I certainly understand how you feel! But here's a few things I'd like to add to the conversation.

Your mom's money is her own, and she is under no obligation to loan it out, and doesn't really even need to make excuses. If she doesn't want to loan you bike money, that's the end of it. Whining and arguing will only make you seem childish, which will enforce her ideas about your not necessarily being mature enough to be trusted to pay her back or drive a fast vehicle safely.

On the other hand, I don't necessarily agree with the "Buy what you can afford" people, either. You will just feel as if you've settled for something you don't want.

I say, keep plugging away and sack away every penny until you can afford the bike you really want! Firstly, you will take more pride in owning your bike knowing that YOU paid for it with no help from anybody....secondly, the ability to postpone gratification is a sign of maturlty, and the fact that you waited and saved up the money yourself will probably impress your mom a lot more than promises that you'll pay her back.

(And the fact that your bike cost so darned much is likely to help convince your mom that it should be kept safely indoors! )