Sway bars

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A sway bar (also stabilizer bar, anti-roll bar) is a torsional suspension member.

Contents

Principle of operation

Introduction

A sway bar acts as torsion spring between the left and right independent suspension assemblies. By supplementing the coil springs it provides a higher effective spring rate during cornering (when a torsional load is applied across the bar) without substantially sacrificing straight-line ride comfort. Sway bars can be used to 'tune' the front-to-rear roll couple distribution, and hence the degree of understeer or oversteer.

Since they reduce the degree of suspension independence, stiffer sway bars may be undesirable when driving on uneven and/or low-traction surfaces where a higher degree of independence may help to maintain traction.

Hollow sway bars

Hollow (also tubular) sway bars are available from several aftermarket manufacturers. A hollow sway bar must have a larger diameter to achieve the same stiffness as a solid bar, but may be lighter. It is not possible to compare the relative stiffness of a hollow bar and a solid bar (or two hollow bars) unless the inside diameter (or, equivalently, the tube wall thickness) is known.

Adjustable sway bars

The effective spring rate of a sway bar can be varied by varying the length of the lever arms connecting it to the left and right suspension attachment points. A number of aftermarket manufacturers provide such 'adjustable' or 'blade adjustable' bars in which the flattened ends ('blades') of the bar's arms have several different bolt holes. A longer lever makes the bar easier to twist i.e. the holes farthest from the axis of the bar are the softest setting.

Effect of sway bars on handling

General rules

In general, stiffening the rear suspension or softening the front suspension reduces understeer (promotes oversteer)[1].

Larger front sway bars - preserving camber

The Impreza is usually set up with a certain amount of negative camber to aid in maintaining a good contact patch on the outside tire during cornering. However, the front suspension undergoes a significant change in geometry during compression: this results in a strong positive camber curve i.e. much of the static negative camber can be lost during cornering. In this case, a larger front sway bar can improve front grip (i.e. reduce understeer) by keeping the geometry closer to its static setting[2]. In particular a larger front bar is said to give better turn-in.

Impreza factory sway bar sizes by year and model

USDM MY05-07 Impreza stabilizer bar (sway bar) diameters
STI Sedan Turbo Sedan Non-turbo Wagon turbo Wagon Non-turbo Outback Sport
Front 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in)
Rear 20 mm (0.79 in) 17 mm (0.79 in) 13 mm (0.51 in) 17 mm (0.67 in) 13 mm (0.51 in) 13 mm (0.51 in)
USDM MY04 Impreza stabilizer bar (sway bar) diameters
STi Sedan Turbo Sedan Non-turbo Wagon turbo Wagon Non-turbo Outback Sport
Front 19 mm (0.75 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in)
Rear 20 mm (0.79 in) 17 mm (0.79 in) 13 mm (0.51 in) 17 mm (0.67 in) 13 mm (0.51 in) 13 mm (0.51 in)
USDM MY03 Impreza stabilizer bar (sway bar) diameters
STi Sedan Turbo Sedan Non-turbo Wagon turbo Wagon Non-turbo Outback Sport
Front N/ A in US/CAN 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in)
Rear N/ A in US/CAN 17 mm (0.79 in) 13 mm (0.51 in) 17 mm (0.67 in) 13 mm (0.51 in) 13 mm (0.51 in)
USDM MY02 Impreza stabilizer bar (sway bar) diameters
STi Sedan Turbo Sedan Non-turbo Wagon turbo Wagon Non-turbo Outback Sport
Front N/ A in US/CAN 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in) 20 mm (0.79 in)
Rear N/ A in US/CAN 20 mm (0.79 in) 13 mm (0.51 in) 17 mm (0.67 in) 13 mm (0.51 in) 13 mm (0.51 in)

Note: sway bar diameters are actual, physical sizes - they can easily be measured with calipers, or determined with a set of open-ended wrenches at a push (paint thickness is not enough to spoil the measurement, at least not on the stock bars).

End links

Stock non-STI Impreza rear end links are 'C'-shaped, and made of a nylon[3] (often referred to as 'plastic'). Billet aluminum replacements are available from most aftermarket manufacturers - their decreased compliance sharpens transitional response, especially with larger bars which may overwhelm the stock links. Stock front end links are nylon on wagon models, but steel with spherical bearings on sedan models.

STI rear endlinks are straight steel with spherical ends, but have been known to bend. #Sway bar 'flipping'

Sway bar and end link fitment

Sedans versus wagons

'New Age' (GD body) sedans were given increased track width, and the stock front sway bars are accordingly slightly (~20 mm) longer than those on the GG wagons and earlier Imprezas. Some aftermarket companies provide 'wagon specific' front bars, but others do not, and people have successfully fitted sedan front bars to the GG wagon with only minor differences in the angle of the end links; others have had problems with poor end link alignment and clunking - caveat emptor.

Note that the wagon front end links themselves are different - the wagon links have a smaller (M8 versus M10) lower bolt, to fit through the 'U' bracket on the wagon lower control arm.

In spite of the track difference, non-STI (GDA) rear bars are identical (the difference is taken up by the position of the mounting points on the lateral links, and it is these that are different between sedan and wagon). STI (GDB) rear bars, end links and lateral links are all different. #Using STI sway bars on non-STI Imprezas

Unlike the GD/GG sedan/wagon, GC/GM/GF body sedans, coupes, and wagons all share the same track.

Naturally aspirated versus turbo front sway bars

Front sway bars designed for turbo Imprezas generally do not clear the exhaust headers on the naturally aspirated models. Since several of the larger front bars are only available in a turbo fitment, a few have been made to fit with some effort[4].

Using STI sway bars on non-STI Imprezas

GD-body STI rear sway bars are longer than those on other Imprezas, and the rear end links are straight as opposed to 'C'-shaped. The bars will fit on any non-STI Impreza using non-STI 'C'-style links, but it will be necessary to bolt the links up on the inside of the bar's lever arms (the stock configuration has the end links on the outside of the - shorter - bar). Some believe that this introduces an undesirable lateral force component that prevents the bar from functioning properly[5].

STI front bars fit the same as other GD sedan bars, but offer no stiffness increase over stock. #Impreza factory sway bar sizes by year and model

Common OEM sway bar upgrades

MY02 WRX rear bar (20mm)

The OEM 20mm rear sway bar from a USDM MY02 WRX sedan is a popular mild upgrade for any later WRX (which got a 17mm rear bar), or other Impreza: MY04-07 owners will need the newer 'flanged' 20mm ID bushings from the MY04-07 STI, while MY02-03 should use the 20mm ID bushings appropriate for the bar unless they choose to upgrade to the newer mounts as well. #Subaru mounts

Tribeca front bar (25mm)

The B9 Tribeca 25mm front bar has been fitted by a number of members, mostly to naturally aspirated cars (for which large front bars are not otherwise readily available). It is longer than an Impreza front bar and needs to be attached on the outsides of the end links. It also has to be fitted upside-down (i.e. looped over the tie-rods) and may required longer end links than stock (STi rear end links have been used in some applications)[6]. Part numbers are

  • 1 x 20401XA00A B9 FSB
  • 2 x 20470SA011 STi rear end links

Sway bar mounts

Subaru mounts

Subaru revised the Impreza rear sway bar mounts for the 2004 North American model year. Extra bracing was added to the mounts, and the bushings were flanged to help retain them under the clamps. The new part mumbers are

  • 1 x 20540FE200 BRACKET R
  • 1 x 20540FE210 BRACKET L
  • 2 x 20466FE000 CLAMP STABILIZER

These revised (04-07) mounts are regarded as a worthwhile upgrade for pre-04 Imprezas, especially those fitted with moderately upgraded (20-24mm) rear sway bars.

9dc.jpg

Aftermarket heavy duty mounts

Aftermarket 'heavy duty' rear mounts are available from several manufacturers.

Impreza front sway bars clamp directly to the cross member with little scope for strengthening.

Sway bar problems

Sway bar 'walking'

When a sway bar is paired with spherical bearing-type end links, the bar may move from side-to-side somewhat within its mounts (note that stock and bushing-type end links do not allow this motion). This 'walking' can be prevented by fitting collars around the bar on each side of the mounts. Suitable DIY collars[7] are available from McMaster-Carr[1]; a low-cost solution can be obtained with hose-clamps.

Sway bar 'flipping'

The rear sway bar on the STI has been known to 'flip' when the suspension is allowed to come to full droop and then loaded again - for example when jacking the rear of the vehicle. The result is that the end link can become wrapped around the lateral link. This can bend or break the end link[8]. It is often first noticed as the rear of the car not sitting level[9].

Other Imprezas use 'C'-style end links and are not susceptible to this problem.

Sway bar math

Note that this section assumes a uniform, straight bar: real sway bars may have bends (rear bars must bend around the exhaust for example) or may taper (e.g. certain larger aftermarket bars, for easier bushing fitment). Exact stiffness values require detailed numerical analysis taking into account the bar's shape - or actual displacement versus load measurements. This section is provided for general interest, and to help explain the origin of many of the 'percentage stiffness increase over stock' values found in the forums and in manufacturers' data.

The torque required to twist a straight, solid bar of length l through an angle θ is given by

<math>M = \frac{GJ}{l}\theta</math>

where G is the material's modulus of elasticity in shear (or shear modulus); and J is a torsion constant depending on the cross sectional geometry of the bar.

To get an effective spring rate, we must convert this torque per unit angular displacement into a force per unit linear displacement (lbf/in, or Nm-1 in SI units) at the end of the bar's lever arms. Let the lever arms' length be b: assuming the end links apply a force F perpendicular to and in the plane of the bar's lever arms, then M = Fb. Meanwhile the linear displacement of the bar end is given by y = b tanθ ≈ bθ. Hence

<math>Fb = \frac{GJ}{l}\frac{y}{b}</math>

i.e.

<math>\frac{F}{y} = \frac{GJ}{b^2l}</math>

For a circular bar of outer diameter D, the torsion constant J is equal to the polar moment of inertia (or polar moment)[10]

<math>J = \frac{\pi}{32}D^4</math>

so that the effective spring rate is

<math>\frac{F}{y} = \frac{\pi GD^4}{32 b^2l}</math>

This is the effective spring rate at the end links - the wheel rate will be different. Hence it may be used to compare the relative stiffness of different bars, but not directly to determine the change in overall suspension stiffness. A table of relative stiffnesses derived from the formula is provided below.


Sway bar percentage stiffness comparison - solid
FROM\TO 13 mm 15 mm 17 mm 19 mm 20 mm 21 mm 22 mm 24 mm 25 mm 27 mm
13 mm 100 177 292 456 560 681 820 1162 1368 1861
15 mm 100 165 257 316 384 463 655 772 1050
17 mm 100 156 192 233 280 397 468 636
19 mm 100 123 149 180 255 300 408
20 mm 100 122 146 207 244 332
21 mm 100 120 171 201 273
22 mm 100 142 167 227
24 mm 100 118 160
25 mm 100 136
27 mm 100

Hollow bars

The easiest way to view a hollow sway bar is as a solid bar of diameter D, with a solid bar of diameter d removed from its center. It is easily seen that the torsional stiffness goes as D4 - d4. Hence a hollow bar (d > 0) must have a larger outside diameter in order to achieve the same stiffness as a solid bar (d = 0).

Adjustable bars

Stiffness is made adjustable by providing a means to vary the lever arm length, b e.g. using different bolt holes in the flattened 'blades' of the bar. Note that decreasing b increases stiffness, and that since stiffness ~1/b2 the fractional change in stiffness is twice the fractional change in length.

Notes