FAQs about Sway Bars

  • Technical discussion of how sway bars work.
  • How can I fix a clunking sway bar?
  • Sway bars improved the ride on my Volvo.
  • Equation for the strength of a stabilizer bar.
  • How the laws of physics apply to sway bars.
  • What happens if your AntiSway Bars are too stiff?
  • Swaybars made simple.

  • Technical discussion of how sway bars work.

    Date: Wed, 20 May 92 15:38:24 EDT
    From: wiegman (Herman L. N. Wiegman)
    To: bever@erim.org
    Subject: Sway bars technical
    Cc: wiegman@orion

    >I tried to solicit some hard physics from the net, but I don't think I asked correctly.


    Interesting.. there are some on the net who are more knowledgable in that area than I.. I am a bit suprised that they did not contribute to the quest.

    0  <--- end link          TOP VIEW (half)


    \ ___

    \_____| |_________ <- sway bar

    Arm | |


    bracket w/ bushing

    (redundant explaination to follow... please don't be offended...;)

    the basic premise is this... the sway bar is solidly connected to the frame.. and the two end-links connect to the lower suspension arms. When a car (especially a Swedish Brick) corners, it obviously leans over according to the center of mass being cantilevered out. This causes one spring to compress.. the other to extend. This usually overloads the "cornering" tire and causes it to "roll-under." The flatter a car is the better, usually. Once a car is made relatively stiff as compared to its weight/center of mass... more stiffness will cause _worse_ handling. Ever try to drive a go-kart on a bumpy surface?... don't.

    The torsional force that a sway bar can take is limited by the Bulk Modulus of the material (don't quote me). If you twist the bar too much it then looks like a pretzel and doesn't do squat for you... (don't worry, sway bars often out live accidents...frame rails and end-links usually don't).

    The diameter of the bar and the length (and angle) of the "Arms" determine the "stiffness" of the bar.. (I am assuming the rest of the geometries to be the same.. i.e. overall width..etc.)

    The "stiffness" goes up with the

    1) fourth power of the bar diameter
    2) inverse power of the Arm length
    3) Cosine of the Arm angle (depends on which way it is measured...etc..)

    The counter force of a sway bar is essentially like a spring for small twisting motions... i.e. proportional to the twist angle...

    Some things to note are....

    1) going over a speed bumps does not use the capabilites of the sway bar...both ends rotate up and down simultaneously... there is no twisting involved.

    2) having a large bar gives up the independent suspension travel of the wheels... i.e. driving may be worse on "wash board" roads... If one wants better performance for these conditions, one must move to looser springs and sway bars, but stiffer shocks. (me thinks).

    3) sway bars don't help in the winter on slipery surfaces...

    all the best,


    To: swedishbricks@me.rochester.edu
    Subject: Re: Recc. on Sway Bars 200 series

    On Jun 1, 2:58pm, FINNEY@husc3.harvard.edu writes:

    > So, we bought the '92 240
    > 5 spd wagon. My husband, to whom I regularly deliver hardcopy
    > of Volvo-net wisdom, wants to know about the best width swaybars
    > for improving cornering. He feels it is ironic that my '76 240
    > wagon seems to do better without sway bars(?), and that the new
    > car could use an upgrade. I have the general list from Volvo-net
    > Who's who, but would appreciate elaboration.

    First of all, in the literature that John Rettallack brought back from Europe, there is a disclaimer that the 1992 200 series volvo's have revised springs/shock absorbers and swaybars. I am uncertain as to whether the springs/shocks/swaybars are firmer or softer than an earlier model car. This is an important point....

    Basically swaybars (whether stock or aftermarket) accomplish two things -a sway bar is a torsional spring that connects the left wheel on a car to the right wheel - if both wheels move up or down together it imparts no resistance - if the wheels move in contrary motion the swaybar will resist the motion - first, a larger sway bar will reduce the amount that the car is inclined to lean in a corner - second, the balance between the magnitude of the front and rear "roll stiffness" (a combination of the spring stiffness, shock absorber stiffness and sway bar stiffness) profoundly affects the "feel" of the car. Does the car understeer? (run wide at the front-end) or oversteer? (hang its tail out) An oversteerer will feel more "agile" than an understeering car.

    The IPD sway bar kit for a normal ride height car (rather than one fitted with their stiffer and lower springs) consists of a 1" diameter front A/S bar and a 7/8" diameter rear A/S. These are roughly 3 times stiffer front and rear than stock. However, in my experience, they alter the handling balance significantly towards oversteer (perhaps what your husband finds disconcerting in the 1976 wagon). On "mushy" tires, the oversteer is unpleasant feeling. I would not run a 200 series with IPD sway bars and stock 185R14 Michelin tires.

    The choice of shock absorbers also greatly affect the handling balance. The Bilstein shock absorbers (over which I wax rhapsodically) significantly improve steering response and also reduce understeer.

    For a recent car, such as your new 245, I would suggest the following "improvements" :

    Tires : replace stock 185R14 Michelin with 205/70HR14 tires (see tire chart) less important on a new car, very important after 40k miles

    Dampers : replace stock shocks with iPD "specially-valved" Bilsteins only then

    Sway-Bars : replace stock sway bars with iPD sway bars

    I did not modify my car in this order, in particular I came to regret the setup with KYB-brand shock absorbers and IPD sway-bars. The difference between Roman's 1978 245dl w/ Continental CH-51 tires, Bilsteins and stock sway-bars vs. my 1979 264gl w/ Continental CH-51 tires, Bilsteins and iPD sway-bars is inconsequential.

    > Also, what if we decide to tow something (a small sailboat for
    > instance) -- do the sway bars affect this?

    No. However the IPD "overload" springs (not MOOG) would be beneficial if the sailboat has a "tounge weight" of substance. 205/70r14 tires have an increased load capacity which is beneficial when towing.

    Hope this helps!


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    How can I fix a clunking sway bar?

    Date: Mon, 5 Apr 93 13:08:55 CDT
    From: barnett@mcc.com (Jim Barnett)
    To: juns_ltd@uhura.cc.rochester.edu
    Cc: swedishbricks@me.rochester.edu
    Subject: clunking sway bar

    I had a recent problem with a clunking sway bar. I called up IPD and hey suggested that I make sure that the sway bar is pushed all the ay forward (i.e., away from the panhard rod.) They said that mperfectly bent sway bars can hit the panhard rod if they aren't ushed all the way forward. I was getting a metallic "BONGGGG" sound oing over bumps, and pushing the rod forward fixed it. (Actually, I'm till getting a bunch of other noises, but I suspect the new stiffer ear springs that I installed at the same time.)

    - Jim

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    Sway bars improved the ride on my Volvo.

    From: Maximo Hans Salinas <mhs2z@adminsun.ee.virginia.edu>
    Subject: 745 Sway Bars
    To: Volvo e-mail list <swedishbricks@me.rochester.edu>
    Date: Wed, 14 Apr 1993 08:43:07 -0400 (EDT)

    Hello to all fellow Volvo fans,

    I've been following the sway bar discussion with interest. As it happens, just installed a set of sway bars in my wife's '92 745GL. I had been ed to believe by both the IPD salesman and local Volvo fans that they ould make the 745 handle better than my '84 Honda Prelude. They didn't. t that was only a small disappointment because they did vastly improve he 745's ride. Even after spending two hours beneath the wagon, I still don't quite understand the mechanics of what makes these bars work. The first thing I noticed after putting them on, though, was the more balanced feeling in the steering (a little less understeer as someone already pointed out). The wagon also corners with a lot more stability, but not to the point where I can really throw it into the turns.

    Overall, I highly recommend them on wagons. Note that the IPD instructions state that most wagons don't come with a rear sway bar to begin with. My brothers both have fairly new 240's. Although, I haven't driven their cars that much, my impression (ie. Subjective opinion) is that the ride on the 745 is now comparable to the ride on their 240's.

    I expect this probably doesn't help those who are trying to make up their minds. If possible, try to find someone in your area with the same model you have who has the IPD bars installed, or at least ask opinions from people who have your model. Everything else may be like comparing apples to oranges.

    Good luck,

    Max Salinas

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    Equation for the strength of a stabilizer bar.

    From: George Chow <george@unixg.ubc.ca>
    Subject: Sway Bar Equation
    To: wiegman@orion (Herman L. N. Wiegman)
    Date: Mon, 12 Apr 93 18:18:34 PDT

    > George,
    > >Hmm... to the 4th power. I'm gonna have to do some research on suspensions
    > to learn a bit more about the mathematics behind that.
    > something about a swaybar being stiff because of the bulk modulas of the
    > material, but the rest comes from the volumetric and linear equations..
    > hence the 4th power.. same with spring rates..

    I looked up some books on suspension over the weekend and found a great volume that covers most of the basic: _Automotive Suspension and Steering Systems_ by Birch. It's got an equation for the strength of a stabilzer bar:

    R = 5e6 x D^4 / (0.4244 x A^2 x L + 0.2264 B^3)

    where: (english units feet? inches?)

    R = Torsion rate of bar
    D = Bar diameter
    A = Effective length of lever arm
    L = Length of bar
    B = Length of lever arm

    > You or a small time mechanic should be able to bolt the front bar in..
    > All the parts and peices fit in the OEM holes.. you will only need a ratchet
    > set. It should take the dealer less than 20 minutes... with out coffee
    > breaks. If I had access to an air gun, I suppose I could change out a 240
    > front bar in 4 minutes... but I would need two pairs of hands.

    I suppose I should take a look under the front to see what's involved. The biggest impediment is that I don't have the gear to do the work.

    > The rear bar is a bit more tricky, because it involves muching with the
    > rear shock bolts.


    > >I thought about that. The question is what is the most obvious. Do I feel
    > the sway bars first or the shocks?
    > they both are active over the same region.. but do different jobs. The diff'
    > being dampening and roll stiffness.. btw, a car with great roll stiffness
    > can still oscillate like a pendulum.

    Yeah, I read up on the shocks and sway bar and I see the difference now. Considering that a set of shocks is gonna cost about twice as much as this swaybar and that shocks do wear, I think I'll wait 'till this set goes.

    > that way you don't break the piggy bank too fast...

    I'm gonna skip the Turbo+ in favour of the sway bar because of the piggy bank. But there's a great discount I can get for the T+ kit... :(

    > herm

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    How the laws of physics apply to sway bars.

    From: Ed Moore <edm@hpvcledm.vcd.hp.com>
    Subject: Re: swaybars
    To: maj@frame.com (Michael Jue) (Michael Jue)
    Date: Sat, 10 Jul 93 19:19:07 PDT
    Cc: swedishbricks@me.rochester.edu (swedishbricks)

    > By using larger sway bars than standard, you are able to resist the suspension
    > motion that much more, thereby lessening the amount of body roll (lean).


    > That ultimately decreases the amount of weight transfer from side to
    > side thereby lessening the amount of centrifugal force on the car

    False. The laws of physics still apply. The roll couple is not reduced. Centrifugal acceleration (V^2/R) is not reduced. But the increased roll stiffness reduces the angle change (body roll) caused by the roll couple. Decreasing the roll angle has at least two beneficial effects.

    1) In steady state roll (a long corner) the suspension geometry change is reduced. Most suspension geometries except a solid axle produce some camber change as a result of roll. Camber change alters the tire's footprint, potentially reducing lateral traction.

    2) Since steady state roll doesn't go as far, the duration of the transient while entering a curve is reduced. I think this is whatwe mean when we refer to "crisper" response. My 1985 Ford Crown Victoria SW has such a long transient that I could eat lunch while waiting for steady state cornering behavior.

    Ed Moore
    Hewlett Packard Co.
    Vancouver, WA

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    What happens if your AntiSway Bars are too stiff?

    Date: Mon, 12 Jul 1993 10:15:43 PDT
    From: werner.wbst311@xerox.com
    Subject: Re: swaybars
    To: nessa@mentor.cc.purdue.edu
    Cc: swedishbricks@me.rochester.edu

    >What happens if one has AntiSway Bars that are too stiff?

    The limiting condition is when there is infinite roll stiffness. At this point, you have lost side-to-side independence in the front wheels.

    The stiffer you make the anti-sway bars, the flatter the car's cornering response will be. In race cars, the limiting factor often becomes the tires. Well tuned racing Porsche 911's often have so much front roll stiffness that they pick up the inside front wheel.

    One obvious problem that can occur is that you can lift up more then one inside wheel. Frank Bov complains that to be competitive in his AutoX class on a national level, he is pushing his BMW to the point where he has 2 wheels off the ground. A little bit hairy, because it is the first step to rolling the car. I have even had reports that I got the inside wheels of my 145 off the ground at an AutoX on a go-kart track. I beleive them, considering I drove _over_ (not on) the "speed bumps" in the apex of the corner and didn't feel them.

    The other, less obvious problem is that you start imparting forces on the suspension that were not part of the design intent. Old 140s, 122s, and 1800s run the risk of bending control arms. This is usually only a problem in car that are driven really hard over rough surfaces (can you say 'rally cars?'). The problem arises when one front wheel is forcefully asked to move up while the other is not (say, by hitting a hole, rock, log, etc.). The sway part does its work and imparts a resisting force on the control arm of the wheel trying to move up. If the wheel is really being forced up hard, it will still go up, and the control arm will bend at the sway bar link. (That's why I run rubber sway bar bushing with the IPD bars now. The poly-eurothane ones got taken out when I replaced the bent a-arm and fitted the R-Sport springs.)

    One other important note: The springs, shocks, swaybars, and tires & wheels all effect ride and handling. Only when these are carefully tuned to work with each other do you get the best solution. Suspension tuning is a real science where not all the answers are applicable to all purposes. Also remember, especially with the tall, boxy volvos (120s, 140s, [54]44s, 200s) that the car has such a high center of gravity for its width that there is going to be body roll, whether or not all 4 wheels stay on the ground. That's why the real sports cars are a lot shorter then they are wide.


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    Swaybars made simple.

    Date: Mon, 12 Jul 93 16:53:33 EDT
    From: wiegman@orion (Herman L. N. Wiegman)
    To: dk010b@uhura.cc.rochester.edu
    Subject: Re: swaybars made simple
    Cc: swedishbricks@me.rochester.edu

    Dan, and non-techno nerd netters [i'm getting my nomex on right now..]

    >Could you please translate this into english sure.. (major scientific nerd to the rescue).

    a swaybar is like a moderator of a talk show... it tries to "even out" the forces on either side of him/herself by transitioning conversation one way or the other, trying to give equal coverage of both sides of the issue.

    you will probably note that there aren't many good talk show hosts out there.... and similarly, there are not too many good sway bars out there. that is why IPD sells their beefy sway bars.. so some of us can get a decent, well moderated ride in our cars with a minimum of "to-and-fro" transients.

    a very stiff moderator will not dive far into the issues (the supporters on either side of the issue are never given much time to talk at all..) it is the same with springs and things... too stiff an "anti-sway" bar, and the cornering ability (ability to see rational issues on both sides) tends to go down... to small a sway bar and you tend to be overwhelmed by emotional gibber (precurser to rolling the car over :).

    lucky us. ipd has done their homework and seems to have a decent product which makes your Volvo "feel" much better.. but as Tim pointed out, it does not necessarily make it "faster"... but I do tend to waist less time in the corners and listening to garbage on the radio on my way home.

    sway bars essentially connect to either suspension arm and the front bar is "grounded" to the frame (as a talk show host must be grounded with some basic skills and has be assertive in order to manipulate the often rowdy guests). the rear swaybars on our bricks are often grounded to the "live" rear axle.. which is a bit different, but conceptually it is similar. like a host who plays off the crowds instead of the encyclopedia...

    herm "with a minor in late-rennesance art" wiegman

    p.s. From Scott (megatest!bldg2fs1!sfisher@uu2.psi.com)

    >How To Make Your Car Handle, Fred Puhn, HPBooks. A good general introduction to what we mean when we talk about understeer, oversteer, camber curves, weight transfer, slip angles and all that stuff.

    I found my copy at a Salvation Army outlet.. highly recommended for the expert and novice. available at Classic Motorbooks.. along with the latest Volvo's buyers guide.. etc.. etc...

    Classic Motorbooks, P.O. Box 1, Osceola, WI 54020
    (800) 826-6600 ; Fax: 1-715-294-4448 car literature & most everthing...

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