To fully comprehend, the effect of the seat tube angle on a bike’s reach, it’s necessary to first become acquainted with the involved terms.
Reach. The term reach refers to the horizontal distance from the center of the bottom bracket to the middle of the head tube. (image above).
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.
Effective Top Tube (ETT). 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.
Note: If you want to learn more about the difference between the effective top tube and the reach, consider reading the dedicated post.
What is the effect of the seat tube angle on a bike’s reach?
If the length of the top tube is changed accordingly, there’s technically no effect.
The reach measures the horizontal distance between the middle of the head tube and a vertical line passing through the bottom bracket.
The STA has no virtual effect on the bottom bracket position and therefore doesn’t impact the reach of a bicycle when the top changes accordingly.
It’s also important to note that the reach property is in full effect only when the rider is standing on the pedals.
The actual property that matters when the rider is seated is called effective top tube length (explained above).
This is the property that successfully expresses the distance between the seat post and the front end of the bike.
The Effects of Seat Tube Angles On Top Tube Length
The relation between the seat tube angle and the effective top tube is as follows:
For every degree of seat tube angle change, the effective top tube changes by approximately 1cm.
At the moment, a standard seat tube angle will be about 73°. If the angle is larger, the STA is classified as steep, if it’s smaller, the angle is getting slacker.
Hypothetically, if a bike has a 73° STA and a 57cm top tube, and the STA is “slackened” to 72°, the top tube length will have to increase by approximately 1cm, and with it the effective top tube length.
Of course, this will take place only if the rest of the frame remains the same. If the new STA is coupled with a shorter downtube, then it won’t be necessary to increase the length of the top tube for it to reach the new position of the seat tube.
In that particular situation, the seat angle has an effect on the reach because it shortens the downtube tube and with it the distance between the bottom bracket and the head tube.
In the opposite case, namely steeping the STA to 74°, the top tube will have to be shortened by approximately 1 cm if the down tube and the frame angles are preserved.
However, if we want to preserve the top tube the same length as before, it will be necessary to increase the length of the downtube so that the top tube can fit.
In that particular situation, the STA will technically increase the reach due to the longer down tube positioning the head tube further away.
Additional Explanation With Images
I understand that the above paragraphs could be slightly complicated to understand and will try to restate the dependencies one more time.
The three basic principles are:
a. If the length of the top tube is kept the same and the seat tube angle is made slacker, the reach will decrease.
b. If the length of the top tube is kept the same and the seat tube angle is made steeper, the reach will increase.
c. If the top tube length reflects the changes in the seat tube angle, the reach will remain the same, but the effective top tube length will increase (if the STA gets slacker) or decrease (if the STA gets steeper).
Option A: The Seat Angle Gets Slacker
The first graph shows the normal 73-degree STA.
If we slacken the STA by 1 degree and keep the original top tube length, the new STA will technically “pull” the top tube towards the rear wheel and reduce the angle formed by the top tube and the seat tube.
This change requires a shorter down tube. And if the down tube gets shorter so does the distance between the head tube and the seat pot. Thus, the reach will get shorter too.
A degree of STA change results in roughly 0.93cm of reach change. In this case, we have a decrease.
The last graph shows a steeper 74-degree STA angle. If we keep the top tube length unchanged, the new STA will position the top tube further away from the rear wheel. Consequently, the downtube will have to be longer. A longer down tube increases the reach.
Summary: What You Need To Know
- Changing the seat tube angle affects a bicycle’s reach only if the top length is kept the same.
- If the top tube gets longer or shorter as required for the new seat tube angle, the reach remains the same.
- The downtube is the tube that has a direct effect on reach.
- If we keep the top tube the same and make the seat tube angle smaller (slacker), the downtube will have to get shorter. This will effectively decrease the reach.
- If we keep the top tube the same and make the seat tube angle larger (steeper), the downtube will have to be longer. This will increase the reach.
Summary of the Summary
When the top tube length is unchangeable:
Steeper STA = Longer Reach
Slacker STA = Shorter Reach
1-degree of STA change triggers about 1cm reach increase or decrease.