I was reading the Rivendell blog today, where Grant Petersen revealed a "mystery bike" that has been in the works for some time. It looks like this. And aside from the obvious oddities, it is endowed with an unusually long top tube for its size: a 62.5cm top tube on a 54.3cm frame. Apropos this geometry, Rivendell notes that the bicycle is "basically a flat-to-rolling land bike that, by virtue of it’s superlong top tube ...locks you into a sweeepyback bar." In other words, it would be difficult to set this bike up with drop bars due to the excessively long reach that would create.
This got me thinking again about my own Rivendell bicycle: a Sam Hillborne that is a 52cm frame with a 57.5cm top tube. That is also an unusually long top tube, given the frame size. Aside from the standover, the 52cm frame fits like a much larger one - possibly too large for someone of my stature to set up as a roadbike.
When I first got the bike, I could not ride with drop bars and so we did everything possible to ease me into it. The bike was built up with an extremely short stem (5cm) and the bars were set up considerably above saddle level. I rode it that way at first, gradually lowering the bars until finally they were level with the saddle. That felt fine for a while, and then came a time when I was ready for a longer stem (the 5cm was always meant to be temporary). The typical stem length for a roadbike is 9-11cm, but I soon understood that this range was out of the question given the long top tube - I would have to be lying down over the bike in order to reach the bars. So we replaced the 5cm stem with what we thought was an 8cm, but was actually a 7cm - and even that feels like a stretch. Now I find myself in bike fit purgatory: From the standpoint of how the bicycle handles, I feel as if I am not forward enough and would like a longer stem. But from the standpoint of reach, even the current stem is too long (and I have already shoved the saddle forward and replaced the seatpost with one that has as little setback as possible).
According to Rivendell's sizing guidelines I belong on a 52cm frame, if not larger. However, it seems to me now that these guidelines are optimised for setting the bicycle up with upright handlebars (even though they do not explicitly say that). Otherwise I do not know how to interpret the sizing.
Long top tubes are good for eliminating toe overlap. They are also ideal for fitting a bike with swept back handlebars, so that the bars don't hit your knees. But if you plan to set up a bicycle with drop bars at or below saddle level and use a standard length stem, a long top tube could be problematic - unless you have a long torso. If you own a Rivendell and have it set up as a roadbike, I would be interested in your take on this.