3 days ago in New Car Reviews
We recently reviewed the sports version of Mazda's third-generation roadster, the MX-5 Club edition but we thought the more tame ...
Written by David S. Wallens
Any enthusiast worth his salt knows that tires have arguably the biggest impact on a vehicle's handling. Obviously, however, there are 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-roll bars.
Properly chosen (and installed), anti-roll bars will reduce body roll, which in turns leads to better handling, increased driver confidence and, ultimately, lower lap times.
Chances are, you've experienced the effects of body roll every time you're behind the wheel. 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're allowing a couple of thousand pounds of vehicle, fluids and passengers to build momentum in that straight line.
When you tell everything to change direction suddenly, through input at the steering wheel, the front tires may change direction thanks to the mechanical advantages of the steering system, but 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 lose 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.
We often hear that preventing body roll is "so important" that we must all 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 on only the tire edge during body roll.
The resulting loss of traction can allow the tires to more easily give way 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.
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 suspensions 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 or shock absorbers) 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-roll bars--also known as anti-sway bars, roll bars, stabilizer bars or sway bars--provides a more cost-effective reduction in body roll with minimal negative impacts upon ride quality.
Put simply, an anti-roll bar is a U-shaped metal bar that links both wheels on the same axle to the chassis. Essentially, the ends of the bar are connected to the suspension 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-roll 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 its 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.
There are two primary factors that determine an anti-roll bar's torsional stiffness: the diameter of the bar and the length of the bar's moment arm. 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 the 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.
For example, to compare the rigidity of a stock 15mm bar to an aftermarket, 16.5mm one, simply use the equation 16.54/154. Some quick math yields the figure of 1.46. In other words, a 16.5mm bar is 1.46 times as stiff-or 46 percent stiffer-than a 15mm 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 percent stiffer than the stock 15mm 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-roll 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 towards the middle and you stayed out on the end. You enjoyed increased leverage at the end, while he suffered from reduced leverage near the middle.)
Because an anti-roll 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-roll bar. As this distance is reduced, torque is reduced-making it more difficult for a given amount of energy to twist the anti-roll bar.
It is this lever law that is applied during the design of an adjustable anti-roll 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 farther 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, say the center-to-center distance of the stock rear anti-roll bar is 200mm. We can compare this to the 160mm distance of the firmest setting of a four-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-percent 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-roll bar by an extra 20 percent.
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. It is commonly used to compare the rate of lateral traction loss between the front and rear tires.
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," or 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 percent indicates that the chassis is balanced-or both the front and rear tires tend to lose traction at roughly the same time. A front TLLTD value greater than 50 percent indicates that the front tires lose traction more quickly than the rear tires-resulting in understeer. And a front TLLTD value lower than 50 percent 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 only considers steady-state cornering maneuvers, such as a long 270-degree 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.
Ideally, you now understand how an anti-roll 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-roll bar changes upon TLLTD (understeer and oversteer.)
In fact, given the above information, one might even assume that a firmer anti-roll bar, which leads to better camber control, would lead to better traction. If we add a firmer anti-roll bar to the front, traction loss diminishes, so understeer is reduced, right?
Wrong. Let's evaluate more closely the meaning of TLLTD-tire lateral load transfer distribution. Stated 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-roll 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-roll 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-roll bar in the rear will increase the rate of lateral load transfer, placing more demand upon the rear tires, accelerating lateral traction loss and creating more oversteer, holding all else constant.
This is why blindly adding parts to your car may not produce the desired results. A wise consumer consults with-and buys from-knowledgeable experts that have the tools to make informed tuning recommendations.
Since on paper a 50-percent TLLTD indicates a balanced chassis, many enthusiasts are tempted to jump to the conclusion that this is therefore desirable. They may think that 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-percent 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-a common mistake among novice drivers. 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, automotive design 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, a stock DOHC Saturn is tuned to produce a front TLLTD of approximately 63.4 percent-a relatively conservative target. (But give Saturn some credit, as this is on the aggressive end of the conservative spectrum, especially compared 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 percent, a value that is considered aggressive, but suitable for street driving.
Since most enthusiasts do not have the knowledge or software needed to calculate chassis characteristics such as TLLTD, the responsibility falls upon knowledgeable tuners.
Obviously, TLLTD and body roll will both be affected by changes to springs and anti-roll bars. While understanding the effects of multiple changes can get confusing, the answer is usually only a phone call away.
By John Comesky. Comesky is the owner of SPS, the largest Saturn tuning company in the U.S. SPS covers all aspects of Saturn performance: engine, suspension, brakes and wheels and tires. They can be reached by phone at (937) 296-1417, and their Web page may be viewed at http://www.spswebpage.com.
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