Kyoto Cruiser with 2 Adults and 1 Child

Joined
Oct 19, 2012
Messages
2,384
Location
Wakefield, UK
I would be very wary of using aluminium. When used in a regular bike it's fully triangulated so all the forces go through the tubes in compression or tension. With a Kyoto and with pretty much all recumbents you are using the frame as a beam with the forces being fed into the welds. Aluminium makes a poor beam with next to no give before failure. Also, unlike steel, when it does fail it will give no warning and fail completely whereas a failing steel frame would bend first or the weld would slowly peel away giving some warning of imminent catastrophe. Recumbents can and have been made from aluminium but it comes with substantial worries for a one-off builder, a fully loaded tandem probably more so than most designs.
 
Joined
Feb 28, 2022
Messages
12
Location
Indianapolis, IN USA
I understand your concerns and Aluminum certainly needs to be engineered differently than steel. You need to take into account the extra deflection as well as about 55% of your strength at HAZ from native 6061.

Not welding in key areas where sheer strength is needed to be max is one factor as well as increase size for stiffness. The failure modes for aluminum are cycle-based, where steel is near infinite cycles.

A trike designed with a 1.5x1.5 16G steel tube will have about 1/5 of the beam strength as a 2 x 2 .120 6061 tube. Static load analysis is coming in at around 6000 psi, which is far under even welded 6061 at 24,000. I'm seeing a 1/3 strength at about 10_000_000 cycles for the 6061 in bending fatigue.

So I understand the issues and believe I'm over-engineering the heck out of it. The only failure I'm concerned with is main beam dynamic loading cycles and working out if I need to add a lower truss for that. I actually like the extra challenge of this one. :) My measured deflection with near 600 lb point load is not concerning at this point
 
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