After growing through their first couple of bikes, most people start looking at specs pretty carefully before making a purchase. This is particularly true of transportational bicyclists who may not have access to a good selection of potential models at their local bike shop. Because of this lack of support at the local level, online comparison shopping takes on an important role in the process of identifying just the right bike for our intended uses. And because bike design in the commuting/transpo realm is still all over the map, and not anywhere near as codified as it is with road and mountain bikes, understanding spec sheets and geometry charts is an important part of the selection process. And yet, while I acknowledge that geometry charts can tell us a fair amount about how a particular bike may ride, I’m rather dubious about putting too much emphasis on one particular number. Let’s look at the seat tube as an example.
In the above diagram, the seat tube length is the same for all three geometries. When looking at numbers in a chart, it would be easy to assume that different bikes with the same seat tube length also have the same standover height. But as is illustrated above, standover height, as well as saddle height, saddle set-back, and top tube length are all affected by seat tube angle. In other words, assuming the other dimensions remain the same, as the seat tube angle slackens, a bike becomes “shorter”, and as the seat tube angle steepens, a bike becomes “taller”¹. Other specifications such as bottom bracket drop and wheel diameter also affect standover.
The same type of thing happens at the front of the bike. Again, assuming the other dimensions remain the same, as the head angle slackens, the top tube becomes shorter and the handlebars move closer to the rider, whereas a steeper head angle results in a longer top tube and more reach to the bars.
Unfortunately, none of this provides a definitive answer to the most important question, “What is best for me?” But, it’s important to remember that isolating a particular number in a geometry chart, and using that number as a benchmark in the selection of a bike or bike size, is not a good idea. It’s better to take a holistic look at frame geometry and try to comprehend how all the numbers work together, which, unfortunately, is not a simple thing to do.
Of course, the best bet is always a real test ride on a real road. Alas, if only we had more widespread support for commuter and transpo bikes at the local level, this problem with numbers would be moot!