I've been wondering, I'm 5' 11" about 200lbs my bike is a 17" cheapo mtb until I can afford something better, is this bike too small for me? It's comfortable, even on long rides. No cramping, spasms, or pain to speak of. But as I read more about bike fit, I've noticed most quick sizing graphs say I should be on a 18.5" to 21", and some say that it can be hazardous to ride a bike so undersized. For instance my knee is about 3" forward from being directly over the pedal spindle. Am I OK or should I get a new bike ASAP?