This post will reveal how one can affect the effective seat tube angle without getting a new frame.
First, let’s present a couple of important definitions that will make the process easier to follow.
What is a seat tube angle?
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.
Effective Seat Tube Angle
The STA definition above applies only when the seat tube is straight. However, in many cases, the seat tube won’t be fully straight.
In those scenarios, we rely on the so-called effective seat tube angle (ESTA).
This is the angle between a horizontal line (parallel to the ground) running through the bottom bracket and a line running through the reference point on the saddle and the bottom bracket.
As you can see in the image above, the actual seat tube is ignored due to its curve. Instead, we use the middle of the connection point between the saddle and the seat post to guide a line through the bottom bracket.
In the case of full-suspension bikes, the same method is used because the seat tube is pointing in front of the bottom bracket (if we extend it with a line).
What is a steep seat tube angle?
The larger the seat tube angle is, the steeper it becomes because the saddle gets closer to the handlebars.
In general, the most common seat tube angle is 73-75°. If the STA is larger than those numbers, it’s on the steeper side; if it’s smaller, it’s considered “slacker“.
On average, road bikes tend to have a seat tube angle of about 73.5° whereas that of MTBs is slacker at 74.5° or more.
How To Steepen The Effective Seat Tube Angle For Free
The seat tube angle is set in stone and cannot be changed unless the frame is rebuilt. This is too costly and more often than not it’s better to buy another frame.
The effective seat tube angle (explanation above) can, however, be affected by the position of the saddle and the seat post itself.
Making the Seat Tube Angle Steeper Via The Saddle
- Option 1: Slide The Seat Forward
Saddles have rails that connect to the seat post via a clamp secured with a 6mm Allen key. If you untighten the clamp, you can slide the seat forward (or backward).
Sliding the seat forward will steepen the effective seat tube angle. (Sliding it backward will slacken the seat tube angle).
In the images above, the saddle is positioned maximally forward resulting in the steepest possible effective seat tube angle that the bike can have.
Usually, the saddle provides about 2cm of movement which adds or subtracts about 2-4-degrees from the effective seat tube angle.
Note: Alternatively, you can get a saddle model with a rail structure that gives you a longer range of motion. For better or worse, similar models are hard to find these days as they are in the retro category.
Option 2: Add a Different “Forehead” to The Seat Post
Another option that could steepen (or slacken) the effective seat tube angle even more is a product known as Fall Line R Forehead. This is an add-on that could add 25mm of additional forward or backward seat movement.
However, the “forehead” is compatible only with seat posts made by the same brand (Fall Line).
Also, the forehead is designed only for forward movement (steeping the STA) or backward (slackening the STA).
What Are The Advantages of Steeping The Seat Tube Angle
- Improved Front Traction
A steeper seat tube angle positions the rider closer to the front end of the wheel. The shift increases the weight on the front end and improves front wheel traction.
- Unweighing The Rear Wheel
The new geometry “frees” the rear wheel by placing more of the rider’s weight forward. As a result, it’s easier to pedal on ascents.
- Lower Chances of Looping Out
If the seat tube angle is extremely slack, the front wheel could raise unexpectedly. In extreme situations, the bike may loop out. When that happens, the rear wheel slides under the rider as it often happens during a failed wheelie attempt.
- Reduction of the Effective Top Tube
In this case, making the effective seat tube angle steeper will directly impact the effective top tube.
The effective top tube is the horizontal distance from the point where the top tube and headtube meet and an extended line from the seat tube. The term has become widely used due to the popularity of sloping top tubes.
By sliding your saddle forward, you will be shortening the effective top tube. In some cases, this will be beneficial because the handlebars will get closer to you.
And if the frame is somewhat larger for you, or maybe you have a long inseam, but shorter arms, the change will be beneficial.
Note: Many think that the seat tube angle directly affects the reach of the bicycle.
(The reach is the horizontal distance from the center of the bottom bracket to the middle of the head tube. (image above).)
Technically, in this particular situation, namely sliding the saddle forward, the seat tube angle has no effect on reach.
First, the reach only applies when the rider is pedaling outside of the saddle. If the rider is seated, the correct value to look at to express the distance between the rider and the handlebars is the effective top tube.
Second, the seat tube angle affects the reach but only when designing a frame. If the frame is already built, the reach is set in stone.
You can read more about the effect of seat tube angles on reach in the dedicated post.
Mini Summary: What You Need To Know
- The position of the seat affects the effective seat tube angle.
- If you slide the seat forward, you will steepen the seat tube angle.
- If you slide the saddle backward, you will slacken the seat tube angle.
- A steeper seat tube angle is beneficial for climbing thanks to the increased front traction, the reduced weight on the rear wheel, the lower chance of accidentally lifting the front wheel, and the more vertical leg push.