unfortunately people have little understanding of lateral flex, torsional flex and radial/vertical frex.
unfortunately, if i were to tell you that the primary purpose of that shape is to provide the frame with a dynamic rigidity and have some sideway (lateral) flex in some moments of the ride it would make many of you say i'm doped.
unfortunately most of you would have a headache if i were to explain how tensile strain affects torsional rigidity for the tube the tensile stress has produced the strain to.
so, maybe it's better just to describe the frame design as some engineering witchcraft that we should not investigate for fear of being struck by lightning bolts.
and we should not ever dare to say that lateral flex in some parts of the bike frame has some purpose to it.
right?...
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long story short but not too short:
with that frame design the seatstays flex under pedalling torque. the compressive stress they experience makes them not only more compressed but also further curved as to permit the chainstays to flex upwards. well, not both of the chainstays (and seatstays) in the same ratio but depending on which side of the bike you press down when pedalling. that happens to a greater effect when climbing out of the saddle. the (undectectible to the eye) flexing of both chainstay and seatstay make the rear triangle have a dynamic rigidity concerning the lateral flex and torsional flex. the chainstays are not as compressed as with the case of straight stays but are also expressing both flexural AND TENSILE strain and that, my friends affects the lateral and torsional rigidity of the tube. as a result the wheel will magically be in a better alignment with the rider's behind. the wheel/tire will have a smoother, better traction without you even knowing why. oh, but of course, you are obsessed with the inherent loss of power, right?... you wouldn't want a bike that was designed to have a variable lateral/torsional rigidity on the rear triangle, influenced by the pedalling torque, right? you'd be rather worried that the flex would make a slower ride, that you'd be robbed of power and not be able to ride with the champs or at least with the same gear as the champs.
important point: the design some dude had in mind was to have the rear triangle stiff enough when going downhill or when leaning into a corner but also having a slightly lower lateral and torsional rigidity for the rear triangle when pedalling hard. so, the rigidity is dynamic. the invention is not new, it's been around for a while. it started with hetchins. very few people cared for this mystery to solve or tell others about.
maybe the engineer that designed/copied the curved stays understood the trick but he certainly doesn't have much say as to what other marketing people tell you.
they just sell you the magic ride and abstain from giving that kind of information that would scare you away. they'd better let you think that rear triangle design offers you comfort instead of telling you there'd be a very, very little loss of power from frame flex but also the loss of power from the tire skidding on the road without you even knowing would be lessened to a greater extent. you would not believe that. so you are not told.
so, again, that rear triangle curved seatstays design provides very, very little comfort overall. it's all about smoothening the transfer of power to the tire and lowering the risks of skidding and accidents.
it is impossible to have satisfyingly high lateral and torsional rigidity of the main triangle tubes AND have too much vertical/foreandaft flex - with regards to the main triangle tubes. even if you would try to do that you couldn't. nobody can.
and again, rear frame triangle is a diferent thing. the frame design not always focuses on having maxed lateral and torsional rigidity for that part.
it's just cheaper bikes/frames gets more sold if people are indoctrinated as to believe the marketing buzz they are thrown in their face.
on good quality, heat treated steel and titanium alloys, flex is more of a concern regarding stability and NOT regarding fatigue or power loss/efficiency. you have to differetiate between the main triangle flex and the rear triangle flex. not only understand vertical, lateral and torsional but also main triangle and rear triangle flex.
people sell all sorts of books making you feel good. likewise, bikes are sold with a placebo effect. but if johnny were to sell you some well designed bike and be 'honest' about it when he tells you what reasons were those that made the engineer design it that way... screw it. he wants to sell the bike, not educate you. at least when he's on the job. almost everybody thinks this way. you either make the money you're supposed to make or you get dumped by the competition selling crap but being 'wise': forgetting about moral principles. seriously now, how many people do you know to afford 'keeping a conscience'?
which of these two would you rather like?
"four legs good, two legs bad"
or
"nomina rutrum rutrum"