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Descent speed

The speed you roll to on a descent without pedalling, and what your position is worth.

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Descent speedFondoLab

75

km/h

Time
8:01
Drag area
0.222
Vs sitting up
+8 km/h

On a 7.0% descent you roll to about 75 km/h without pedalling (back down, hands on the drops).

Corners and caution hold most riders well below this: race pacing's normal descending style tops out at 65 km/h. The time assumes you are at top speed from the start.

The positions compared over 10.0 km
PositionCdAkm/hTime
Sitting up0.277678:58
Back flat0.236738:16
Back down, hands on the drops0.222758:01

How we got this

The formula with your numbers in it, then every constant we used and where it comes from.

The working

  1. Rider and bike: 80.5 kg
  2. Drag area: 0.222 m² in this position, scaled to 72 kg
  3. Air density: 1.068 kg/m³ at 1000 m and 20 °C
  4. At top speed, gravity's push equals air and rolling: m × g × sin θ = ½ × ρ × CdA × v² + m × g × Crr × cos θ

The numbers and their sources

A number marked FondoLab default is our judgement, not a study's finding; we say so rather than borrow authority.

NumberValueBasis
Drag area when descending back up 0.277 · back flat 0.236 · back down, hands on the drops 0.222 m², scaled to your weight Simulations checked in a wind tunnel, for a 72 kg rider sitting on the saddle. Positions on the top tube were faster, but the authors found positions with the weight over both wheels safer, and those are the ones we offer. Scaled to your weight as in the rider profile. [1][2]
Power and speed rolling + gravity + air drag, less drivetrain losses The standard model of road cycling power, which predicted measured road power with a standard error of 2.7 W. We leave out its smallest terms, wheel bearings and the energy to speed the wheels up, and ride each piece at a steady speed. [3]
Air density pressure of the standard atmosphere at the road's elevation, temperature from the forecast The gas law: thinner air at altitude and in the heat means less drag. Without a forecast we use the standard atmosphere's temperature too. Humidity changes it by under 1%, so we leave it out.
FondoLab default
Rolling resistance from tyre pressure Crr = 0.1071 × kPa^−0.477, between 1.5 and 12 bar Fitted to coast-down measurements of one tyre on a smooth indoor floor. Real roads are rougher and modern tyres differ, so treat it as a starting point: if you know the Crr of your tyres, enter it. [4]
Descending cautious 55 km/h, 17° lean · normal 65 km/h, 22° · confident 75 km/h, 27°; braking 3 m/s² The speed a corner allows follows from its radius and how far the rider leans: tan(lean) = v² ÷ (g × radius). We read each corner's radius from the course every 25 m and brake for it in time. The top speeds, lean angles and braking are FondoLab defaults. [5]

References

  1. Blocken B, van Druenen T, Toparlar Y, Andrianne T (2018). Aerodynamic analysis of different cyclist hill descent positions. Journal of Wind Engineering and Industrial Aerodynamics 181:27–45. doi:10.1016/j.jweia.2018.08.010

    Computer simulations (CFD) validated with wind-tunnel tests · One rider (183 cm, 72 kg), 3D-scanned in 15 positions; quarter-scale models in the wind tunnel, legs and wheels not moving, still air, smooth clothing. · Read by us: Full text (open access)

    Drag area (CdA) of rider and bike in descent positions, for a steep, straight descent where the rider does not pedal or steer hard:

    • Sitting on the saddle: back up 0.277 m², back horizontal 0.236, back down with hands on the drops 0.222, back down with forearms on the bar ("puppy paws") 0.214, elbows on the bar 0.258.
    • On the top tube: the "Froome" position 0.233; the fastest, sitting at the rear of the top tube with the torso flat and head down, 0.201. The "Pantani" position behind the saddle 0.212.
    • The back-up position had 38% more drag area than the fastest. Results were nearly the same at 54, 72 and 90 km/h.

    The authors' choice where pedalling and steering are needed: back down with hands on the drops, or back horizontal. Positions with weight more evenly over both wheels are safer. Limits: one body shape, no crosswind, no pedalling.

  2. Heil DP (2002). Body mass scaling of frontal area in competitive cyclists not using aero-handlebars. European Journal of Applied Physiology 87(6):520–528. doi:10.1007/s00421-002-0662-9

    Cross-sectional measurement study · 21 competitive male cyclists (mean 74 kg, 1.82 m, 24 years), photographed on their own bikes on stem, hoods, drops and aero positions with drop handlebars. · Read by us: Abstract

    Projected frontal area grows with body mass, but less than in proportion to it:

    • The body's frontal area scaled with body mass to the power 0.720.
    • Rider and bike together scaled with body mass to the power 0.594.

    So a rider twice as heavy does not present twice the frontal area. The study measured frontal area from photos, not drag; drag area also depends on the drag coefficient, which the study did not measure.

  3. Martin JC, Milliken DL, Cobb JE, McFadden KL, Coggan AR (1998). Validation of a mathematical model for road cycling power. Journal of Applied Biomechanics 14(3):276–291. doi:10.1123/jab.14.3.276

    Validation study (model against measured road power) · Road cycling on a flat course with a bicycle-mounted power meter, itself validated against a laboratory ergometer. · Read by us: Abstract

    Derived a mathematical model of the power needed to ride a road bike, determined each of its parameters, and compared its predictions with power measured on the road.

    • Measured power was highly correlated with the model's (R² = 0.97) and not different from it.
    • The standard error between modelled and measured power was 2.7 W.

    The model is the standard one for road cycling, used by later pacing studies such as Atkinson et al. (2007). We have read the abstract only, so the terms we use are listed on the method page in our own words, not attributed to the paper beyond its validation.

  4. Grappe F, Candau R, Barbier B, Hoffman MD, Belli A, Rouillon JD (1999). Influence of tyre pressure and vertical load on coefficient of rolling resistance and simulated cycling performance. Ergonomics 42(10):1361–1371. doi:10.1080/001401399185009

    Field measurement study (coasting deceleration) · One rider and bike coasting in a level indoor hallway, at five tyre pressures and four loads. · Read by us: Abstract

    The coefficient of rolling resistance (Crr) fell as tyre pressure rose, following Crr = 0.1071 × P^−0.477 with P in kPa (r² = 0.99):

    • 150 kPa (1.5 bar): Crr 0.0101
    • 1200 kPa (12 bar): Crr 0.0038, 62% lower

    Carrying 15 kg more raised Crr by about 11% (0.0035 to 0.0040).

    These numbers come from one tyre on a smooth indoor floor. Rough roads, and tyres of other widths and constructions, give other values; the relationship with pressure is what carries over.

  5. Sakai D, Uchida N, Enomoto S, Iwata S, Kawakami Y, Wesugi S (2020). Improving Road Bike Leaning Skills on Downhill Corners (Developing a System for Detecting Curvature Change Points and the Angle of a Road Bike while Riding). Proceedings 49(1):153 (13th Conference of the International Sports Engineering Association). doi:10.3390/proceedings2020049153

    System development with a small field test (conference paper) · One expert rider on a campus downhill with corners of 30 and 50 m radius, speed held at about 25 km/h by an automatic brake, three runs. · Read by us: Full text (open access)

    Describes downhill cornering as three skills: anticipating hazards and judging the situation, controlling the bike's lean, and controlling speed.

    • The theoretical lean angle for a corner follows from speed and radius: tan θ = v² / (g R).
    • The expert's measured lean followed the theoretical lean reasonably well, and he started leaning about 5 m before the start of the corner.
    • An earlier study by the same group found that some beginners leaned their bikes more than the expert in the same corners, which the authors link to tyre slips and falls.

Read more: Descending faster and safer: position, corners and cold

On a descent your position is worth more than your watts. Which tuck saves the most drag, where the time is won, and why to bring a jacket.

On your race

Race pacing: this physics on your own course, with race-day wind and air

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