It is perfectly reasonable to be 5'6" and ride a 50cm bike. It all depends how the 50cm is measured

SADDLE FORE-AFT = is used principally to position your knee in relation to pedal spindle. This affects your centre of gravity, your core and power efficiency. You may have read somewhere your knee cap should be over the pedal spindle at a 9 & 3 pm crank position. This is just a general guideline and NOT an inviolable rule. Use that understanding as a starting point and fine tune accordingly.

A more forward position has the effect of making the the seat tube more vertical (closer to a triathlete's setup). Setting the seat further back makes the angle more relaxed. Generally speaking shorter people will like to have their seat more forward, while riders with longer legs and femurs (thigh bone) will prefer more set back in the seat.

At the correct position for you, you will not only feel more comfortable, but be able to generate more power resulting in less effort for the same speeds you used to ride, or better climbing on hills.


SADDLE HEIGHT.
A good foundation is to measure the height of your inseam from the floor while standing in your usual cycling shoes. Call this your inseam measurement.

INSEAM X 0.883 = DISTANCE FROM MIDDLE OF THE BOTTOM BRACKET TO THE TOP OF THE SEAT.

If the seat is too high, your hips will rock from side to side, and your maximum cadence will be less than if the seat was lower. If the seat is too low, you will lack power and joint injuries may occur. Using the above formula as a starting point, you can tweak it 1 cm way to settle on something that works for your unique measurement. When the height is right, you will be able to spin freely without rocking thereby maximizing power and efficiency while minimizing injury. At the right height, you should be able to lock your core and only have the legs pumping without the body moving side to side.

You are your own. At the right height, your leg should have the kind of bend which resembles this picture when the crank is at 6pm.



Shorter riders will have less of a saddle to bar drop than taller riders.


STEM LENGTH
Set the saddle fore-aft and saddle height first before dialing in the stem length. The stem length should allow you to balance yourself, crouch into the drops and remain balance in an out-of-saddle climbing hill position with relative ease. A shorter stem will make the steering more twitchy\sensitive, while a longer stem will make the steering feel more stable or slower.

Read more here: http://davesbikeblog.blogspot.com/20...rearm-and.html




STEM HEIGHT
Stem height is often overlooked in bike fit and can be a source of cantankerous debate. Aesthetically, young people seem to like to show a huge saddle to bar drop leading to a slammed stem look (see here: http://slamthatstem.com/)

While this may be applicable for racing and short sprints, riding for an hour or longer at a time may require a different stem height. Depending on age, flexibility and intended use, a stem height that sets the bars at saddle height and plus\minus an inch from saddle height will yield more comfort while preserving aerodynamic options to ride fast or combat windy conditions. I ride my bikes with the bars about 1 inch below saddle.



Dave Moulton on Bike Fit:
http://www.prodigalchild.net/Bicycle6.htm

Dave Moulton on bar height:
http://davesbikeblog.squarespace.com...-position.html