Measuring the size of a bike can feel daunting. The numerous angles and tubes vary dramatically from bike to bike.
Fortunately, by understanding a few key measurements, we can quickly understand what size bike we have and what the different measurements might mean for riding.
Mountain bike frames are typically measured by the height of the seat tube. The seat tube is the vertical tube where the seat post is inserted.
In order to measure the frame, we take measurements from the top of the seat tube, or where the seat post clamp sits, down to the center of the bottom bracket. The bottom bracket is the axle where the crank arms are connected to the frame.
Measuring this distance indicates the frame size and can help riders choose the proper bike size.
Some bike frame manufacturers use centimeters while others use inches, typically measured to the nearest half-inch.
This measurement is important because the ideal bike will offer ample stand-over height for the rider. Meaning, you should be able to stand over the top tube comfortably based on your inner leg length.
While there are numerous other measurements on a bike, the seat tube measurement is how most manufacturers indicate the size of the bike.
Measuring the seat tube will give you a sense of the size of the bike based on how the manufacturer divides up the different sizes. However, there are a few other measurements that can be helpful in understanding the fit of your bike.
The top tube, or the horizontal tube at the top of the bike, can change how the bike fit feels. In order to measure the top tube, we typically use an “effective top tube measurement” which measures the length of the top tube as if it were perfectly horizontal as opposed to sloping down towards the back of the bike.
To measure the effective top tube length, we measure from the center of the head tube to the center of the seat tube in a line that is horizontal to the ground. The head tube is the vertical section of the frame at the front of your bike and the seat tube is the horizontal tube where your seat inserts.
The best tool for measuring the top tube is a large level. In order to find the effective top tube length, we hold a level from the head tube where the top tube inserts, to the seat post, moving the level until it is horizontal.
From here, we can measure from the center of the head tube where the top tube inserts to the point on the seat tube where the level is flat. This measurement will tell you the effective top tube length of your mountain bike.
It is important to note that the top tube measurement is primarily used for comfort on the mountain bike. Typically, when mountain biking we practice bike and body separation which does effectively negate the specifics of the top tube length in terms of bike performance.
Instead of looking at the effective top tube length, it is more helpful to consider the reach of the bike.
Unlike the top tube length, one of the most effective measurements for understanding how a frame will fit is the reach. This measurement is taken by drawing a line straight up from the bottom bracket, then intersecting this line by measuring horizontally to the top of the head tube.
This distance will dictate how the bike feels when you ride it and how comfortable you’ll feel climbing and descending.
Often, riders will use this measurement to ensure that when they move from bike to bike, they maintain a similar bike fit.
Seat Tube Angle
In addition to the lengths of our bikes, we can look into the angles on certain parts of the frame. One important angle to consider is the seat tube angle.
This angle is measured from the ground near the back wheel, up to the line which connects the seat post and the bottom bracket.
A steeper seat tube angle is more efficient in terms of using big muscle groups to pedal; however, for steeper seat tube angle does make it more challenging to descend because your weight is pushed towards the front of your bike.
In context, a time trial road bike frame will have a seat tube angle of nearly 90 degrees whereas a downhill mountain bike will have a significantly lower seat tube angle.
Head Tube Angle
The next angle to consider is the head tube angle. We can find this angle by drawing a straight line through the stem and the head tube and down the fork and then measuring the acute angle from the ground.
Similar to the seat tube angle, a larger head tube angle provides greater pedaling efficiency while a smaller head tube angle provides stability on technical terrain and descents.
Bottom Bracket Height and Drop
Moving down, we can look at the bottom bracket height and drop.
The bottom bracket height is the measurement from the center of your bottom bracket to the ground. When this distance is shorter, the bike typically feels more stable, particularly in corners.
If the bottom bracket height is higher, the bike can feel more playful and agile.
The second measurement in regards to the bottom bracket is the bottom bracket drop. This measurement can be taken by measuring the difference in height between the axles of the wheels and the bottom bracket.
Often, 29” wheels offer a greater bottom bracket drop which can make feel more secure for riders or offer a sense of sitting “in” the bike rather than “on top of” the bike.
A smaller bottom bracket drop can make the bike feel more agile. Often, bikes with 26” or 27.5” wheels will have a shorter bottom bracket drop than bikes with 29” wheels.
With just a few measurements, we can start to understand how different mountain bike frames will feel. Be sure to measure slowly and accurately in order to understand the size and fit of your bike.