Fill the Gaps, Lose the Drag: Aerodynamic Tips for Triathlon Gear Placement
When it comes to free speed on the bike, aerodynamics is everything. But it’s not just about the frame or your helmet—it’s about how you use the space around you. By understanding how high and low pressure zones form around your body and bike, you can strategically place bottles, kits, and gear to reduce drag, smooth airflow, and even fill in the gaps that slow you down. In this guide, we break down the aerodynamic fundamentals of triathlon bike positioning and how to make smarter choices for speed and storage.
1. The Basics of Aerodynamic Drag
In triathlon, the majority of drag (up to ~80%) comes from the rider, not the bike. Drag is the combination of:
Pressure Drag: Caused by the difference in air pressure between the front (high pressure) and rear (low pressure) of the rider and bike.
Skin Friction Drag: Air rubbing against the surface.
Induced Drag & Turbulence: Mainly applies at yaw (crosswind angles), affecting how air moves off the body/bike.
To reduce drag:
Minimise high-pressure zones (smooth airflow over the front).
Fill low-pressure zones (areas where turbulent air swirls behind objects).
Maintain laminar flow as long as possible before separation.
2. Key Pressure Zones on the Triathlon Bike
A. High Pressure Zones (HPZ) – Where Air Crashes In
What’s Happening Physically:
As you ride forward, you're pushing into still air. That air doesn’t have time to move out of the way smoothly. Instead, it crashes into the front of your body and bike, piling up. This creates a high-pressure bubble — like water building up against a dam.
Where They Form:
Helmet and face
Shoulders and upper arms
Hands and forearms on the extensions
Front of the bike frame and forks
Visual Analogy:
Imagine running into a pool of water at full speed. You hit resistance immediately — that’s HPZ. It’s that initial impact zone where the airflow compresses.
Why It Matters:
This is where most of your drag is created. The bigger your frontal area (head up, arms wide, chest exposed), the more air crashes into you. Narrowing the front profile and adding things like a BTA bottle between your arms can actually help reduce this crash zone by filling in the void and smoothing the shape.
Air builds up in front.
It can’t stay compressed forever, so it tries to flow around.
The sharper, smoother, and narrower your front profile, the better air flows around — reducing the size of the HPZ.
B. Low Pressure Zones (LPZ) – Where Air Loses Contact
What’s Happening Physically:
After the air crashes into your front, it needs to flow around your body and bike and rejoin smoothly at the back. But it doesn’t — especially in fast motion. Instead, it separates and leaves a “void” behind you. In this void, the air becomes turbulent, swirling chaotically — like water behind a fast-moving boat. That’s your low-pressure wake.
Where They Form:
Behind the head and helmet (especially if there’s a gap)
Behind your arms or between them (if arms are spaced apart)
Behind your back and hips
Behind your saddle and seatpost
Under the arm cups (if there's an empty space)
Visual Analogy:
Think of a speedboat. Behind it, you see that foamy, messy trail in the water — that’s turbulent wake. Now imagine that happening in the air, behind your body.
Why It Matters:
Turbulent air = suction drag. It’s like the air behind you is trying to suck you backwards. You can reduce this drag by filling in some of that empty space — for example:
Tucking a bottle between your arms (BTA)
Adding a bottle behind the saddle to streamline your wake
Narrowing your position so the airflow doesn’t detach early
🔁 Putting It Together
🟥 High Pressure Zones (HPZ)
Where air crashes into you and compresses.
→ Think front-facing surfaces like your helmet, shoulders, and arms.
→ More surface = more resistance = more drag.
→ Avoid broad, exposed shapes. Narrow your profile. Smooth transitions help air flow around you.
⚫ Friction Drag (Surface Drag)
This occurs along your skin and bike surface as air moves across them.
→ Even if air flows smoothly, it still rubs against you, creating resistance through skin friction.
→ Friction is influenced by your clothing texture, skin position, and the surface smoothness of your gear and bike.
Example: Loose or wrinkled kit, hairy legs, or a rough bottle surface = more friction.
Fix: Use tight, smooth clothing, clean transitions, and aerodynamic surface finishes (like smooth paint or matte vs gloss where relevant).
🟦 Low Pressure Zones (LPZ)
Where air gives up behind you, separating into swirling turbulence (the “wake”).
→ Think what you leave behind—the back of your helmet, your upper back, behind the saddle.
→ This creates suction drag pulling you backward.
→ Clean up the wake by filling gaps and shaping airflow behind you. Tuck bottles, kits, or mounts into those zones smartly.
3. Strategic Gear Placement
Here’s how to use pressure zones to your advantage:
🚫4. Common Mistakes in Triathlon Bike Positioning
1️⃣ Gap Between Arms Without a BTA Bottle
Problem: Creates both High Pressure Zone (HPZ) and Low Pressure Zone (LPZ).
When your forearms are spaced apart with nothing in between (no bottle or bridging element), air:
Crashes into the chest → creates a HPZ (more resistance from the front).
Breaks apart behind the arms → leaves a swirling LPZ (aerodynamic wake).
Visualise It: Imagine holding your arms out like a tunnel. Air rushes into the tunnel, hits your chest, then gets tossed around on the way out. That void creates chaos = drag.
Fix: Place a BTA (Between-the-Arms) bottle or other bridging object in the gap. This smooths the front profile and helps air reattach more cleanly behind.
2️⃣ One Bottle Only Behind the Saddle
Problem: Creates asymmetric airflow and a messy wake.
When you only have one bottle hanging off the saddle, especially on one side, it:
Sticks out into turbulent airflow.
Unbalances the rear wake, making one side worse than the other.
Increases drag without offering any aerodynamic benefit.
Visualise It: It’s like having a spoiler on just one side of a race car. Instead of managing airflow, it makes it worse.
Fix: If you're mounting bottles behind the saddle, use two bottles evenly spaced to help shape the wake more symmetrically — or tuck one close behind the seatpost in the centre.
3️⃣ Rounded Helmet With a Gap to the Back
Problem: Creates a Low Pressure Zone right behind your head.
If your helmet:
Has a round tail or ends abruptly,
Or doesn’t follow the contour of your back closely...
...then air coming off your head detaches immediately, forming a turbulent wake behind it.
Visualise It: The air rolls off your helmet and drops into a “void” between it and your shoulders — creating a swirling LPZ.
Fix: Use a teardrop helmet that matches the angle of your back, especially in your normal riding position. The goal is to let the air glide off the helmet and onto your back.
4️⃣ Poorly Tucked Head Position
Problem: Breaks up smooth airflow over the back and shoulders.
When your head is too high or bobbing:
It catches air, increasing the HPZ on the face and helmet.
It prevents airflow from hugging the back, leading to a larger LPZ behind you.
Visualise It: A lifted head acts like a brick wall for air. Then the air has to figure out how to reattach to your back — and it usually fails.
Fix: Practice riding with a low, tucked chin (sometimes called “turtle-ing”) where the helmet meets the shoulders or back smoothly. This helps keep airflow attached longer.
🔧 Quick Summary:
🌀 5. Other Aerodynamic Considerations
1️⃣ Yaw Angles – Real-World Wind Isn’t Head-On
What it is:
In a wind tunnel, airflow is usually simulated as coming directly at the rider (0° yaw). But in the real world, you’re rarely riding straight into a headwind. Most of the time, wind hits you at an angle — known as yaw angle (anywhere from 5° to 20° is common).
Why it matters:
Certain gear setups or bottle placements might be aero in a direct headwind but cause turbulence when the wind hits from the side.
For example, a side-mounted bottle may seem tucked, but in a crosswind, it sticks out like a sail.
Visualise it:
Imagine slicing through the air with a knife straight-on: clean. Now tilt the knife slightly—suddenly it's catching and disrupting more air.
Takeaway:
Test or evaluate your setup with yaw angles in mind. Side airflow can drastically change drag. Position gear where it will still behave aerodynamically in crosswinds.
2️⃣ Reynolds Number – Speed Changes Everything
What it is:
Reynolds number is a fancy way of describing how airflow behaves depending on your speed and the size of the object.
At high speeds, airflow becomes more sensitive to surface shapes and sharp transitions (more potential for turbulence).
At low speeds (like climbing), airflow is slower and drag has a reduced effect compared to gravity or rolling resistance.
Why it matters:
On fast, flat courses, aero is king. Every decision around gear placement counts.
On hilly or technical courses, you can relax aero priorities a bit and focus more on weight, comfort, or accessibility.
Visualise it:
Think of riding at 40kph like slicing through thick air with a razor—every bump matters. At 15kph uphill, it’s more like pushing through jelly—less resistance from the air, more from gravity.
Takeaway:
Don’t over-optimise aero in zones where you’ll be riding slowly. Prioritise aerodynamics where you’ll spend the most time at speed.
3️⃣ Integration – Smooth Is Fast
What it is:
Integration refers to how well your gear blends into the bike and body shape. Smooth transitions keep air attached longer, reducing LPZs and drag.
Why it matters:
A bottle that's flush with the frame or tucked behind your arms creates less disruption.
A tool kit bolted under the saddle is better than one flapping in the wind.
Even cable routing, bolt heads, and the shape of your mounts can influence separation.
Visualise it:
Think of a plane’s wing—it’s all about smooth contours. Now imagine strapping a shoebox to the front. That’s what non-integrated gear can do to your airflow.
Takeaway:
Choose or design mounts that look like they belong there. Avoid anything that sticks out at a right angle. Use custom mounts, flush bolts, and contoured surfaces to keep airflow clean.