40K TT Finish Pacing: How Wind Resistance, Gradient, and Corners Affect Pacing

The 40K TT (25 mile TT) is the classic distance in road time trialing, typically taking 55–70 minutes. On the surface, it looks like simple arithmetic of “average power × time = distance,” but in reality, every change in wind direction, every gradient shift, and every corner rewrites your pacing strategy.

1. The Non-Linear Relationship Between Power and Speed

On flat roads with no wind, the relationship between power and speed is approximately:

Power ≈ 0.5 × CdA × ρ × v³ + Crr × m × g × v

The key is the v³ term—for every 10% increase in speed, aerodynamic drag increases by 33%. Real-world data:

(Assuming CdA = 0.23 m², rider + bike 80kg, Crr = 0.005)

So pushing an extra 20W might only gain you 1.5 km/h—this is why “average power” matters more than “peak power” in TT pacing.

2. The Effect of Wind: Headwind and Tailwind Are Not Symmetrical

A 40K TT is usually an “out-and-back” or “loop” course. In theory, tailwind and headwind should cancel out, but in practice they don’t.

Assume you can ride 40K @ 250W in 60min with no wind. With a 5 m/s cross-tailwind / cross-headwind (i.e., a 10 m/s wind differential):

• Tailwind section: speed increases by 4–5 km/h, saving 3–4 minutes
• Headwind section: speed drops by 5–6 km/h, costing 5–6 minutes
• Net result: 1.5–2.5 minutes slower overall

Headwind pacing strategy:

• Increase power by 5–10W in headwind sections (from 250W → 257W)
• Decrease power by 5–10W in tailwind sections (from 250W → 243W)
• Use a compromise between “constant speed” and “constant power”—the theoretical optimum is “constant normalized power (NP)”

3. The Effect of Gradient

The steeper the gradient, the greater the share of gravity resistance, while aerodynamic drag becomes relatively less significant.

Relationship between gradient and required power (speed 30 km/h, rider + bike 80kg):

Pacing strategy:

• Start “building power” 200m before the climb to avoid a sudden drop in speed
• Briefly increase power by 10–15% on the climb (to avoid rapid lactate accumulation)
• Maintain cadence after cresting the hill—don’t ease off immediately

4. Corners: The Underestimated Time Killer

Each 90-degree corner costs an average of 3–5 km/h, plus 15–25 kJ of energy for re-acceleration.

Estimated loss from 8 corners in a 40K TT:

• Speed loss and recovery: 60–90 seconds total lost
• Extra energy expenditure: 120–200 kJ (equivalent to 5–8 minutes of FTP)

Cornering technique:

• Look ahead 100m to assess the corner and choose the widest-radius line
• Brake to a safe speed before entering the corner—do not brake while in the corner
• Immediately resume the aero position after exiting the corner and accelerate back to target speed

5. Pacing Distribution Recommendations (40K TT, FTP 280W)

Assume a target power of 250W (89% of FTP), paced in 4 segments:

6. Conclusion

The 40K TT is not as simple as “average power 250W.” Actively adjusting power with terrain, compromising pacing with wind direction, and handling corners strategically—these three things determine whether you finish in exactly 60 minutes or break your personal record in 58.

Related Reading

• Time Trial Power Pacing: The Science of Power Distribution in a 40K TT
• Time Trial (TT) Power Pacing: Why TT Pacing Isn’t FTP × 100%
• The Science of Time Trial Pacing: Trade-offs Between Constant Power, Negative Splits, and Gradient-Adjusted Pacing
• Flat Road TT Training Plan: The Dual Pursuit of FTP and Aerodynamics