In other words, you want to put training wheels on?
I think that you will find them unnecessary very quickly, but I guess if you feel you need the reassurance, they may work.
How are you at riding a bicycle? Any stability problems there?
Maybe your 650 is just too big for you?
Or maybe you just need to get on it and get going, the longer you wait the harder it gets. Go slow, stay out of traffic and develop your confidence.
