Everyone is talking about bike geometry. What exactly is a fork offset or a trail, and why do we need a steep seat angle? Learning geometry basics will help you understand how a bike feels and handles.
Bike geometry can be confusing to a lot of bikers, particularly to the new ones. And reading all these profound biking terms on the internet makes it a lot more unbearable!
The good news is that we already researched everything for you! All you have to do is read and understand. Want to become a better biker? Here’s all you need to know regarding bike geometry.
Stack and Reach
In the past, bike brands used the head tube and top tube to measure how long and low, high and short, and how aggressive a bike was. You can get a pretty accurate idea using these metrics, but it could be problematic.
Top tubes can be measured from various points. You can measure it from the center top tube or actual top tube. It wasn’t clear which brand was using which metric. This has made measurements incomparable across bikes. Also, due to the emergence of disc brakes and tire clearance, fork lengths now vary. This change made measuring headtube particularly useless to customers either.
Since the head tube and top tube aren’t that accurate, stack and reach came up. Stack and reach is the newest and easiest standard to start comparing bike fit. They have become so far the most relevant measurements when determining bike handling and bike fit. Stack is the distance from the center of a bottom bracket going up to the center of the headtube. Reach is the measurement from the middle part of the bottom bracket to the head tube’s center.
Stack and Reach Ratio
A short stack and a long reach place a biker in a fast and aggressive position. This position also allows the rider to be more aerodynamic, making it suitable for racers.
Suppose you want a comfortable ride, a shorter reach, and a higher stack. This is mainly favored by daily bike commuters and those who want to ride all day at slower speeds.
Stack and Reach ratio is stack divided by ratio. Getting a lower number means a more aggressive stance. A higher number, on the other hand, would result in a more relaxed and comfortable posture.
Stack and reach can be instrumental in determining bike handling and fit. But, there are warning signs you should bear in mind. Stack and reach do not consider different bike components like the stem length and handlebar reach. Thankfully, brands started solving this issue by placing stack plus and reach plus (reach +) in their spec sheets, considering the stem and the handlebar reach.
Fork Rake and Offset
Fork Offset is the measurement of the distance between your bike’s steering axis to the front axle. Fork Rake Offset is a measurement that’s essential to know to figure out your bike’s trail measurements. The headtube angle and the fork offset can both affect the length of your trail. Decreasing the angle of your headtube results in a long trail. Similarly, reducing your fork offset will also result in less trail. It would be best to remember that decreasing your fork offset will slow the ride down, while increasing the fork offset results in faster steering.
Head Tube Angle
The head tube has two key measurements influencing a bike’s handling; the angle and the length. Headtube is the angle from the fork measured horizontal, and it is described by these two terms: slack and steep.
Bikes with less travel, tamer terrain and mountain bikes will likely have steeper head angles, meaning they’re meant to handle quicker. That means your bike will demand lesser effort to steer. But when the terrain gets steep, a slacker angle should come into play. A slacker angle is much more stable at speed but can feel a bit lazy at lower speeds and harder to steer.
Some key measurements determine the way your bike handles and the way it feels to ride. The first of which is the trail.
The trail is essentially the product of the head tube angle and the fork rake. It refers to the distance between your front tire’s contact patch and the steer tube angle.
If your bike has a steeper head angle, that means it has less trail and forks with less rake. Bikes with less trail often are fast and twitchy. Also, shorter trails are better for commuters who usually take tight turns. With more trail, that means you’ll have slacker head angles and forks with less rake. And these bikes can typically feel stable at high speeds.
Bottom Bracket Drop
The bottom bracket drop is a static and fixed measurement that tells the distance of the cranks from the ground while you pedal. The lower the bottom bracket gets, the lower your center of gravity will be. That means you can corner your bike perfectly.
Off-road touring bike manufacturers often provide a high bottom bracket, while others offer a lower bottom bracket for better stability, common in road-oriented touring bikes.
Going too low the bottom bracket will cause your pedals and cranks to go low to the floor, which risks you striking your pedals and blocks or stones. That’s why we recommend that you get the right balance between a bike that corners well but is high enough off the floor.
Seat Tube Angle
To simply put, seat tube angle is the angle of your seat tube in reference to the ground. There are two different ways to measure this. There’s the actual seat tube angle and the effective seat tube angles.
The actual seat tube angle is exactly what it is: the exact angle of the seat tube. The issue with seat tube angles is it often starts inches away from the head of the bottom bracket, and sometimes it starts right above it. This doesn’t tell where the seat will end up where you need it to be, and that’s where the effective seat tube angle comes in.
The Effective Seat Tube Angle can be measured by drawing a line from the center of the bottom bracket up to a specific height position. The actual seat post angle is the measured angle between the seat post and the floor. It is usually based on a bike’s layout and frame design.
ESTA is an adjustable measurement. If you want to alter your ESTA, move your seat forward or backward. Your ESTA will also vary depending on the position, whether it’s a fully extended, middle, or dropped position.
Chainstay Length is affected by the wheelbase of your bike. Chainstay length is the distance between your front and rear wheel axles, which significantly influences the length of the wheelbase and the bike’s handling. A longer chainstay would mean more stability, while having short ones leads to sharper handling. Also, according to Bowman Cycles, short chainstay on larger bike frames pushes you further back than when in smaller frames.
Another market for how stable a bike will feel is the wheelbase. You’ll get a larger turning circle for longer wheelbases, while a shorter wheelbase has a shorter turning circle. The wheelbase has two parts: the front center and the rear center.
A higher number for both the front center and rear center will feel more stable and comfortable at lower speeds, and a lower number will feel nippier at a faster pace. Some bikes that offer long wheelbases are touring and endurance bike roads, while ross country bikes offer shorter wheelbases.
Other measurements to consider when looking at your bike’s geometry are stem length and bar reach. Stem Length measures from your steer tube’s center to the handlebar clamp’s center. A longer stem will significantly slow the handling of your bike. Thus, it requires more effort from your hands to get your front wheel to move. This means you can affect how your bike handling depending on whether you are using a short or a long stem.
Stem length differs depending on the brand and frame. It is also one of the major parts that can determine a rider’s style and flexibility. That is why, before choosing a stem, consider your flexibility and style. If you’re a rider with poor flexibility and want to ride upright, you may need to dump the standard stem and customize it for a short one.
How wide should your handlebars be? Road handlebars come in different sizes from 38cm-46cm. Traditionally, bikers choose a handlebar that is the same width as their shoulders. The most common methods in measuring handlebar width are the center to the center (C-C) or outside to outside methods (O-O).
Center to center measures horizontally from the middle of the lever hoods. Outside to outside is the measurement from the outside of the lever hood, resulting in a larger size. Note that like how stem lengths work, the smaller the frame, the narrower the bars measure. Narrower handlebars may improve aerodynamics though it can result in “twitchy handling.” On the other hand, getting wider bars can result in more stability, perfect for climbing and sprinting.
Crank length is probably the most crucial adjustment you never thought about. It’s also one of the most expensive adjustments to make. So, what is the crank length, and why is it important?
Crank length is measured by getting the distance between the middle of the pedal axis and the center of the bottom bracket. Some of the most common sizes you’ll encounter are 170, 172.5, and 175 mm. But, some cranks range between 165mm and 180mm.
What is a Bike Geometry Chart?
A bike geometry chart is a table where you can get all the critical measurements of a bike model. Measurements may vary from one bike manufacturer to the other. But generally, it will include all the essential things you need to know, like the main tube lengths, headtube angle, seat tube angle, stack and reach and others.
Bike geometry charts also show measurements of the rake and trail to assist in knowing what the bike’s geometry and handling characteristics are.
Is the Bike Geometry Chart Useful When Choosing a Bike?
Bike Geometry charts have a long list of uses. Knowing how to read a bike geometry chart correctly can assist you in learning your most suitable frame size, bike fit, bike handling, and bike component compatibility.
But remember that some geometry charts aren’t created equal. Some can assist you with everything you need to know, and some can’t tell you anything other than how models and sizes are different from each other.
Proper Bike Fit
Setting up your bike correctly is the main difference between a comfortable and efficient ride and an inefficient and uncomfortable one. You can adjust these bike parts to ensure a proper fit—the stem, saddle, handlebars, and cranks. Make sure you get your bike set up correctly if you’re buying it from a bike shop. Having it correctly set up can save you from costly adjustments.
Adjusting your stem and bars determine how stretched out you are on the bike. It is by far the most important change you can make to get proper fit and handling. You may not feel the need to adjust, but if you feel too stretched out, you should change it to control your steering appropriately. It also removes unnecessary strain and pressure on your neck and shoulders.
The proper bike fit and handling would highly depend on your riding style and how you feel during the ride. That is why we highly suggest that you test the bike first before closing the deal. As we said earlier, bike adjustments can be costly, and not having them adjusted according to your riding style and preference will cost you a lot of money. Are you buying a new bike? Check these best budget full suspension bikes!