Mountain bike reviews often seem to focus on the latest and greatest in drivetrain technologies, fancy suspension components, frame linkage designs and carbon fiber wheels, but often gloss over what is arguably the single most important factor in understanding how a bike will actually handle on the trails: frame geometry. This article is meant to offer a short and concise guide to understanding modern mountain bike geometry charts, and how to read and understand geometry tables when shopping for a new bike.
What is frame geometry?
Simply defined, the geometry of a bicycle is the distances and angles between the various tubes and other elements of a bike’s frame. These measurements, when tabulated precisely, helps make sense of how a bike will ride, the terrain it can be used to tackle and the essential handling characteristics it will have, at least theoretically, without actually riding the bike.
Why understanding frame geometry matters?
Frame geometry determines the main handling characteristics of the bike. With how diverse modern mountain bike disciplines are (cross country, downhill, enduro, freeride, etc.), it is important to be able to tell at a glance if the bike you are considering buying is suitable for your intended riding type. One of the quickest ways to tell how a bike will handle on the trail is to look at the geometry table. It also helps you understand the nuances of bike fit and accordingly make an informed decision about the frame size you would need. Nothing beats real world experience, of course, and the best way to make decisions on a potential bike purchases is to go for a demo ride, but that is not always possible and this is one way having a good understanding of mountain bike geometry can be immensely helpful.
Bike geometry is a rather complex topic, and we encourage you to pursue a deeper dive into it once you have a basic understanding of the essentials. That said, the intention of this article is to highlight some of the most important elements of modern mountain bike geometry that we deem essential knowledge for all mountain bikers.
Head Angle:
What is it? The head angle is the angle of the steer tube of the bike, measured from horizontal.
What does it tell us? The head angle determines how well a bike climbs or descends. Slacker head angles (lower numbers) place the front wheel further away from the rider, and as such enable the rider to have a rearward weight bias that enables descending steep trails with more confidence. Conversely, a steeper head angle allows the rider to put more weight on the front wheel while climbing steep trails, and accordingly decrease the tendency of the front end of the bike to wander, wobble or otherwise lift on steep or technical climbs.
What to look for: For XC bikes, you want head angles between 67-69, for trail bikes 65-67, enduro bikes 63-64 and DH bikes <63.
Seat Tube Angle:
What is it? The angle of the line from the center of the bottom bracket to the top of the seatpost when set at pedaling height.
What does it tell us? The seat tube angle determines where the weight of the rider is placed relative to the front and rear axles, and as such how weight distribution will be affected with changes in the grade of the trail.
What to look for: Modern full suspension bikes have steep seat tube angles (72-73 degrees) to place the rider’s center of mass closer to the suspension pivot point, which minimizes saddle vertical movement with suspension compression, resulting in a more comfortable ride and better weight distribution with grade changes.
Reach:
What is it: The horizontal distance measured from the center of the bottom bracket to the center of the top of the upper opening of the head tube.
What does it tell us? Reach is a critical measurement to understanding modern mountain bike geometry. It is a proportional measurement (i.e. it changes with frame sizes) and is the single most useful number to understand how a bike will fit. The reach number also gives you a good impression as to how much seated/standing room you have between the saddle and the handlebar. On modern mountain bikes, the reach numbers have been getting longer along with shorter stems, which makes for roomier cockpits and better handling on technical terrain.
What to look for: As mentioned, reach numbers are proportional to frame size. The trend in modern mountain bike design is towards roomier front ends, and to get a sense of this you can compare reach numbers of various bikes (you can use an online geometry comparison tool like https://geometrygeeks.bike)
Stack:
What is it: The vertical measurement from the center of the bottom bracket to the center of the top opening of the head tube.
What does it tell us? Stack essentially determines the minimum bar height. If you prefer a more upright position on the bike, you are going to want a longer stack. If you are an XC racer with preference for a more aggressive position, you would probably want a lower stack.
What to look for: Stack numbers are proportional to frame size. Expect to see longer stack figures on bikes oriented for gravity riding, like downhill racing and enduro bikes than on XC/trail bikes designed for all-around riding.
Final words:
The current mantra in mountain bike geometry seems to be “long, slack and low”, which holds true if you look at contemporary mountain bikes in various categories. Bikes are getting lower centers of gravity and longer wheelbases and reach numbers, which make them highly capable in high-speed and technical riding situations. That said, it is important to first consider your local terrain and your own riding preferences. If you prefer long endurance rides with a lot of climbing, a bike with an XC-oriented geometry would probably be your best bet. Are you a gravity fiend? Then a super-slack enduro sled should be your weapon of choice. Getting better acquainted with deciphering geometry tables will help you pick your next bike to suit your local trails and style of riding.
