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13/12/2012
GIVE ME A LEVER AND ...

FROM THE ENGINE TO THE WHEELS

Tyres must be properly “guided” to make them move as precisely as possible

Nicodemo Angì

TYRES ARE ALSO part of a car’s image: low profile tyres give an idea of bold sportiness, whereas coloured tyres give off smoke when drifting and are decidedly spectacular. The list could continue but, at the moment, what interests us is the tyre’s primary function: to ensure optimal contact with the road in terms of comfort, handling and safety.

Tyres are part of a well-integrated team, with the star players being the rims and the suspension. 

The suspension is anything but taken for granted in the sense that it meets all needs, but raises not inconsiderable questions with regard to mass production because it can be fairly complex and expensive.

In this article we will take a close look at rear suspension which currently has a wider variety of layouts.

We all know that in order to give their best, tyres must always have a certain amount of longitudinal and vertical “deviation” from the vehicle axis and be perpendicular to the ground. The inclination with respect to the direction of movement is the convergence (or toe), whereas the deviation from vertical is the camber. These static angles must (should) be maintained also when the wheels are turning, but this is not always the case.


The angles are best if they don’t change

Let’s suppose that the characteristic angles remain the same when the wheels are jolted while driving on a straight road – for example when going over a pothole: this is an excellent result, but what happens when cornering?

The parts of the suspension are fixed to the body and when it tilts to take a corner, the parts follow the movement, altering the angles and the law of wheel motion.

If the suspension is compressed when the body is horizontal, wheel movement is generally not the same as when the suspension is tilted by the cornering vehicle.

The angle that suffers most from tilting is the camber, with consequences that cannot be ignored. This angle is usually negative (the wheel’s rotational planes meet at a point above the car) but if the suspension cannot compensate for the tilting of the body, the external wheel angle becomes positive with serious consequences for grip. 

Equally negative in terms of handling and safety is the decrease in rear wheel convergence. The rear tyres are not in fact parallel  but – as the name indicates  – their rotational planes converge at a point towards the front of the vehicle.

If the wheel further from the corner (the one providing the most support for the rear axle) should straighten up – in other words, if its rotational plane aligns with the longitudinal axis of the vehicle  – or even passes that point - its grip on the road would be dangerously affected.

To sum up the concepts: the rear suspension should withstand lateral and longitudinal stresses  in order to maintain convergence and keep the camber as constant as possible when the suspension is compressed on the straight and when tilted.

The classic (but now little used) rigid rear axle is excellent as far as camber is concerned (it varies very slightly) but it does very little to prevent variations in convergence.

Driving comfort is a lot less satisfactory – the wheels are not independent -  as is the dynamic behaviour because the considerable non-suspended mass reduces the ability to absorb surface irregularities that cause jolting.

More comfortable is the rear drivetrain with semi-independent suspension  (also called interconnected wheels) which has a C-shaped crossbeam that supports the hub housings, the springs and the shock absorbers. It also connects the two wheels with a looseness calculated to allow them to move with considerable independence over rough surfaces and give them sufficient solidity to avoid the use of an anti-roll bar.

However, its behaviour with regard to variations in characteristic angles is still not very satisfactory; the parts attached to the body which are furthest from the hub housings bend considerably and if they are prevented from doing so by bushings that are too rigid, comfort is affected. On the other hand, its lightness and low cost make it almost obligatory for front-wheel drives that are not very powerful.

Although it is expensive and not very light, the multilink family is very effective and intelligent simplifications have enabled it to be used also in mid-size vehicles.


The lever game

The concept is to give each lever the task of guiding only one wheel movement, with the result that the law of wheel motion can be determined a priori. The “real” multilink was introduced by Mercedes in 1982 for its 190: it has 5 arms and, in theory, controls all wheel movements except the vertical movement, which is the most useful.  Camber and convergence are maintained and longitudinal stresses generated by accelerating and braking are effectively prevented. The 5 levers work exclusively on traction-compression and therefore their flexible bushings can be optimized to give even more comfort.

In effect an independent  wheel has 3 levels of freedom because it could move freely along the three Cartesian axes: the 5-arm multilink leaves only the vertical axis free, which is the one that is needed.  Some might object that ESP makes all cars extremely safe. And this is certainly true, just as it is true that it is better to start off with a “healthy” base and then let electronics take care of final adjustments rather than intervene too frequently. The simplifications mentioned above reduce the number of levers by giving up strict control of movements, but they make construction lighter and more economical. We can see that these layouts include “half levers”, arms that are articulated at one end and attached at the other end to the hub housing, for example.  Layouts with two-and-a-half and three-and-a-half levers are used fairly frequently and are an excellent compromise in terms of weight, complexity, performance and comfort. They give high levels of handling and good driving comfort in the Alfa Romeo Giulietta, Ford Focus and VW Golf (in the VII series of the mid-size German car they are mounted only in the versions of over 90 kW), just to mention a few.

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