I would like to see you turn a bike with front-wheel chainstays. Straight-only driving, turns are obsolete! :D
Also, the front wheel is connected to the frame in a very different way than the back wheel: single strong tube, only diverging further on; and there is no requirement for the front wheel to be powered - therefore the front wheel can (and does) move WRT the frame, something the back wheel must not do. Yes, they're both wheels, but that's where the similarity ends; the rest of the requirements are very different.
Of course - note that this, too, requires a front wheel with two degrees of freedom (although it's possible to turn a no-steering two-wheel vehicle by leaning, the effect is minimal compared to one steerable wheel); front chainstays would prevent this.
It's the usual way to turn when using your hands too - in fact, you (subconsciously) turn your handlebars the "wrong" way when cornering on a bike (https://en.wikipedia.org/wiki/Countersteering). This is very counterintuitive and discussing it is a good way to start arguments...
Countersteering is becoming more well known know amongst motorcyclists (thanks to more rider training and the internet), but some riders go there whole lives without knowing about countersteering. This is unfortunate because learning to consciously countersteer makes you a much safer and more skilled rider. The same holds for bicycles too I suppose.
I thought that countersteering is only for motorcycles?
But then , cite "The entire sequence goes largely unnoticed by most riders, which is why some assert that they do not do it", end cite, is most probably true.
No. The effect is, of course, much more pronounced with motorbikes, given the greater mass and velocity - but anything resembling a bike can be countersteered.
Front wheel is attached to a thick tube that is almost vertical (weight tries to compress the tube), and that tube is attached to another thick tube throught a wide strong pivot that keeps the angle between the tubes constant.
Back wheel is attached to two thin rods at 45 degreee angle. Weight tries to push the seat down and push the back wheel away (to increase the angle). That's why we need another tube to hold the wheel at constant disntance (and to keep the angle fixed).
You could probably make it work by creating similar strong joint keeping the angle between seat tube and wheel rods constant for the back wheel, but why? Hm, maybe to make a bicycle with both wheels turning :)
Sorry for my English, never had to describe bicycles and probably all the techincal terms are wrong :)
Also, the "chainstay" provides rigidity against the tension created by the rider on the top half of the chain, and the force of the wheel pushing the bike forward, which can be prodigious!
PS your English is fine! Only noticed a throught->through typo.
It doesn't really _need_ it; the seat stay could be designed to be stronger, but there's more of your weight on the back wheel than on the front wheel, so the seat stay would have to be stronger and, hence, 'fatter' than the fork.
And, since the angle between vertical and the seat stay tends to be larger than that between vertical and front fork, the seat stay already would need to be a bit fatter (the fork is relatively more compressed and relatively less bent than the seat stay)
Also, the force you exert on the chain when riding the bike tries to bend the seat stay, too. That, too, would require a stronger seat stay.
However, there is a better (in the sense that it makes the frame stronger at the same weight or gives you a lighter frame of the same strength): the chain stay. It introduces a triangle in the structure, and triangles are strong.
Edit: implying: if the front wheel doesn't need it, why would the rear wheel?