This post compares the differences between the actual seat tube angle and the effective seat tube angle.
Seat Tube Angle (STA).
The seat tube angle (STA) is the angle between the seat tube and a horizontal line (parallel to the ground) running through the bottom bracket. (image above)
Note: It’s easy to conclude that the STA is the angle formed by the seat tube and the chainstays, but that is incorrect. The chainstays are irrelevant in this case.
Effective Seat Tube Angle (ESTA).
The effective seat tube angle is the angle between a horizontal line (parallel to the ground) running through the bottom bracket and a line passing through the bottom bracket and the middle of the junction where the seat meets the seat post. (image below)
In the graphic above, you see the effective seat tube angle of a road frame with a curved seat tube.
The next graph shows the effective seat tube angle of a full-suspension MTB.
Those are the two most common instances when the effective seat tube angle is discussed.
The Main Differences Between STA and ESTA
The seat tube angle is constant as it’s formed by the position of the bottom bracket and the seat tube – both of which are unchangeable once a frame is built.
Meanwhile, the effective seat tube angle is more “adaptive/dynamic” and “accurate” for the following reasons:
- It reflects the position of the saddle. (If you move the saddle forward, the effective seat tube angle gets steeper; if you move it backward, it gets slacker.)
- The effective seat tube angle provides valuable data regardless of the frame’s shape and is, therefore, more “universal”.
- Accounts for saddle height too.
The height position of the seat influences the effective seat tube angle. This happens because the ESTA uses the mid-saddle as a reference point.
By elevating the saddle, one is technically slackening the angle; by lowering the saddle, the effective seat angle gets steeper.
The graph below illustrates the phenomenon.
I know that the above graph appears a bit complex, but it’s not all that difficult to grasp the concept once you know what you’re looking at.
- The black lines represent two positions of the saddle – low and high.
- The curved pink line is the seat tube of the bike. (The curve is exaggerated on purpose).
- The grey line is the seat post in the two positions (low and high).
- STA (L) indicates the effective seat tube angle in the low position of the saddle; STA (H) indicates the effective seat tube in the high position of the saddle.
By elevating/extending the seat post, we’re slightly pushing the saddle towards the rear wheel. The new position alters the ESTA.
The more we extend the seat post, the slacker the effective seat tube angle gets.
By lowering the seat post, we are doing the opposite – namely steeping the effective seat tube angle.
Those changes indicate that the same bike would have different effective seat tube angles for riders with different inseams. The taller rider would have a slacker ESTA; the shorter rider would have a steeper ESTA.
When companies present the effective seat post angle of a bike, they use a conservative seat post exposure to make the calculations.
If you have selected a frame that’s slightly smaller for you, and you have to extend the seat post more, you will have a slacker seat post than what the manufacturer indicates.
Note: The non-effective/standard/actual seat tube angle (image below never changes even when the seat post is extended because the seat tube’s position is constant. Therefore, even if we extend the seat post to the sky, that angle will not be altered
For that reason, many people consider the effective seat post angle a more accurate representation of how a bike would feel.
The Position Of The Saddle Matters Too
It’s also necessary to mention that the horizontal position of the saddle matters too.
Summary + Conclusions
- The actual seat tube angle is formed by the seat tube and a horizontal line going through the bottom bracket. The actual seat tube angle is constant once the frame is built.
- The effective seat tube angle is dynamic. This is the angle between a straight line from the middle of the seat and seat post junction and a horizontal line passing through the bottom bracket.
- Since the effective seat tube angle uses the saddle rather than the seat post, its values are a more accurate representation of the bike’s real geometry. In the case of a road bike with a curved seat tube or a full-suspension MTB, the effective seat tube angle is the go-to measurement because the seat tube doesn’t make a straight line to meet the bottom bracket.
- The height of the saddle affects the effective seat tube angle. More height results in a slacker effective seat tube angle; less height steepens the effective seat tube angle.
- The actual seat tube angle and the effective seat tube angle will coincide only when the frame has a straight seat tube and the saddle’s horizontal position isn’t altered. Modern geometries, however, often prevent that outcome.
- Ultimately, the effective seat tube angle is the more accurate value to look at when analyzing a bike’s geometry.
Obsessing Over Seat Tube Angles Is Not Recommended
It’s important to note that there isn’t a “perfect” seat tube angle.
The seat tube angle is just one part of the equation. There are many other factors that matter and greatly influence the performance of a bicycle.
Those would be:
- Overall fit (A frame with perfect angles that doesn’t fit you well is no good.)
- Bike Type (Getting the right bike for your goals is way more important than playing math with its frame.)
- Quality (A quality bike with descent suspension will always outperform a bike of poor craftmanship but with an ideal seat tube angle).