The A Arm Suspension ( A brief Overview)

In this brief article, we will discuss the A-Arm suspension, the types of A-Arm suspension and the different types of A-arms.

What is an A arm Suspension?

In the context of an Automobile, the A-Arm suspension refers to any suspension that has an A-Arm.  It refers to the Wishbone type of suspensions, where the Wishbone is shaped like the letter “A”, hence the name. 

What are the Different Types of A-Arm Suspension?

The 2 Types of A-Arm suspension are:

  • Double A-Arm or Double Wishbone suspension
  • MacPherson Strut Suspension

The A-Arm is a main component of the Double Wishbone type of suspension and the MacPherson Strut Type of suspension. You could therefore consider the Double Wishbone and the MacPherson as the two variants of A-Arm 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, there is only one A-Arm, which 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 Types of A-Arms?

In the initial days where the wishbones had just become popular, the shape of the wishbone closely resembled the letter ‘A’.  As the Double Wishbone and the MacPherson setups got more sophisticated, several optimizations have happened to the actual shape of the wishbone. Each manufacturer would have its own unique strategy for developing a particular model’s chassis and suspension.

The different types of A-Arms are:

  • Single Piece A-Arm
  • Two-Piece and 3-Piece A-Arm
  • Straight rod A-arm
  • Sheet metal formed A-Arm
  • Forged A-Arm

Single Piece A-arm

The Single Piece A-Arm is the most common of A-Arms. It is probably the most widely used among all the different suspension components. The Shape of the A-Arm has evolved over time. In the initial days, the A-Arm actually looked like the letter ‘A’. But nowadays, due to optimized geometries and weight reduction, the A-Arm is more resembling the letter ‘L’. This is the case almost always with the MacPherson Strut suspension. A majority of OEMs use this shape. The reasons being:

  • The reduced weight due to eliminating the horizontal line connecting the two legs of the A-Arm
  • As per the Kinematics and Compliance requirements, the actual hard-points of the MacPherson setup have moved such that the in the top view the line joining the Ball-joint to the A-Arm chassis mount is perpendicular to the axis of rotation of the A-Arm

2-Piece and 3-Piece A-arm

The 2-piece and 3-piece A-Arm design came about in order to enhance serviceability. A-Arm bushings are made of rubber and would need replacement every one in a while. The trouble is that the bushing would be molded into a metal ring and the bush outer ring would need to be press-fitted into the A-Arm. Traditionally, this press-fitting is done only by the OEMs because of the jigs and fixtures that enabled them to do so in an accurate manner. Replicating this is difficult in a garage setup and as a result, whenever the A-arm bushings wore out, the whole arm assembly with bushings fitted would need to be replaced, even if the A-arm itself was fine.

The solution to this was to have an A-arm where the Ball-joint was a separate piece, that bolted onto the main A-Arm. The A-Arm bushing can also be made as a separate piece with a lock-and-key type of joint. The main A-arm had a square profile protrusion, which was the key. In this manner, the Bush could become a separate part that had a square groove matching the A-Arm key. The two could be assembled easily without the need for any special tools. In the case of 3-piece design, the Bush, the Ball-joint and A-arm main body are all separate.

The straight Rod type of A-Arm

This sort of A-Arm is not very common in Modern cars. There are, however, a few exceptions like the front Macpherson Control arm in the previous generation Suzuki Alto 800 sold in the Indian market. The straight rod will have the lower ball-joint at one end and the cylindrical chassis bushing at the other. There is an additional arm called the ‘Strut Rod’ that is connected to the A-arm and forms the missing leg of the letter ‘A’.

Sheet-metal formed A-Arm

The ‘L’ shaped A-arms are mostly of Sheet-metal construction manufactured using ‘Stamping’ die’s. A hydraulic Stamping machine with a capacity ranging from 50-100 tonnes would stamp a sheet metal using Die tools that have the same inner and outer surface profiles of the A-Arm.

Forged A-Arm

The Forged A-arm is probably the best type of A-Arm design from a strength and weight point of view. The Forged A-arm would mean more strength from a leaner-looking shape. Because of the special high-temperature process in Forging combined pressure applied during the Forging operation, the outcome is a stronger A-Arm that has a better molecular arrangement internally.

Conclusion

In this brief article, we have discussed the A-Arm suspension, the types of A-Arm suspension and the different types of A-arms.

In case of any questions or queries, please feel free to ask.

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