What is mountain bike geometry and why is it important?
Mountain bike geometry is a list of specifications that describes some of the most important angles and lengths on a bike. The average mountain biker shouldn’t be bothered by most of the measurements on a geometry table but some are absolutely critical to performance and can vary greatly between bikes.
The main purpose of a frame manufacturer publishing the mountain bike geometry is to give customers an idea of how a bike performs, what its intended purpose is and how the bike is sized.
For the customer, mountain bike geometry tables can be useful when comparing a bike you already own to a bike you might want to buy. Once you have a good understanding of how geometry figures translate to how a bike rides, you’ll be able to focus on what you find lacking on your current bike and figure what you want in your next bike.
This is especially important for riders who don’t have access to demo bikes from their favorite brands. When you’re laying down thousands of dollars on a new bike, you want the most assurance that you’re going to like the bike you’re buying.
Below is a table of most of the measurements you will commonly see on a frame or bike’s geometry table.
What It Describes
|The angle between an imaginary line through the middle of a frame’s head tube and the horizontal plane. Lower angles are a slacker and higher angles are steeper.|
Seat Tube Angle
|The angle between a line that goes through the middle of the seat tube and the horizontal plane.|
|The length of a line between the head tube and an imaginary point above the bottom bracket at the same height as the headtube. The imaginary point above the bottom bracket is measured where it assumed the bike is built and standing.|
|The length of a line between the bottom bracket of a frame to the same imaginal point described in the reach measurement.|
Bottom Bracket Height
|The measurement between the middle of the bottom bracket and the ground, assuming the bike is already built up and standing. It can change when the head angle, fork height, and even tires change.|
Bottom bracket drop
|The measure of how far below the middle of the bottom bracket sits from an imaginary line between the rear axle and where the front axle is assumed to be. This is a measurement often seen on frame only builds.|
|The distance between the middle of the bottom bracket and the rear axle of the bike.|
|The distance between the rear axle and the front axle of a bike.|
|The distance from the middle of the bottom bracket to the front axle of a bike.|
Seat Tube Length
|The distance between the middle of the bottom bracket and the top of the seat tube.|
|The distance between a point on the physical top tube of the bike, a few inches in front of its junction with the seat tube, and the ground.|
Top Tube Length
|The length of a straight line between the top of the seat tube and the middle of the head tube.|
The list above can be a lot to take in but it can there is plenty of room to break it down. Although all the components of bike geometry affect each other, they can be roughly divided into two categories, the measurements that determine performance and the measurements that describe the sizing of the bike.
Getting a mountain bike that fits for you is the most important first step in choosing a bike. Bicycles aren’t like motorcycles which usually don’t have size options.
Sizing is important in all form of cycling because, unlike motorcycles, the weight of the rider is typically over 5 times heavier than the weight of a bike. This means that the center of gravity of the bike-plus-rider system is mostly dependent on the position of the rider.
The bike size determines where a rider is positioned on the bike and how much freedom they have to move around while riding.
The first figure I always look at when I look up the mountain bike geometry table of a bike I am interested in is the reach. When figuring out what size bike you need, ignore any other measurement go straight to looking at the reach.
I suggest following these steps to figure out what reach measurements you feel comfortable on. If you’ve had a bike before, try and find its geometry table and find its reach to give you a baseline.
If your bike is too old to have this measurement listed or you’ve never had a mountain bike before, try visiting a local LBS and give some of the new bikes they have on display a sit for sizing.
When you have a short list of bikes and sizes that felt good under you, look up their geometry chart online when you get home and take note of their reach measurements. Now that you know what reach numbers you like, you can check out any frame online and know which size fits you.
In the past two to three years, bikes have been having a little revolution in geometry which can lead to quite a bit of confusion to someone who hasn’t been kept up to date.
Many frame manufacturers are making their bikes much longer, along with other adjustments to make the increase in length comfortable. But as a consequence, there are some companies who are leading the charge and making extremely long bikes and others that are conservative and haven’t lengthened their bikes drastically.
This is why you should ignore the “small, medium, large” nomenclature and look straight at the reach. It’s not uncommon for one company’s large to be the same physical length as another company’s medium.
The stack is the other measurement to consider in modern sizing but it’s a much smaller factor. It’s not as important as the reach when it comes to sizing but it can be a good reference to how high or low the front end of a bike feels when you’re standing on it.
This can also be adjusted and compensated for easily with stem height, bar rise, and fork length.
Seat Tube Length
This has become less of an issue lately with the advent of long travel dropper posts. The only time where you might have to worry about this is when you’re eyeing a bike from a frame manufacturer who hasn’t really caught on to the longer reach trend.
You may want a reach that would put you one or two sizes up relative to other companies. The consequence with regards to the seat tube length is that companies often put longer seat tubes on larger sized bikes because they assume that these are going to be ridden by taller riders.
You may get stuck with a higher seat tube than you’re used to. Remember, you can raise the seat on a bike with a low seat tube height but you can’t reduce the seat tube height on a bike with a high seat tube.
Stand Over Height
This measure is often published but should only really be an issue with riders who are on the shorter side with inseams below 32 inches. Your inseam should have a good 3 or 4 inches over the standover height to stand over your bike comfortably with your feet flat on the ground.
Even after looking up the figures, I still recommend taking a good look at the picture of the actual bike and see if there is a decent amount of space above the top tube.
Some manufacturers measure the stand over from the crevice at the junction of the seat tube and the top tube, which is not a comfortable place to stand.
If you’re a shorter male rider, I highly recommend considering female-specific models. Their design comes with a more generous standover height than their non-female specific frame models. They usually come in better colors than the normal models, too!
Top Tube Length
Before the reach measurement was adopted across the industry, top tube length was the go-to measurement for sizing your bikes and it still is in road biking. The problem with this measurement is that it’s only a valid dimension if you’re sitting down.
When a mountain biker is squeezing the performance out of their bike, they are usually standing up. This metric is also easily adjusted with seat position and different seat tube styles.
On the other hand, you have very little wiggle room with reach adjustment, making it a set dimension of a frame. This is why reach became the dominant way of sizing a mountain bike.
After figuring out what the right size is for me, I look at the geometry measurements that determine which is the bike´s main purpose. Looking at a geometry table may not tell you exactly how a bike will behave but it can definitely tell you what the bikes intended purpose is and whether it checks all the boxes of things prefer in a bike.
Head Tube Angle
The head angle is the first one I look at. A slacker head angle will give you more stability at speed, it will help you stay behind your bars on steeper trails and pushes your weight further back on the bike.
Consequently, the slacker the head angle the more sluggish a bike feels at lower speeds, it also makes the bike difficult to handle on flat ground or climbs. A common description of this behavior is that the front of the bike “wanders”.
It makes it difficult to control because on flat terrain or when climbing, the slack head angle pushes your weight so far back that the front gets unweighted, and the unconscious actions of balancing yo
ur bike become more exaggerated making it difficult to keep the front pointed in the same direction while maintaining balance at low speeds.
Steepening the head angle of a bike fixes this issue and makes low speed handling much better at the cost of high-speed stability and prowess on steeper trails.
This is why you typically see downhill bikes, built for high speed and steep trails, with the slackest head angles, around 63°-64°, enduro bikes with 64°-66°, trail bikes with 65°-67°, and cross-country bikes, built for climbing and long distance, with 67°-71°+.
Part of the revolution in mountain bike geometry I mentioned earlier is slacking out the head angles of trail bikes or endure bikes to angles that are almost as slack as downhill bikes.
This is one of the biggest reasons why modern trail and endure bikes perform so much better than their counterparts from just a few years ago.
They have been able to take advantage of the benefits of a slack head angle while keeping the climbing and low-speed performance excellent by lengthening the bike and pushing the rider’s weight forward into the new space they created using steeper seat tube angles.
Seat Tube Angle
Which brings us nicely to the next thing I look at on a geometry table, the seat tube angle. This is a specification that impacts on how rideable a slack bike is at low speeds.
Having a steep seat tube angle along with a long reach and slack head tube angle seems to produce that “in the bike” (rather than “on the bike”) feeling that was so difficult to reproduce in the past.
This is what the revolution in mountain bike geometry is all about and it is the reason why companies are pushing for longer reaches, slacker head tube angles, and steeper seat tube angles.
It culminates in bikes that perform magnificently.
Some companies are catching up to the trend slowly by adjusting one geometry component at a time and not the whole package. Unfortunately, it’s often the seat tube that changes at last and there have been companies which have come up with long slack bikes that have yet to embrace steeping the seat tube.
I, personally, would stay away from full suspension bikes with anything less than a 73° seat tube angle and definitely lean towards bikes with a 74°+ seat tube angle.
Sure, there are bikes out there with slacker seat tubes that perform great, but there are already so many very good options now that there’s hardly any reason to compromise in such a critical area.
Bottom Bracket Height
The bottom bracket height of a bike is something I look at but is not much of a deciding factor anymore because just about all frame manufacturers are getting it right.
Bottom bracket heights are generally where they should be these days, around 13 to 13.5”. This is usually a bit higher on long travel bikes and a bit lower on short travel bikes.
It’s counter-intuitive because longer travel bikes will normally sit deeper. A lower bottom bracket height means that a lower center of gravity because almost all your weight centers around this point.
Even tenths of inches here can make a significant difference in how a bike hugs the grounds, especially in corners. The reason it isn’t lower than this is that the lower it is, the more likely you are to hit your pedals on the ground or on trail-side obstacles while pedaling.
If you have lots of tight, low-speed technical trails where you ride, a higher bottom bracket might actually save you a few a headaches.
The Chainstay length is another measurement with no right or wrong value. Relatively long and short chainstays both have their merits. A shorter Chain stay usually makes a more playful bike.
They are easier to maneuver in tight corners, easier to do manuals and wheelies on and generally makes it easier to throw the bike around.
On the other hand, longer chain stays make the bike much more stable and much better traction at high speeds.
For most 27.5” bikes, it’s fairly typical to have a chainstay length of around 17”, with downhill bikes usually having higher lengths at 17” to 17.5”.
The wheelbase is basically the measure of the length of a bike. Generally, a longer wheelbase bike will be more stable at high speed and a shorter wheelbase bike will be more maneuverable, especially at low speeds.
The wheels base is basically the sum of several factors which have the same effects on bike handling, namely: chainstay length, headtube angle and the reach. Don’t worry about this too much since it is dependent on other factors that we’ve already gone over.
This is similar to the wheelbase in that it is an important component of mountain bike geometry but it is also just a summation of effects from other geometric components, namely the reach and the head angle.
It can give some valuable insight, especially when looked at with the chainstay length, towards how a is the bike´s balance.
How can mountain bike geometry help you?
Mountain bike Geometry can be overwhelming, but this is your bike´s language and learning this language can be the source of some valuable insights.
The first, and sometimes only, figures I look at when I browse a mountain bike geometry table are the head tube angle, seat tube angle and reach.
This trio lets me know how adventurous a manufacturer is with adapting to the new geometry trends and whether or not one of their sizes fits me bang on.
Being in between the sizes of a certain company’s sizing scheme is often a deal breaker for me. Sometimes, I also look at the chainstay length to get a gauge of how playful or stable a bike is.
This is just my preference as a gravity rider, but a trail rider or a cross-country rider might be looking at other measurements more relevant to their needs.
If you’re still having trouble making sense of these numbers, I suggest looking at your friends’ bikes and see if any have the different year releases of the same bike model and see if you can do parking lot test.
Frame manufacturers tend to update the geometries of their bikes incrementally making small changes over the years, mostly longer reach figures and slacker’s head angles.
Translating numbers on paper to familiar feelings can be difficult, especially without much experience, but getting time on different kind of bikes can help a great deal helping you, not only understand, but feel the effects of changes in mountain bike geometry.
Understanding mountain bike geometry is the first step in knowing how to make a bike “feel right” for you personally. It isn’t the only factor, but it is by far the most important one.