The Importance of Mountain Bike Geometry
For some, the word “geometry” can trigger a minor anxiety attack, reminding them of their days in the classroom sharpening a #2 pencil in preparation for a math test. Don’t worry, mountain bike geometry is a whole lot more fun, and we won’t ask you to solve for X. Erase that flashback from your mind and rethink what you know about the word “geometry.” Understanding mountain bike geometry will help you predict how a bike will ride long before you even toss a leg over it. Here, we plan to show you how to more effectively understand the numbers that make up the bike you ride. Without further ado, let’s dig deep into the design of our bikes to better understand their intended purpose.
Head tube angle: Head tube angle is the measurement in degrees at which the head tube points towards the ground. A 90-degree head tube angle would be straight up and down. Think of a shopping cart with the wheels spinning in circles. Bikes use head tube angles ranging from 63 to 72 degrees. The difference in degrees causes a dramatic difference in the way the bike handles. Bikes with a steeper angle (higher number) will have quicker steering and better climbing capabilities. A bike with a slacker angle (lower numbers) will have slower steering and more stability at high speeds. Think of a chopper versus a high-performance street racing motorcycle. Cross-country bikes benefit more from steeper angles ranging from 68 to 72 degrees, while a full-on downhill bike will have a slacker angle between 63 and 65 degrees. A trail bike’s head tube angle can vary from 66 to 68 degrees, depending on its intended purpose.
Chainstay length: A bike’s chainstays are the lower part of the rear triangle with a horizontal connection to the rear wheel. Chainstay length plays a major role in the playfulness of a bike. A bike with shorter 16- to 17-inch chainstays will wheelie and manual better than a bike with longer stays. Cornering is also improved with short chainstays due to a shortened wheelbase. If the chainstays are too short, however, it can cause problems. Slightly longer chainstays will help keep the front wheel planted on the ground during steep climbs, and the increased wheelbase can add stability at high speeds. Chainstay length should be chosen carefully in order to achieve the right balance.
Bottom bracket height: Bottom bracket height is the measurement from the center of the bottom bracket to the ground. A lower height will result in better cornering performance due to a low center of gravity. Bikes with super-low bottom brackets, however, will often have clearance issues with the crankarms. When a full-suspension bike compresses, its bottom bracket height is lowered, moving the crankarms closer to the ground. This can cause a rider to bash the pedals on rocks and roots during technical sections of the trail. Cross-country bikes generally have a slightly higher bottom bracket height compared to trailbikes, enduro or downhill bikes to help the rider tackle difficult climbs. On the other hand, trail, enduro and downhill bikes will handle better on descents and corners due to the rider’s lowered center of gravity.
Wheelbase: A bike’s wheelbase is the distance between the front and rear axles. Bikes with a longer wheelbase may have a harder time cornering, but will track better at high speeds. Short-wheelbase bikes tend to have snappier handling and maneuver better through tight turns and switchbacks. A downhill bike built solely for high-speed runs will often have the longest wheelbase. Since trail bikes are designed for everyday riding, their wheelbases are a happy medium, allowing for better maneuverability at most speeds. An XC bike often has the shortest wheelbase of the bunch. These cross-country race rigs are designed to get in and out of tight corners as quickly as possible to shave seconds off lap times.
Standover height: Back in the days when bikes commonly had horizontal top tubes, a bike’s standover height was much more relevant than today. Modern bikes generally have sloping top tubes that allow riders to get on and off of their bikes more easily. Sloping top tubes also provide extra clearance for cornering by allowing the rider to move his or her legs and hips more freely above the bike. The important thing to keep in mind with standover height is that the rider’s inseam should be longer than the bike’s standover measurement. Fat bikes, such as the Chumba Ursa Major, which is designed for snow riding, have super-low standover heights to compensate for when riders are standing in deep snow. Meanwhile, cross-country bikes tend to have a slightly taller stance since putting a foot down is less common.
Top tube length: It’s important to understand the difference between actual top tube length and effective top tube length. A bike’s actual top tube length is a fairly irrelevant measurement on a bike with a sloping top tube. Effective top tube length is measured by making an imaginary straight line from the center of the steer tube to the center of the seatpost. This measurement is the most accurate way to determine a bike’s size.
Reach: A bike’s reach measurement is also a popular way of determining its fit. Reach is measured from the same point on the head tube but meets with an imaginary line coming up from the center of the bottom bracket. Many modern bikes offer lowered seat-tube heights to allow for additional dropper-post clearance. This means more riders can stand over frame sizes that they might not have been able to before. The reach measurement of a bike will indicate how leaned forward a rider will be. Some riders may want to try a frame sized up or down depending on their preferred fit. A longer reach may provide more stability, while a shorter reach gives a bike a snappier feel.
Seat tube angle: The angle of a bike’s seat tube is measured in degrees, usually ranging from 72 to 78 degrees. Seat tube angles are designed to center a rider’s weight over the bike’s crankset. A common design for modern bikes is making the seatpost angle steeper, putting additional weight on the front end during seated climbs. A forward (high number) seat tube angle also moves the saddle out of the way better when using a dropper post. Keep in mind that most bike saddles are adjustable, which can help the rider find a comfortable position that takes weight distribution and knee alignment into consideration.
Wrapping Up The Numbers:
As promised, we didn’t make you do any math problems, but we still hope you learned something. When you’re considering a new bike and comparing multiple options, make sure to not only look at the components and prices, but also the numbers behind the bike. Learning how geometry affects the riding characteristics of a bike will help you make a more informed decision when you purchase your next bike.