answers for why the bike wobbles back and forth when standing on the pedals and working it So I read all these answers about the bike moving underneath the rider back and forth when standing on the pedals.  So how much is actually effective and how much is for "show" (looks cool)  I have a friend that ocassionally uses my bike and throws it violently back and forth when getting on the pedals.  I don't think he is strong enough to really radically make the bike do this but after the rides the back wheel becomes untrue and the brakes rub because the wheel is that much out of line.   I have to take it for the wheel to be trued.  What advice should I give him about properly riding on the pedals so that I don't have to have the wheel trued everytime?
I am fairly powerful on the pedals but I don't have that much movement.
The way he's riding is bad because it damages your bike. That in itself ought to be enough reason to stop. You could point out that all the effort spent pushing the bike from side to side is wasted; he could use he energy more effectively to push the bike forwards using the pedals. 

I suspect that if your friend was riding his own bike, he'd be less cavalier about his treatment of it.

I'd be inclined to say one of the following things to him:


"Please return my bike in the condition it was in when I lent it to you; either don't damage it, or get it properly repaired before you give it back", or
"You're not borrowing my bike any more"


Note that different types of bike are designed for different purposes. You've tagged this as "road"; we generally want road bikes to move forward efficiently, but BMX riding (for example) is very different. It sounds a bit like your friend isn't used to this kind of distinction.