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.

6B: Springs and Roll Bars

Racetracks are, in general, smooth surfaces. A driver wouldn't say that because he feels every tiny bump in the surface, but that's not the track's fault! The engineer wants the car to perform well, which means driver comfort takes a back seat to keeping the wheel on the racetrack. The engineer has two concerns. He must primarily create a suspension, which obtains the best response and reaction to load transfers. Any small change in ride height will cause changes in the aerodynamics, so suspension must aim to minimize the cars eagerness to dive (nose down) under braking, squat (sit down) on acceleration, and roll. Secondly, he will wish to keep the contact patch on the track as much as possible, which means the driver will have grip and confidence, even if he doesn't have comfort!

Three situations put forces on the car. Braking and accelerating causes load transfers which lead to the vertical movement of the sprung mass (car, diver, fuel, etc.), centrifugal acceleration in cornering causes the sprung mass to roll, and the bumps on the road surface cause the unsprung mass (wheels, hubs, etc.) to vertically deflect. All of these lead to a camber change relative to the road surface, and therefore to the contact patch not performing at its best. The amount of vertical movement or roll depends on the wheels Ride or Roll Rate resistance, and this resistance force comes from the springs and roll bars in the suspension.

The springs are connected to the push or pull rod, which in turn connects to the lower wishbone of the suspension, so that when the suspension moves up or down, the spring is compressed or extended, and the shock loads aren't transmitted directly to the vehicle. Included in this system, however, is a damper. Without this, when the car traveled over a bump, the spring would compress under the full force of the bump, and all the energy from this force would be transferred to and stored in the spring. It would then have to be released in some way, and this would come in the form of an opposing extension. The spring would then continue to compress and extend until it was naturally damped out whilst the car bounced along like an over energetic bunny rabbit - clearly not ideal!

The damper consists of a cylinder of oil through which a piston moves, pushing the fluid through various holes. The piston is connected to the spring, and the cylinder is attached to the lower wishbone by the pushrod. The damper takes some of the kinetic (movement) energy from the bump and transfers it to thermal energy in the oil. This means there is less energy stored in the spring, so reduces the spring-back problem, and removes the tendency for the car to become a bucking bronco! The bigger the acceleration (or g-force) created by the bump or weight transfer, the faster the piston will move, and, by the laws of physics, the larger the damping will be. There are two strokes in the damper - bump and rebound. The bump stroke damps the movement of the unsprung mass due to the bumps, and the rebound damps the reaction of the sprung mass to the spring compression. Thus the damper, spring and suspension system acts as a force filter between the car, wheels, and racetrack.

The springs and dampers for the front suspension are usually hidden away in the front chassis, and most often placed on top of the chassis, above the driver's legs, covered by a removable access panel. The configuration of the dampers tends to be chosen for packaging reasons, and a rocker arm is used to activate them in any position required.

Ayrton Senna at the 1987 Monaco GP in his Lotus 99T One system, which was developed and banned in the 80's and 90's, was active suspension - it has now found its way onto road cars. The system is an electronic replacement for the conventional suspension described here. It uses a one sensor mounted in front of the suspension, and another on the wishbone itself, whilst the conventional dampers are replaced with hydraulic actuators. The forward sensor tells a computer the distance it is from the track, and from this, the computer will move the actuator milliseconds later. In doing this, the car effectively predicts where the bumps are, and moves the wheels vertically to move over them without reacting to them. The system allows a constant ride height to be maintained, which therefore gives more consistent aerodynamics and a contact patch, which is always on the road. This all leads to a better performing car - shame it's banned really!

The choice of springs and dampers goes a long way to determining the stiffness of the suspension, but one more contributor is the roll bar. In roll, the spring doesn't compress much at all, so the cars have 'Anti-Roll Bars' front and rear. These limit the chassis roll to levels at which the suspension can keep control of wheel camber changes. Each one is basically a torsion bar, which is mounted to the chassis (but allowed to rotate in this mount), and connected to the wheels at the hub on either side of the car. If both wheels deflect vertically at the same time, it simply rotates in its mountings.

If the chassis rolls, the bar resists this to some extent (in a similar way to the springs, the kinetic energy is transferred from the wheel chassis to the bar), and transfers the load across from the unlaiden wheel to the loaded one. The roll bar allows the car to cope with this motion, but makes the suspension no longer independent. However, this is a preferable compromise to the problem, and also allows engineers to cancel out understeer and oversteer quickly. If the car is oversteering, the rear roll bar can be softened so the bar can transfer less load, and the rear wheels will generate more traction, thus reducing the problem.

Suspension set-up is an exact art. If it's too soft the car will wallow over the bumps and be sloppy around the corners, but if the engineer takes it too far the other way, an over-hard suspension will lead to a loss in car sensitivity. The soft versus stiff compromise is very delicate.

  • Next Week: Part 7a: Wind Tunnel Technology

    Previous Parts in this Series: Parts 1 & 2 | Part 3 | Part 4A | Part 4B | Part 4C | Part 5A | Part 5B
    | Part 6A

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