TT and Triathlon Bike Fit: How to Set Up an Aero Position
Updated · Written by Stas Bashylov, Ihor Twin
A time-trial or triathlon position trades some comfort for a lower, narrower body on the bike. There is no single correct aero position: what you can hold for the whole race, while still producing power, matters more than how low you get.
This guide gives you starting points — the formulas and target ranges the bike fit calculator and the position check use — and explains where each one comes from and how far to trust it.
Saddle height for TT and triathlon
The calculator starts from the same LeMond/Hinault formula as for road — inseam × 0.883, from the centre of the bottom bracket to the top of the saddle — and then adds +8 mm for TT/triathlon. The methodology it follows gives +5 to +10 mm for an aero position; the calculator uses +8 mm. This is the calculator’s method, not a fixed biomechanical rule.
Dan Empfield’s static tri-fit numbers
For TT/triathlon the calculator also shows contact-point numbers from Dan Empfield’s static tri-fit system. D is saddle height and H is your height:
- Seat angle: 79.5°, measured from the bottom bracket to the centre of the saddle rails — the median of Empfield’s trials.
- Cockpit (E) = 0.72 × H − 0.67 × D: from the saddle nose to the shifter pivot bolt at the end of the extensions.
- Saddle nose to armrest (F) = 0.58 × E.
- Armrest drop (C, cm) = 0.005·D² + 0.0075·79.5·D − 0.7775·D, with D in cm: the vertical drop from the top of the saddle to the top of the armrest. Most riders in Empfield’s data fell within ±1.5 cm of this equation.
Worked example (computed with the calculator): a rider 180 cm tall with an 85 cm inseam gets a saddle height of 759 mm, a cockpit of 787 mm, saddle-to-armrest of 457 mm and an armrest drop of 150 mm.
Target angles for an aero position
These are the ranges the calculator and the position check use for TT/triathlon:
| Angle | Target range | Source |
|---|---|---|
| Torso, from horizontal | 10–25° | The calculator’s TT range |
| Shoulder (hip–shoulder–elbow) | 80–105° | The position check’s TT range |
| Elbow (shoulder–elbow–wrist) | 75–110° | The position check’s TT range |
| Knee at the bottom of the stroke | 145–155° (photo) / 140–150° (video) | The same as for every discipline |
In the position check, a smaller shoulder angle points to a cramped cockpit (consider more reach) and a larger one to a stretched-out cockpit (consider less reach).
How to dial it in
- Start with the calculator’s numbers and a torso angle towards the upper end of the range.
- Lower the front end gradually, over several rides, and stop when you can no longer hold the position or produce power comfortably. Hold each change at race effort before going lower.
- Check your angles with a side-on video in the position check, choosing TT/triathlon. Follow how to check your position at home.
- Check knee angle and saddle height first: an aero front end does not fix a saddle that is too high or too low. See knee angle in bike fitting.
Frequently asked questions
What torso angle should I aim for in a TT or triathlon position?
Is there one correct shoulder angle for an aero position?
Can I use these numbers to choose a TT frame size?
Why is the TT saddle height higher than for road?
How can I check my aero position at home?
Sources
- Empfield D. Devising a static tri-fit system. Slowtwitch; 2009-06-29.