TT and triathlon bike fit: the aero position by the numbers

Rotating forward keeps the hip-to-pedal relationship (and your power) while the torso drops toward horizontal. That is why steep seat angles and forward saddles define TT bikes: they let you get flat without folding the hip shut.

The one number that does not change is knee flexion at the bottom of the stroke — 25–35° in both disciplines. Saddle height relative to the pedals follows the same rule everywhere.

The torso is where TT aerodynamics live: 10–25° from horizontal is the target, against 40–55° on a road bike. Every degree flatter saves drag, but only if you can hold it — a position you abandon after 20 minutes, or that stops you breathing and producing power, is slower than a slightly higher one you can race in.

Get lower gradually: drop pad height in small steps across training sessions and confirm you can stay aero for your full race duration before dropping further.

The hip angle usually decides how low you can go. TT targets are 35–55° — deliberately more closed than road. Below about 28°, most riders lose meaningful power and breathing room; that is the signal to stop lowering the front end and instead move the saddle forward (or steepen it) to re-open the hip at the same torso angle.

On the extensions, the elbow should sit close to a right angle — 85–105°. This stacks the forearm under the shoulder so the skeleton, not the muscles, carries your upper body. Elbows more closed than ~75° cramp breathing and shoulder movement; elbows opening past ~120° mean the pads are too far away and you lose the aero benefit the extensions exist for.

Forearms should be roughly horizontal or slightly tilted up at the hands ('praying mantis'), never pointing downhill.

A KOPS offset of −2 to +12 mm — knee at or ahead of the pedal spindle — is expected in a TT position; the road ±5 mm rule does not apply. If you race under UCI rules, note the regulation that the saddle nose must sit at least 5 cm behind the bottom bracket, which caps how steep you can go (triathlon has no such limit).

Remember that moving the saddle forward effectively lowers it — re-check knee flexion after every fore-aft change.

Can I get a decent TT position with clip-on aerobars on a road bike?

Partly. Clip-ons fix the arm support but a road seat angle keeps you rearward, so flattening the torso closes the hip below its workable range. A forward-offset seatpost or a saddle slid forward recovers most of it — accept a torso angle at the high end of the TT range.

Why do I lose power in the aero position?

Usually a hip angle that is too closed: the torso dropped but the saddle never came forward or up to follow. Some loss versus your road position is normal (a few percent); the aero saving usually outweighs it at race speeds above roughly 30 km/h.

Are the target ranges different for triathlon vs a pure TT?

The geometry targets are the same; the choices within them differ. Long-course triathletes typically sit at the more open, comfortable end of the hip and torso ranges to protect the run; short-course TT riders push the aggressive end.

Should I measure my TT position on the base bar or the extensions?

On the extensions, in your actual racing posture — that is the position the TT ranges describe. A photo taken on the base bar tells you about a climbing/handling posture, not your aero fit.

• Knee angle in bike fitting
• KOPS explained
• Road bike fit angles
• How to take bike fit photos

• Saddle setback calculator — See how far behind the bottom bracket your saddle sits, and how that compares with the range road and time-trial riders use.
• Stem length calculator — Compare two stem setups and see how much reach and height you gain or lose from stem length, stem angle and spacers.