[ajuc]

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 :)

[LeifCarrotson]

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.