Modern Mountain Bike Geometry

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.

Bike Geometry Explained

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 only focusing on the geometry numbers. Following each geometry term listed below is a section which explains how the term affects the ride of a 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.

How it affects the ride:  As the seat tube length increases, it may affect how long of a dropper post you can fit on the frame. Riders typically want to have as much drop as possible, but not all frames can fit a 180mm or 200mm dropper post without the saddle being too tall for the rider. 

B) Effective Top Tube Length:  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 use when determining the size of a bike.

How it affects the ride:  In general, as the top tube length increases, the overall length of the bike increases. The top tube measurement is relative per bike, so do not use this number when comparing the length of two different models of bikes. 

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.

How it affects the ride:  In layman’s terms, stack is how tall the bike feels. The lower the stack number, the more bent over the handlebars you will become. A low stack height is great for climbing and a high stack height is great for steep descending. Most riders try to run a stack height that is between these two extremes. 

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. 

How it affects the ride:  In layman’s terms, reach is how long a bike feels. As reach increases, the distance from your stance to the handlebars increases. Unlike top tube length, the reach measurement is standardized across all bikes. This is the best measurement to uses when comparing bike sizes regardless of brand or model. 

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.

How it affects the ride:  As the bottom bracket height decreases, the bike will become more stable, especially at high speeds. The trade-off of having a low bottom bracket height, especially on a full suspension bike, is the increased likeliness of pedal strikes. Typically, as a bike has more rear travel, the vertical distance in which the bottom bracket drops because of the suspension increases. 

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.

How it affects the ride:  Bottom bracket drop is just another way of measuring bottom bracket height. Bottom bracket drop and bottom bracket height affect the ride in the exact same way. Reference term "E" for more information.

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. 

How it affects the ride:  A longer headtube increases the stack height of the bike. For more information on how stack affects the ride, reference term "C".

H) Head Tube Angle:  The head tube angle is the degree measurement between the angle of the fork and the ground. Many modern mountain bikes have adjustable geometry that allows you to adjust the head tube angle.

How it affects the ride:  Generally, a bike with a steep head tube angle will steer more sharply, while a bike with a slack head tube angle will be more inclined to travel in a straight line. Riders like a slack head angle because it feels more at home on steep terrain and when riding at fast speeds. A steep head angle has more responsive steering and climbs more efficiently. 

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.

How it affects the ride:  As effective seat tube angle gets slacker; the climbing efficiency of the bike typically diminishes. Modern bikes are typically receiving steeper and steeper seat tube angles to improve climbing ability without necessarily hurting descending ability. Some companies match a steep seat tube angle with a long reach number to create a bike that has a long reach for stability at speed but maintains a comfortable pedaling position.

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. 

How it affects the ride:  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. Riders tend to like a bike with a short standover height because it improves the mobility of the bike beneath you. There is more clearance to maneuver the bike around. 

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.

How it affects the ride:  A shorter chainstay length will make a bike more playful and maneuverable. A longer chainstay will make a bike more stable and planted, especially at high speed. Typically bikes designed for having fun and popping off features have a shorter chainstay. Bikes that are designed for racing typically have longer chainstays to provide the rider with more control at high speeds. Many companies are now changing the chainstay length on different sizes of the same bike. The small sizes will have a shorter chainstay than the large sizes. This helps the bike feel more balanced. 

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.

How it affects the ride:  The wheelbase measurement increases as the size of the bike increases. Other measurements such as reach, chainstay length, and headtube angle all play a role in the overall wheelbase of the bike. A bike with shorter chainstays and a steep headtube angle will have a shorter wheelbase than a bike with longer chainstays and a slack headtube angle. As the reach number of a bike increases, the wheelbase number will also increase proportionately.

Bike Geometry by Discipline

Geometry numbers that change per discipline

Head Tube Angle:  The head tube angle is the main measurement that drastically changes between each discipline of mountain biking. The list below shows the range of head tube angles per discipline. Some bikes fall outside of these ranges, but the majority of bikes are represented in the list below.

Cross Country Bike 67 degrees or steeper
Trail Bike 67 degrees to 65 degrees
Enduro Bike 65 degrees to 63.5 degrees
Downhill Bike 63.5 degrees or slacker

Geometry numbers that stay "relatively" the same per discipline

Bottom Bracket Height and Drop:  Bottom bracket height/drop measurements stay relatively similar across all disciplines of mountain bikes. Typically bikes with more travel have higher bottom bracket heights because the bottom bracket lowers further as the bike cycles through its suspension. 

Seat Tube Angle:  Seat tube angle measurements vary a fair amount, but it is hard to distinguish a pattern based on discipline. Typically bikes better for climbing such as cross country and trail bikes have steeper seat tube angles, but now a lot of enduro bikes have very steep seat tube angles as well.

Chainstay Length:  Chainstay length measurements are relatively similar across all disciplines of mountain bikes. Chainstay length is more dependent on wheel size than discipline. Some bike companies now increase chainstay length as the frame size increases. This helps the larger frame sizes feel more balanced.

Geometry numbers that change based on frame size

Seat Tube Length:  As a bike frame increases in size, the seat tube length typically increases. 

Effective Top Tube Length:  As a bike frame increases in size, the top tube length always increases. 

Reach:  As a bike frame increases in size, the reach always increases. 

Stack:  As a bike frame increases in size, the stack usually increases as a result of the head tube length typically increasing as well. 

Head Tube Length:  As a bike frame increases in size, the head tube length typically increases. This results in the stack increasing as well. 

Standover Height:  As a bike frame increases in size, the standover height typically increases. 

Wheelbase:  As a bike frame increases in size, the wheelbase always increases.