Mobility Scooter Chassis Optimization Considering Fatigue Strength According to FKM Guideline

| Technical Article

As part of this work, the chassis of an existing Merlin-Mobil wheelchair was optimized in terms of weight and payload, based on FEM analyses and evaluation of the fatigue strength in accordance with the FKM guideline.

This article shows how the S-Life FKM software from PART Engineering can be used to easily design and optimize the shape, material, weight and load capacity of an electric mobility scooter chassis in accordance with the FKM guideline. With obese users in mind, on the one hand the scooter was to be modified so that its maximum load capacity could be increased from 150 kg to 200 kg. On the other hand, the aim was to reduce the overall weight of the vehicle from 87 kg to a maximum of 65 kg in conjunction with other optimizations so that it can be transported on a standard car trailer hitch rack.

Load case determination and analysis of the current chassis

The first step was to determine which accelerations and forces are to be assumed in the event of a curb crossing at a maximum speed of 15 km/h and a ride over rough cobblestones. The latter load case served primarily as the basis for the fatigue strength analysis, whereby a payload of up to 200 kg was considered.

Based on these load assumptions, the current frame shown in Figure 2 was then analyzed regarding the static stresses and safety factors to be encountered. The current version of the scooter has already been in use for several years so that its operational stability was considered proven. Thus, the new frame was initially designed in such a way that the resulting safety factors at least corresponded to the pre-determined ones.

Development of a new frame concept

The design of an optimized frame variant was based on the results of a software-supported shape optimization analysis and ultimately resulted in a frame consisting of hollow aluminum profiles measuring 50 x 25 x 1.5 mm, which was reinforced in critical areas by insert profiles. It can be implemented either as an exclusively welded construction or as a predominantly bent construction (Fig. 3). For both variants, it was observed that the maximum stresses are in the range marked in Fig. 3. After the static strength verification had been provided, this region was therefore investigated regarding the fatigue strength.

Practical use of S-Life FKM in the context of frame optimization

In accordance with the handling of S-Life FKM, the frame shown in Figure 3 was also examined (Figure 4). Specifically, it was investigated whether a fatigue strength assessment result is uncritical if  a person, weighing 200 kg, sits down and stands up multiple times. For this purpose, only one load case had to be considered, which was included in the calculations with a stress ratio of R = 0.

In addition, a ride over cobblestones was examined for both 150 kg and 200 kg payloads.

Even if "stresses from special events or misuse", as is the case with the frontal curb crossing at maximum speed, are only to be considered in the static strength assessment in accordance with the FKM guideline, a fatigue strength assessment was also carried out for these.


Subsequently, for the individual load cases a fatigue strength assessment (N=106) was carried out, which produced results in accordance to Figure 5. Even if the limit stress amplitude (N=108) is decisive for the material used, the fatigue strengths were also examined for completeness.

Where the cyclic utilization ratio was higher than 1 (=100%), i.e. the fatigue strength was not given, the "finite life strength" assessment was used to investigate how many cycles can be tolerated. As expected, this was the case with the frontal curb crossing, as these are individual cases with massive stress peaks, which are usually only subject to a static strength assessment.

The frame meets all the usual criteria for fatigue strength. Also, no signs of fatigue are expected if a person weighing 200 kg sits down as often as required.

Regarding the frontal crossing over a curb at maximum speed, in the case of a person weighing 50 kg, no cyclic failure is to be expected even after 108 cycles, corresponding to the limit stress amplitude. In the case of a load of 150 kg, up to 1 million crossings can be carried out without damage; with a load of 200 kg, the number is reduced to N = 725,000, corresponding to the fatigue strength.


Overall, the S-Life FKM software was successfully used in the investigation of fatigue strength. By using the software, calculation errors could be avoided when the detailed and complex FKM guideline is considered. In addition, S-Life FKM made it possible to carry out an analysis covering every node of the model in a time-efficient manner, whereas manual application of the FKM guideline would only lead to selective assessments.

Author: Timm Piper, HTWG Konstanz - University of Apllied Sciences, Konstanz, Germany