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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: email@example.com. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
WHY USE SWAY BARS AT ALL?
In the last few issues, you have been talking about anti-roll bars and how they affect mechanical traction.
My question is why do we use a sway bar at all? What effect will removing front and rear sway bars have on the behavior of the car? Sure the car will roll on the corners, but what about traction on the bends and overall performance?
I would definitely not advise taking the sway bars off an existing car, and then taking it for a lap at speed. On most cars today, the front/rear roll stiffness distribution is tuned with the bars, and the car will not have the same oversteer/understeer balance if the bars are removed.
It is quite true that for vehicles with the sprung mass c.g. heights commonly encountered in cars, roll has only a small effect on overall load transfer. It is also true that we can achieve any roll gradient (how much the car rolls per unit of lateral acceleration) using only the ride springs. And finally, it is true that we can get any front/rear distribution of load transfer using only the ride springs. So why not just do that?
In fact, many cars have been built with no sway bars (or anti-roll bars – I use both terms interchangeably). I'm not sure when the first anti-roll bar appeared, but before World War II they were practically unknown, both on passenger cars and on race cars. But cars without any anti-roll bars basically fall into three categories:
· ones that roll a lot;
· ones that ride really hard and don't absorb bumps well;
· ones with beam axles at both ends.
Some cars are in more than one of these categories at once.
Anti-roll bars and other interconnective springing devices offer the following advantages:
· they let us control wheel rates in the four modes of suspension motion – roll, pitch, heave, and warp – independently, or at least independently within certain constraints;
· they let us achieve better control of roll for a given ride quality;
· they afford us a way to readily adjust front/rear load transfer distribution, even from the driver's seat, with a minimum of effect on other things.
What's wrong with just letting the car roll?
With independent suspension, the wheels inescapably lean with the sprung mass to some degree. We can compensate for this by using geometry that makes camber go toward negative in compression in ride, but we can't get 100% camber recovery in roll without excessive camber change in ride. Generally, we have to accept 50% camber recovery or less: the wheels lean at least half as much as the body. So roll hurts camber, and consequently reduces grip, with any independent suspension. This is not true with beam axles, but there are other disadvantages to roll even with beam axles.
Roll uses up suspension travel and, on the outside, uses up ground clearance. Depending on the amount of travel available, roll can bottom out or top out the suspension, or lead to the bodywork or skirting dragging, or create a situation where undetected bottoming out or topping out occurs on bumps. In cars with ground effects or very low front wings, roll can disrupt the under-car aerodynamics enough to cause problems. These disadvantages occur even with beam axles.
Roll does increase lateral load transfer, directly and indirectly, although the amount of this is quite variable. There is a small lateral migration of the car's c.g. with roll, relative to the tire contact patches. In tall vehicles, this can become significant. In cars, it is relatively small, but still present, and undesirable. In addition to this direct effect, there can be an indirect increase in load transfer if roll forces us to run the car at greater ride height to avoid bottoming.
The whole question of whether using anti-roll bars (or stiffer ones, or various blends of spring versus bar) causes the car to run higher or lower is fairly complex, and will be addressed further below.
So far, we have been discussing effects of roll in steady-state cornering. In abrupt transient maneuvers, greater roll displacements imply greater roll velocities and accelerations. Roll acceleration will add to lateral load transfer when the direction of roll acceleration is into the turn. This occurs as the vehicle approaches maximum roll displacement, and roll is outward, and still increasing, but slowing. In taller vehicles, this added component of load transfer can sometimes be enough to make the difference between the vehicle staying upright and having all wheels on the ground, or overturning or lifting wheels. This phenomenon explains why some SUV's will lift wheels or roll over in a lane change test, even though they will stay right side up and slide controllably in a skid pad test. Reducing roll displacements in such a vehicle will improve overturning resistance in abrupt maneuvers.
We generally will not encounter this in racing, but the same effects are present, in lesser magnitude. We can often observe that softer setups favor smoother drivers, and vice versa.
For road use, and generally for real-road competition, ground clearance requirements are mainly established by the need to clear driveways, snow, and obstacles at low speed. Bottoming of the
suspension or the underside of the car on bumps and dips is usually not the main constraint. We will also want to choose ride stiffnesses and front/rear natural frequency relationships that will make the car ride well and take bumps well. We will need to balance these objectives against a need to keep roll within reasonable limits. For this, we will do well to get between 30% and 60% of the overall elastic roll resistance from the bars, and the rest from the springs.
For a pure racing car, the constraints are different. We will generally be prepared to put up with a fair amount of hassle loading and unloading the car, and having it unable to negotiate driveways, if that's what it takes to win races. We will then be concerned about how low we can run the car without having the underside hit the track (much). We may also need to limit heave and pitch displacements very tightly to control under-car aerodynamics.
Those constraints will dictate that we have the car very stiff in the heave and pitch modes. When we want the car that stiff in heave and pitch, adequate roll stiffness almost comes with the package automatically. Even then, however, we will generally run some anti-roll bar, for the following reasons:
· to provide fine adjustment of roll resistance;
· to provide quick adjustment of roll resistance;
· to provide driver adjustment of roll resistance;
· to allow steeply rising ride rate, or pitch and heave rate, via a third spring, while maintaining linearity or whatever rate curve is desired, for the roll and warp rate.
With a bar and a third spring, we can actually have a setup where the wheel rate in roll exceeds the wheel rate in ride at or near static ride height, while the ride rate is the higher of the two once the suspension is compressed a bit.
There are peculiar circumstances, generally imposed by peculiar racing rules, that may favor setups with really soft springs and stiff bars, or with a stiff wheel rate in ride and little or no wheel rate in roll.
Big bar/soft spring front end setups have been popular for some time in American stock car racing, although this trend is fading somewhat. This approach makes sense, at least up to a point, for stock car front ends, because the rules impose a minimum ground clearance requirement that is considerably higher than where we'd like the front of the car to be on-track, and the turns are banked. Soft springs let the front end compress due to banking and aero forces, while the bar maintains enough roll resistance to keep the car from being loose.
The opposite extreme exists at the rear of a Formula Vee car, where the rules require swing axle suspension, which has an extremely high roll center and a tendency to jack up in cornering. Here, it works best to use springing that only works in ride, and have a wheel rate of zero in roll. The last thing we'd want for this application is an anti-roll bar.
Cars with beam axles at both ends can have very ample geometric anti-roll (high roll centers) at both ends, and can consequently have moderate roll gradients without anti-roll bars. Any beam axle suspension without an anti-roll bar has a smaller wheel rate in roll than in ride, because of the springs inevitably being inboard of the wheels. However, high roll centers carry some penalties of their own, even with beam axles, in the form of large lateral movement of the contact patches with respect to the sprung mass on one-wheel bumps. Beam axle suspensions generally perform much better without anti-roll bars than independent suspensions do, but there is nothing wrong with using an anti-roll bar with a beam axle either, and indeed this is very common on roadgoing beam axles today, and on racing ones as well, where the rules don't prohibit anti-roll bars.