Condensed Answer: The average seat tube angle of a dirt jumper is 71.3º
This post examines the seat tube angle of dirt jump bikes. If you are not familiar with the terms seat tube angle and effective seat tube angle, consider reading this article.
The table below contains the seat tube angles of many popular dirt jumpers.
|Model||Seat Tube Angle|
|Saracen Amplitude CR2||73°|
|Santa Cruz Jackal||72.7°|
|NS Bikes Metropolis 3||70°|
|Scott Voltage YZ 0.1||71°|
|Canyon Stitched 720||72.5º|
|NS Bikes Movement 3||72º|
|NS Bikes Zircus||70°|
|Haro Steel Reserve 1.1||70°|
|Polygon TRID 2021||72º|
Conclusion: The average seat tube angle of a dirt jumper is 71.3º. This is a slack number in comparison to what we find on road, gravel, XC, and enduro models. However, it’s very close to the average seat tube angle of a BMX.
Why Do Dirt Jumpers Have A 71.3º Seat Tube Angle On Average?
А 71.3º seat tube angle is technically slack, albeit not extremely. The reasons behind this choice are:
- Slack Head Tube Angle
Dirt jump bikes are designed for jumps and tricks such as bunny hops and manuals. There are two geometrical properties that facilitate that task – a slack head tube angle and short chainstays.
The head tube angle is the angle formed by the head tube and the ground. Dirt jump bikes have a fairly slack head tube angle at about 69º.
A slack head tube angle is beneficial because it raises the front end, reduces the amount of weight over the front wheel, and positions the rider close to the rear wheel. When you add the short chainstays, you get a bike that allows you to lift the front wheel with minimal effort.
The slack head tube angle, however, cannot be combined with a very steep seat tube angle. If that’s done, some of the positives (e.g., less weight on the front wheel) will be negated. Thus, the angle needs to be fairly slack.
If the angle is too slack, however, the top tube will have to be longer and will make the bike feel less agile – an integral property of a dirt jumper.
Consequently, the seat tube angle is kept in a conservative range that’s neither extremely slack nor steep.
The Seat Tube Angle Matters When Building The Frame
Dirt jumpers are not designed for seated pedaling. The saddle is there for brief rest periods between attempts. Thus, you will rarely see someone commuting on a dirt jumper.
Therefore, the seat tube angle doesn’t play an essential role when it comes to performance. The head tube angle is more important.
However, the seat tube angle influences the overall geometry of the frame before it is constructed.
For example, if you have a frame designed with a 71º seat tube angle and slacken the seat tube angle to 69º, the rest of the frame will have to change too.
What are the options?
The first option is to simply extend the top tube so that it can reach the slacker seat tube. That move will keep the bike’s reach the same because the distance between the bottom bracket and the head tube will not change. However, the longer top tube will add weight and make the bike feel longer.
The second option, in this case, is to keep the top tube the same length. If that’s done, however, the downtube will have to be shorter or else it won’t reach the seat tube.
If the downtube is shortened, the reach will decrease and the bike could feel too small.
The seat tube angle’s effect on reach is explained in greater detail in the dedicated article.