2nd Amendment Cyclist
Join Date: Oct 2010
Location: Cary, NC
Bikes: Schwinn 2010 World Street, Handsome Speedy w/ SRAM Apex
Sadly, many bike salespeople have you straddle the top tube, see that you have an inch or so of clearance, and call it a good fit.
You need a professional bike fitting. Be prepared to pay a hundred bucks or so.
To get a good fit, you need to properly position your saddle with respect to your pedals - that's where EVERYTHING starts, because pedal position is the one thing on the bike that can't (easily) be changed. Is you saddle high/low enough? Is it too far forward/backwards - not with relation to the handlebars, but to your feet. Saddle nose too far up/down? Sliding off or gripping on for dear life?
That step done... you start looking at your position with regards to the handlebars. Too far forward/back, too narrow/wide bars, etc...
As you go on, you may need special things to handle little irregularities of your particular body.... short leg? Maybe you need a shim under that shoe. Shorter arm? maybe you need to slightly twist your saddle from true center to subtly shorten the reach of one arm while extending the other. Maybe your legs bend oddly... and one or both pedals may need to be shimmed further out away from the center of the bike.
Once you've been riding regularly for a few years, you can probably dial in your own bike fit pretty well. But for a new rider, there's so much involved that it's just difficult to even understand what may be causing you discomfort.