Condensed Answer:
A 73° seat tube angle is 1° to 4° slacker than what we find on modern MTBs. In the world of road bikes, 73° is a common middle ground.
What is a seat tube angle?
Many assume that the seat tube angle is formed by the seat tube and the chainstays, but this isn’t correct.
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.
What is a Slack Seat Tube Angle?
The smaller the STA, the “slacker it is”.
In different, if the angle gets larger, it is considered “steeper”.
Why Do Modern MTBs Have Steeper Seat Tube Angles?
The main reason for the steeper tube angles on modern MTBs is actually another angle part of the frame geometry – namely the Head Tube Angle or HTA.
The HTA is the angle formed by the head tube and the ground.
Modern MTBs have slacker (smaller) HTAs in comparison to the MTBs of the past. The advantages of a slacker HTA are:
- The front wheel is positioned further in front of the frame and has an easier time overcoming obstacles.
- The frame can be combined with a long fork with a massive travel.
- The rider is less likely to go over the handlebars. In other words, slacker HTAs are safer when going downhill.
The slacker head tube angles of modern MTBs require steeper seat tube angles for balance.
If the HTA of an MTB frame is slackened and the STA remains the same, the rider will be too far back.
As a result, climbing will be more difficult, and the chances of lifting the front end of the bike accidentally will be high.
Ultimately, the steeper seat tube angles today are simply a necessity resulting from the slacker head tube angles rather than a goal.
So, Is a 73 Degree Seat Tube Angle Too Slack?
To answer the question, it’s necessary to examine the STAs of modern XC and trail bikes.
| Model | STA |
| Specialized Epic World Cup PRO | 74.5° |
| Cannondale Scalpel HT Hi-MOD Ultimate Carbon | 74.5° |
| FOCUS Raven 8.8 | 74° |
| Specialized Chisel Comp | 74° |
| Specialized Epic Expert | 75.8° |
| Santa Cruz Highball 3.0 | 73° |
| Scott Spark 930 | 75.9° |
| Santa Cruz Blur CC X01 | 76.3° |
| YT Izzo Pro Race | 77° |
| Merida Ninety-Six RC9000 | 74.5° |
| Giant Anthem Advanced Pro | 75.5° |
| BMC Fourstroke 01 One | 75.6° |
| Trek Supercaliber 9.8 | 74° |
| Giant XTC SLR | 74° |
| Average: | 74.9° |
| Model | STA |
| Canyon Spectral CF 7.0 | 76° |
| Focus JAM 6.8 Nine | 76° |
| Giant Trance 29 2 | 76.3° |
| Merida ONE-FORTY 800 | 75° |
| Scott Genius 950 | 75.3° |
| Stumpjumper Comp Alloy | 77.7° |
| Trek Remedy 8 | 74.7° |
| YT Jeffsy 27 AL Base | 75° |
| Cannondale Habit Carbon 1 | 77.5° |
| Nukeproof Reactor 290 | 75.5° |
| Norco Optic C1 | 76° |
| Average: | 75.9° |
Conclusions:
- The average seat tube angle of modern XC bikes is 74.9°.
- The average seat tube angle of trail bikes is 75.9°.
Given the data, a 73° seat tube angle is indeed too slack by modern standards.
That being said, the seat tube angle is just one part of the equation and should never be studied in isolation.
There are other factors that hold a higher importance. Those would be:
- Frame quality (it’s better to have a well-built bike with “too slack” STA than an optimal STA and a badly built frame).
- Suspension quality and travel
- Wheels
- Overall comfort (Angles don’t matter when you feel weird and uncomfortable on your bike).
Mini Summary: What You Need To Know
- Modern MTBs have slacker head tube angles which demand steeper seat tube angles for optimal balance and performance.
- The modern seat tube angle is in the 74-77° range.
- A 73° is technically too slack by modern standards, but it’s not outdated to the point where the bike can’t serve you well.
- If the bike feels good and is built well, then 2-3 degrees don’t matter. Just ride.
