Bikes – and especially bike frame – are often referenced by a size, like 56cm or 27 inches. For those “in the know” this is a good approximation to indicate if a particular bike will generally fit you. While this may not be all that critical when you are in a bike shop and can actually throw a leg over the bike, this number can be important when you are looking on Craig’s list, for example. So what does this number actually mean??
It actually is simply a measure of the length of the seat tube, from the center of the bottom bracket to the very top. It is important to note that this is different from the saddle height. While a particular person can actually ride frames of different sizes, generally the saddle height is constant for a given rider on any bikes that fits them. However, when you hear the size of a bike mentioned, it is almost always this seat tube measurement that is being referred to. The one place where will sometimes hear something different is when a bike is referred to as a “650.” 650’s are actually a reference to the size of the wheel – 650c – and not a particular frame tube.
You may wonder why, if saddle height it what matters more, the seat tube length is so often mentioned as the size of a frame. This is mostly due to the fact that it is truly a measurement of the frame, and not the overall fully equiped bike size. It is also worth note that historically road bikes have usually had a frame geometry where the top tube is parallel to the ground, as in the Cannondale R300 pictured at right.
But many modern bikes don’t have a horizontal top tube. Instead many manufactures are shipping frames that have a sloped top tube, like the Look pictured at left. The attentive among you may immediately see the issue – if our top tube is sloping down doesn’t that make the overall frame size measure smaller than it will feel when we are on it? Indeed it can – and how this is dealt with can vary from manufacturer to manufacturer. By the book, however, the frame size should be exactly the seat tube length, regardless of the relative angle of the top tube. While you may be expecting an imaginary line going from the top of the head tube and intersecting an imaginary line running through the seat tube, that’s generally not how bike frames are measured.
Another complication is that some manufactures don’t actually publish the exact measurements of their frames, but use more generalized size classifications like small, medium and large. In these cases you can sometimes get your hands on a sizing chart from the manufacture that will give the size range in cm for a particular frame.
To illustrate how confusing frames sizes can sometimes bike, consider that both of the bikes pictured in this article are mine and basically fit me. The Look, with its sloping top tube, is labeled as a “Small.” It technically measures only 48cm. The Cannondale, with its straight top tube, measures at 56cm. If you look at the common “rule of thumb” sizing charts, someone like myself that is 5’7″ with a 30.5 inch inseam should expect to be comfortable on about a 53-54cm frame. It is no coincidence that bikes labeled as small, medium or large are very commonly the models that do not have a straight, horizontal top tube.
And of course if you look at bikes like folding commuter bikes or mountain bikes, that don’t even have a front triangle (the triangle formed by the seat tube, down tube and top tube), then this whole frame measurement becomes completely irrelevant.