Frame geometry, loosely defined, is all about the lengths and angles of a bicycle frame. It’s important because it determines how the bicycle will perform as well as how it will fit you. Sometimes it’s easy to look at the most obvious aspects of frame geometry (how long is the top tube and what is the standover height) and forget about the rest. Understandable, because we (the manufacturers) don’t always do a good job of explaining the rest to you and what it means. This gets a little dry, but bear with me.
So let’s start with frame angles: the seat angle and the head angle. The seat angle is the angle between the top tube and the seat tube. The head angle is the angle between the top tube and the head tube. For typical road bicycles, the head angle is between 71 and 74 degrees. The head angle, in combination with the rake, determines how the bicycle handles. Steeper head angles, like 74 degrees, are usually reserved for bikes that are very manuverable, like criterium bikes. On the opposite end, shallower head angles are found on touring bikes where, in combination with a long rake, they provide stable handling.
The seat angle isn’t quite as sacred as the head tube angle, but it sure has important implications for fit because it determines whether or not you’re going to be able to get in the right position relative to the pedals. Seat angles range from 72 to 74 degrees with 73 degrees being the most typical. Note that by moving the saddle back and forth on the rails, you can effectively change the seat tube angle a couple of degres. Once the seat angle is greater than 74 degrees, the geometry really puts the rider in an awkward position relative to the pedals. Unfortunately, some designers manipulate the seat angle to make the top tube shorter. To easily visualize this, imagine a bike with a 90 degree seat angle (i.e., the seat tube is vertical). Yes, the top tube is really short, but at the expense of a good fit. Steep seat angles also tend to give a rougher ride: imagine sitting on a pile driver.
Another dimension most geometry charts mention is the bottom bracket height. This is the distance from the ground to the center of the bottom bracket. You’ll find higher heights on bikes where pedaling through corners is important (like bikes used in criterium races) and lower heights on bikes where a low center of gravity is important for maintaining stability (like touring bikes). Because the bb height can vary a little depending on the tires, some manufacturers also publish the “drop”. This is the vertical distance from the wheel axle to the center of the bottom bracket. It’s a fixed number, so in manufacturing, bicycle frames are built to drop, not to bottom bracket height.
No geometry chart would be complete without rake, which describes how much the end of the fork blades deviates from a straight line drawn through the head tube. Rake is usually in the 2 to 6 cm range. Rake doesn’t mean a lot by itself, but when manipulated in a trigonometric relationship with the head angle and the circumference of the front rail, you can derive the trail and the caster angle of the bike. When all bikes had 700c front wheels, trail used to be a hallowed indicator of how the bike would handle, but with the advent of 650c and 24″ wheels, it’s lost its luster. Caster angle is probably a better measure; caster angles in the 80 to 82 degree range give neutral steering regardless of the wheel size.
Finally, there’s the chainstay length. You’ll find them in the 39 – 40 cm range on racing bikes, since this makes for a stiffer rear end that won’t twist under explosive acceleration. Bikes made for touring usually have the longest chainstays (43 cm +) for better shifting with wide range gearing and heel clearance for panniers.
Hopefully this has given you a little more cycle savvy about frame geometry!