Triumph's Swing Spring: An Innovative Solution

The humble Triumph small-chassis cars, beloved by many enthusiasts, often find themselves at the centre of discussions regarding their rear suspension. A recurring topic, particularly on automotive forums, revolves around the swing axle and a specific innovation Triumph engineers implemented: the 'swing spring'. This article delves into the 'why' behind this modification, its intended benefits, and how it aimed to mitigate the inherent characteristics of the swing axle design, which, contrary to some beliefs, was not exclusive to the Triumph marque.

Understanding the Swing Axle

Before we explore the swing spring, it's crucial to understand the swing axle itself. This type of rear suspension system, where the axle shafts pivot outwards from the differential, was quite prevalent in automotive design during a particular era. It's important to note that Triumph was by no means alone in utilising this technology. Vehicles from well-known manufacturers such as Volkswagen (up to 1967 models), Mercedes-Benz (including the iconic 300SL), Porsche (the 356), Tatra, and Chevrolet (with the Corvair) all featured swing axles at some point. This widespread adoption underscores that while the swing axle had its known drawbacks, it was considered a viable, albeit sometimes problematic, solution for rear suspension.

The primary characteristic, and indeed the most significant challenge, of a swing axle is its inherent tendency to exhibit extreme camber changes. As the suspension compresses (bumps) or extends (droops), the angle of the wheels relative to the vertical plane changes dramatically. This can lead to a phenomenon known as "jacking" during hard cornering. Jacking occurs when, under significant lateral forces, the swing axle's geometry causes the car to lift on the affected side, effectively reducing tyre contact and potentially leading to unpredictable handling. This is compounded by the fact that as the suspension compresses on one side during a corner, the opposite side extends, leading to even more pronounced camber changes.

Triumph's Innovative Solution: The Swing Spring

Triumph engineers were acutely aware of the limitations and potential pitfalls of the swing axle. Rather than ignoring the issue, they sought an innovative solution to improve the handling and stability of their cars. This led to the development and implementation of the "swing spring".

The core concept of the swing spring was to alter the way the transverse leaf spring, which provided the springing and damping for the rear axle, was mounted. Instead of being rigidly attached, the swing spring was designed to pivot. This pivot point was strategically located on the differential casing. The ingenious aspect of this design was that it allowed the car's body to roll more independently of the suspension's angular movement. In simpler terms, while the body might tilt due to cornering forces, the wheels themselves experienced less of the extreme camber change that plagued conventional swing axle designs. This decoupling of body roll from suspension geometry provided a significant improvement in straight-line ride comfort and, more importantly, enhanced stability and control during cornering.

How the Swing Spring Worked

Imagine the rear axle as a rigid beam connecting the two rear wheels. In a standard swing axle, this beam pivots at the differential. The leaf spring is typically mounted to the chassis at one end and to the axle beam at the other. When the axle moves up or down, the spring deflects, and the axle's angle changes. With the swing spring, the leaf spring itself pivots. This means that as the axle moves through its arc, the spring can rotate around its central pivot point. This rotation allows the axle to articulate more freely, reducing the amount of camber change that is directly transmitted to the chassis and body. The result is a more controlled and predictable rear-end behaviour, particularly when subjected to lateral forces.

Addressing Competition and Performance

For those looking to push the performance envelope, especially in a competitive environment, controlling the camber change of a swing axle was paramount. Two primary methods were commonly employed to achieve this:

1. Lowering the Car by Raising the Transverse Spring: This modification involves repositioning the transverse spring higher relative to the chassis. By doing so, the axle is placed in a different part of its operational arc. This typically results in more negative camber when the car is travelling in a straight line. Crucially, during cornering, the reduced travel into positive camber means the tyres maintain a more optimal contact patch with the road, enhancing grip and predictability.
2. Increasing Spring Stiffness: A stiffer spring resists deflection more effectively. For the same cornering side force, a stiffer spring will cause less vertical movement of the axle. This, in turn, limits the extent of camber change, contributing to a more stable platform. An example of this approach was seen in the experimental Silverback, a Vitesse Estate race car. This car successfully employed stiffer springs, resulting in a noticeable reduction in rear suspension movement during cornering, as evidenced by its performance at Castle Combe, where significant body roll was visible with minimal rear suspension articulation.

Beyond the Swing Spring: Further Developments

While the swing spring was a significant step forward, Triumph continued to evolve their rear suspension designs. Later models and modifications saw the introduction of a fully controlled rear axle. This typically involved the addition of a lower wishbone (often reversed) to better locate the axle laterally and longitudinally, and the use of a "Rotaflex" coupling. The Rotaflex, a rubber donut at the outer end of the driveshaft, served to absorb torsional vibrations and allowed for some angular movement, effectively replacing the direct swing axle connection with a more controlled articulation. However, these later iterations, while an improvement, still faced their own set of challenges.

Challenges with Later Designs

Even the Rotaflex-equipped, more controlled rear axles were not without their issues. Common complaints included the continued reliance on a single parallel wheel bearing, which could be a point of failure. The quality of the Rotaflex donuts themselves could also degrade over time, affecting performance. Furthermore, the original cast iron wishbones were considered excessively heavy, adding unsprung mass. Enthusiasts and builders have since developed more advanced solutions. For instance, the "Son of Silverback" (SofS) project incorporated MGF rear uprights, employing doubly opposed conical hub bearings for greater strength and durability, along with CV joints. This was coupled with Volvo half shafts featuring "LoBro" (Low Braid) joints at the differential and fabricated wishbones with fully spherical joints for maximum articulation and minimal play.

Debunking Myths: Snap Oversteer

A common concern often raised about Triumph swing axles is their propensity for "tuck under" and "snap oversteer". While these phenomena are indeed characteristic of poorly managed swing axles, the severity and inevitability of these issues have, in the opinion of some, been somewhat overstated. Snap oversteer typically occurs when a driver enters a corner too quickly, becomes unsettled, and lifts off the throttle mid-corner. This abrupt weight transfer to the front wheels can indeed cause the rear wheels of a swing axle car to tuck under, leading to a sudden and often dramatic loss of rear-end stability. However, with careful driving, understanding the car's dynamics, and potentially the aforementioned suspension modifications, these tendencies can be managed and mitigated.

Table: Swing Axle vs. Swing Spring Benefits

Frequently Asked Questions

What is a swing axle?

A swing axle is a type of independent suspension where the axle shafts pivot outwards from the differential, allowing the wheels to move up and down independently.

What is the main problem with a swing axle?

The primary issue is the extreme camber change that occurs as the suspension moves, which can lead to "jacking" and unpredictable handling during hard cornering.

How did the Triumph swing spring solve this problem?

By pivoting the transverse leaf spring on the differential, the swing spring allowed the car's body to roll more independently of the suspension, thus reducing extreme camber changes at the wheels.

Were swing axles only used by Triumph?

No, swing axles were used by many manufacturers, including Volkswagen, Mercedes-Benz, Porsche, and Chevrolet.

Can a swing axle be improved for competition?

Yes, improvements can be made by lowering the car (raising the spring) or increasing spring stiffness to reduce camber changes.

What is a Rotaflex coupling?

A Rotaflex is a rubber donut used on the driveshaft that absorbs torsional vibrations and allows for some angular movement, often used in later, more controlled rear axle designs.

Is snap oversteer inevitable with a Triumph swing axle?

While a risk, especially with aggressive driving, the severity of snap oversteer has been debated, and with careful driving and potential modifications, it can be managed.

Conclusion

The Triumph swing spring was a testament to the ingenuity of automotive engineers seeking to overcome the inherent challenges of existing suspension designs. By introducing a clever pivot for the transverse leaf spring, Triumph significantly improved the handling and stability of their small-chassis cars, making them more enjoyable and predictable to drive, especially when pushed. While the swing axle's reputation sometimes precedes it, the evolution of designs like the swing spring demonstrates a commitment to refinement and a desire to offer a superior driving experience. For enthusiasts today, understanding these design principles is key to appreciating the engineering prowess of these classic vehicles and exploring the myriad of further modifications available to enhance their performance even further.