51cm top tube too small for someone who normally cycles frame with 55cm top tube? I'm looking to buy a second bike and have found a really nice frame. The bike I cycle at the moment, which fits me perfectly (I am 5'11" (180cm)), has a 55cm top tube. The frame I'm looking at has a 51cm top tube. Is this difference too much?
There are two lenghts on a bike that are crucial for bike fit:


Seat tube length, which tells you how much of your seatpost will be outside the frame;
Effective top tube length, which is the horizontal distance between the centerlines of seat tube and head tube. This tells about your reach to the handlebars, how much the bike will feel "long" or "short" to you.


Some time ago, almost all the bikes had an horizontal top-tube, and due to manufacturing constraints (lugged frames using lugs with fixed angles) they tended to have a certain proportion between frame "height" and "length" in order to fit the average rider with a certain height.

Today we have a widespread adoption of welded frames, sloped top-tubes, mirabolant geometry designs, and we should always consider variations in leg-torso proportion for people with the same height. 

That's why I think is safer not to consider the relation between  one of length/height of two frames to have a correspondent variation of the other measure. In other words, is perfectly plausible to have two frames with same seat tube lenght and significantly different effective top tube length, and vice versa.

So, to answer your question: Yes, you can ride a frame with a 51cm seat tube lenght, achieving a similar fit, IF the effective top tube length of the frame is similar to the corresponding measure of the former 54cm frame.

EDIT: Unfortunately, since the frame you are looking at is a 51cm top tube, it is unlikely to be a good fit.

(EDIT: I thank Zenbike for editing my question, because I had mistaken top tube for seat tube and didn't even notice. Usually only the seat tube length is informed, and end up being considered as "THE" frame size).