Compliant mechanisms in suspension frame design?

Seems like manufacturers are already dabbling in this arena. From flexy seat and chain stays to compliant carbon layups that are flexier in one direction than the other. I do wonder what the limits are though. Complaint mechanisms as my non-engineer brain understands them, are predominantly used as hinges in light or non-weight bearing systems. 

Spot bikes has been dabbling in this for a while as well, they use a composite leaf spring instead of a second link/pivot in their full suspension design. I'm a composites guy professionally and i've spent a few years working at companies that make compliant composite mechanisms for space structures (fancy hinges and booms like daneb referred to). Based on my experience in that arena, as well as what I know about how bikes are currently designed, manufactured and the margins on their sale (used to work in a bike shop) i'd venture to say we are a ways out from the bike industry functionally adopting the use of flexible composite mechanisms in place of "traditional" pivots w/ bearings and axles. The costs associated w/ properly engineering completely flexible mechanisms w/ adequate safety margins (requisite for bikes) are rather large due to the cost of those specialist engineers and the specialist software required to actually model large deformation, elastic structural behavior. On top of that, the bike industry would need to make a switch to higher end carbon fiber prepregs to facilitate the desired structural behavior as your standard T300/T700 carbon fibers aren't strong enough (they have a relatively low strain to failure compared to other fibers, namely IM7, IM10, T1000, T1100 etc.). Some companies currently leverage those higher end fibers, but they do so sparingly. Flexstays are probably here to stay (pun intended), and they are indeed a first step into the use of flexible mechanisms, but I wouldn't expect leaps and bounds of advancement from them anytime soon. For most bikes companies, the cool factor juice won't be worth the financial squeeze. 

Spot's flex plate is titanium (unless they changed it recently). Do you see any potential in unidirectional flex joints made out of alloys, like steel or titanium? Would such a design solely rely on local tubing geometry, or is there any way to get metal grain alignment to promote flex in one directional plane and not another? Is there better vocabulary to describe the question I'm asking?

Interesting, maybe there was a change at some point in the past, I could have sworn the "OG" Spot Living Link was composite, I think it still is on their 115mm travel XC bike but it does appear the living link is titanium on their Mayhem bike. Anyway, I assume when you say unidirectional flex joint you mean a joint that is compliant in one direction but not in the other directions. If my assumption is correct, i'd guess that it's difficult to do that in a meaningful way without compromising stiffness in the other directions for a given frame weight spec, or, compromising weight and aesthetics for a given stiffness spec. Frames will always be driven by some target weight requirements, and they've always been more akin to fashion/clothing than other sporting goods, so aesthetics are a big deal. OG roadies get real fired up over that stuff haha. More reasons why I think we won't see anything fancy in the near future. As far as your question on metallurgy goes, i'm a fibers/plastics guy and not a metal guy so who knows hahahahaha. I'm not aware of anyone doing local stiffness tailoring of terrestrial (i.e. not in space, subjected to gravity loads) metallic structures in that way, you usually change the shape of the thing before you change the material microstructure as it's much easier to design, analyze, produce and implement. Fiber reinforced composites are unique compared to metals in this regard, where you have an "intermediate" knob you can turn between just tube shape and full on microstructure change. That "intermediate" knob is changing ply materials, thicknesses and orientations. Also know as the "black magic" behind composites.

Some more fodder - I recently crossed paths professionally with an industry veteran who has done composite design and analysis for a bunch of bike companies (he still does it now), most of which you have definitely heard of. He and I nerded out about frame/layup tuning for a little while one day and what he communicated to me was that most of that tuning happens on road bikes. Frame loading is (insert my opinion now) better defined on the road side of things than it is on the MTB side of things, so engineering localized compliance into road frames is easier to accomplish for a given amount of safety/failure margin. They also have thinner wall thicknesses than mountain bikes which opens the door to using fancier, thinner prepregs which again, enable you to tune the bike much more. Mountain bikes are tricky because the wall thicknesses are way larger and the layups more complicated - sure you could throw that thin fancy prepreg in the downtube of a Nomad or an Enduro, but now it takes 2-3 times longer to make a frame than it did before, and your average Larry McJerry trail rider won't feel the difference but he will certainly complain about the increased cost passed onto him from the frame manufacturer to account for the costlier materials and added mfg. time. Bikes are business, and business ain't boomin like it was a couple years ago. Big investments in new technology to squeeze out small improvements are probably going to slow down for a little while. Also, no shade on Larry McJerry, i'm Larry McJerry most days hahaha. 

I've been down a whole bunch of rabbit holes looking at ways to apply compliant mechanisms to a suspension layout. Countless hours in linkage, numerous scientific papers read, and much daydreaming. 

Some applications. 

https://www.instagram.com/p/Cq2pj7SJ-Xb/

Two pivots replaced with the flex of a tube. 

https://www.instagram.com/p/CtPAC95p1qy/?g=5&img_index=1

3d printed flex dropouts

https://www.instagram.com/p/CeT-oDVBcpN/?img_index=1

Possible application would be to replace the slider mech on Arc8 with linear motion flexures. 

The best optimization is where you can limit down the degree of movement needed or the distance travelled. The lower a bearing would rotate the higher it's probability for replacement with a flexure.

Whats old is new again! 

https://theradavist.com/1998-ibis-bow-ti-mtb/

Keith Bontrager made a prototype FS bike with a composite main flex/pivot in the 90s. Parent company Trek/Fisher started using flex stays in 2000 on the Sugar and then the first Fuel.

The one in the background has the flex pivot.