Geometry numbers on a modern mountain bike play a huge role in how the bike rides and handles. As your skills improve as a rider, you will become more aware of how geometry relates to the feel of a bike. Bike geometry also typically correlates with the type of terrain you ride. If you ride steep and rocky terrain, a bike with slack and low geometry is the best bet for you. If you enjoy casual rides on smooth flowy trails, a bike with steep and high geometry might be your preferred choice. As a bike's geometry becomes slacker and lower, it also usually becomes less efficient at climbing. The most popular category of mountain bikes, trail bikes, feature geometry that is between super slack/low and super steep/high. These bikes are both decent at climbing and descending and are typically great bikes for both beginners and experts alike.
To start off with, let's get acquainted with all the terms you will see on a bicycle geometry chart. These are the universal terms that the cycling industry uses to explain the numbers and angles of their bikes. While you are looking at the terms, you can also hoover your mouse over the diagram in the upper right to see where each term is represented on a bike.
A bike's rear suspension design also plays a huge factor in how it rides, but today we are focusing on the geometry numbers. Following the definitions listed below is a section which explains how geometry numbers change depending on which type of bike you have and how those changes affect the overall ride quality of the bike.
A) Seat Tube Length - Most manufacturers measure seat tube length from the center of the bottom bracket to the top of the seat tube. In road cycling, this measurement indicates frame size. For example, a 56cm road bike will have a 56cm seat tube. In mountain biking, the frame size is usually not determined by the seat tube length.
B) Effective Top Tube - The effective top tube is measured by a horizontal line from the top of the head tube to the seat post. Since seat tube angles on modern mountain and road bikes vary quite a lot, effective top tube length isn’t as accurate of a measurement for determining bike size as it used to be. With modern bikes, the reach and stack measurements are the best numbers to used when determining the size of a bike.
C) Stack - Stack is the vertical distance from the center of the bottom bracket to the top of the headtube. The true stack height of a bike will be increased by spacers under the stem and the rise of the stem and/or handlebars. In layman’s terms, stack is how tall the bike feels.
D) Reach – Reach is measured from the center of the head tube to the center of the bottom bracket on a horizontal plane. Many consider reach to be the most important measurement when determining the size of a bike. In layman’s terms, reach is how long a bike feels.
E) Bottom Bracket (BB) Height – Bottom bracket height is the distance from the ground to the center of the bottom bracket. The lower the bottom bracket is, the more stable the bike will feel. Many modern mountain bikes have adjustable geometry that allows you to adjust the bottom bracket height.
F) Bottom Bracket (BB) Drop – Bottom bracket drop is the distance from the center of the bottom bracket to the horizontal line between the axles of both wheels. Some bikes will have a “negative” bottom bracket drop number. This means the bottom bracket lies under the horizontal line between the wheel axles.
G) Head Tube Length – As it sounds, the head tube length is the length of the headtube. Typically, as the size of the bike increases. The length of the headtube also increases. A longer headtube also increases the stack height of the bike.
H) Head Tube Angle – The head tube angle is the degree measurement between the angle of the fork and the ground. Generally, a bike with a steeper head tube angle will steer more sharply while a bike with a slacker head tube angle will be more inclined to travel in a straight line. Many modern mountain bikes have adjustable geometry that allows you to adjust the head tube angle.
I) Seat Tube Angle – The seat tube angle you see in a geometry chart is typically the “effective” seat tube angle. The effective seat tube angle is the angle measured between a horizontal line running through the center of the bottom bracket with a imaginary line that runs from the center of the bottom bracket to the intersection point of the seat tube and the horizontal line that runs across the top of the headtube. As effective seat tube angle gets slacker, the climbing efficiency of the bike typically diminishes.
J) Standover Height – The standover height is the vertical distance from the ground to the top tube that runs through the center of the bottom bracket. The standover height is especially important for shorter riders to ensure that they will be able to straddle the bike with both feet flat on the ground.
K) Chainstay Length (Rear Center) – The chainstay length is the distance from the center of the bottom bracket to the center of the rear axle. This measurement is typical done with a horizontal line that intersects the vertical lines of the bottom bracket and rear axle. Some companies measure the chain stay length as the direct distance from the center of the bottom bracket to the center of the rear axle, regardless if that measurement is perfectly horizontal.
L) Wheelbase - Wheelbase is the horizontal distance between the front and rear wheel axles. Shorter wheelbases can make the bike feel more lively, while a longer wheel base improves stability and comfort. Typically, as a bike increases in size, the wheelbase increases as well.
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