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This post compares the advantages and disadvantages of different seat tube angles on performance and overall riding experience.

What is a seat tube angle?

Many people think 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.

Steeper Seat Tube Angle

• A steeper or larger seat tube angle positions the saddle closer to the handlebars.

Slacker Seat Tube Angle

• A slacker or smaller seat angle positions the saddle away from the handlebars and closer to the rear wheel.

The Advantages of a Steeper Seat Tube Angle

• Easier Climbing

A steeper seat tube angle positions the saddle and respectively the rider closer to the handlebars. As a result, there’s less weight over the rear wheel and it becomes easier to spin the pedals when climbing.

The extra weight on the front wheel is also beneficial during technical climbs as it results in greater traction and reduces the chances of lifting the front end accidentally.

• More Powerful Pedaling

A steeper seat tube angle puts the rider in a more vertical position in regard to the bottom bracket. As a result, the power transfer from the legs to the cranks increases.

I will use a somewhat alternative way to explain how this leverage system works.

Imagine that you want to squish an aluminum soda can with your foot. What would you do? You will more than likely position your foot directly over the can and then press down aggressively.

You wouldn’t put the can further away from you because you instinctively know that a vertical leg push will produce the largest amount of power and thus make it easier to smash the can.

Bringing the hips closer to a vertical line going through the bottom bracket has a similar effect. And a steeper seat tube angle makes that geometrical change a possibility.

• Balances out a Slacker Head Tube Angle

No angle in a bike frame should be studied in isolation. They are all connected and when the engineering is good, the combination results in better performance in the particular discipline.

The head tube angle (HTA) is the angle formed by the head tube and the ground.

Currently, MTBs have slack head tube angles. This means that the front wheel is positioned further away from the rider.

The advantages of a slacker HTA are:

1. Less weight on the front end makes it easier to lift the front wheel.
2. The bike has an easier time overcoming obstacles, especially on descents. Hence the extremely slack HTA of downhill bikes.
3. Forks with a longer travel can be installed.

If a slacker HTA is taken to the extreme, it will put the rider too far away from the bars and over the rear wheel. The position is uncomfortable, makes pedaling inefficient, hurts climbing, and increases the chance of accidentally lifting the front wheel.

A slack HTA requires a steep STA. Otherwise, the bike’s geometry will become too imbalanced.

The Downsides of a Steeper Seat Tube Angle

• Uncomfortable Pedaling on Flat Ground

If a seat tube angle is too steep, pedaling on flat ground becomes weird and uncomfortable due to the position of the body in relation to the bike’s bottom bracket. Knee and hip pain can also occur.

That said, such an extreme, will not take place on a well-engineered frame.

• Demands a Long Front Center and a Slacker HTA

A steeper head tube angle will require the front center of the bike to be further away from the rider. Otherwise, the reach of the bike will be poor, and the rider could be thrown over the handlebars very easily.

FAQ: What would be considered a steep seat tube angle?

73° STA is considered the happy medium – it’s neither too steep nor too slack.

If the angle is larger than 73°, it’s on the steeper side; if it’s smaller, it’s getting slacker.

At the moment, the vast majority of MTBs have an STA of around 73-74°.

The Advantages of a Slacker Seat Tube Angle

Slacker seat tube angles are currently not the norm for MTBs or road bikes. However, they’re often found on bikes designed for long distances (e.g., touring models).

The advantages of a slacker STA are:

• Less weight on your hands

A slacker seat tube angle positions more of the rider’s weight behind the bottom bracket and closer to the rear wheel. As a result, there’s less strain on the wrists, elbows, and shoulders. This is considered beneficial for touring – a discipline where comfort for long hours is crucial.

• Compliments a Relaxed Upright Position

A slacker seat tube angle fits well into upright geometries that make the rider’s back angle more horizontal.

The Disadvantages of a Slacker Head Tube Angle

• Not ideal for climbing

When the center of gravity is shifted towards the rear wheel, it gets harder and harder to maintain traction during climbs. Also, the rear wheel becomes harder to spin as it’s “suffocated” by the rider’s weight.

For that reason, some riders instinctively bend over or stand up to place more of their body weight over the front end during difficult and/or prolonged climbs.

• Not ideal for aggressive road riding

If an extremely slack seat tube angle is put on a road bike, for example, the rider would feel extremely stretched when pedaling in the aero position.

A “normal” or even a steeper STA is more beneficial to the common road bike stances.

This is also observed in Time Trial (TT) bikes. TT models have the steepest STA (e.g., 78° ) as they’re built for minimum body drag and maximum power transfer to the pedals. Comfort is not at all the intent behind such STAs.

Comparison of STAs

The tables below compare the STAs of XC, trail, and road bikes.

Modern XC Bikes

Modern Trail Bikes

Modern Road Bikes

Touring Bikes

Conclusions:

• Trail bikes have the steepest seat tube angles of the bicycles in the tables (76° on average). Then, seat tube angles get progressively slacker. The gradation is Trail bikes > XC > Road > Touring

Summary: What You Need To Know

• Steeper STAs are the norm in the case of modern MTBs, primarily to balance out the slack HTAs.

• A steeper STA makes climbing easier but could hurt pedaling comfort on flat terrain.

• Steeper STAs compliment aggressive riding.

• Slacker STAs are more common for bicycles designed for long riding when comfort is a high priority.

• Ultimately, the STAs should not be analyzed in a vacuum. Instead, one should take into account the entire geometry of the bicycle.