Building a safe bike is our top priority. Some things are straightforward such as fitting efficient brakes and choosing good tyres. However, being sure a frame and fork design is strong enough to survive a lifetime of hauling cargo, takes a little more thought.
With this in mind; we've used some computer wizardry to measure stresses and deflections of our box bike frame.
We took our CAD model and applied some forces to simulate some cargo and the rider weight. In this case a rider weight of 130Kgs as a point load on the seat post and an Uniformly distributed Load (UDL) of 70Kgs along the load area.
Finally we placed reaction (supporting) loads to represent both axles.
The computer did the maths to indicate expected deflection of the frame and also highlight any stress concentrations. This process (known as the Von-Missers stress) takes the ultimate tensile stress of the material and shows where that stress might be exceeded. Simple and clear.
The results were good. Our step over cross bar was shown to take some strain from the bottom bracket and bracing at the front end was keeping things in check. Of course none of this simulation is a substitute for empirical testing, but it is an indication that we are on the right track.