In this series, we’ve separated all the parts of a mountain bike into six “systems,” which we'll break down into easily digestible parts. With a good understanding of how all the components of a bicycle work together, you’ll be able to more confidently approach mountain biking, no matter how long you've been riding!
Part 1 of Understanding Mountain Bikes discusses the component system at the heart of a full-suspension mountain bike: The frame and suspension. These parts are the foundation for the rest of the build and determine how the bike will fit you and handle on the trail.
Let's start by exploring the frame
Viewed from the side, you can see that a frame consists of two triangles: Front and rear. The three connection points a rider has with a bike are located on the front triangle: bottom bracket shell, head tube, and seat tube
The distances between these three points is crucial in determining how a bike will fit you, which we’ll talk about more in a moment. Connecting these three anchors are the Downtube, which goes “down” from the headtube to the bottom bracket shell, and the Top Tube which is on the top of the frame.
With that understanding, let’s look at some of the most important aspects of bike geometry and fit.
You’ve probably heard the terms Reach & Stack. These measurements can help give you an idea of what a bike will feel like when descending, because they tell you how far, vertically and horizontally, your hands are from your feet.
Next, seat angle & effective seat tube angle. You can measure your seat angle with the angle finder on your phone… BUT that’s not what you’ll see on a geo chart. Instead, Effective seat tube angle is measured using the imaginary line from your bottom bracket to your saddle, and will be a larger angle than the one you’d physically measure.
Lastly, we have “Effective Top Tube.” This isn’t the actual length of your top tube, but the horizontal line from the center of your headtube to your seatpost. It dictates how stretched out you’ll feel while pedaling in a seated position, because it is the main determinant of how far your grips are from your seat.
Attached to the front triangle is the Rear Triangle. On hardtails it is welded to the front triangle, but on full suspension bikes the rear triangle is an entirely separate unit that moves independently of the front, and has two sets of tubes:
- The Seat Stays are called that because on hardtails they serves to support, or "stay", your Seat Tube. On full suspension bikes, they most often act as an attachment point for your rear shock linkage.
- The Chainstays are the bottom of the rear “triangle”. They are often referred to in geometry charts to illustrate how long the rear end of a frame is, since this measurement spans from the bottom bracket to your rear axle.
To give you an example of why these numbers are worth considering, let’s look at the current trend of giving bikes a longer reach and slacker headtube angles. This lengthens the whole front end of the bike in an effort to give us more downhill speed and stability. On the other hand, it also moves our handlebars further away from our saddle, and our front tire further away from our center of mass, which can make the bike wander while climbing. To counteract that, we’ve seen seat tube angles get steeper to bring our bum forward. This creates a more comfortable pedaling position, and helps weight our front wheel to keep it from wandering while we climb.
That covers the frame and leads us nicely to our first suspension component, the Rear Shock.
Rear shocks are what make full suspension bikes so capable. They consist of a spring and damper, and serve to control the travel of the rear wheel. This helps maintain traction for climbing and descending, improves braking, and makes for a more comfortable ride. The two types of springs you’ll see in a mountain bike shock are a coil spring or an air spring.
Coil springs are simply a metal spring, something we’re all familiar with. An air spring is pretty simple as well, and at its most basic is just a compressed chamber of air. They both serve the exact same purpose, which is to absorb energy directed at the wheel. To avoid having a pogo-stick of a bike, we need to manage the action of the spring. That leads us to the other part of our shock: the damper. The damper lets us slow down the rate at which the spring can compress and expand. If you want to learn more about all these terms and how they affect your ride, check out our recent suspension set-up video, below.
As far as the physical dimensions of a shock, there are two primary measurements. The eye-to-eye, which is the total length of the shock, and the stroke, which is the distance the shock can compress. The stroke will always be less than your rear wheel travel because the frame’s suspension linkage multiplies the shock’s travel. This is what the term “leverage rate” describes: it is the ratio of vertical rear wheel movement to your shock’s stroke. For example, an average leverage rate of 2 means that for every 2 inches your rear wheel moves your shock will move an average of 1 inch.
Like the rear shock, the fork allows your wheel to move independently of the rider, but it also has the task of steering your front wheel. It is connected to your frame with your headset, which is simply a set of bearings and the cups they rotate in, which can be seen below.
The fork itself consists of four main parts: Uppers, Lowers, Spring, and Damper. The uppers start at the steer tube, which passes through your frame’s headtube and then attaches to your stem, as seen above. This is what allows you to steer your bike. At the bottom of your steer tube is the crown, which serves to connect to the sliding element of your fork: the stanchions. This whole assembly is often referred to as the CSU - Crown Steerer Unit. The remaining visible part of the fork is called the lowers. Here we have the fork arch, which helps to stabilize your fork and is where the front wheel dropouts are located. Like a shock, the functional parts of a fork are a damper, housed in the left stanchion, and a spring, housed in the right stanchion. They function in much the same way as they do in a shock.
That finishes off our fork, and leaves us at a good point to talk about one of the most widely cited geometry measurements in mountain biking: head tube angle.
Head Tube Angle
When viewed from the side, this is the angle in between your fork and the ground. One way it affects how a bike handles is by altering how impacts with trail features compress your fork. The front wheel of a bike with a smaller headtube angle will more easily move out of the way of incoming obstacles than one with a larger headtube angle. That is one reason why bikes designed for downhill speed have smaller headtube angles, whereas bikes designed for uphill speed have larger head tube angles.
Head tube angle comes into play in several other ways that are beyond the scope of this article, but you can check out our blog post titled The Relationship Between Fork Offset, Headtube Angle, and Wheel Size for more info. Your headtube angle will determine the location of your front axle, which leads us into the last geometry measurement we want to touch on: Wheelbase.
Wheelbase is simply the horizontal distance between the two axles but it is one of the most important geometry numbers in terms of determining how a bike handles. As a general rule, long bikes will feel more stable at high speeds, and less sensitive to rider inputs. Shorter bikes tend to feel more nimble. They are more responsive to rider input, making them easier to get around tight corners, but they get bucked around more in rough terrain.
That concludes Part 1 of Understanding Mountain Bikes. Now that you are familiar with the frame, suspension, and basic geometry of a mountain bike, we can move on the components, starting with the drivetrain. If you have any questions, please send us an email, start a chat on the website, or let us know in the comments. Don’t forget to try out our Custom Bike Builder and see what your dream build could look like! Thanks for reading, we’ll see you next time.
- Dan at Fanatik