Stress and how to avoid it!

The bikes built by us at Rodford are very much built by hand, the initial designs for our bikes begin life drawn out on envelopes and tea ring stained A4 paper. This is not to say that we don’t employ some modern techniques to check out engineering hunches and intuitive thinkings. Since we began, we have been feeding figures and each small detail of the frame into a CAD drawing and making some stress calculations using computer wizardary.

The XL single spar bread delivery bike we are building for Southvilles ‘Marks Bread’ is a one off, so before we built our constructors challenge entry we put the changes we were making to our ‘standard’  frame in to our CAD and applied some forces to simulate the cargo and a rider weights, and identified reactive (support) forces at both axles.

Simple enough. 

Then the computer did the maths to show us deformations and stress distributions across the frame.

The results were good. We already knew that introducing some more material in the form of our step over cross bar was shown to take some strain from the bottom bracket and this new analysis showed us that the additional spar bracing at the front end was keeping things stiff and flex in check. In addition to this our frame is looking pretty light – made from Spitfire grade chromolly steel (that’s a supermarine Spitfire not a British Leyland one) which has resulted in a frame that is light but strong. 

Of course none of this clever stuff is a substitute for a “build and test”, but it is a great tool to double check our design and indicates we’re on the right track. Meanwhile our prototype frames are in the process of being ridden around and generally abused by us to confirm the theory. 

Oh and always remember : Sigma = F/A


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