Hard-core mountain bikers love to geek out about mountain bike geometry, spouting off numbers about things like reach, head tube angles, and wheelbase – but what do these number mean, and how do they affect your ride? We’ll help break down a couple of the most important specs, to get you up to speed with the bike nerds, or at least help you understand what to look for when choosing a new bike.
It’s easy to tell that a lot of engineering goes into a mountain bike. They’re complicated machines after all, but it can be confusing to figure out how that translates into the quality of the ride. This is where geometry numbers come into play. They are a standardized set of measurements that can tell you how aggressive a bike is, how it will fit, and what kind of riding it will be best for.
Size: Mountain bikes come in standard sizes from Small to XL, but it’s important to note that these sizes are not always consistent between brands. For instance, a large Santa Cruz mountain bike may fit like a medium Giant bike. This is why it is important to compare other specs when choosing the right size for you. There are a number of measurements that are important in determining mountain bike sizing, two of the most important are reach and stack.
Reach: The reach measurement is the horizontal distance between the bottom bracket and the center of the head tube. In layman’s terms, it’s the distance from the bike saddle to the handlebars. This is arguably the most important figure for fit because it affects the length of your bike’s cockpit when you’re standing on the pedals, as well as how much range of motion in your hips you’ll use up in order to achieve a good, strong riding position. Too long of a reach and you’ll be stuck leaned over and stretched out. Too short, and you’ll be stuck in a position that’s overly upright and uncomfortable.
Stack: While reach is a horizontal measurement, stack is the vertical distance between the bottom bracket and the center of the head tube. This is primarily a gauge of seated pedaling position and relative handlebar height. This can be adjusted to a degree, with headset spacers and handlebar rise for increased stack height. For mountain bikes, where geometry is based on the aggressive riding (standing) position with the seat down, reach trumps stack as the primary fit dimension.
Head Tube Angle: Head angle, or head tube angle, is the angle between the front fork of your bike and the ground. Although there are other design aspects that affect the front wheel, (fork offset, trail, etc…) this is the key metric that the industry uses to determine front-wheel characteristics. A “slack” mountain bike head angle is a lower number (eg: 65°) relative to a “steep” head angle (eg. 70°). Think of the angle of a chopper bike vs a scooter. The chopper is super slack, while the scooter is as steep as it gets. A slacker head angle will, in general, be more stable at high speeds as well as feel more comfortable on steeper (downhill) terrain. Conversely, your bike will steer less precisely on uphill terrain. Bikes with steeper head angles feel like the front wheel is planted and can steer nimbly on uphill terrain, while feeling twitchy and unstable on fast downhill terrain. If your inclinations (no pun intended) are towards pedaling uphill, you’ll pick a bike with a steeper head angle and vice-versa if you like to head down.
Wheelbase: Wheelbase is simple, it’s the distance between your bike wheels. There are a lot of factors that contribute to the wheelbase length on a bike including head tube angle, reach, and chainstay length. Total wheelbase is something of a culmination of all of these factors. This is also one of the biggest differences you get when sizing up or sizing down your mountain bike. The gist of wheelbase is that increasing the distance will increase stability, whereas shortening wheelbase will make a bike more maneuverable. Take note of where that increased wheelbase is coming from (Head tube angle? Chainstays?) as these measurements will also affect the overall ride of your bike.
What does this mean for me?
Now that you have a general idea of what these specs mean, you can better narrow down the field, finding the bike that will work best for your style of riding and the trails that you’ll hit. These numbers are just the beginning though, the best way to figure out what will be most suited to you is to get out there and ride! Taking a look at a bike size chart can help, but there is no substitute for trying it out for yourself. You may size up with one brand and size down for another. So go out there with an open mind, a good idea of your riding style and how you’ll use the bike, and find the best fitting bike for you!
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