by John Comeskey of SPS and James Walker of scR motorsports
Hopefully, after reading the scR Tech Article on tires, the value of tire traction toward constructing a fun-to-drive performance-oriented Saturn has become clear. But there are areas of chassis dynamics that extend beyond the realm of tires. Once you increase the traction threshold at the road surface, then you may be ready to take the next step into improved vehicle handling – reducing body roll through the use of anti-sway bars.
This article will explore the concept of body roll, explain how an anti-sway bar works to reduce body roll, define the factors that determine the effectiveness of an anti-sway bar, and introduce the concept of tire lateral load transfer distribution and how this variable is affected by changes to anti-sway bars.
What is Body Roll?
The chances are high that you have experienced the effects of body roll during the course of your normal driving. It happens during almost every turn when one side of the car lifts, causing the entire vehicle to "lean" toward the outside of the turn.
The cause of body roll is simple physics: an object in motion tends to stay in motion until acted upon by an outside force. So in practical terms, as you drive ahead in a straight line, you allow 2300-2600 pounds of vehicle, fluids, and passengers to build momentum in a straight line. Suddenly, through input at the steering wheel, you tell everything to change direction. But even though the front tires may change direction, thanks to the mechanical advantages of the steering system, the momentum of the vehicle, fluids, and passengers continues in the original direction. The tires are the only element capable of generating an outside force that can act against this momentum and change its direction.
At this point, one of two scenarios is most likely to occur. If enough momentum exists in the original direction, and the tires lack enough grip to act against the original forward energy, then the vehicle will slide out of the turn as the tires loose traction. However, if the tires have enough grip at the road surface, then instead of sliding, the vehicle’s traction at the road surface will overwhelm the original forward momentum and act upon the original forces to induce a change of direction. Hence, a cornering maneuver.
But what happens to that energy? Even though we may have had enough grip to hang-on through the turn, we know that the momentum of the vehicle mass will continue in the original direction. The result is a weight transfer toward the new outside edge of the vehicle – the same direction as the original forward momentum. If enough energy is behind the weight transfer, then this energy will cause the outside suspension (in this case, the spring and strut assembly) to compress while the other side lifts and extends. An engineer-type likes to describe this by saying that one side moves into "jounce" while the other moves into "rebound." The rest of us call it "lean" or "body roll."
Why is Body Roll a Bad Thing?
We often hear that preventing body roll is "so important" that we all must rush out and buy "this" product or "that" product in order to prevent it. And many enthusiasts have consequently accepted that body roll is therefore "bad." But what exactly does body roll do to negatively affect vehicle handling?
For starters, it disrupts the driver. This is probably the effect that most drivers can see and feel during their own driving experiences. And while this is not the most important negative effect of body roll, it is true that the car does not drive itself – no matter how many aftermarket parts you install. So keeping the driver settled, focused, and able to concentrate on the task of driving is a foremost priority for spirited vehicle handling.
However, the most often misunderstood effect of body roll upon vehicle handling is the effect of body roll upon camber - and the effect of camber changes upon tire traction. Put simply, the larger the contact patch of the tire, the more traction exists against the road surface, holding all else constant. But when the vehicle begins to "lean" or "roll" to one side, the tires are also forced to "lean" or "roll" to one side. This can be described as a "camber change" in which the outside tire experiences increased positive camber (rolls to the outside edge of the tire) and the inside tire experiences increased negative camber (rolls to the inside edge of the tire.) So a tire that originally enjoyed a complete and flat contact patch prior to body roll must operate only on the tire edge during body roll.
The resulting loss of traction can allow the tires to more easily give away to the forces of weight transfer to the outside edge of the vehicle. When this happens, the vehicle slides sideways – which is generally a "bad" thing.
How to Prevent Body Roll
By definition, body roll only occurs when one side of the suspension is compressed (moves into "jounce") while the other extends (moves into "rebound.") Therefore, we can limit body roll by making it harder for the driver side and passenger side suspension to move in opposite directions.
One fairly obvious method to achieve this is through the use of stiffer springs. After all, a stiffer spring will compress less than a softer spring when subjected to an equal amount of force. And less compression of the suspension on the outside edge will result in less body roll. However, stiffer springs require the use of stronger dampers (struts) and have an immediate and substantial effect on ride quality. So even though handling is improved, they may not be the easiest or most cost-effective way to achieve the objective of reducing body roll.
For many enthusiasts, the use of anti-sway bars – also known as "anti-roll" bars, "roll" bars or "sway" bars – provides a more cost-effective reduction in body roll with minimal negative impacts upon ride quality.
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