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).
Depends on the bike geometry. I'd say that's a pretty big difference though. I would only vary a few cm in either direction from your ideal frame size. I'm sure you could ride it, but I think you'd feel kind of cramped on it. Different geometry may have an impact on top tube size though. I would hold out for a frame that suits you well and you'll be happy with for years.
Frankly, it's better for the bike to be a bit too small (within reason) than a bit too big. The main question is whether you can get the seat up high enough (without "cheating") to get full leg extension, and whether the "reach" of the bike and the handlebar height are then satisfactory.
Measurements only get you so far -- you need to ride the bike to be sure. (Which is a little hard to do with only a frame, sadly.)