No, That Is Not a 27 Inch Bike

I spend a fair amount of time sifting through the 'Bicycles For Sale' section of Craigslist looking for potential projects, bike parts, and accessories on the cheap.  I've noticed that a lot of sellers will include information on the "size" of the bicycle.  It usually says something like this:  "For sale is a 27" road bike..." and so on and so forth.  And when I look at the accompanying image, it looks like a perfectly average size bicycle.  The problem is that this is what a 27" bicycle looks like:

That's a ridiculously huge bike frame.  I wasn't even able to find a frame size chart that went to 27", but if you extrapolate from the chart here, a rider requiring a 27" frame (or approximately 68 cm) would be approaching 7 feet tall.

So when someone says they have a 27" bike, where are they getting this number?  This is most likely the wheel size of the bicycle.  Older road bikes were commonly equipped with 27" wheels, while modern road bikes normally have 700C rims and mountain bikes have 26" wheels - or sometimes 29" or 650B.  And if this is not the correct way to describe the size of a bicycle, what is the correct way?  The generally accepted frame size measurement is the seat tube length.  To do this correctly, the measurement should start at the top of the seat tube where it intersects with the centerline of the top tube, and should end at the center of the bottom bracket.  For more information on bicycle sizing, check out Sheldon Brown's thorough discussion of the subject.

As one might imagine, the best way to determine whether or not a bike is a good fit is to take it for a test ride.  This is especially true since you most assuredly can't count on Craigslist sellers to provide you with an accurate frame size.