This topic is quite controversial and has been a source of endless debate since the early days of biking. Especially when the topic is mountain biking wherein the bike geometry and sizing is forever evolving, many people tend to get lost it comes to bike sizing.
Bikes come in various forms, shapes, and sizes depending on the type of riding that you do. You pretty much cannot be efficient on a road bike if you take it mountain biking. The same is true for a downhill bike if you take it on endless uphill rides.
The Importance of Bike Sizing
Many years ago, people do not care about what bike size they ride. As long as it has 2 wheels then it is good to go. As biking evolved, so did the sport.
As of today, efficiency is the key even in the most extreme aspects of biking. And aside from having the perfect frame and effective shocks, have the correct bike fit is also a must.
If you opt to ride a bike that is sized way smaller, your pedaling efficiency would dwindle significantly. If you use a bike that is way too big, then it might be harder to handle. But if you use a bike that fits you perfectly, that is where comfort and efficiency would agree.
How are bikes measured?
Different mountain bikes and bike disciplines have different sizing needs. Older bike sizing was based on how long the seat tube length is. This length refers to the length from the center of the bottom bracket.
Before, frames differ mostly because it is based on this measurement scheme. However, this sizing scheme is still being used today but with changes to the following:
- Reach and stack height
- Top tube length
- Chainstay length
- Head angle angle
- Bottom bracket height
Many of these aspects change as the bike frame size changes as well. Many people also have the wrong misconception that bike measurement is based on wheel size. And that 29er mountain bikes are only for those that stand 6 feet and above.
This is completely wrong as wheel bike size is a matter of preference, and bike sizing is based on modern-day geometry.
Measuring the Bike
Reach and stack height
The reach and stack are determined by doing the following. For the stack, you must measure the height from the center of the BB to the horizontal level of the center of the top tube.
Now to determine the reach, you must get the horizontal distance from the center of the top tube to the point that is perpendicular to the center of the bottom bracket.
Top tube length
This is the horizontal length between the centerline of both the head tube and seat post. Your measurement must be parallel to the ground to get the right measurement of the top tube.
The wheelbase of the mountain bike is determined by measuring the distance from the center of the rear hub to the center of the front hub. This is important if you are buying kids’ bikes. Kids’ bikes need to be the right size for the child to ensure utmost safety.
Also called back center, the chainstay length is determined by measuring the center of the dropout or hub towards the center of the bottom bracket.
These two are probably one of the most significant measurements in modern geometry. You can measure this by getting the inside angle of the seat tube and head tube relative to the ground. You can use your smartphone’s inclinometer in doing so. A mountain bike frame has a slacker head angle compared to a road bike. This is because a road bike is made for flat roads while mountain bikes are for uphills and downhills.
Bottom bracket height
Bottom bracket height is determined by measuring the distance off the bottom bracket’s center from the ground.
Sizing Guide for Mountain Bike
There are many factors that you need to consider when it comes to bike sizing. This would include your height, preferences, and the type of bike your riding needs.
As discussed earlier, having the right bike frame size, whether it be road bikes or a mountain bike, will make your experience comfortable, fun, and very efficient.
To proceed with the sizing guide, you must first determine your riding style, your height, and the type of bike that you would ride.
Getting Your Height Measurement
The most common way to get the right bike size for you is to determine your height. To do this you will be measuring your height and the height of your inside leg. The first step to get the right size bike is to stand flat against a wall and mark the top of your head with a pencil.
Make sure that it is level so that you will get your exact size bike height. If it is difficult for you to do so, you can ask for assistance from someone.
With your back still against a wall, get a book and let it stand in between your legs. This will simulate the top tube measure the height of the book from the floor.
However, if your height is at the borderline of the bike size range, the reach measurement will be used as the deciding factor. This is determined by measuring your ape index.
What you need to do is to measure your arm span then subtract your height and compare it with the bike size. If your ape index is positive then a larger bike size is the way to go for you. If it is negative, then a bike size smaller would work best. This is especially true for road bikes that have a small frame size
Knowing Your Stand-over Height
This aspect is significant because it allows your groin some clearance once both your feet are planted on the ground. Normally around 2cm of clearance is enough.
To determine the perfect standover height for you, this is technically getting your inside leg measurement minus 2 cms. You can use this as a guide when you are looking at geometry charts.
Knowing how bikes are measured means it would be easy for you to choose the right bike for your size. Especially if you are opting to engage in racing or aggressive riding, having the correct bike geometry will make you more efficient while on trails.