Mountain Bike Geometry Explained

We at 99 Spokes believe the frame geometry is one of the most important things to consider when purchasing a new bike for two reasons. Firstly, geometry is free, meaning you don’t have to pay more for a bike with geometry better suited for your riding style. That makes it the cheapest way to ensure your new bike rides as good as you imagined. Next up, unlike other parts on your bike, geometry cannot be changed. With the exception of a few devices that can marginally modify things like your headtube angle, you’re essentially going to be stuck with the same geometry for the duration of the bike’s life.

All this means that it’s a good idea to have a solid grasp on how various geometry figures affect how a bike feels and handles, but just because it’s important doesn’t mean it’s easy! Instead of getting overwhelmed by the dozens of measurements listed on some geometry charts, we recommend you become familiar with the following 8. To further simplify these, we’ve split them into two categories; “fundamental,” which determine the bike’s size and terrain it’s designed to be ridden on, and “stylistic,” which will further finetune how a bike feels and handles. Lastly, keep in mind that this article is not intended to be a comprehensive bike sizing guide, and there are dozens of philosophies regarding what geometry is best for riders of a given size, skill level, and discipline. Instead, our hope is that by understanding the following measurements you will be better equipped to make your own decisions around what model and size bike is best for you. 

Fundamental Geometry

The following five measurements are the first terms you should be familiar with because they determine how you will be positioned on the bike (essentially, the bike’s size) and the type of terrain it’s designed to be ridden in.

Reach

These days, reach is the most commonly agreed upon measurement for defining the size of a bike. A bike’s reach is the horizontal distance from the bottom bracket to the top of the head tube. This will determine how long and roomy the front of the bike feels while in a standing position, and there’s a surprisingly large range of accepted reach numbers for a given size across models and manufactures. In general, a short reach will make the bike feel a bit more compact while standing, making it more agile and maneuverable at slow speeds. A longer reach will feel more stretched out and roomy, leading to more stability at high speeds and through steep, rough sections of trail. Reach can be marginally adjusted by changing stem length, but this will also affect other handling characteristics of the bike.

Effective Top Tube Length (ETT)

Similar to reach, the effective top tube (ETT) measures how long or roomy the front end of a bike will feel, but this time in the seated position. The ETT is the length of a horizontal line drawn from the top of the head tube to the seat post. A longer ETT will make for a more stretched out, aggressive seated riding position, whereas a bike with a shorter ETT will be more upright. Like reach, ETT can be adjusted by stem length. Reach and ETT are closely connected by the effective seat tube angle, discussed below.

Stack

Stack height is the vertical distance from the bottom bracket to the top of the head tube. This effectively determines how high the handlebars will sit in relation to the rest of the frame. In general, a shorter stack will result in a more hunched over, aggressive riding position while seated and while out of the saddle. A taller stack, on the other hand, will provide a more upright riding position. While stack height itself can’t be changed, overall handlebar height can be increased in two ways; stem spacers and high-rise handlebars. Stem spacers sit between the top of the frame and the stem and in turn raise the contact point with the bars but also decrease reach/effective top tube. As the name would imply, high rise handlebars have increased rise (up to 100mm) built into the bar, increasing your contact point by the same amount. This does not affect reach.

Effective Seat Tube Angle (STA):

The effective seat tube angle (STA) is the angle of an imaginary line connecting the bottom bracket to the top of the center of the saddle when set at pedaling height. This can become a bit confusing because the angle changes as the seat is raised and lowered as well as with forward and backward adjustment of the saddle. That said, its effect on how a bike will feel is relatively straight forward. A steeper (higher number)  STA will position the rider’s weight forward, closer to the center of the wheelbase, improving front end grip. It also places the rider's weight more directly over the bottom bracket which can help improve pedaling power and efficiency. A slacker (lower number) STA will have the opposite effect. As mentioned earlier, STA is also connected to reach and effective top tube, and bikes with a steeper STA will have a shorter ETT for a given reach. This is why bikes with a longer reach for a given size will tend to compensate with a steeper seat tube to prevent the seated riding position (or effective top tube length) from feeling overly stretched out. It’s also worth noting that on a hardtail, STA will steepen under sag.

Head Tube Angle (HTA):

If reach is said to be the most important parameter in sizing a bike, then the head tube angle (HTA) is the biggest determiner for the type of terrain that the bike was designed for. The HTA is the angle, measured from horizontal, of the steerer tube of the fork. This affects many things including descending capability, steering response, and even suspension performance. A slacker (lower number) HTA puts the wheel further out in front of the bike. This makes the bike less likely to pitch down (and throw you over the bars) while descending. It also slows the steering response and changes the suspension path of the fork, helping the bike track more smoothly over obstacles. As you’d imagine, a steeper (higher number) HTA has the opposite effect, bringing the front wheel closer to the center of the bike. This increases steering speed and makes the bike easier to control and maneuver at lower speeds and while climbing. In general, bikes designed for steep terrain with an emphasis on descending speed will have a slacker HTA, whereas bikes built for more rolling terrain and with an emphasis on climbing efficiency will have a steeper HTA. Lastly, just like STA, the HTA on a hardtail will steepen under sag.

Stylistic Geometry

Once you have a grasp on the fundamental geometry it's time to start thinking about stylistic geometry, and this is where it starts to get fun. These geometry numbers are what give a bike personality, and will differentiate bikes designed for all out speed, comfort, playfulness, or stability.

Wheelbase

A bike’s wheelbase is the distance between the front and rear axle. Wheelbase is affected by several factors; chainstay length (more on this later, head tube angle, and reach. In general, longer wheelbases increase stability by dissipating how forces like braking or impacts affect the bike as well as giving the rider more room to maneuver over the bike. Shorter wheelbases will make the bike feel more maneuverable and tend to be preferred for riding tight twisty trails or jumps.

Chainstay Length (CSL)

Chainstay length (CSL) is the distance from the bottom bracket to the rear axle. This has a big effect on two things; how a bike corners and how easy it is to lift the front wheel off the ground. A shorter chainstay makes it easier to maneuver a bike through tight corners, largely because of the way it reduces the wheelbase but also because it allows the rider to more easily “pivot” around the rear wheel. It also makes it easier to lift the front wheel off the ground for bunny hops, manuals, and wheelies. On the flip side, a shorter chainstay means that the rider’s weight is shifted further to the back, which will require placing more weight on the grips to maintain a balanced position over the bike.

Bottom Bracket Height (BBH)

Bottom bracket height (BBH) is the distance from the bottom bracket to the ground. This affects the bike's overall center of gravity, and a bike with a lower bottom bracket will often feel more stable at high speed. This is often described as feeling more “in the bike” vs. “on the bike.” Higher bottom brackets, on the other hand, tend to make the bike feel a bit more maneuverable. The biggest consequence of a low bottom bracket height is pedal strikes (hitting the ground while pedaling). These days many mountain bikes come with shorter crank arms to allow for a lower BBH without increasing pedal strikes. Full suspension bikes will often have higher static BBH to compensate for it lowering under sag.

Conclusion

One of the best ways to get an idea for how these various numbers affect how a given bike will perform is to try out as many bikes as you can! Next time you go out on a group ride, try swapping bikes with a fellow rider for a section of trail. Take note of what you feel, and when you get home compare the geometry of both bikes using our bike comparison feature on our website. With time, you’ll begin to find what measurements you prefer, making your mountain biking experience that much more enjoyable! If you have any specific questions regarding mountain bike geometry feel free to shoot us an email at hi@99spokes.com or DM @99spokes on your favorite social media platform.