The traditional bicycle frame, known as diamond or double triangle has changed very little since its introduction in the 1890s. It was revolutionary then and was known as the safety bicycle. It is composed of eight tubes, or eleven if you count the fork. The intersection of these tubes, know as frame angles, and the lengths of each tube are the minute details that custom frame builders dwell on. This article is not intended to explore these details, instead I will discuss the common variations, and why you might want one or the other.
Most variations on the frame geometry are dictated by wheel and rider size. Other variations are designed for specific uses. You don’t have to know all of the minor variations in frame geometry - we can help you find the right frame design for you. If you are purchasing a major brand and can tell us how you plan to use your bike, we’ll get you set up with the right geometry.
However, these subtle differences are some of what distinguishes brands. A frustrating difference between brands is a lack of consistency of measuring standards. Brand “X” 55cm can be bigger than brand “Y” 56cm. Bicycle frame size has always been described by the height of the seat tube. This is too ingrained in the bike world to change, but the horizontal length of the frame affects your riding position at least as much as the height does.
A very low head tube on a triathlon bike gets your body’s head lower. This is better for aerodynamics, but worse for comfort for most people. A longer rear triangle on a touring bike will provide better clearance between your heels and panniers. This also increases the wheelbase length, which increases stability. A bike with a short wheelbase and steep steering angle will be more agile. This is the frame geometry preferred for criterium racing where racers are in a tight pack going fast through hard turns. A slightly “relaxed” steering angle and clearance for medium width tires on a hybrid bicycle will be a stable ride with predictable handling.
At County Cycles, we are proud to represent Bianchi, which is the oldest bicycle company in the world, Felt who have done major research in aerodynamics and Trek who have one of the largest engineering departments in the world of bicycles. All three of these companies know bike geometry!
I hope you can imagine that what makes the functional difference between a mountain bike, a road racing bike, a triathlon bike, a touring bike and a BMX bike is all in the geometry. Did you think that geometry was just a class in school?