Tech guide: anti-roll bars. We take a look at what keeps your corner lean in check. Springs and dampers are all well and good but without an anti-roll bar, you’ll be all at sea. We explore how they work. Words: Gerry Speechley. Photos: BMW, Eibach, SuperPro, Schaeffler.
The links themselves on most production cars are of a fixed length and are known as drop links, roll bar links etc. However for a degree of tuning, they can be adjustable in length and may be installed either singularly, or in pairs.
So why would you require an adjustable drop link? With a vehicle perfectly loaded and standard set up, there will be no load whatsoever on the antiroll bar. However, as soon as the driver gets in the car, as they sit to one side (assuming it’s not a McLaren F1), the suspension compresses from their weight on that side and applies a preload to the roll bar. The car will then have a preloaded anti-roll bar even when driving in a steady straight line. To correct this, one of the drop links is removed from the car and the driver, or a weight equivalent to them, is installed in the driver’s seat, and then the new adjustable drop link is adjusted to fit to the roll bar without any preload. Clearly, when the driver gets out of the car, or any other loading is applied to the car, the roll bar will be preloaded, but for optimum setup for the vehicle with a specific driver, this is fine tuning for a totally neutral roll response.
WHAT IS AN ANTI-ROLL BAR?
Following on from our recent article on vehicle damping control, we will now look at the systems used to control vehicle body roll. For the vast majority of us, this will involve front and rear anti-roll bars, known as sway bars or stabiliser bars in many parts of the world.
The reason for the installation of a roll control system on a vehicle is to control/reduce the amount of body ‘lean’ during vehicle cornering by essentially adding a torsion spring between the left- and right-hand suspensions, both on the front and rear axles. Whilst the amount the vehicle suspension compresses on the outside of the vehicle during cornering is somewhat controlled by the spring stiffness, and to a slight extent, the damper compression settings, if either of these are too great, the suspension will be uncomfortably stiff during normal driving, and so we need a system to control this vehicle ‘lean’ during cornering.
The anti-roll bar connects the two front suspension units together using an arrangement of a bar mounted across the vehicle (usually anchored to the chassis), a pair of arms (in most cases part of the main bar but can be separately fitted to the bar in more advanced systems), and a pair of links connecting the ends of these arms to the lower suspension arms or lower section of the shock absorbers. Whilst there are countless shapes of anti-roll bar made to fit around the suspension, steering, engine and exhaust components, the bar and arms can be simplified for this article as a squared-off ‘C’.
The stiffness of the bar is controlled mainly by the diameter and length of the bar across the vehicle, and the length of the arms. The bar is usually a solid, heat-treated steel bar (similar to a suspension spring), although some manufacturers increase the diameter slightly and use a hollow tube, reducing the weight whilst retaining the stiffness of the solid bar. As the bar diameter increases, and overall length decreases, so does the stiffness. In motorsport applications, there can be a whole range of bars available for specific track conditions, such as three-piece bars, which use a single main bar with the side arms mounted separately and often with different side arm lengths. As the side arms increase in length, the roll stiffness reduces because the suspension has a greater leverage ratio against the main bar. However, in a single piece, bent roll bar, this is can be achieved by offering a choice of mounting holes to enable a degree of adjustability and the further out the link is attached to the arm, the softer the setting.
HOW DOES IT WORK?
Well, during cornering, the weight transfers to the wheels on the outside arc of the cornering vehicle, the body starts to roll, compressing the suspension on the outside of the turn and unloading the inner suspension. The anti-roll bar will resist this roll by acting on the vehicle’s body, through the bar mounts on the body, and on the opposite suspension leg according to the stiffness of the bar, pulling down on the body to limit the roll.
The anti-roll bars will not offer any resistance to suspension motion, or alter stiffness or harshness to the vehicle’s suspension when both wheels on the same axle are responding to the same bumps or undulations, or pitching of the vehicle because the main bar is free to rotate in its bushes/mountings on the chassis. It only affects the ride when a load on one side only acts on the bar, adding the bar stiffness/spring rate to the overall suspension spring rate. This then increases the force on the outer tyre, increasing grip whilst decreasing the load on the inner tyre. If this increase is too high, then the tyre can lose traction due to overloading its grip capability. There will also not be any vehicle ride height change. Utilising varying anti-roll bar stiffnesses, you can tune the car’s overall handling, adjusting the tendency to oversteer or understeer. The effect of the bars is to allow the vehicle to corner flatter, improving the car’s weight distribution, and therefore cornering speeds.
THICKER ISN’T ALWAYS BETTER
It would appear at first glance that the thicker and stiffer the roll bar system the better. This is not the case, though, because whilst the diameter of the bar will give an indication of the overall stiffness it will deliver, there are other factors such as bush material and arm lengths to consider, as well as the fact that the system can be so rigid, the roll bar will lift the inner wheel completely off the road in heavy cornering leaving the total vehicle traction limit to just three tyres.
A stiffer bush, such as a urethane bush over the standard rubber bush, will transmit more force through the bar to the vehicle, effectively increasing the efficiency of the system whereas race cars may even use a Delrin bush, that due to the inherent lubricity of the material, gives virtually a metal-to-metal stiffness in the bush and maximum efficiency. It would, however, transmit a great deal of noise through the vehicle as it does not have the NVH (Noise, Vibration and Harshness) favourable characteristics of rubber or urethane. You will always require a greater roll stiffness on the front of the vehicle to have a front/rear roll ratio that will encourage understeer or neutral steer because excessive rear stiffness will promote oversteer which can go very badly, very quickly. The ideal neutral steering is the aim but this would require a variable roll bar system to work in all situations, so aiming for neutral to understeer is the far safer option.
This shows the direct effect of altering the roll stiffness to the front and rear of a vehicle. Increasing the roll stiffness at the front will increase the vehicle’s tendency to understeer and increasing rear roll stiffness will induce oversteer.
ACTIVE BODY CONTROL
The ideal solution, then, is a system that will alter the stiffness of the roll bar depending on the driving conditions, known as an active roll control system. This system will allow the vehicle to alter the understeer/oversteer characteristics of the vehicle, controlled by the steering/suspension ECU with inputs from steering input angle from the driver measured at the steering column steering angle sensor, vehicle lateral acceleration, vehicle speed and road conditions. It has another advantage over the passive roll bar system in that with a traditional anti-roll bar system, any motion on one side of the vehicle will transfer through the roll bar to the other side. In an active roll control system, the two sides of the suspension are isolated from each other by uncoupling the connection along the two roll bar sides. This system will therefore give a greater comfort level at low speeds.
Initially introduced on the E65 7 Series and known as ‘Dynamic Drive’, the system has naturally been improved and refined over the last 15 years and uses a hydraulic pump and valve block complete with separate left turn and right turn valving, controlled by an ECU to send high pressure oil to a pair of rotational hydraulic actuators that connect the two sides of the roll bars to a variable degree, similar to a clutch, to alter the effective stiffness the vehicle ‘sees’ from the bars and maintain the ideal neutral steering response.
In operation, the ECU determines the ideal roll rates for the front and rear roll bars in any given driving situation and sets them using control of the valve block to alter the effective pressure in the torsional actuators between the two halves of the roll bars. This principle is also used in the Mercedes Active Curve System to decouple the two halves of the roll bar.
The new G11 7 Series has an updated electro-mechanical version of Dynamic Drive where the chassis is adjusted to any driving situation in as little as 20 milliseconds absorbing road irregularities. This new system reduces emissions and improves fuel economy because being electrically based, it only draws power on an as-needed basis. A control motor with three-stage planetary gears loads the separate halves of the roll control ‘bars’ and a torque sensor regulates the actuator quickly and precisely to optimise roll control.
Toyota and Lexus have a similar Active Power Stabiliser Suspension System (APSSS) for controlling the roll by installing electrical motors and reduction gears to the separate sides of the vehicle roll bars and using similar inputs from steering angle and lateral acceleration to actively preload the roll bars to a level for optimum body roll control. Another active system used by Porsche called PDCC (Porsche Dynamic Chassis Control) uses hydraulic roll bar drop links that are ECU controlled to alter the mechanical load applied to the roll bars front and rear according to the driving conditions.