Can lightning be attracted to you if you are holding something metal? Can you die?
get hydraulics yo! its really fast the whole car can jump a foot in the air. you could get going about 30 the hit it, and clear a speed bump completely. otherwise, i dont want to hear about your big problem with you Mercedes touching the ground. my geo hits bumps too.
Talk to your parents about knocking first. In some situations they need to know what your up to, but you deserve a little privacy at least that much privacy. As far as a lock goes you really should just get an actual lock and put it on. Locking doorknobs are not expensive or difficult to install. If you can't do that I have one good idea, but it won't work if the doorframe is solid. If there is a separate piece of trim inside the door you can stick a butter knife under it so that it also sticks out in front of the door. Be careful though. If the trim is not nailed on tight it might come of when someone tries to come it. If it is thin it will probably break, and if they really want in this won't stop them. Good luck.