Solid Axle ( A Brief Insight)

In this brief article, we will discuss the Solid Axle, the types of Solid axles and Solid axle suspension, their advantages and disadvantages.

What is a Solid Axle?

From the Oxford definition, the word Axle means –

“..a rod or spindle (either fixed or rotating) passing through the center of a wheel or group of wheels…”

In a vehicle suspension, a solid axle is also called a ‘beam axle’, or a ‘rigid axle’. Solid axle is  basically a lateral structure holding the left and right wheels. 

In the earliest forms of 4 -wheeled vehicles, the front wheels, and the rear wheels were held together by the front axle and the rear axle respectively. The axle was a solid beam that had the knuckle attached at each end, in the case of the front suspension. In the case of the rear, the axle consisted of 2 tubes connected at the center to a differential.

What are the Different Types of Solid Axles?

There are basically 2 types of Solid Axles:

  • Live Axle
  • Dead Axle

Live Axle

A Live axle means that apart from the structural support that it provides to the left and right wheels, the axle also houses the Differential Gears and axle shafts that transmit power to drive the wheels. A live axle is most common on vehicles that are rear wheel drive. Vehicles with 4-wheel drive have a live axle both in the front and the rear. 

Dead Axle

A Dead axle can also be called a beam axle. A dead axle is mostly found in the front suspension of trucks and is of the steerable type. The axle beam is either straight or “Gooseneck” shaped. The knuckles and wheels-ends are attached at each end using a Kingpin joint which is serviceable. The knuckles are connected to each other by a Steering Tie-rod through ball-joints.

What is a Solid Axle Suspension?

A Solid Axle suspension can also be called a dependent suspension. The earliest form of suspension for the solid axles was the leaf spring type. Because the left and right wheels were connected rigidly, the vertical motion of the left and right wheels was always coupled. In other words, when the left wheel went upwards over a bump, it pushed the right wheel to go downwards.

What are the Different Types of Solid Axle Suspension?

The different types of Solid Axle suspension are:

  • Leaf-spring
  • Multi-link suspension
  • Watts Linkage

Leaf Spring Solid Axle Suspension

The Leafspring-type suspension is the most popular among all solid axle suspension systems. A majority of rear wheel drive trucks and commercial vehicles have a leaf spring rear solid axle suspension.

The most common type of leaf spring is a Semi-Elliptic leaf spring. It is basically a stack of slender steel strips stacked one above the other and clamped together. The chassis is hinged to the 2 edges of the leaf spring, whereas the middle of the leaf spring is clamped to the Solid Axle using a U–Bolt Clamping arrangement.

The leaf spring started out in the early 20th century as the most common type of suspension and can be found on the front suspension of almost all cars of right upto the 1940’s. Later on, in recent times, the leaf spring setup became more extinct and are found only on Pickup trucks like the Ford F150 and the GMC Sierra.

The multi link rear suspension can be of the 4-Link or 5-Link Type. The suspension would consist of:

  • 2 upper control arms
  • 2 Lower Control arms
  • A panhard rod (in the case of a 5-Link type)

The upper and lower control arms take up the braking and acceleration and Roll and cornering loads coming from the tires in the case of the 4-Link setup. In many cases, a 4-link setup would flex under heavy cornering loads for vehicles that are performance-oriented. The 5-Link setup basically evolved to overcome this limitation. The Panhard Rod takes up Lateral inputs from the tires.

A well known example of a current day 5 -link solid axle rear suspension is the third generation (D23 platform) Nissan Navara, also known as the Nissan Frontier.

The Panhard Rod had the disadvantage that it constrained the axle to move up and down in an arc when looking from the vehicle rear. This would cause sideways movement of the axle from bump to rebound. The sideways movement leads to issues in handling predictability. In order to overcome this limitation the Watts Link was developed. 

The Watts link did the same job as a panhard rod but by splitting the panhard rod into 3 effective linkages. This would allow the axle to move up and down in a straight line. Cars that came with a factory fitted Watts Link were the Ford Mustang (2005-2014) and the Dodge PT Cruiser (2001-2007).

Solid Axle Suspension vs. Independent Suspension

An independent suspension is defined as any suspension in which the left/right wheel’s vertical motion does not affect the vertical motion of the opposite side (left/right) wheel. Unlike in the case of the dependent suspension, there is no solid structure that directly connects the left and right wheels.

Advantages of a Solid Axle Suspension

  • A Solid axle’s biggest advantage is in its simplicity of design, Robustness and relatively low manufacturing cost
  • There are a lesser number of CV joints required in a Solid axle. The driveshafts for both the wheel ends are always straight and in line, as opposed to an independent suspension where separate driveshafts and joints are required at both wheel ends.
  • A Solid live axle housing is very rigid casing that protects the driveshafts and give them longer life
  • A Solid live axle would be cheaper and faster to develop, as opposed to a new Independent suspension which requires a comparatively higher engineering effort and expense. 
  • The number of parts and different types of joints involved in a solid live axle is lower and hence the number of maintenance issues and associated costs in comparison to an independent live axle.
  • The Solid axle Camber and Toe settings are fixed by the axle geometry and hence there is no change in the wheel alignment as the suspension moves from bump to rebound.
  • Because of the constant wheel settings, there are much lesser wheel alignment issues as compared to an independent suspension.

Drawbacks of the Solid Axle Suspension

  • Independent suspension typically used where the vehicle’s ride quality and handling characteristics are a high priority
  • Due to the higher unsprung mass and the inability of one wheel to follow the unevenness of the road without affecting the opposite wheel on the vehicle, the ride and handling quality in a solid axle suspension is much less as compared to an independent suspension.
  • In extreme off-roading conditions, where wheel articulations are important, an independent suspension is more capable of ensuring contact on all 4 tires as compared to a solid axle suspension.
  • The axle beam becomes the lowest point in the underbody of the vehicle and therefore the primary influence in ground clearance. In extreme off-roading conditions, this is a limitation that is overcome in an independent suspension setup.

Conclusion

In this brief article, we have discussed the Solid Axle, the types of Solid axles and Solid axle suspension, their advantages and disadvantages.

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