Originally Posted by digibud

At 5' 11", for somebody to be on a 18" frame (46cm), they would have to use a freakishly long stem. They could not have long legs or the seatpost would not go high enough. I'm 6' and a 56cm frame will typically just barely allow me to ride it if I raise the seatpost right to the max. A 56cm frame is a pretty good size for most people around 5' 11". To ride a 46cm frame....well...I say no. No way is somebody 5' 11" riding a 18" frame. Pictures and video or it didn't happen . And if it did....BLOW MY MIND (said like Kramer).
You don't need to know the inseam to know that a 46cm bike would only allow the seat post to be raised enough to accomodate a 5' 11" person -ONLY IF THEY HAD SHORT LEGS. My wife is 5' 7" and rides a 50 or 52cm bike (I forget which). For somebody 5' 11" to ride a 46cm bike....they would HAVE to have short legs, or they might sit way down low, I guess. Put a stingray seat on it . Then their long torso would require a crazy long stem. So I just don't see it and think the OP simply must be somehow mistaken. Perhaps the bike is not 18". Maybe he's not 5' 11". lol