Mountain Bike Company Uses FEA to Design Fast Bikes Faster

By Joe Graney, Engineering Director, Santa Cruz Bicycles

Santa Cruz Bicycles makes best-in-class, fully suspended mountain bikes.  Located in Santa Cruz, CA, the company was started to make suspension bicycles more efficient to ride uphill and faster downhill without sacrificing durability.  Now we are using FEA to analyze stresses and design bike frames that won’t break under brutal competitive stresses.

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FEA images display areas on bicycle frames where test lab failures corresponded exactly to the analysis results, allowing frame weaknesses to be corrected. Image shows peak stresses near the head tube when loading the fork from the front.

Many of our bikes are ridden in competitive events.  They must be lightweight, yet strong and durable.  Manufacturing facilities are in China and Taiwan, but the bikes are designed in California, using five PTC CAD products, including Wildfire 4.0 MCAD and Pro/Engineer Mechanica for FEA.  Loading is calculated with Pro/Mechanism, then stress analyzed with Pro/Mechanica.  Pro/E Mechanica analyzes native Pro/ENGINEER models and stores the analyses in the model files. This means no data translation, and data management is streamlined.

At Santa Cruz we have a battery of tests in our domestic factory for testing strength, toughness and fatigue life of our frames.  These tests evolved over many years of selling aluminum frames, and each frame made must survive this battery of tests before it can be sold. Traditional aluminum frames tested with the new impact tests became the “baseline” for the new composite frames.

These calculations are intricate, because they don’t take into account the complexity of the major mass on the bike (the rider) and how energy can be absorbed during impacts with the rider.  Nevertheless, all frames have to pass these tests.  So we use Mechanica to test the frames “on the desktop” then compare the results with the physical texts we observe.

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Stress on the top tube resulting from front loading of the fork.

Doing FEA on aluminum welded frames can be a delicate exercise.  First a merge model is built from the assembly so the tubes can all be integrated into one another without little shards where the tubes meet.  Then some rounds are inserted at tube junctions to “model” welds.

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This image evaluates the impact of the angle of the cut at the base of the seat tube insert on peak seat tube stresses. This image specifically shows peak stresses in the seat tube, modeled with a solid seat post used in the test rig.

But even when you do all that you don’t have varying weld profiles and penetration like real world welds.  We find that Mechanica is able to work with our long thin-wall tubes well because we can set the mesh to start with a small average size.  Meshing very thin wall (0.8 mm) tubes up to 750 mm long can be problematic for some packages, but Pro/Engineer actually has a setting that enables the meshing and analysis on a complete frame to go very quickly.

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Design exploration of the impact of modifying the angle of the seat tube insert on peak stresses in the seat tube.

With the new carbon frames, we are able to replicate the loads in our tests by doing a simple force balance using the mechanism function in Pro/Engineer.  We feed those forces into Pro/Mechanica, which does the FEA.  Then we communicate results to our manufacturing partner’s R&D engineers, who use the analysis along with other factors to determine how/what/where to locate individual plies.  The images show different areas where we had test lab failures that corresponded exactly to the analysis results, and were then corrected.

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Another design exploration evaluating the impact of seat tube insert angle on peak stresses in the seat tube.

We were able to try six different scenarios for machining/insert length combinations in a couple of hours using Mechanica, chose the best, and prototyped that — all while we had one of our engineers at the factory in China.  The revision he carried back passed testing, so we were able to go to production and hold the production schedule.

Results:
— Product design time shortened from 22 to 14 months
— Suspension design iterations increased from <10 to >100 per model
— Warranty claims decreased by 70%
— Distinctive look makes Santa Cruz unique in a competitive market
— First of a series of carbon fiber models being announced 1/20 — lightest and strongest on the market.

Santa Cruz Bicycles

www.santacruzbicycles.com/home/

Parametric Technology Corp.
www.ptc.com

::Design World::