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10 Basic Bike Geometry Terms You Need to Know

Fixed gear bike indoors.

If you have ever shopped around for a new bike, you probably heard the term “bike geometry” But what exactly does that even mean? Are we going back to a high school math class? In a manner of speaking, yes. Let’s take a closer look at this term and see if it reminds you of the study of polygons.

Man having a mathematic meltdown.

Bike geometry is a collection of measurements in lengths and the angles that define a bikes configuration. These measurements play a critical role in how a bike performs. 

Is that all there is to it? Absolutely not. But don’t worry, because in this post, we will discuss bike geometry along with names and definitions every cyclist should know. 

What Is Bike Geometry 

Bike geometry is a collection of measurements in lengths and the angles that define a bikes configuration. The key areas of focus are the wheelbase, steering axis angle, fork offset, and trail. These measurements all play a significant role in how a particular bike handles. Pretty much everything on a bike can be customized, and these measurements help a person do just that. Some people will call this bike geometry.

Bike geometry schema.
A common bicycle frame schema.

Basic Bike Geometry Terminology

When we talk about bike geometry, these are the specific things you should consider. Each of these has an impact on the design of your bike. You may want to adjust and customize these to fit your needs.

Frame Size

This is one of the most significant parts of a bike’s anatomy. It’s the most commonly known aspect of bike geometry. The frame of a bike is what everything else is based upon. Without the frame, there is no bike. 

The bike size is usually measured from where the seat post starts and to the crank, measured in centimeters, however, you might occasionally run into a manufacturer that refers to their bikes as small, medium, or large — double-check what that size means to that manufacturer to avoid confusion.

Below is a simple breakdown of frame sizes and how they relate to a human’s height. This is an excerpt from a post on bicycleguider.com. Their full guide on bike frames can be found here.

Stack and Reach

Rider heightLeg inseamFrame
Feet/InchesCentimetersInchesCentimetersInchesSize
4`10”-5`1”147-155 cm24-29”61-73 cm14”XS
5`1`-5`5`155-165 cm25-30”63-76 cm15”S
5`5`-5`9`165-175 cm26-31”66-78 cm16”M
5`9`-6`0`175-183 cm27`-32`68-81 cm17″L
6`0`-6`3`183-191 cm28`-33`71-83 cm18″XL
6`1`-6`6`191-198 cm29`-34`73-86 cm19″XXL
A basic bike frame guide.

the stack refers to the vertical portion of the bike when measured from the center of the frame up to the top of the head tube. This helps determine how tall a frame is. The reach refers to the horizontal measurement of the frame from the center of the bottom bracket to the top-center of the head tube. This helps determine how long the frame is. If a bike is too tall or long for an individual, that means they won’t be able to ride it comfortably, and they should find a bike with shorter dimensions of these metrics. 

Head Tube Angle, Fork Rake, and Trail

The head tube angle deals with the direction of the head tube relative to the ground. On a road bike, the standard head tube is at a 73-degree angle. The steeper (or higher) the number, the less difficult the bike is to steer. And the bike will also be more agile. 

The fork rake (also called the offset) is the distance between the steering axis and the wheel center. 

The trail is best described as the tire patch behind the steering axis. Its size is determined by the head tube angle and fork rake. A smaller trail means a bike with better handling. A bike with a lot of trail is going to be better for high speeds.

An excerpt from a paper showing the mathematics behind bicycle trail.
An excerpt from a paper showing the mathematics behind bicycle trail.

The whole thing involves a lot of math. But you don’t have to go through to be a rocket surgeon to figure allot of this stuff out. Now you can just check out this nifty online trail calculator!

Bottom Bracket Drop

This measurement comes from how far the bottom bracket sits below the wheel axis. This measurement translates into how low you sit on the bike. It’s something to be aware of if you want or need to be elevated on your bike. Also, the lower this number, the faster the bike is to respond to changes in speed and steering. The average drop averages out to about seven centimeters. If the drop gets much lower, it can be challenging to pedal around corners.

Red Royal Enfield Revelation bicycle bottom bracket.
Red bicycle bottom bracket.

Seat Tube & Seat Tube Angle

The seat tube angle refers to the angle of the seat tube in relation to the ground. If you change the seat tube angle, you can alter how steep or “slack” the bike is when in the saddle position. This angle tends to stay between 71-74 degrees. The seat tube is the portion of the bike that holds the seat/saddle of the bike.

Chain-stay Length

You can get this measurement by taking the horizontal distance from the bottom bracket to the center of the rear axle. The length of the chainstay has an impact on the length of the wheelbase and how the bike ends up handling overall. The longer the chainstay, the more stable the bike. This design works well on an endurance bike. Shorter chainstays work better on performance bikes and allow for sharper handling.

Cyclist with Stiff handling.

Wheelbase, Weight Distribution, and Front Center

The wheelbase is the horizontal distance between the two wheels of a bike. It is measured by determining the distance between the center of the two wheels. A long wheelbase is ideal for high speeds but can get in the way otherwise. The front center isn’t as crucial as other bike measurements. It is more of a nice-to-know measurement. This measurement can help judge toe overlap between bikes. 

Weight distribution helps make sure the weight of a rider is distributed correctly on a bike. Under ideal circumstances, 45% of both a bike and rider’s weight should be distributed on the front wheel. The remaining 55% should be on the back wheel. You can check this on any type of scale. Proper weight distribution is crucial for optimal performance. Too much weight on either wheel makes it difficult for a bike to turn and/or climb.

Silver bike on bridge with high top tube bike geometry.
Silver fixie with a very high top tube.

So What Bike Geometry Mean to Me?

This is a lot of information to take in, especially if you aren’t familiar with bikes or what any of these terms mean. The best thing you can do is to first determine what you need the bike for. Once you know the purpose of the bike, then you can do some research on what an ideal bike is for that category. And don’t forget to take a few out for a test ride. There is no better way to know what kind of bike feels best for you than to give it a spin. 

Conclusion

We hope this post taught you some bike geometry terminology you can use next time you need to upgrade or adjust your bike. 

In this post, we covered:

  • What bike geometry is
  • Frame size
  • Head tube angle 
  • Fork rake, and trail
  • Bottom bracket drop
  • Seat tube & seat tube angle
  • Chainstay length
  • Wheelbase, weight distribution, and front center

So have you ever heard of bike geometry? Is there something you’d like to add to this post? Let us know in the comments! Click here to check out all of our blog posts. Thanks for reading and stay fixed.

Written by Yerain Abreu

Yerain is the owner and operator of this fine website. As a native New Yorker, Yerain is no stranger to the fixed gear scene. He was there in 2008 when it was hot, and he's still riding fixed–day in, day out. When he’s not on the bike, you can find him practicing his many hobbies including, cycling, video production, and photography.

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