There is nothing more demanding than lateral frame flex when designing a tandem. If you've ever experienced two people out of sync on a tandem its really quite un-nevering.
Frame flex is one of the key design parameters we consider when we designed the ZXT. We carried over the design criteria to the ZR. There is a real trade off to having the lightest bike and an efficient one. You can design the lightest bike in the world, but its almost guaranteed to be a noodle, when it comes to hammering up a hill. Further that light bike isn't going to last a life time either.
The above FEA deflection plots are rear views of the ZXT on the left and a titanium tandem on the right at a scale of 25 times normal. The titanium tandem design is based on the available literature and the tubing specifications from Ancotech, one of the largest US titanium tube suppliers to the bicycle industry.
The ZXT and Ti frames have exactly the same geometry in regards to seat and head tube angles, and the loading conditions are the same. The loading conditions are in general terms two large people who are stalled on a hill standing on the cranks as hard as they can, but we've put them on the seats to increase the frame loads.
Typically we hear some body say "it feels like a noodle", the FEA deflection plots let you see what these people are talking about. Those shiny unpainted Ti-frames are not always the best choice of material for a frame. For one thing, titanium has a lower modulus (16 MSI) in comparison to carbon (33 to 45 MSI). The Ti-tandem designer is at the mercy of the tube manufacturer and has to design around available tubing. In the composite design world its very easy to add a few more plies here and there to stiffen things up.
YOU WANT YOUR BIKE TO GO FORWARD! Lateral frame flex robs you of your limited supply of energy to propel the bike forward. You do not want your physical effort going into flexing the frame laterally. You push on the cranks and you want the wheels to turn, not flex the bike sideways.
We call it :
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