What a Sway Bar Does

A sway bar ties the left and right suspension together so the car resists leaning in a corner. A stiffer bar means less body roll, a flatter stance, and steering that answers sooner. It does not add grip by itself. It moves grip from one end of the car to the other, and it keeps the tires working on more of their tread.

WRX and STI cars come with a soft setup that understeers. That is the factory choice for a driver who is not at the limit on purpose. A performance-minded driver has room to take some of that understeer out. The bar is one of the first suspension parts that does it, and on a smooth road it usually does it without extra noise. You feel it in a corner, not in a straight line.

There is no single best diameter. The right bar depends on the roads, the tires, the springs already on the car, and whether the day is a commute, an autocross, or a track day. Bigger is not automatically better. A car with no suspension at all would not be the one with the most grip.

Chassis and Diameter

The 2002–2007 WRX and the 2004–2007 STI share a chassis and answer the same bar pairs in a similar way. The 2008-and-later WRX and STI do the same inside their own generation. A millimeter on one generation is not a millimeter on the other. The mounting points and the effective length of the bar are different, and the front control arms and the rear suspension are different parts. Moving the mounts on one rear bar can change the feel by at least 2 mm even before you change the tube.

Read the table as a rough match, not a part number. A 22 mm bar on a 2008 WRX is in the same neighborhood as a 24 mm bar on a 2004 STI. Ordering the 2004 size for the 2008 car is a much stiffer bar than the label suggests.

These cars use a MacPherson strut in front. The top of the strut is fixed, so the outside tire does not gain negative camber the way a short-upper-arm suspension does. Past a point in the travel it gains positive camber, and the load walks onto the outside edge of the tire. Less body roll keeps more of the tread on the pavement. That is the WRX-specific reason a bar helps, beyond “the car leans less.”

Bar Combinations

The sticky that collected these sizes runs from a mild street pair to a pair that belongs on a smooth race surface. Front is listed first. A dash means that pair was not offered for that generation in the write-up. Stiff coilovers already fight roll, so they want a softer bar than a car still on factory springs.

Equal bars were the sticky author’s choice for a daily driver and for street-tire autocross, because the matching rear bar takes out more of the factory understeer. A front bar one step larger, with a smaller rear, turns in harder and understeers more. On a smooth track with sticky tires that larger front is useful. On street tires in autocross, national-level STI cars in this discussion were faster with a soft front bar and stiff coilovers, not with the biggest front bar in the catalog.

Track days are not autocross. You want less rotation, and you want the front bar a bit stiffer because the suspension is loaded harder. For casual track days, a little understeer is the safer side of neutral. On street tires the suggested pairs were 27 mm front and 24 mm rear for 2002–2007, and 24 mm front and 22 mm rear for 2008–2013. Equal bars can still work on track if the front is set to its stiff hole and the coilovers have spring rates that already control roll. Then the front suspension stays more independent, and the springs, not an oversized front bar, keep the rear from stepping out.

Adjustable Settings

An adjustable bar does not change the metal. It changes which hole the endlink uses, which changes how long the lever is. Inner holes shorten that lever and make the bar act stiffer. Outer holes lengthen it and make the bar act softer. The middle hole, on a rear bar, acts like the diameter printed on the bar.

The front pattern depends on who drilled the arm. Some bars treat the soft hole as the real diameter and the stiff hole as about 2 mm thicker. Others split the difference, about 1 mm softer and 1 mm stiffer. On a rear bar, a 24 mm tube on the middle hole acts as a 24. The stiff hole acts nearer 25 or 26 mm. The soft hole acts nearer 22 or 23 mm. That spread is why an adjustable bar is worth the extra cost: changing your mind means moving a bolt, not buying another bar.

On an equal 24 mm pair, the sticky author’s street and track setting was front stiff and rear middle. For autocross he moved the rear to stiff, and sometimes the front to soft, depending on the course. He did not start on the soft rear hole. Soft was not the autocross setting, and it only meant unbolting the links again.

Endlinks

Endlinks connect the bar to the suspension. Without them the bar hangs and does nothing. Factory links will bolt to many aftermarket bars. They were built for the factory diameter, so they flex. In about the first inch of suspension travel the bar is softer than its size, and it comes in less linearly, because the link is bending before the bar is twisting. A stiffer link lets the bar do its job sooner.

Broken or flipped factory links are not the usual story. The person who wrote the sticky, and who sold these bars regularly, rarely saw a failure except on 2002–2007 rear links. Front factory links on 2002–2014 cars, and rear factory links on 2008–2014 cars, can break with a large bar and aggressive driving, but that was the exception. The everyday loss is the flex, not a snapped stud. A bar without new links is still an upgrade. It is a better one with links that match the bar.

Urethane resists any angle that is not square to the bolt. Mild angles often stay quiet. A bad angle loads the bushing, the hardware, and the mount, and the clunk gets worse with miles. Spherical bearings move freely and then knock once grit is inside. Sealed ball links showed up around 2009–2010 as the middle path: they move freely, they do not use a soft bushing as the joint, and they are less likely to chirp. They cost a little more than urethane. A hybrid with a ball on one end and urethane on the other, usually seen on the rear of a 2008-and-later car, is a fair daily driver that also sees an occasional autocross. It is not the choice for a stiff bar on a track car.

Mounts and Harshness

The rear mounts were designed for the factory bar. A much larger bar loads them harder. On 2002–2007 cars the upgrade replaces those mounts. On 2008–2017 cars it is a brace added to the factory points. Neither is required for the bar to function. Both are recommended once the rear bar is well above stock, because they cut flex and they are what stands between the bar and a torn bracket. The difference is easier to feel when the bar and the links are already stiff. A soft bar does not stress the mount enough for the brace to announce itself.

One 2002 wagon owner broke both ears off a factory rear mount, and snapped a thin rear endlink, on a 22 mm bar. That is one car, and 22 mm is not a huge bar for that generation. It is the reason the mount is part of the job on the early cars, not an optional dress-up piece. Later STI mounts were the beefier factory alternative that owner pointed at.

On a smooth road, a stiffer bar is mostly silent. Harshness shows up when the left and right wheels have to move separately, which is a broken or crowned surface, not a sweeper. If that is the commute, choose a smaller bar than the track table suggests. You still get a flatter corner. You do not have to buy the stiffest pair to feel it.

2015+ Cars and Wagons

The size ladder above stops at the 2008–2014 shape. A 2015 WRX already has a stiff front bar: a 24 mm solid, in the sticky author’s reading of the factory part. Replacing that with a 25 mm hollow bar was judged a step backward. The hollow tube did not make up for the extra millimeter. For a street 2015 WRX, the useful front work is fresh D-bushings and stiffer endlinks so the factory bar reacts sooner. The rear is where a new bar belongs, typically a 22 mm or 24 mm adjustable, with matching links and a mount brace. A stiffer front than that is a track car on stiff coilovers, not a commute.

Front endlink bolts changed with that larger factory bar. The thread’s fitment note is a 12 mm bolt on 2015-and-later cars and a 10 mm bolt on the earlier Impreza, WRX, and STI. Links and bars listed for 2008–2014 will not necessarily bolt to a 2015. Order the pair for the chassis you have.

Wagons in the 2002–2007 generation use narrower arms than the sedan. Urethane links on a slightly lowered 2006 WRX wagon in this thread leaned inward, popped, and eventually clunked hard enough to feel in the brake pedal. Washers bought a little time. Ball-link ends were not made for that wagon mount at the time, and the thread never published a fix. If the car is a wagon, the endlink has to be the wagon part, and the joint has to sit near square. A sedan link forced into a wagon arm is how that clunk starts.

A lifted car is outside this ladder. One lifted 2006 STI bent heavy street endlinks from the spring angle alone, before those links saw dirt. Street links are not a lift kit.

This packet does not size bars for a 2022-and-later WRX, or for a base Impreza that left the factory with no rear bar. Those cars need their own fitment, not a diameter copied from the table above.

How to Choose

Work in this order. Skipping a step is how a street car ends up with a bar that only makes sense on a smooth track.

1. Name the job. Daily driving, street-tire autocross, and track days are three rows in the table, not one “handling” upgrade.
2. Name the chassis. Use 2002–2007 sizes on a 2002–2007 car, and 2008–2014 sizes on a 2008–2014 car. Measure the factory bar if you are comparing to stock.
3. Pick the pair, then soften it if the springs are already stiff or the roads are rough. Equal bars are the neutral street starting point in this thread. A bigger rear than front is an autocross tool on stiff coilovers, and a poor surprise in an emergency lane-change.
4. Spend the adjustable money on the rear if you cannot do both. Set the middle hole first. Move one end at a time after you know the car.
5. Match the endlinks to the chassis and the bar. Plan on new links with any bar worth buying. On a 2015-and-later car, confirm the 12 mm front bolt.
6. Reinforce the rear mounts when the new bar is much stiffer than stock. Replace the brackets on 2002–2007 cars. Add a brace on 2008–2017 cars.
7. Drive it easy, then adjust. The limit moves the day the bolts are tight. Learn the new breakaway before you use it.

FAQ

What size sway bar should I put on a WRX or STI?

Match the chassis first. A common street pair in this thread is 24 mm front and rear on a 2002–2007 car, or 22 mm front and rear on a 2008–2014 car. Rough roads want the milder step. A smooth track wants a larger front. A 2015 WRX’s factory front bar is already stiff enough for the street.

Do sway bars and endlinks have to be replaced together?

The bar still helps on factory links, and those links rarely snap. They flex, so the bar is softer for the first bit of travel than the diameter suggests. New links are how you get the stiffness you paid for.

Will a bigger sway bar always handle better?

No. Past a point the front tires skip on bad pavement, or the inside rear tire lifts. Stiff coilovers need a softer bar, not a larger one, because the springs are already resisting roll.

Can I compare a 22 mm bar on a 2008 WRX to a 22 mm bar on a 2004 STI?

Not directly. A 22 mm bar on the later car feels closer to a 24 mm bar on the earlier car. The mounts, the arm length, and the suspension around the bar are different.

Should I upgrade the front sway bar on a 2015 or newer WRX?

Not for the street. The factory front bar on that car is a 24 mm solid, and a 25 mm hollow was judged softer than what was already there. Refresh the bushings and endlinks, and put the new bar on the rear.

Are hollow sway bars as stiff as solid bars of the same size?

Do not assume they are. This thread never settled a conversion that works for every tube. The one concrete case is the 2015 WRX: a 25 mm hollow front bar was judged less stiff than the factory 24 mm solid bar.

Will stiffer sway bars make the car harsh?

On an even road, usually no. You notice them in corners. Harshness shows up when one wheel hits a bump the other wheel misses. That is the reason to choose a smaller bar if the roads are rough.

What if the rear bar is stiffer than the front?

The car rotates more. That is useful in low-speed autocross when the coilovers are already stiff. On the street it is extra oversteer in a sudden turn, which is the opposite of what most daily drivers want.