Suspension System

Suspension is the combination of energy storage part and energy absorbing (dissipation) part and the parts that connect the wheel to the body of the vehicle. The purpose of suspension is to protect the vehicle from vibrations, sudden shocks (to protect from fatigue failure and to give comfort and driving pleasure) and provide max contact of wheel to the road (for dynamic stability of vehicle)

Two main parts

• Energy storage part (coil spring or leaf spring)-The jerk or force on the tyre due to ill road condition, bumps or cornering is stored up by the spring so that it may not be transmitted to the vehicle. It gives comfort by storing the sudden shocks but it only stores the shock rather than absorbing it.
• Energy dissipation part (damper or shock absorber)-The energy that is stored in the spring is damped up by the damper. The compressed spring when released starts oscillation. The damper reduces this effect or prevents oscillation hence giving smooth restore to spring without oscillation. Modern Energy dissipater damp according to the velocity of the vehicle as the strength of shock depends on the velocity of the vehicle.

Internal components of the damper

The dampening action in hydraulic shock absorbers occurs by transferring oil under pressure through the valves that restrict the oil flow. The twin tube type shock absorber is most common in which the cylinder is inside another cylinder and the piston is inside the inner cylinder. There is the space for oil in the cylinder below the piston and also above the piston. The piston rod occupies the volume above the piston as to compensate that volume the space between inner and outer cylinders is used. The oil seal prevents the oil leakage. The rod guide keeps the rod in alignment as it moves in and out the cylinder. The piston valves transfers fluid from below piston space to above piston space. The bottom valve transfers fluid from below piston space to space between cylinders.

Working of damper

Under normal condition the oil is present below the piston and to some level between the cylinders. When the compression phase occurs the oil is forced under pressure to transfer through piston valves to the space above piston and through base valve into the space between the cylinders. This high pressure oil transfer through these valves causes oil to heat up hence dissipating the shock into the heat energy. Vice versa in the rebound phase

Macpherson Strut

It’s the type of suspension system usually used in the front wheels of the front wheel drive car. In this type of suspension system we have one A-arm and vertical strut having damper and surrounded by coil spring. The single A-arm is pinned with chassis and top of strut is fixed with body.

Internal components

Advantages of Macpherson strut

• Best for front suspension in FWD as it has the space for drive shaft into the tyre.
• Cheaper lighter than double A-arm.

Disadvantages of Macpherson strut

• It’s taller than double A-arm.
• During cornering (heavy body roll) the tyre does not adjust the camber or does not maintain much contact to ground as the strut restricts its camber motion.

Solid Axle/Beam Axle

The axle that connects the two wheels rigidly is known as solid axle (left and right wheels connected by single rigid axle). The solid axle suspension is usually used in rear wheels. You might have seen solid axle with differential in the backside of the trucks. The vehicle rests on the axle through the coil spring or leaf spring and the damper are connected parallel to springs. There are many ways for restricting the lateral and longitudinal motion of axle from the body, the leaf spring is the cheapest way. The other way is trailing arm that connects axle to chassis preventing lateral and longitudinal motion, Pan hard rod also prevents lateral motion.

Advantages

• Extremely simple,strong and durable.
• Good track/drag applications when there is no road imperfection.

Disadvantages

• It affects the clearance of vehicle due to large differential hub.
• Traction and handling problem as motion of one tyre over bump affects other (the axle is solid) If independent suspension would be there one tyre on bump will not affect other and hence will maintain traction.
•  Mass of the solid axle is part of the unsprung weight of the vehicle.
• Cornering ability is typically worse than other suspension designs because the wheels have zero camber angle gain during body roll.

Double wishbone suspension

It has two control arms having the shape of wishbone or letter ‘A’ hence correspondingly known as double wishbone suspension or double A-arm suspension. These two arms are connected to frame by the pin joint. Initially both the A-arms had equal length and it has the advantage that the camber angle does not change when the suspension had positive or negative travel due to the bumps on the road but, the disadvantage was during the cornering of vehicle when the body rolls the angle of tyre with respect to vehicles vertical axis couldn’t change hence the camber on outer tyre becomes positive (the rolled car’s tyre angle with ground) and the camber angle of inner tyre becomes negative. These shift of camber angles are imperfect for the cornering as there is less grip of tyre with road. What we want is the camber on the tyre having much weight should be negative and the camber on the tyre having less weight should be positive for the perfect grip. So engineers decided to have shorter upper arm so that as there is positive travel of suspension the camber changes to negative and vice versa. Now when vehicle goes around the corner the outer tyre has positive suspension travel due to lateral weight transfer which suddenly changes its camber to negative hence giving perfect grip on road (as the vehicle is rolled but tyre is still vertical with respect to the ground). Similarly, the inner tyre has negative suspension travel has changing its camber to positive. So double A-arm suspension with unequal arms is a good suspension system

Advantages of double wishbone suspension

1)      Good camber adjustment on body roll.
2)      As it has double arm so the suspension travel is more than single arm and solid axle suspension. It can handle large bumps also.
3)      Can be used in both front and back tyres.
4)      The drive shaft can also be attached as placement of shocker is adaptive.

Pushrod Suspension System

figure 1

figure 2

This type of suspension can usually be seen in formula-one cars and are often used with double wishbone system. The main components of pushrod suspension can be seen in figure 1. When vehicle goes over a bump the pushrod gets push from the wishbone. The pushrod is pivoted to a rocker and rocker rests on the torsion bar. The torsion bar is a long cantilever circular bar fixed at one end, made of alloy steel which elastically deforms to resist and store the twisted motion when twisted thus gives the spring action to the jerk. On the other hand the rocker is pivoted with the damper so that the stored force is damped out into heat. The heave spring is also pivoted to rocker which provides the extra resistance and prevents excessively low ride height (pushes the car up when the suspension travel decreases the clearance). So whenever the car is in heave (no roll) the heave spring comes into play. Anti-roll bar (ARB) as the name indicates that it prevents the car roll during cornering. There are two basic designs of ARB i.e. simple pivot type and U-type connected by drop links. What happens is when the car goes around the corner the car body rolls and the outer tyre suspension travels up pushing the drop link down as shown in figure 2. The ARB (U-type) pulls the inner tyre up also by pulling down the drop link of inner tyre due the torsion resistance of ARB hence decreasing the roll angle and straightening the car body over the corner.

In the fig 1 the simple pivot type anti-roll bar is used which is pivoted with the chassis at centre and at other two sides the drop links are pivoted to ARB as shown is figure 3. When the left rocker in fig 1 is rotated by pushrod of outer tyre it pulls the drop link rotating the ARB. The ARB pushes the rocker on the other side lifting the inner tyre up.

Advantages

• Good aerodynamics as there is no coil spring or damper in contact with air that can create turbulence (all components are inside the body). The pushrods are smooth circular aerodynamic in shape.
• Due to heave spring the ride height does not change hence does not affect the aerodynamics of car.

These are the basic reasons why this type suspension system are commonly used in formula 1 car.