"Why is it that many shops that would like to project an image of being professional and knowledgeable are so bad at 'guesstimating' bike sizing?"
This is a question recently posted on a cycling forum I frequent. Here's my response:
That there is the $64,000 Question. And for those who have no idea what that references, then you have a clue as to the problem [the forum member above] talks about. It's a generational thing – most of those old guys, who had an idea of how to match up a rider to their optimal size frame, are gone. Moved onto jobs that actually paid the bills or moved to that velodrome in the clouds (no, not the internet service). Many of those still left have a difficult time dealing with frames lacking a horizontal top tube; they just don't get it; and that's the biggest problem most dealers have with putting customers on optimal size frames.
Compact Geometry = downward sloping top tube. A couple of years ago Giant Bicycle asked for my opinion on why a large dealer in Michigan argued that "Giant bicycles never fit any of our customers…." This was a very large, well respected dealer that had been in business for many years. After getting a few more details, it appears the dealer was looking at the center-to-center physical length of the seat tube instead of the effective (virtual) length. For example, a Medium Giant TCR road frame has a seat tube with a CtC physical length of 50cm. The Michigan dealer was calling this a 50cm size bike. The frame also has a top tube listed at 55.5cm (although Giant still doesn't label it as "effective"). You'd need the body proportions of a FERRET to need that 50/55.5 frame design. To compare the current crop of sloping top tube frames with classic designs using horizontal top tubes, you need to measure the new designs as if the top tube was actually horizontal; that usually means measuring the frame size somewhere on the seat post. Thus the effective (virtual) size of the Medium Giant TCR is really 56cm (+/-1cm for practical purposes), not 50cm. In reality, all the sloping top tube does is uncover seat post and deliver a larger adjustment range for saddle height. That means each compact geometry size will optimally fit a wider range of cyclist proportions compared to traditional horizontal top tube designs. Everyone wins, unless the shop fails to consider the effective seat tube length rather than the physical seat tube length.
For those shops still looking for the traditional one inch of stand-over clearance – they just do not understand. Forget about the one inch clearance standard; it is totally meaningless with sloping top tube bicycles – you're more likely to have 4-8 inches of clearance. As an aside, that one inch clearance thing was always more a creation of lawyers than anyone in the cycling industry.
Hope that helps.
Several years ago, BGI started using propreitary software developed for the specific purpose of eliminating the confusion of translating body proportions into frame proportions. While manufacturers label frame sizes based on seat tube length, the more important consideration is top tube length. The reason is simple – there's a lot more acceptable adjustment available with the seatpost (saddle height) than with stem length (reach to bars). This gives BGI the ability to be much more precise and consistent when advising customers about their optimal frame size. The software also compensates for the different size labeling systems used by our various bicycle manufacturers. This "Sizing" step is free for BGI customers and takes just a few minutes.
After finding your optimal frame size (which could be different for each manufacturer), be sure to have it adjusted to your body with a BGI Bicycle Fitting. Most people wouldn't consider buying a tuxedo that wasn't tailored to fit. Why buy a nice bicycle and fail to have it tailored to your body? Think about it – a tuxedo is mostly for standing around in, whereas your bicycle is for hours of active moving excercise.