ENGINEER SOLVES MYSTERIOUS FAILURE OF MOUNTAIN BIKE COMPONENT DESIGN USING ALGOR FEA

The Rond mountain bike in action.

June 16, 1998, Pittsburgh, Pennsylvania - Professional mountain bike riders must develop a strategy for how best to maneuver through courses of steep hills, boulders and ravines and do it faster than their competitors. While the best cyclists are skilled, experienced and physically fit, they also depend on reliable, lightweight equipment. Rond Products B.V. of Ingen, Holland, creates quality mountain bike components to help the best cyclists cross the finish line first.

Rond Products develops components made of aluminum and magnesium alloys for top European bike manufacturers and riders, including Koga-Myata, Focus, La Pierre, American Eagle and Vario, and the first Dutch Olympic gold winner Bart Brentjes. Rond Products uses physical prototyping in conjunction with industry and manufacturer guidelines to optimize its bike component designs; however, recently they enlisted the expertise of Gerhard Bosch, president of Bosch Engineering.

Based in Oldenzaal, The Netherlands, Bosch Engineering specializes in product development using Unigraphics Solutions' Solid Edge CAD system and FEA software from Algor, Inc., Pittsburgh, Pennsylvania. The firm was asked to model and analyze a new front fork design.

Before the computer analysis of the design, Rond Products tested a prototype of the front fork made of a magnesium alloy. Cracking occurred under normal loading conditions after only a few rough rides. Because magnesium had not been used for this purpose previously, it was unclear whether the yielding occurred due to high stresses at normal loading or the stress corrosive properties of magnesium. The critical loading type also was mysterious: was it braking, horizontal/vertical loading of the wheel on the fork or internal pressure by damping?

Converting the Existing Design to CAD and FEA

To solve the problem, Bosch modeled the front fork as a 3-D solid model in Solid Edge and then transferred it to Algor using Houdini, a CAD-to-FEA solid modeling tool. He optimized the model's surface mesh to create a finer mesh around small holes located at the brake and front wheel connections and then created a solid brick mesh with Houdini. The final model had more than 12,000 nodes and 20,000 elements; 75 percent of the surface mesh was eight-node bricks.

A finer surface mesh was applied around critical areas of the front fork to obtain more detailed, accurate analysis results.

"I use Algor's Houdini to automatically create solid meshes so I can focus on improving the overall design instead of spending hours meshing a model," Bosch said. "Algor produces high quality brick elements that I find to be more accurate than other element types."

Next Bosch had to account for the loading applied by the brakes and the horizontal/vertical loading of front wheel, which are connected to the front fork in actuality, but are not part of the component. To do this, he used beam elements to guide loading to the front fork. Bosch also applied pressure loading using separate load cases in the X-, Y- and Z-directions to the inside wall of the front fork to determine internal pressure.

Technical information about mountain bikes and their loading and analysis methods was obtained with the assistance of the research and publications of the Arbeits Konstruktionstechnik II of the Technical University of Hamburg-Harburg.

Solving the Mystery through FEA Analysis

Bosch conducted a series of linear static stress analyses to determine the cause of the cracking. His first run showed areas of high stress beyond the critical failure point at nodes where normal loading was applied. Bosch performed four iterations of his first design, making modifications to the shape of the design and wall thickness.

"It was difficult to optimize the design without changing the actual look and weight of the part," Bosch said, adding that sleek, durable and lightweight parts are a requirement to be competitive in the bike manufacturing industry. "It's very important that the complete front fork weigh no more than 1.5 kg. If a customer sees that your part outweighs another part, even by five grams, they will likely choose the other part."

High stresses, indicated in red, on this brake attachment result from loading applied by the brake assembly, which is not part of the front fork design.

The analysis determined that the cracking experienced in the initial prototype testing was due to the part design, not the material properties of magnesium. The critical loading type was primarily a function of braking and horizontal/vertical loading of the front wheel. The internal pressure had little bearing when considered in the overall design, according to Bosch.

After completing the stress analysis, Bosch performed a fatigue analysis using unit stresses established through Algor. The analysis determined the frequency of maximum peak stresses over a defined period of time. According to Bosch, determining fatigue is extremely important because of safety concerns.

The entire front fork analysis design.

Speeding to the Finish

Rond Products is just beginning the final prototype testing phase of the new front fork design and is finalizing the permanent manufacturing mold of the part ahead of schedule. According to Bosch, Rond Products' confidence in the Algor analysis results is speeding up the final design process.

"Because they (Rond Products) trust the analysis so much, they have chosen to create the final casting without producing a temporary mold," Bosch said. "In August, the first series of front forks will be tested in the field."

Because Bosch could make quick changes to the design using Algor FEA, he was able to produce a realistic, accurate representation of the actual part roughly 10 times faster and cheaper than Rond Products could produce a physical prototype. This shortened Rond Products' design process and time-to-market. Further, Bosch was able to optimize the design without increasing the product weight or compromising strength. The end result: a safe, lightweight and durable product that a professional cyclist can rely on from start to finish.

The front fork prototype.