Ive been 45 on a bike, no motor. No im not lying, and no i dont care if anyome believes me. I was young, dumb and riding on a state highway early in the morning, long down hill grade pedalling for all i was worth, in the fastest gear, with a tail wind. I couldnt even see the speedo until i got it home in the light to see the highest speed saved. I will tell you this, it was scarry and fun. And loud. Very loud. I can tell you that i realized i was going too fast because it FELT wrong, it was just too much for that bikes equipment. Even with moderate upgrades a bike should only be taken so far, after that, you are building a small displacement motorcycle for all intents and purposes. Even if its still technically a bike, physics is physics. Agreeing with the OP, its definitely not something that should be done with a cheap bike, frame, wheels, hubs or spokes, or brakes. Speed is all good and fine, but if you do it wrong, bad sthings can happen.