New design approach for leaf-springs in motorcycles

Type of the Paper: Conference Paper

New design approach for leaf-springs in motorcycles

Hannes Fellner¹

¹ Department Suspension – Advanced Engineering, KTM Forschungs & Entwicklungs GmbH, Austria, Hannes.Fellner@ktm.at, ORCID 0009-0007-0182-3526

Name of Editor: Jason Moore

Submitted: 05/09/2023

Accepted: 07/09/2023

Published: 07/09/2023

Citation: Fellner, H. (2023). New design approach for leaf-springs in motorcycles. The Evolving Scholar - BMD 2023, 5th Edition.

This work is licensed under a Creative Commons Attribution License (CC-BY).


Keywords: Motorcycle, Leaf Spring, Composite

Abstract:

A leaf spring is a very simple type of mechanical spring which is commonly used for heavy duty suspension systems. In single-track vehicles, such as motorcycles or bicycles, the coil and air spring are most widespread as well as current state of the art. However, the application of a leaf spring is nothing new on these types of vehicles. Various design approaches can be admired in museums. A century ago, it was even more common than the coil spring, but history shows us that this spring system has gradually been replaced due to its inherent disadvantages. So why should we deal with it again now? Because we believe that the leaf spring is not yet old news and has the potential to improve current vehicles. In this document, we want to introduce an alternative leaf spring design and the associated benefits. One that at the core is old and simple in form but utilizes new approaches and technologies to meet the demands of modern motorcycle and improve riding behavior.

Introduction

If we observe the early days of motorbikes, namely 50 years ago and earlier, we will notice that a lot has changed since then. If we now only look at the last 30 years, we will notice that although there has been progress in the details of individual parts the basic concept has been consolidated. Figure 1 shows three motocross vehicles from the last three decades. The basic concept of the chassis has remained largely the same. At the front the telescopic suspension fork and at the rear the wheel is connected to the frame via the swingarm. The shock absorber with spring sits either directly between the swing arm and the frame or is connected with a lever mechanism. In order to continue improving year after year, the focus in recent years has been on optimizing the details.

Figure 1:KTM 250 SX 1993 (left), KTM 450 SX 2003 (middle) and KTM 450 SX-F 2023 (right)

The motivation to deal with this topic is that the degree of optimization is now very high, and the improvement steps are becoming smaller and smaller. That is why we have been considering an overall different spring system. Our hope was to find a system that would more easily meet the set requirements from the ground up. Surprisingly, we ended up with an idea for a new type of leaf spring. Applications of leaf springs in motorcycles were plentiful in the past, but nowadays they have been replaced by the coil spring. However, this spring system has its merits and an earlier project by a competitor also shows that we are not alone in this conviction. Before we explain our approach, we would like to look at one existing design to better explain the differences.

Figure 2: Yamaha YZM 250 1993 (Oku & Sasaki 1993)

The companies Yamaha and Öhlins jointly developed a motocross bike with a leaf spring rear suspension in the early 1990s. The arrangement and operating principle are shown in Figure 2. The spring, made of glass reinforced plastic (GRP), was designed as a straight bar, and was located under the engine. Since it offered very versatile adjustment possibilities, it was a good system from a development perspective. The driving characteristics were certainly not bad either, shown by the fact that it was a winning vehicle. However, this system did not make it into series production nor did it last long in racing. Complexity and cost were a disadvantage and the durability was a problem. Stone impacts as well as touchdown due to reduced ground clearance damaged the spring and led to premature failure. It shows once again that for a new system to establish itself, it is necessary to think holistically. So not only how well it performs, but also that it has no major disadvantages in any area.

A new spring concept was needed to eliminate the systematic weaknesses of the common 3-point bending beam. Our approach: two attachment points, one at the top of the vehicle frame and one at the bottom of the swing arm. Both decoupled from moments by means of pivot bearings. This results in a very simple mechanical system, namely a double-jointed bar. This has two essential features. Firstly, it cannot transmit moments and secondly, it can only transmit tensile or compressive forces in the rod direction. The bar direction is the direct connection line between the two attachment points and has nothing to do with the actual shape of the bar. In our example, the actual leaf spring (shown in red) is placed approximately parallel to the line of action in the direction of the gearbox housing. The connection between the spring and the mounting points is made via aluminum clamps, and an adjustment mechanism is integrated in the upper mounting to set the spring preload or spring rate. All these parts together make up the spring system and close the circle to the simplified mechanical element, the double-jointed rod.

Figure 3: Schematic diagram of the leaf spring concept (left) and the simplified mechanical element (right)

In principle, simple concepts are preferred to complex ones. However, the difficulty is usually to meet all requirements without compromises in terms of performance. Especially the requirement of a progressive force curve was a difficult and sometimes insurmountable hurdle for other concepts, like the torsional bar. In current off-road vehicles, the progressive force curve is a basic requirement in order to offer sufficient comfort with low vehicle excitations and at the same time sufficient reserve with large excitations, e.g. when landing after a jump, and to prevent the chassis from sagging. On current motocross vehicles, this progressive force curve is realized with the help of an elaborate extra lever mechanism, as shown in Figure 7.

In our concept, a lever mechanism can be omitted because a continuous progressive course is achieved in the spring system itself by a simple constraint. Figure 4 explains this. The figure shows the two extreme states of the spring system. Fully extended (blue) and fully compressed (red), where the two systems are aligned according to their effective force line. In the vehicle, the spring system would rotate around the upper attachment point due to the compression movement. For the sake of simplicity, this rotation is omitted here. The spring compression movement at the rear wheel leads to a reduction of the distance between the spring attachment points Δl. The displacement of the spring leads to a deformation and respectively to a reaction force in the spring system, whereby mainly the spring element is deformed due to the large differences in stiffnesses of the leaf spring and the connecting parts. The bending moment that occurs in the leaf spring can be described by a simple relationship. Namely, the spring force multiplied by the distance to the line of action.

With this formula, the acting bending moment can be determined very easily at every point of the spring. To illustrate how the progression occurs during the spring compression movement, two freely selected points P1 and P2 are defined along the spring contour in Figure 5. Point P1 is in the lower area of the spring and point P2 in the upper area. The deformed state of the spring with the corresponding points P1` and P2` is shown slightly reddish. At the lower end of the spring the distance to P1 remains constant during the compression movement a_p1 = a_p1`. Here, according to formula 1, there is a linear relationship between spring force and bending load. In comparison, the situation is different for P2. Due to the protrusion of the spring over the upper attachment point, the upper part of the spring deforms in the direction of the effective force line and the distance becomes smaller a<sub>p2</sub>`< a_p2. This means that due to the deformation of the spring, the spring force F_Spring increases disproportionately compared to the bending moment M_b. In other words, the bending line of the spring is a result of the external load, and the deformation of the spring is decisive for the progression. This effect is the key to the force progression without the previously mentioned lever mechanism.

Figure 4: Leaf spring assembly in fully extend (blue) and fully compressed (red) position.

How can the force curve be influenced for tuning a vehicle? The first and most important parameter is the basic shape of the spring. If you change this (e.g. more or less protrusion upwards), you can have a great influence on the characteristics as described above. Next, the cross-section of the spring should be mentioned. Our spring has a rectangular cross-section of about 12x65 mm. A larger spring cross-section is harder to deform and offers more resistance. In order to make it easy to manufacture, the spring cross-section should have the same width over the entire length of the spring. A greater variation of the spring thickness is possible by inserting short layers (UD layers that do not run the entire length of the spring) and is quite common. However, there is always a small weak point when a layer ends and should be avoided if possible or not absolutely necessary. If we look at the deformed state of the spring, the lower and middle areas of the spring are largely parallel to the spring's effective line and a similarly high bending load occurs as described above. Therefore, a variation of the spring's cross-section is not necessary. Another parameter influencing the force curve is the E-modulus. The fiber volume content, i.e. the ratio of how many fibers and resin are used, influences the E-modulus of the composite material. However, the manufacturing specifications must also be taken into account considered here.

Figure 5: Force curve at the rear wheel

Finally, the integrated adjuster should be mentioned in the explanation of functions. In the current stat of the art, it is common to be able to adjust the spring preload to adjust the height of the vehicle. This has an influence on the geometry and subsequently on the handling of the vehicle. In order to achieve neutral handling for both heavy and light riders, the coil spring can be pre-tensioned on the shock absorber by means of a threaded collar mechanism. Figure 3 shows the adjuster in our concept which is similar to the current system, a thrust piece with thread. The big difference, however, is that with this arrangement the spring rate can be changed in addition to the spring preload. Until now, the spring rate could only be changed by replacing the coil spring but can now be adjusted continuously simply by turning a knob. This is based on the same principle as described before, that the spring is simply placed further away or closer to the spring action line by the adjuster. If the adjuster is parallel to the spring's effective direction, only the spring preload is changed. If, on the other hand, the adjuster is placed at an angle to the spring's effective direction, the stiffness of the entire system changes as described above. In our application, the mechanism is mounted at an angle of about 28 degrees and there is a superposition of spring preload and spring rate.

Let us now turn to the consideration of weight savings. Figure 6 shows an overview of the spring weights. The percentages in the bars refer to the reference (OEM spring made of Cr-Si alloy spring steel). In the overview it can be seen that high-strength materials, such as special steels or titanium, can significantly reduce the weight of the spring. By using hollow wires or oval wires, a further reduction of the spring weight is possible and should be mentioned for the sake of completeness. In Figure 6, this was deliberately omitted as we did not have any physical prototypes with comparable spring rates available here and there are only theoretical weight values.

Figure 6: Overview graphic of current spring weights

All the options mentioned so far have one thing in common a reduction in weight is more or less "bought" by a more expensive material or by more complex production. There is nothing wrong with that. The fact is, however, that although these options have been known for some time, they usually do not find their way into OEM applications because they usually do not perform well enough in the cost/benefit analysis. This becomes particularly clear with the GRP coil spring. Here, the basic materials are comparatively inexpensive to obtain. However, the processing procedure to position the fibers in the component in a way that is suitable for the load is very complex and this is reflected in the high production price. This is also the big difference to leaf springs. The layer structure is very simple and consists of unidirectional layers that are all aligned in the longitudinal direction of the spring. Our prototypes were produced using the prepreg process. The spring has largely the same cross-section and no short layers (or outgoing layers) are necessary. Furthermore, a base material with a large semi-finished thickness can be selected, which in turn results in a simple and inexpensive layer structure.

Weight reduction is of course always desirable, but in current vehicles the installation space and the arrangement of the components is at least as important. A critical area is currently in the middle of the vehicle (shown in Figure 8). From the rider's point of view, a slim silhouette is desired in this area. At the same time, however, one wants a large airbox to supply the combustion engine with sufficient air. In addition, the exhaust is located here and, due to the proximity to the overall center of gravity, the battery and electrical components are placed there. And last but not least, the shock absorber including reservoir and spring is also located in this area. Due to the coaxial arrangement of the coil spring and the shock absorber, the spring appears to be integrated in a very space-saving way at first glance. On closer inspection, however, the wire diameter of the spring defines the distance of the gas reservoir to the damper tube, which makes the shock absorber very wide exactly where the installation space is most limited. The reservoir contains the necessary gas volume to ensure constant damping characteristics. Previously shown spiral springs made of titanium or GRP are lighter, but due to the lower modulus of elasticity, a larger wire diameter is necessary to achieve the required stiffness, which further exacerbates the installation space problem.

Figure 7: Side view of a KTM 450 SX-F 2023 (left), without cover parts (middle) and the rear shock absorber (right)

With the leaf spring concept, the coil spring is completely omitted, and this means that the gas reservoir can be arranged directly on the damper tube. A coaxial arrangement of the reservoir above the damper tube is then also applicable. This allows a big step forward in terms of space for the engine components, which is not only advantageous for the power delivery, but is also important to meet the noise requirements. As described before, the deflection mechanism is no longer necessary. The shock absorber is placed directly between the swingarm and the frame while the leaf spring is responsible for the progressive spring rate. Eliminating the baffle leads to an increase in ground clearance, which is an advantage for this type of vehicle when driving over large obstacles such as stones or tree trunks.

Compared to steel springs, another advantage is the corrosion resistance, durability and emergency running properties. Our years of experience have resulted in very few spring fractures. However, analysis shows that corrosion is very often the cause. Unfortunately, the applied spring end is a systematic weak point, as the paint is ground away here and the spring begins to rust and is weakened in this area. In combination with the mechanical load, this leads to breakage, which usually happens very abruptly and without advance notice. The leaf spring made of fiberglass does not rust and in case of any overload it has a very good breaking behavior. The leaf spring consists of a large number of small glass fibers. In the event of an overload, the fibers with the highest load will break first and the stresses will automatically be transferred to the lower loaded fibers. This leads to a gradual fracture behavior and no abrupt drop in force, which in turn can increase driving safety.

Compared to steel, GRP has a high degree of damping inherent in the material. Experience from the automotive industry has shown that this internal friction, the reduction of the unsprung mass and the change of the natural frequency has a positive effect on driving comfort. (Gardiner 2019)

When developing a new vehicle, the tuning of the lever mechanism is an important adjustment variable. There is no clear answer to the question of how much progression is necessary in which range of the stroke, and it varies according to the subsequent area of application. Due to the coaxial arrangement of spring and damper, overall characteristics could be influenced by the lever mechanism up to now, but spring and damping characteristics were always changed at the same time. The optimum characteristic of the spring differs from that of the damper, so the aim has always been to find the best possible compromise between the two. With the leaf spring, the spring and damper are separated, and it is for example possible to combine a strongly progressive spring with largely linear damping.

Conclusion

As an Engineer the focus is usually on improving the performance of a product. For a successful series production implementation, however, many more factors are important and must be considered early on in the project. It was very important for us to take a holistic view, considering the manufacturability and economic aspects as well as the effects of serial tolerances, the assembly process and the serviceability at the dealer or customer. If you look at existing concepts and evaluate the riding performance in isolation, some of these systems have high potential. The fact is, however, that many of these concepts have not been implemented, which is usually due to factors other than performance. The goal was to learn from the past and do it better.

In this paper, a new spring system with a leaf spring was introduced. The mode of action is not a 3- or 4-point bending beam, as was usually the case before, but a mechanical double-joint bar in the form of a bending beam. This simple approach is at the heart of this concept and is the cornerstone for meeting all the necessary requirements. The leaf spring is therefore not yet old-fashioned but offers the possibility of re-sorting a motorbike with all its components and achieving an improved overall package.

Figure 8: Overview of influencing factors for concept decision

References

Gardiner, I. (2019). Kordsa works with Ford Otosan to develop composite leaf springs for heavy trucks. Gardner Business Media Inc. https://www.compositesworld.com/news/kordsa-works-with-ford-otosan-to-develop-composite-leaf-springs-for-heavy-trucks

Oku Y. & Sasaki K. (1993) Rear wheel suspension device for motorcycle (JPH05178264A). Japanese Patent and Trademark Office. https://sobj.orbit.com/sobj/servlet/get_pds/JP05178264A.pdf?userid=ORBITRS&type=0&pdfid=123108617&ekey=1285