If youโve ever shopped around for a new bike, you probably heard the term โbike geometry,โ But what exactly does that even mean? Are we going back to a high school math class? In a manner of speaking, yes. Letโs take a closer look at this term and see if it reminds you of the study of polygons. But firstโwhat is bike geometry?
Bike geometry is a collection of measurements in lengths and angles that define a bikeโs configuration. These measurements play a critical role in how a bike performs.ย
Is that all there is to it? Absolutely not. But donโt worry, because, in this post, you will learn the basics of bike geometry, along with some terms and definitions every cyclist should know.ย
Editorโs note: This article was updated on October 14, 2021, to include additional information regarding bike geometry.
What is bike geometryย
Bike geometry is a collection of measurements in lengths and angles that define a bikeโs 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.
Basic bike geometry terminology
When we talk about bike geometry, there are a few things you should consider. Each of these has an impact on the feel of your bike. You may want to adjust and customize these to fit your needs.
1. Frame Size
This is one of the most significant parts of a bikeโs anatomy. The frame is what everything else is based upon. Without the frame, there is no bike.ย
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.
If you want some help in determining your ideal frame size, check out our guide on how to choose a fixed gear bike. You can also check out the simple breakdown of frame sizes and how they relate to a humanโs height below. This is an excerpt from a post on bicycleguider.com. Their full guide on bike frames can be foundย here.
| Rider height | Leg inseam | Frame | |||
|---|---|---|---|---|---|
| Feet/Inches | Centimeters | Inches | Centimeters | Inches | Size |
| 4โ10โ-5โ1โณ | 147-155 cm | 24-29โณ | 61-73 cm | 14โณ | XS |
| 5โ1โฒ-5โ5โฒ | 155-165 cm | 25-30โณ | 63-76 cm | 15โณ | S |
| 5โ5โฒ-5โ9โฒ | 165-175 cm | 26-31โณ | 66-78 cm | 16โณ | M |
| 5โ9โฒ-6โ0โฒ | 175-183 cm | 27`-32` | 68-81 cm | 17โณ | L |
| 6โ0โฒ-6โ3โฒ | 183-191 cm | 28`-33` | 71-83 cm | 18โณ | XL |
| 6โ1โฒ-6โ6โฒ | 191-198 cm | 29`-34` | 73-86 cm | 19โณ | XXL |
2. Stack
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.
3. Reach
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 wonโt be able to ride it comfortably, and they should find a bike with shorter dimensions of these metrics.ย
If you want to determine your stack and reach measurements, check out our guide on how to calculate your bikes stack and reach.
4. Head Tube Angle
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.ย
5. Fork Rake
The fork rake (also called the offset) is the distance between the steering axis and the wheel center.ย
6. Trail
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.
As you can see from the image below, the trail can get very complicated very fast.
The whole thing involves a lot of math. But you donโt have to go through to be a rocket surgeon to figure this stuff out. Nowadays, you can just check out this niftyย online trail calculator!
7. 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. Itโs something to be aware of if you want or need to be elevated on your bike.
And, 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.
8. 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 โslackโ 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.
9. 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.
If youโre in the market for a new chain, check these out.
10. Wheelbase
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 isnโt as crucial as other bike measurements. It is more of a nice-to-know measurement. This measurement can help judge the overlap between bikes.ย
11. Weight distribution
Make sure your weight is distributed correctly on a bike. Under ideal circumstances, 45% of both a bike and riderโs 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.
12. Standover height
The standover height is the distance between the ground and the top tube of the bike. This is one of the most important measurements to consider when purchasing a bike. Below is a video on how to determine your standover height from Pure Cycles.
Conclusion
This is a lot of information to take in, especially if you arenโt familiar with any of these terms. To find your ideal geometry, take a few bikes for a spin, and note the one that you like best. There is no better way to know what kind of bike feels best for you than to give it a spin. Then have the shop provide you with info on that bikeโs measurements.
We hope this post taught you some bike geometry terminology you can use next time you need to upgrade or adjust your bike.ย
In this article, we covered the basics of bike geometry and provided some of the definitions you should know. Here are some key takeaways:
Key takeaways
- Bike geometry refers to the the angles on a bikes frame and fork.
- Finding your perfect mesurmets can help you pick the perfect bike for you.
- To find your ideal geometry, take a few bieks for a spin, and note the one that you like best, then have the shop provide you with info on that bikes mesurments.
So, have you ever heard of bike geometry? Is there anything youโd like to add to this post? Let us know in the comments below (we read and reply to every comment). If you found this article helpful, check out our full blog for more tips and tricks on everything fixie. Thanks for reading, and stay fixed.
