Why are the front forks on a bicycle usually curved ?
Forks don’t have to be curved, but as you noted non-sus forks usually are. Trek, Giant and many other companies do make strait forks as well. The curve in the fork provides rake and a little suspension. The curve is also a nice design element. Both the curved and strait forks have a rake. Rake is the offset measurement from an imaginary line going through the head tube and the fork axle center. Along with head tube angle, the rake provides a certain ride or steering quality. The more the rake the slower, more stable the steering. The overall steering has gotten a lot faster in the last 20 years. The strait forks are actually bent forward, this provides the rake. On MTB sus forks the crown is offset forward, again providing rake.
