Difference Between Independent & Torsion Beam Suspension, Why Mazda & Mercedes-Benz Use Latter?

The difference between a torsion beam suspension and an independent suspension is significant. Strictly speaking, a torsion beam suspension is a semi-independent suspension. As for the claim that Mazda and Mercedes-Benz insist on using torsion beam suspension, it is a generalization. In fact, many car companies design A-class and A0-class cars using torsion beam suspension, such as Volkswagen, Honda, and PSA.

The torsion beam suspension is limited by its own structure and cannot constrain too many degrees of freedom. Therefore, its potential from a hardware perspective is not as good as independent suspensions like McPherson struts, let alone larger suspensions like double wishbones and five-link suspensions. Therefore, it is destined to be used in most small cars because small cars are lower and smaller, so torsion beam suspension can cope with them. This is like solving a four-step mixed operation problem, which doesn’t require college students, but elementary school students are enough.

The torsion beam suspension is not the same as the leaf spring suspension that is commonly referred to as “board suspension”. In fact, the torsion beam suspension is a completely different type of suspension. The true “board suspension” , as shown in the above picture, uses layers of stacked steel plate springs, also known as leaf springs. This type of suspension is typically only found on heavy-duty vehicles such as trucks, where the leaf springs are combined with a solid axle to form the “board suspension”.

However, in the passenger car industry, leaf springs are not used, and therefore, the “board suspension” does not exist. The torsion beam suspension is also not the same as the “board suspension”. Even in off-road vehicles that extensively use solid axle suspensions, such as the one shown above, leaf springs are not used.

Therefore, it is not logical to refer to the torsion beam suspension as “board suspension”. There are many different types of suspensions, and even the simplest non-independent suspension, the solid axle, has its own suitable applications. As shown in the above picture, when one wheel on a solid axle suspension is lifted, the other wheel is pushed downwards, ensuring that the tires remain in contact with the ground and providing traction.

Torsion beam suspension is a semi-independent suspension.

Why is torsion beam suspension considered a semi-independent suspension? As shown in the figure above, the structure of torsion beam suspension is simple, consisting of only a beam (torsion beam), a spring, and a shock absorber. Although torsion beam suspension is not as independent as independent suspension and does not have interference, torsion beam does twist. That is to say, when one wheel is impacted by a force from the ground, the force will be transmitted to the beam through the trailing arm, but it will not directly affect the other wheel.

The beam undergoes torsional deformation when it bears external forces. It uses this torsional deformation to offset the external forces, thereby ensuring that the external forces do not interfere with the other wheel. Many car enthusiasts like to use the figure above to evaluate torsion beam suspension, that is, when one wheel is impacted and raised, the other wheel will also be affected at the same time. This statement is completely true for solid axle suspension, but it is not entirely true for torsion beam suspension. Because the beam undergoes torsional deformation when it is impacted, it can absorb some of the force and reduce or eliminate the impact on the other wheel.

Therefore, torsion beam suspension has both independent suspension characteristics and non-independent suspension characteristics. The characteristics of torsion beam suspension depend on the load and impact it is subjected to. When the beam uses torsion to offset the load and impact from one wheel, torsion beam suspension exhibits independent suspension characteristics. On the other hand, once the motion load exceeds the geometric constraints of the beam, torsion beam suspension will exhibit non-independent suspension characteristics. The key point lies in the hardware structure design and the load that the beam can offset.

Disadvantages of Torsion Beam Suspension

It is undeniable that there are advantages to torsion beam suspension, but there are still many disadvantages. Since the crossbeam connects the left and right wheels, torsion beam suspension is ultimately not as independent and free as independent suspension, which does not create problems of mutual interference under any circumstances. However, the drawback of torsion beam suspension is that it cannot absorb all of the impact when the vehicle’s motion load exceeds the maximum bearing capacity of the torsion beam (torsional limit), and the unreduced impact will still affect the other side wheel.

In other words, when the vehicle is heavy enough, large enough, and traveling fast enough, the drawbacks of torsion beam suspension will be completely revealed, just like a pure non-independent suspension. Conversely, if the vehicle is small, light, and not driven to the limit, then torsion beam suspension can use torsional deformation to absorb the impact, and the suspension will exhibit completely independent suspension characteristics. Therefore, we can see that most cars equipped with torsion beam suspension on the market are A and A0-class models, with a weight of around 1.3 tons, and these types of cars have a light self-weight and short wheelbase, even when traveling at high speeds, the motion load generated is often not large.

Therefore, the performance limit of torsion beam suspension is very low and is limited by its simple structure. However, a low performance limit does not necessarily mean it is bad. For some car models with low requirements for suspension performance limits (A-class, A0-class), its economic and practical characteristics are completely advantages. Therefore, many car manufacturers like to use torsion beam suspension when designing A-class cars, because the drawbacks of torsion beam suspension are not easily apparent and are not noticeable for A and A0-class models. If torsion beam suspension is installed on B and C-class cars, it is not as good as independent suspension and the difference is more obvious.

Because the twisting of the torsion beam is limited, the maximum amount of impact absorption that the beam can provide through torsional deformation can be increased by increasing its torsional rigidity. However, the stiffness of the suspension during smooth driving is already too high, which can actually make the vehicle feel more like a non-independent suspension in normal conditions, making it difficult to ensure comfort. Therefore, the strength of the torsion beam is often limited and not worth further development, which is why B-class or larger models rarely use torsion beam suspensions. Even PSA, which prefers to use torsion beam suspensions, rarely uses them on medium or large-sized cars.

This is the difference between torsion beam suspension and independent suspension. Independent suspension is where each wheel has its own suspension unit and there is no interference between them. However, in a torsion beam suspension, the two wheels are connected by a beam that can undergo torsional deformation. While the beam can absorb some of the impact, there is a limit to how much it can absorb. In summary, the simple structure of the torsion beam suspension limits its maximum potential, while the complex structure and ability to easily control multiple degrees of freedom of the independent suspension makes its maximum potential higher. It is really difficult to optimize the torsion beam suspension, requiring strong structural design capabilities (key) and strong tuning capabilities in the later stages, whereas optimizing the independent suspension is relatively easier.

Add Comment