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How an Anti-Sway Bar Works

Put simply, an anti-sway bar is a "U-shaped" metal bar that connects to both wheels on opposite sides of the car at the ends and connects to the chassis in the middle. Essentially, the ends of the bar are connected to the wheels while the center of the bar is connected to the body of the car.

In order for body roll to occur, the suspension on the outside edge of the car must compress while the suspension on the inside edge simultaneously extends. However, since the anti-sway bar is attached to both wheels, such movement is only possible if the metal bar is allowed to twist. (One side of the bar must twist upward while the other twists downward.) So the bar’s "torsional stiffness" – or resistance to twist – determines that bar’s ability to reduce body roll. Less twisting of the bar results in less movement into jounce and rebound by the opposite ends of the suspension – which results in less body roll.

Factors that Determine Sway Bar Stiffness

There are two primary factors that determine an anti-sway bar’s torsional stiffness: the diameter of the bar and the length of the bar’s "moment arm" (more commonly known as the amount of leverage that the vehicle is able to apply against the twisting motion of the bar.)

Diameter is generally the easiest concept to grasp, as it is somewhat intuitive that a larger diameter bar would have greater torsional rigidity. Torsional (or twisting) motion of the bar is actually governed by the equation:

twist = (2 x torque x length)/(p x diam4 x material modulus)

And since "diameter" is in the denominator, as diameter gets larger, the amount of twist gets smaller. Which, in a nutshell, means that torsional rigidity is a function of the diameter to the fourth power! This is why a very small increase in diameter makes a large increase in torsional rigidity.

To compare, for example, the rigidity of a Saturn’s stock rear 15.0mm bar to a larger 16.5mm bar, simply use the equation, 16.54/15.04 which yields 1.46. In other words, a 16.5mm bar is 1.46 times as stiff – or 46% stiffer – than a 15.0mm bar of the same design.

Add just one more millimeter to the diameter of the bar – for a total of 17.5mm – and the torsional strength skyrockets to 85% stiffer than the stock 15.0mm bar. (17.54/15.04=1.85.)

However, in addition to the diameter of a bar, there is another very important factor that determines an anti-sway bar’s torsional rigidity. This factor is known as the "length of the moment arm" – or in common terms, the amount of leverage between the vehicle and the bar.

As with anything, an increased amount of leverage makes it easier to do work. This is governed by the "lever law," force x distance = torque. As "distance" - or the length of the lever - increases, the resulting amount of torque also increases. (This is why it was easier to move your big brother on the teeter-totter when he moved into the middle and you stayed out on the end. You enjoyed increased leverage at the end, while he suffered from reduced leverage in the middle.)

Because an anti-sway bar is shaped as a "U," the ends of the bar that lead from the center of the bar to the end-link attachment serve as a lever. As the distance from the straight part of the bar to the attachment at the end link becomes longer, the torque applied against the bar increases – making it easier for a given amount of energy to twist the anti-sway bar. As this distance is reduced, torque is reduced - making it more difficult for a given amount of energy to twist the anti-sway bar.

It is the lever law that is applied during the design of an adjustable anti-sway bar. By using multiple end link locations, the distance from the point of attachment to the straight part of the bar can be altered. Or in engineers’ terms, the length of "the moment arm" can be increased or reduced in order to make more or less torque against the bar. Using a setting further from the center of the bar increases the length of the moment arm, resulting in more torque against the bar, allowing more twisting motion of the bar, creating more body roll. Using a setting closer to the center of the bar reduces the length of the moment arm, resulting in less torque against the bar, allowing less twisting motion of the bar, creating less body roll.

The actual impact upon torque can be compared by dividing the center-to-center distances of the end-link attachment points. For example, the center-to-center distance of the stock rear anti-sway bar is 200mm. We can compare this to the 160mm distance of the firmest setting of the 4-way adjustable 17.5mm bar by simply dividing the distances, 160/200 = .8. In other words, a 160mm center-to-center bar produces only 80% of the torque that would be produced by a 200mm center-to-center bar of the same diameter. Or simpler yet, by using the 160mm end-link attachment points, we increase the stiffness of the anti-sway bar by an extra 20%.

(Note: by now you may have realized that the 17.5mm, 4-way adjustable bar found on the scR ITA racecar in the example above is as much as 105% stiffer than stock. 85% of this is a result of diameter, while the remaining 20% is a result of moment arm length. The dramatic increase in stiffness is why it is recommended for race-use only!)

So, in summary, less twist = less deflection = less body roll.

What the Heck is "TLLTD?"

TLLTD stands for Tire Lateral Load Transfer Distribution. While this term may sound complex, it simply measures the front-to-rear balance of how lateral load is transferred in a cornering maneuver and is commonly used to compare the rate of lateral traction loss between the front and rear tires. You probably understand this already as the concept of "understeer" and "oversteer."

Put simply, there is only so much force that a tire can handle. When we ask more of the tire than the tire can deliver, it saturates and loses traction. If the front tires saturate before the rear tires, then we call this "understeer" or "push" – which means that the car tends to continue moving in the original direction, even though the wheels are turned. If the rear tires saturate before the front tires, then we call this "oversteer" or "loose" – which means that the rear of the car tends to swing around faster than the front, causing a spin. When neither of these conditions prevail consistently, then we describe the chassis as "balanced."

We can measure and compare the steady-state understeer and oversteer characteristics of a vehicle by assigning a lateral load transfer percentage of the front relative to the rear. A TLLTD value equal to 50% indicates that the chassis is balanced – or both the front and rear tend to lose traction at roughly the same time. A front TLLTD value greater than 50% indicates that the front tires lose traction more quickly than the rear tires – resulting in understeer. And a front TLLTD value lower than 50% indicates that the rear tires tend to lose traction more quickly than the front - resulting in oversteer.

(It is important to note that our discussion of TLLTD is only considering steady-state cornering maneuvers, such as a long 270 deg. on-ramp or off-ramp. Moderate-to-aggressive throttle or brake application can upset this "balance" during a transient condition, briefly transitioning a vehicle from understeer to oversteer.)

The Effect of Anti-Sway Bars Upon TLLTD

You now understand how an anti-sway bar can be used to limit body roll. And you understand that reduced body roll can lead to a reduction in adverse camber changes for better tire traction. But what may not be obvious is the effect of anti-sway bar changes upon TLLTD (understeer and oversteer.)

In fact, given the above information, one might even assume that a firmer anti-sway bar, which leads to better camber control, would lead to better traction. So if we add a firmer anti-sway bar to the front, traction loss diminishes, so understeer is reduced, right?

Not quite. Let’s evaluate more closely the meaning of TLLTD – tire lateral load transfer distribution. If said another way, we might describe TLLTD as the relative demand of side-to-side energy control that is placed upon the tires. Because a firmer anti-sway bar allows less deflection, it will transfer side-to-side energy (lateral loads) at a faster rate. As the rate of lateral load transfer increases, additional demands are placed upon the tire. So if we install a firmer anti-sway bar in the front, then we increase the distribution of lateral load transfer toward the front tires. This increases the front TLLTD value, which will result in additional understeer, holding all else constant.

The same logic also holds true in the rear. A firmer anti-sway bar in the rear will increase the rate of lateral load transfer, placing more demand upon the rear tires, accelerating lateral traction loss, creating more oversteer, holding all else constant.

"I want a 50% TLLTD on my car, right?"

For many enthusiasts, it is tempting to jump to the conclusion, since a 50% TLLTD indicates a "balanced" chassis on paper, that this is therefore desirable. And all cars "should obviously come this way from the factory." Unfortunately, this is not the case – and the considerations are not that simple.

In reality, a car with a 50% TLLTD is literally on the constant brink of oversteer. And there are many factors that can quickly and easily take the car from the brink into a full-scale, out-of-control, spinning-in-circles disaster!

For starters, consider the effects of weather conditions that might create a wet or icy road surface…or imagine that the driver happens to apply too much brake late into a turn …or consider the effects of varying tire temperatures, tire pressures, or tire wear – all of which will have major impacts upon lateral traction thresholds. And of course, varying weight distribution, as a result of changing fuel tank levels, passengers, or the number of subwoofers in the trunk, will also impact TLLTD.

With all of these things to consider, the engineers are forced to create a more conservative TLLTD. As a result, they intentionally target higher front TLLTD values so that stock vehicles will be prone to understeer - the assumption being that understeer is safer and more predictable for the average driver. For example, the stock Saturn SC2 is tuned to produce a front TLLTD of approximately 63.4% - a relatively conservative target. (But give Saturn some credit – this is on the aggressive end of the conservative spectrum, especially relative to other front-wheel-drive economy cars!)

As a general rule, an average street-driving enthusiast is probably willing to accept some compromises – within reason – of a more aggressive TLLTD in exchange for better handling. A suitable target is probably a front TLLTD value of approximately 58% - a value that is considered "aggressive," but suitable for street driving.