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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?

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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).

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  • Very nice answer. We get folks that come into the shop that NEED a 54" Tri bike because some chart said that for their height that was the correct size. May have to print your answer and hang it somewhere in the shop.
    – Ken Hiatt
    May 31, 2012 at 19:30
  • This is a good answer in general. It ignores the relevant sizing information in the OP's question, however. "The frame I'm looking at has a 51cm top tube..."
    – zenbike
    Jun 2, 2012 at 3:10
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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.

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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.)

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    While this is true as far as it goes, it implies that a 51cm top tube frame is a possibility for this rider. It isn't.
    – zenbike
    Jun 2, 2012 at 3:07
  • I've seen neither the rider nor the frame, and the rider has not ridden the frame, so I couldn't say, especially given that frame member measurements are far less meaningful/useful than they used to be.
    – Daniel R Hicks
    Jun 2, 2012 at 12:16
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Simply put, yes, a 51cm frame is too small, assuming it uses traditional geometries. Since you state that the top tube is 51cm, you can assume that it uses traditional geometries.

My wife, 5'2" and 118lbs, rides a 51cm top tube.

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It's within the range where you can adjust the fit to work with things like longer stems and seatposts and have a comfortable ride, but it will handle differently. You should probably install a fork with longer rake than what you currently have to avoid overlap.

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