Anti-Sway Bar Bushing Q&A
by James Walker, Jr. of scR motorsports (2000)
Q: I have heard that the purpose of stiffer anti-sway bar bushings (Delrin bushings, for example) is to offer less flex, limiting the movement of the anti-sway bar. Is this correct?
A: Not quite. Anti-sway bars, by design, are supposed to counteract body roll during cornering. they do this by mechanically tying the body of the car (in this case the rear suspension crossmember) to the vehicle unsprung mass (suspension). The movement in the suspension during cornering creates a “twist” in the bar. The bar reacts to this “twist” by “pulling down” on the body of the car, resulting in less body roll. The bushings only acts as a link between the bar and the body. The bar, by design, is supposed to twist and rotate in its mounts. In fact, during an impact like a speedbump where both wheels move up and down together (as opposed to during cornering where one wheel moves up as the other moves down), the bar NEEDS to rotate in its bushings or else the suspension movement will be compromised by the binding caused by the interference in the bushings.
Look at a real race car sway bar and you will find that the anti-sway bar is not even mounted in a bushing…they use spherical rod ends as ‘bushings’ to hold the bar in place in order to REDUCE friction between the bar and the ‘bushing’. The only forces which should be impacting wheel motion should be the force generated by the suspension spring as it is compressed, the resistance generated by the damper internals, and the TORSIONAL (twist) resistance of the anti-sway bar – not the friction between the bar and the bushing.
As a general rule, a less compliant (stiffer) material = less bushing deflection = more efficient transfer of forces = sometimes better handling. The bar will still twist the same as it did before and will still “pull down” as much, but if the bushing material deflects under this load it will not transfer the “pulling down” force as quickly or efficiently as a “stiffer” bushing will to the body structure.
In summary, the anti-sway bar determines the total amount of twist. The bushing determines how fast the force is transferred to the body.
Q: So, should I lubricate my anti-sway bar bushings?
A: In a perfect world, lubricating the anti-sway bar bushings would reduce the parasitic friction between the bar and the bushings, but there is another factor here. By design, the Saturn rear anti-sway bar bushings DO in fact ‘hold’ the bar in place cross-car (side to side) to keep the bar from running into other suspension components. Although this contradicts the statements above about wanting ‘zero’ friction in this interface, Saturn chose the design to eliminate the need for an additional motion restrictor.
By lubricating the anti-sway bar bushings (the rear bushings, anyhow), you may decrease the friction to the point where the bar could ‘react’ to the twist force by moving side-to-side instead of creating the ‘pull down’ force on the body. The result could be the bar whacking other hard parts and ultimately your road wheel.
Q: What if I want to change the bushing material of the front anti-sway bar only?
A: While the rear Saturn anti-sway bar is ONLY an anti- sway bar, the front bar is also an integral part of the suspension geometry. In other words, by removing the rear sway bar the wheel will not flop around uncontrolled,
but if you were to remove the front bar the car would be undriveable because the front lateral link would be unrestrained.
So, what does this mean? Well, the bushings used to mount the front bar not only impact the force transfer rate to the body as described above, they also impact front wheel caster, camber, and toe changes during dynamic maneuvers. In English, if you change the bushings you also impact ‘where the wheels are pointed’.
This can have pretty noticeable effects! The ‘factory’ bushings are designed to provide certain camber, caster, and toe characteristics during dynamic maneuvers…and by changing all – or even worse – some of the bushings, you can impact these characteristics. Think of it like putting really sticky tires on the front of your car and leaving the stock tires on the back. Not the best ‘system’, right? You can run into the same trouble by replacing ‘some’ of the bushings – and even if you were to replace all the FRONT bushings, you still have the ‘stock’ bushings in the rear. This can lead to undesired steering effects, and there is really no way to know in advance just how significant these effects will be.
Q: What have you chosen to do with the bushings on your ITA racecar?
A: In ITA we are allowed to replace any or all of our suspension bushings, and yet we have chosen to leave them all stock. Why? Because most of the bushings on the market are for the ‘front’ of the car…and on a vehicle which already understeers as a rule, this would simply make the matter more pronounced. Remember, rear stiffness = a more balanced package while front stiffness = more understeer.
Of course, we inspect our bushings frequently to make sure that they are in proper operating condition, but when we do replace our bushings, we use factory replacement parts.
Q: So, why would anyone WANT to replace their anti-sway bar bushings?
A: Anti-sway bar bushings certainly have their place, but without careful selection of components and bushing rates you can do much more harm than good – especially with the Saturn front bar interactions. Unfortunately, most people only see the low-speed, non-limit handling effects of the changes (the steering might feel quicker for example) and swear by the bushings. However, if you spend any time driving at the limit, trial and error will ultimately tell you if you made the right choice.