So far our discussions have not resulted in cooperation on the plywood velomobile. But we agreed to analyze the possibilities for a Z-crank. Johan is interested in a Z-crank with chain wheel at the left side. Here you can see why. However the long path the load on the pedal travels through the crank to the chain wheel will cause large deflections of the crank. You can see this in the figure below.
|A finite element simulation of the Z-crank. The crank was modeled with tubular aluminum 35x28mm Dxd. The crank radius is 150 mm. A load on the rigth pedal of 1000 N causes deflections of 8 mm !|
I have googled material data of steel, aluminum and carbon-epoxy. The specific flexural modulus of carbon-epoxy is ~3 times higher than that of steel or aluminum (materials: steel, aluminum, carbon epoxy, densities: 8100, 2700, 1600 [kg/m3], moduli 210, 69, 125 [GPa], specific moduli 26, 25, 78 [Nm/kg]). Carbon-epoxy is the right material for this part.
Experience of others show that it can be done: Harald Winkler's 12 kg velomobile. Harald explains that his carbon crank design is at least as stiff as the conventional crank. In his design the pedal bearing has an inside diameter of 35 mm. This gives you some idea of the size of the crank.
|The carbon crank of Harald Winkler's 12kg Velomobile|
|Miles Kingsbury shows a Z-crank with a central chain wheel. The deflections of this construction will be much smaller than with the chain wheel aside.|