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Old 07-10-18, 04:39 AM
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PMK
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Location: Royal Palm Beach, Florida
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Bikes: 2006 Co-Motion Roadster (Flat Bars, Discs, Carbon Fork), Some 1/2 bikes and a couple of KTM's

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[QUOTE=merlinextraligh;20411753]
Originally Posted by OneIsAllYouNeed View Post
Tandem frames made of steel, titanium, or carbon fiber can all be made "whippy", overly stiff, or somewhere in between. Historically, many titanium tandem frames have been on the whippy end of the spectrum. Tubing and couplers weren't readily available in sufficient diameter for boom tubes; chainstays were the same as single bikes; bottom bracket standards limited space in the most critical location. Comparing a modern, well thought-out Ti frame to an older design, there are substantial differences in lateral stiffness. All the Ti frames mentioned in this thread -- Santana, Granite, DaVinci -- shouldn't disappoint on lateral stiffness. I haven't ridden a Calfee, but I've piloted quite a few Ti tandems. Ti tandems can definitely be made stiff enough for two powerful 200lb athletes; they can also be made to mimic a vintage noodley frame. A good builder should be able to make the bike feel the way the client wants.[/QUOTE]


Not at the same weight. You can make a Ti Tandem as stiff as Calfee Dragonfly, but it would weigh a lot more. Carbon Fiber has a stiffness to weight ratio 4 times higher than TI:




To build a latereless open frame Ti tandem that approached the rigidity of a Calfee Dragonfly with the extra stiffness option, you'd be adding some seriously heavier tubes.



There's a reason that Ti has pretty much vanished from the high end racing market.
These charts and graphs are sometimes cool to see.

I enjoy the way the carbon is so superior, or is it...This chart is in regards to tensile strength, merely one stress load direction induced into the material.

If you find charts representing compressive strength, the carbon will be whale turd low compared to the others.

Carbon is a wonderful material when utilized in specific load directions or stresses. In order for a carbon fibre flat sheet to have what is known as balance, equal tensile strength for all directions, in the ply layup, assuming we are using woven or unidirectional fibres, we must use many plies. At a minimum it will be 4 unidirectional plies at 4 different and specific angle orientations.

To build a carbon flat panel or shaped part and not have induced stresses from succesive plies, the ply stacks must be laid symmetrically.

So, with carbon, or actually most stacked ply composite layups, to mimic and compare to metallics, the composite must have balance and symmetry, or as is commonly referenced, quasi-isotropic.

One of the true great features of carbon composites, when designed properly, is not so much the lightweight, but more importantly, the fatigue resistance.
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