Features - Technical
MAY 21, 1997
Formula 1 Suspension Trends
BY PETER WRIGHT
The suspension determines the attitude at which the tyre is presented to the track, and the vertical load on it, both quasi-steady-state and oscillating due to track undulations. It also feeds back to the chassis the loads that the tyres generate in response. In turn the chassis responds, taking up an attitude that, in turn, determines the aerodynamic loads on it and alters the attitude and loads on the tyres. Both messages and responses, from chassis to tyres, go by way of the suspension and are shaped by it.
The tyre war between Goodyear and Bridgestone is fast changing both the construction of the tyres and the forces that they are able to generate through the development of tread compounds. Tyre characteristics and their interaction with the dynamics and aerodynamics of a car are not a precise enough science, even in Formula 1, to look at the characteristics of a new tyre and dial straight in the appropriate changes to the suspension etc. that are needed to re-optimise the whole system. Instead the engineers must dig deep into their boxes of tools and go round and round the loop of changes until they find out just what the new tyre wants, and tune out the negative side effects of those primary changes. If they are lucky, the changes will be within the adjustment range of the suspension geometry and spring and damper settings. If not it may require a whole new geometry (new pickup points in the monocoque or on the gearbox, or new uprights) or, worse still, a different weight distribution or aerodynamics.
In V7N3 I described how suspension engineers were equipping themselves with the option to run a third spring and/or damper on either axle, in order to separate out roll characteristics from those controlling vertical motion. Behind this trend there is a more fundamental suspension/tyre issue that really came to a head when Jacques Villeneuve joined Williams last year. Since running active suspension, Williams had been pursuing a policy of developing suspension and aerodynamics to allow the suspension to be as soft as possible for mechanical grip. Active suspension had allowed the compromise between suspension stiffness and controlling the ride height to be resolved in the favour of both features. Since it's ban in 1993, they have worked hard to ensure that the aerodynamic demands did not limit how soft the suspension was set up. In early 1994, when Senna first drove for them, the FW16 initially possessed less than ideal aerodynamics, preventing Senna from dominating in the way that people had expected. The problem was identified and resolved and, guided by the smooth driving style of Damon Hill, they have set the standards for both high and low speed grip since that time.
For Villeneuve ultimate grip is less important than the ability to place the car exactly where he wants it, when he wants to. For this he needs a highly reactive car, and that means one that has stiff suspension. It is a question of achieving a balance between the greatest potential performance and realising as much of that potential as possible. Conventional theory indicates that ultimate grip comes from varying the load on the tyre as little as possible i.e. a softly sprung and optimally damper suspension. If the oscillating load has an amplitude equal to the steady vertical load on the tyre, the load will be zero, once per cycle. At that point the tyre cannot grip, and will slide. The analysis becomes very complex at this stage, but in essence the conditions at the contact patch are "stick-slip", rather than steady "sticking". Oscillating loads of the same magnitude as the steady load are common on a stiffly sprung car at low speeds, and this condition usually appears to reduce the grip.
However, there are characteristics of the tyre that, under certain conditions, prefer the "stick-slip" situation. For a start, it heats up the tyre faster and, when optimum tyre temperature is not attainable, may raise the temperature and so achieve higher grip. For Qualifying, when overheating the tyres is not a terminal problem, many drivers stiffen the suspension compared to race settings. Forcing a race compound to behave like a qualifying one, by treating it harshly to achieve optimum temperature, and a more responsive car are both desirable for a single quick lap. For the race however, drivers tend to adjust to a smoother driving style, and a set-up that looks after the tyres on a car that is heavy with fuel, is of paramount importance.
There are two aspects of the tyre war that emphasise these differences. Firstly, the compounds being developed are more marginal, and so need careful treatment during the race. Secondly, with two types of rubber from different manufacturers, scattered in small lumps all over the track, there is a real problem with tyres picking up this incompatible debris and ceasing to perform as advertised. A stiff car, which is more aggressive towards it's tyres, seems to have less problem in scrubbing off any pickup. A typical case of real life interfering with theory, and one that keeps the race engineers busy during the free practice periods, changing springs and damper settings to suit track, tyres and driver.
Most of the top teams develop their own dampers, working with damper manufacturers to make the more specialised cylinder and valve components. Because there is still considerable mystique surrounding damper technology, details of what goes on inside them is well guarded. Apart from the introduction of the third damper, described in V7N3, there are two main areas of damper R&D. The first is to develop a development tool or procedure to assist in the quick setting up of dampers, and their associated springs, for a given circuit. The rate at which circuits are being altered and re-surfaced these days means that databases, gleaned in the preceding years, are not much help. After the 50% wind tunnel, the next "must have" is a 4-post, electro-hydraulic, road simulation rig. With the car placed on it, and aerodynamic loads imposed via two further servo-actuators, it is possible to excite the suspension with an actual track profile, at any simulated speed. The ability to measure any parameter and study particular conditions in the laboratory is a valuable tool in understanding suspension dynamics and validating models. In particular, it can be used to study the second issue:
Without active control, it is not possible to change the damping to suit varying conditions. With rising rate suspensions, the stiffness increases as the suspension is compressed by the aerodynamic downforce and ideally the damping would increase too. For a long time, a way of achieving this has been sought and a clever mechanism is bound to appear soon, if it has not already done so.
Two regulations new this year, have had an influence on suspension linkage design. In 1996, Tyrrell decided to test the FIA's interpretation of "aerodynamic effect", in regard to components that moved relative to the entirely sprung part of the car. They made the entire top front wishbone into a streamlined shape, forcing a new regulation to define suspension linkages as having no greater aspect ratio (defined, in this case, as chord to thickness ratio of a transverse section) than 3.5:1. There is a sufficient aerodynamic advantage in streamlining the linkages, especially the top one, that there became an even greater reason to manufacture them from individually shaped CFRP mouldings, rather than standard section streamline, steel tubing.
There are a number of constructors however, who prefer to retain steel for most if not all suspension members, adding a CFRP sleeve to streamline them. This may have something to do with the second regulation, which states that the forward, bottom wishbone strut must be the strongest in tension, and be able to articulate, without necking out the inner joint, at least 60¡ or until the wheel hits something solid on the car (i.e. the side impact structure for the front wheel). This regulation is a reinforcement of the need to try and prevent the front wheel striking the driver's head, in the event of a front/three-quarters impact into a barrier. CFRP linkages tend to shatter in an impact, and are unlikely to retain the wheel. Steel with sufficient ductility stands a chance, and provided that the front lower link is intact, will stop the wheel swinging up into the cockpit. It has not been possible to define a test that will ensure success under all possible types of impact, and so a sensible requirement has been stipulated. Only time will show whether it is the right one. Once again one is reminded that safety is a statistical science, based on experiment.