## The Optimal Seat Tube Angle of a Tri Bike

The vast majority of tri bikes have a seat tube angle of around 78°.

This angle allows the rider to assume the horizontal back position that tri bikes are built for.

If the angle gets any slacker, the legs of the rider will hit the chest, and pedaling efficiency and comfort will be lost.

Note: The seat tube angle varies depending on frame size. The table below focuses on medium size frames.

Conclusion:

The average effective seat tube angle of triathlon bikes is 77.28°. That number is much steeper than what we observe on road bikes. (The average effective seat tube angle of a road bike is about 73-74°).

## What Are The Advantages of Steeper Seat Tube Angles In Triathlon

• Maintaining The Standard TT Aero Position

Tri bikes are designed to be maximally aerodynamic. That position is achieved with a massive saddle-to-handlebars drop that makes the rider’s back as horizontal as possible. The result is minimal body drag.

The horizontal back angle, however, decreases the hip angle. (The angle between the torso and the femur).

If the effective seat tube angle is made slacker instead of steeper, the hip angle will close even more, and the rider’s legs will begin hitting the chest during pedaling.

Consequently, the angle is steepened via the frame. A steeper seat tube angle opens up the hip angle and gives the legs enough space to go up and down during the pedal stroke.

Note: Modern road bikes have a massive saddle-to-handlebars drop too, but the geometry isn’t as aggressive and there are no aero bars.

The aero bars on tri bikes result in an even bigger effective saddle-to-handlebars drop because the rider rests his elbows on the pads.

Road bikes do not rely on aero bars and thus an extra steep ESTA is not needed to prevent contact between the chest and legs.

• Power Transfer

The position on a tri bike allows the rider to generate a ton of force via the strong hip extensors (glutes) in a straight downward line.

If the effective seat tube angle was slacker, the rider’s hips would be further behind the bottom bracket and the power transfer would not be as efficient.

• Unweighing the Rear Wheel

A steeper seat tube angle shifts the rider’s weight towards the front end of the bike. As a result, the rear wheel becomes easier to spin whereas the front wheel gains traction which is beneficial during climbing.

## What About The Head Tube Angle?

The extra steep STA of tri bikes sometimes requires a slacker head tube angle. Otherwise, the steering of the bicycle could get “twitchy”.

The table below compares the head tube angles of road and tri bikes.

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

Conclusion:

Some tri bikes have a slacker head tube angle than road bikes, but the difference is quite small and sometimes completely absent.

## FAQ: How can I mimic a tri-bike position with my road bike?

While this practice is not exactly ideal, it’s not entirely out of merit either.

The changes below could help, although a good final result is not guaranteed as tri and road bikes are after all built for different disciplines:

• Steepen the seat tube angle by sliding the saddle forward. (Note: This move shortens the effective top tube and may therefore require a longer stem. Otherwise, you may feel compressed on the bike.)
• Add aero bars to your handlebars

## Summary: What You Need To Know

• TT bikes have a steeper seat tube angle (about 78°) so that the rider can assume the aero position that the frame is designed for.
• Without the steep seat tube angle, the rider’s knees will hit his chest.
• The position allows the rider to heavily utilize his glutes (the most powerful muscle in the lower body).
• The position spares the quads. This is beneficial because the saved energy makes running easier later on.
• The head tube angle of tri bikes is sometimes slacker than what we see on road bikes but more often than not, the difference is small to non-existent.
• It’s somewhat possible to create a tri-bike setup with a road bike by artificially steepening the effective seat tube angle, lowering the stem, and adding aero bars.