Atlas F1   Taking the Lid Off F1

Formula One Technical Analysis

  by Will Gray, England

Atlas F1 presents a series of articles by certified engineer Will Gray, that investigates in greater depth all the technical areas involved in design, development, and construction of a Formula One car.

6A: Suspension Geometry

Connecting the wheels and tyres to the car is a simple concept, but doing it in a manner which obtains good performance is a different matter altogether. In traveling around the circuit, the car experiences bumps from the track surface, and weight transfers from braking, accelerating, and cornering, and the design of the suspension geometry (relative angles and lengths) determines how the vehicle reacts to these forces.

The first consideration in suspension design is the car's track (the width between the two front or the two rear wheels), and its wheelbase (the distance between front and rear wheels). A long wheelbase creates a car which is stable in a straight line, and has less load transfer between front and rear on braking and acceleration. With a shorter wheelbase, the car is more maneuverable, but less stable. A wide track will give reduced lateral load transfer (transfer from one side of the car to the other), and allow longer suspension links. Go narrow, and you get the opposite. Many teams lengthened their cars during the 1998 season when the rules stipulated a narrower track, and this proved to be a lengthy process - it's something you need to get right first time!

Front suspension on the BAR HONDA 002 The modern Formula One car uses 'Double Wishbone' suspension, which has two (hence double!) v-shaped (hence wishbone!) suspension members (also called suspension links). They are situated one above the other, on each side of the car, and are clearly visible on the front suspension, but hidden by bodywork at the rear. This is a type of independent suspension, which means that the wheels on either side are not connected, and that if one wheel is upset by a bump, the upset will not be transferred to the other wheels. The loads are instead transferred independently to the chassis, which itself is designed to be stiff enough to take them without deforming. Taking one side of the car, each wishbone (upper and lower) attaches to the chassis at two attachment points (inboard), about which they are allowed to rotate up and down. The width between the two points on each wishbone determines the area of the chassis over which the wheel loads are transferred. At the wheel (outboard) there is a single attachment point for each wishbone (upper and lower) on the 'upright' - or 'hub'.

The suspension system described so far will be useless: There is currently no resistance to movement, and the car would just sink to the floor and stay there. We need springs and dampers to provide this resistive force, and to have these, we need a pushrod - the final as yet unmentioned member of the suspension. It connects the lower wishbone to the spring and damper system which is usually mounted in the chassis, just in front of the driver, hidden under the bodywork. The length of this pushrod determines the initial ride height - the height the car sits above the ground - and the spring and damper set up determines how much the suspension is allowed to move under loading. Given this type of suspension, there are hundreds of different design possibilities, and once the track is decided upon - often by the rules - the choice of suspension link lengths, attachment point positions, and wishbone angles must be made, along with the layout.

We will now consider the two types of movement undergone by the wheel: Bump and Roll. As the car travels around the racetrack and encounters these forces, the outboard attachment points of the suspension, (and therefore the wheel), move in an arc about the inboard points on the chassis, and this also works the other way. The most important consequence of this is on the wheel contact patch. As we know, to get the most grip from the tyre, the contact patch must be as large as possible, with an even pressure distribution across it. This is obviously achieved by having the wheel vertical in relation to the track surface, and pushed down onto the track. If the tyre is angled away from the vertical, it has Camber, and this is introduced into a suspension design so that in cornering, the contact patch remains as large as possible.

With a vertical movement of the chassis from a bump, the camber of the wheel will change due to the geometry of the suspension, so the contact patch will reduce, and take the grip with it. It is the suspension's job to minimize this camber change under bump, and carefully designed geometry can achieve this.

Damon Hill not avoiding the new for 1999 profiled curbsRoll (entering a turn, for instance) is a different matter. The whole chassis moves to an angle relative to the ground due to the centrifugal acceleration of cornering. The load will be transferred to the outside wheel (furthest away from the turn), and here the upper and lower chassis attachment points move down and out relative to the chassis centre line. The wheel will again end up at an angle relative to the race track (in this case leaning away from the car), with the opposite occurring on the unlaiden side. However, the camber in this case will be larger and more difficult for the suspension designer to cope with. And guess what....the car is most sensitive to the worst condition - a combination of bump and roll - at the front of the car on turn in to the corner (when the weight is forced forward under braking) and at the rear of the car on exit (as the car squats at the rear under acceleration) - just where car stability is needed most!.

So the desired suspension system must have enough vertical movement to cope with the bumps and loadings, whilst being light and stiff enough to cope with roll. As usual, we can't get all of these perfect so the designers must once again use the art of compromise.

Previous Parts in this Series: Parts 1 & 2 | Part 3 | Part 4A | Part 4B | Part 4C | Part 5A | Part 5B

Will Gray© 2000 Kaizar.Com, Incorporated.
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