Hardtail or Fully (Full Suspension Bike)? This is the crucial question within the cross-country (CC) and marathon mountain bike sector. Caused by the rigid rear end hardtails are stiffer and simultaneously lighter, however the rear suspension of a Fully provides comfort, downhill performance and traction. In order to offer an even better alternative to the ambitious racer, the FlexiFrame project goal is combining the advantages of both frame designs (see Figure 1).
To achieve this goal, the project team will develop a flexible rear end which doesn’t use any bearings and classic spring-damper elements. The foundation for this frame design is a newly developed hybrid laminate, which is a combination of carbon fiber reinforced layers and elastomer layers. This newly developed composite material allows to assign directional stiffness to the rear end, which, on the one hand, enables suspension and damping of the rear end to compensate for bumps, while maintaining a direct driving feeling. In a first step, different laminate designs for the hybrid material are investigated and the mechanical properties are determined. In this case, static characteristic values which are necessary for the following design of the frame are determined, as well as dynamic characteristic. By using elastomeric layers, the damping behavior of the hybrid material can be significantly increased - compared to a conventional composite material (see Figure 2).
However, by simply substituting materials, the challenges of the FlexiFrame frame design cannot be met. Therefore, in addition to the material analysis, a complete analysis of the frame design is carried out. For this purpose, a parametric beam model of the frame was developed (see Figure 3). In this model, the chain and seat stays are divided into four sections each. In addition to the stays, the model contains a leaf spring for absorbing the spring forces which arise during compression. In the different areas of the model, the geometric parameters as well as the material properties can be varied. The variation possibilities are determined according to installation space specifications and the previously determined properties of the hybrid material. The model is clamped in the area of the bottom bracket and at the end of the leaf spring. For the optimization of the design a constraint condition and different optimization goals are defined. As a constraint 25% of the maximum travel under static load is specified, which is a default suspension design for the CC sector. The optimization goals include the weight of the frame, a minimum tension in the area of the bottom bracket (usually the most heavily loaded area in the rear), and the torsional stiffness of the rear triangle. The parametric model is examined by using the optimization software LS Opt. Individual optimizations are carried out and on the basis of this a target function is created with which an optimization of all parameters is calculated.
In order to further ensure the adjustability of the frame to the rider, a highly flexible and fully automated process for driver-specific production is being developed. For this purpose, a tape laying head for towpregs and a molding tool have to be developed. This process will enable the production of hybrid laminate preforms for hollow components with small radii and strong curvatures.
The project FlexiFrame („Highly flexible hybrid composite rear for individual mountain bike frames; Design and process development for an individual mountain bike frame“ is funded by the Federal Ministry of Economic Affairs and Energy (BMWi) within the ZIM program (funding reference: ZF4052316RE7).
Torsten Heydt, M.Sc.
Design of Composite Structures
Institut für Verbundwerkstoffe GmbH
Phone: +49 631 2107 209
Tim Schmidt, M.Sc.
Impregnation & Preform Technologies
Institut für Verbundwerkstoffe GmbH
Phone: +49 631 31607 32