A mountain bike frame is a fascinating thing, and your choice of a certain kind of frame is really very important. It is a very large part of what the bike will feel like while riding. The frame size is a very good starting point. But not everybody who measures 1,80 meters (for example) has the same build: leg length, arm length they all differ. There are many things that determine how the bike feels. Actually, you have to start asking yourself what kind of riding style do I have and what kind of riding do I (want to) do?
Do you like cross country racing, where climbing is relatively more important than bombing downhills, you’ll want to sit a bit more upright and forward, so that you keep enough pressure on your front wheel to keep traction as you climb. Doing that without having to stand on your pedals, is really important, or you’ll see your back wheel spinning and all your energy disappearing in a cloud of gravel and dust.
Are you going to do enduro or downhill riding, where speeds are higher, you’re faced with steeper singletracks sections? You’ll want to lower your center of gravity. You need more stability at higher speed, so the bike may be a bit longer and lower. You also want to sit back on those really steep sections with the center of gravity away from your steering wheel and towards your rear wheel. You want to get your saddle out of the way, to drop your body weight down. (often using a dropper post to lower your saddle dynamically).
There are many lines, measurements, and angles that affect that feeling and the way you sit on your bike. Together, all these details determine the geometry of a bike. In order to know exactly what to look for and what the consequences are of different sizes, angles and measurement, here are explanations of different elements to your bike’s geometry.
This is the distance between your saddle and the headtube (not your steering wheel, which can be further by a few centimeters). Actually, this is a very important measurement and for a large part will determine your body position. If it’s shorter, you’ll be more compact and upright. Is the reach is longer than you’ll find your body a bit lower and slightly stretched. You should really feel comfortable and the reach has to fit your riding style. Take into account that the reach will vary from brand to brand even in bikes that are meant for the same kind of riding. So anfor example, an “enduro” classified bike by Cube will feel very different from the same category bike when made by Canyon.
Head Tube angle
The reach and wheelbase (more about that later) are closely connected to the angle of your head tube. This is an important angle of your frame. It is the angle your front fork makes with the ground. The smaller the angle, the slacker the fork, the farther away your front wheel will be. This is what you want on fast and steep downhill sections, giving your bike a nice, stable feel. Moreover, it prevents your bodyweight from getting too far forward and it’ll be more difficult to go over the bars. The wheelbase generally will be longer and your center of gravity shifts further backwards relative to the center of the bike. And the other way around, if you want uphill, you want a bigger corner with your
Conversely if climbing is your thing, and the trails you ride are bit less steep but more cross country-ish you want the head tube angle to be larger. The tube will be steeper, the wheel will be closer, you’ll be seated more upright. Climbing and steering will be easier and more responsive to what you need.
Seat tube angle
This is the angle of your seat tube makes with the ground. The angle of this tube determines where the saddle is (and thus where you are) in relation to the rest of the bike. And, especially in relation to the crankset (where you deliver your power). The more your hips are placed directly above your pedals, the more power you can put into them.
This is the height of the top tube when measured from the ground. Actually just how high the bike is if you’re standing with both feet on the ground. It doesn’t impact the rest of the frame geometry as much, but if your foot slips from the pedal you will experience the benefits of a lower standover height (or a higher) when you smash your crotch into your top tube. Ding dong. No pleasant experience.
The chainstay length, i.e. the length between the center of the crank to your rear axel is a very important measurement. The shorter the chainstay, the more nimble and maneuverable your bike will be. Your body weight will be closer over the back wheel, helping control there. It influences the total length/wheelbase of the bike as well. A shorter chainstay means a shorter your wheelbase.
The head tube angle, the chainstay, your seat tube angle and your top tube length all affect the length of the wheelbase. The wheelbase is the distance from axel to axel (of your wheels) and largely determines the stability of the bike. The longer the wheelbase, the more stable the bike is, especially at higher speeds. But a longer bike is also a less maneuverable bike. You would expect that a downhill bike needs to be very maneuverable, but these bikes especially need to be very stable at the high speeds that a downhiller will take.
Bottom bracket height
The bottom bracket height is the height of the center of your crankset from the ground. The lower the bottom bracket, the lower the center of gravity of the bike. A low center of gravity is good for grip on the track, especially in corners. This has limits, because if it’s too low, you’ll hit the ground, rocks or routes with your pedals or even your crank itself and that can lead to very nasty crashes.
All in all, a lot of angles and heights and degrees and millimeters and centimeters; if you are that gear nerd you can go crazy with figuring out exactly what you want. But in the end, you really know when you ride a bike and feel if the geometry of that bikes matches your build, your riding style, and ambition.