The Suspension A Arm

In this brief article, we will discuss the suspension A Arm, its design features and applications.

What is a Suspension A arm?

In the context of an Automobile, an A-Arm refers to a suspension Control Arm.  It refers to a Wishbone, shaped like the letter “A”, hence the name. 

The A-Arm is a main component of the Double Wishbone type of suspension and the MacPherson Strut Type of suspension. In the case of the double wishbone, there are 2 ‘A-Arms’, referring to the upper and lower wishbone. In the case of the MacPhserson strut type of suspension, the A-Arm refers to the lower control arm.

What is the use of a suspension A-arm?

According the Oxford Dictionary, the definition of an ‘Arm’ is –

..a thing comparable to an arm in form or function, typically something that projects from a larger structure..

The Suspension A-Arm projects out of the car’s main structure, which is the body or chassis, and hence is called so by definition.

The A-Arm’s basic purpose is to control the motion of the suspension to conform to a certain path or trajectory. The A-Arm , in combination with other members of the suspension, define the path of the wheel, when it is excited by road inputs while driving.

The inboard end of the A-arm is hinged onto the chassis using cylindrical or spherical rubber bushings. The outboard end of the A-Arm is connected to the Knuckle via a spherical joint, also called a ball–joint. The Control arm rotates about an axis, which is the line between the 2 bushings, and allows the ball-joint end to move along an arc.

The A-arm, by itself, cannot constrain the motion of the wheel. Since the A-arm can constrain only 2 degrees of motion, the other components of the suspension like the MacPherson strut, tie-rod and other control arm, will have to constrain the other degrees of freedom.

What are the different Control Arms in a suspension?

In a Double Wishbone type of suspension layout, there are 2 ‘A-arms’, the Upper Control Arm or Upper Wishbone and the Lower control arm or Lower Wishbone.

The Upper A-Arm is connected via a ball joint to the Upper side of the Knuckle, and the Lower A-arm is connected to the Lower side of the knuckle using a ball-joint. Together, the control arms define the path of the knuckle to be upright at all times, while the wheel moves upwards and downwards.

Apart from A-Arms, control arms that have only two attachment points, shaped more like a Rod, are commonly used in multi-link type of suspensions. The different types of Control arms are-

  • A-Arm: Is mostly found in the Double wishbone and MacPherson type of suspension. Apart from this, it is also found in some rear multi-link suspension layouts
  • Panhard Rod: Is a control rod used mostly in a solid live axle type of suspension. Its purpose is to constrain lateral movement of the axle. It is mostly found in rear suspension.
  • Lateral Link: Is a link to constrain the wheel in the sideways direction and is found mostly in a multi-link suspension setup
  • Toe-Control Arm: Is a link that is used to adjust the Toe setting of the wheel in the rear suspension. It is mostly a part of a multi-link suspension setup.
  • Upper and Lower Control Rods: The control rods are used in a general fashion to anchor the top and bottom extremities of the knuckle to the chassis. Depending on the particular multi-link layout, the control rods could be oriented at any angle with respect to the chassis straight-ahead direction.
  • Trailing Arm: Is usually found in the rear suspension multi-link set-up. The trailing arm, usually constraints the rear wheels in the fore-aft direction and are generally oriented parallel to the chassis fore-aft axis
  • Swing-Arm: A swing-arm is a combination of a lateral arm and a trailing arm. It mostly hinges at two points on the chassis and the axis of the hinge is at an angle to the longitudinal axis.

Conclusion

In this brief article, we have discussed the suspension A Arm, its design features and applications.

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