Road bike training: Aerodynamics when riding downhill - Faster downhill on a road bike - Wind tunnel check

Peter Sagan may not have won the 16th stage of the Tour de France, but the images of his impressive descent to the stage finish in Gap have been etched into the collective memory of spectators and cyclists from all over the world more than the name of the stage winner. Or do you remember who it was?* Sagan's par force ride downhill, on the route where Joseba Beloki crashed so badly in 2003, took the spectators' breath away: The way he chased through the bends at maximum speed, crouching on the top tube between the hairpin bends with acrobatic skill - that showed the cycling world what characterises a complete racer: namely the ability to decide races downhill with skill and a willingness to take risks.

Laws of physics

Whether consciously or unconsciously - Sagan utilised the laws of physics. The maximum achievable speed downhill is dictated by the rider's air resistance. From a gradient of around 7 per cent and 65 km/h, a racing cyclist will no longer become significantly faster even by pedalling energetically - but by crouching down and thus reducing the area exposed to the wind. Driven by body mass and gravity, the rolling system accelerates until the downhill force and the braking forces of air and rolling resistance are in equilibrium. If the rider makes himself smaller on the bike without pedalling, he becomes more aerodynamic and faster. The differences in aerodynamics can be so great that a particularly aerodynamically favourable rider simply rolls away from his fellow cyclists because they can no longer stay in the slipstream.

We analysed how big the differences really are using twelve different riding positions on the bike - from relaxed pedalling to the extreme aero position - in the wind tunnel. In positions 1 to 6 (see next double page), the rider grips the upper handlebars or the brake levers. In the upright upper handlebar position, we determine a drag area (cwA) of 0.346. On a seven per cent gradient, the rider would be rolling at a maximum speed of 56 km/h.

Down with the chin

At a gradient of ten per cent, the maximum speed that can be reached without pedalling increases to 68 km/h. The more the rider leans over the bike, the lower the air resistance; viewed from the front, the surface area that the rider offers the wind becomes successively smaller. With the brake handle in a strongly angled position (position 4), the top speed is already 79 km/h. It gets even faster if you grab the top handlebars on both sides of the stem and lean over until your chin is resting on the stem (85 km/h).

Riding on the imaginary recumbent handlebar (position 5) is a tad faster; although the frontal area is larger than in position 6, the body is more stretched and therefore more streamlined. However, this position is too dangerous on fast descents because you don't have the handlebars under control. On the other hand, professionals often adopt this position when riding solo on flat terrain - which is not surprising, as it is aerodynamically better than most other positions (1-5, 6, 7-10). The cwA values were measured without leg movement and are slightly lower than in normal riding, but show the same sequence that would also result when pedalling. To categorise: The aero position (5) saves 28 watts compared to the low handlebar position (10) at a speed of 45 km/h.

The most secure downhill grip is provided by the lower handlebar position. The standard position with slightly bent arms reaches around 75 km/h, but offers the best control.

The speed drops a little further with the elbows out (8). Bent more, with the forearms parallel to the ground, our test rider would reach 80 km/h (position 9). Turning the wrists inwards adds another kilometre per hour. And it gets even faster if you minimise the frontal area in the lower handlebar position and keep your chin very close to the stem (position 11): 86 kilometres per hour is possible.

For our tester Paul Schuler, however, the fastest position is the one that Peter Sagan took several times in the direction of the Gap (position 12): Sitting on the top tube instead of on the saddle, he slips through the wind and mathematically reaches a top speed of 88 km/h. However, this position cannot be recommended, even if the neck is not overstretched so much and the view to the front is slightly better; but it is uncomfortable, the centre of gravity is unfavourably far forward and there is a risk of getting wedged under the saddle nose if you want to get your bottom back into the saddle.

A variable lower link position is therefore recommended, in which you can switch from full control (7) to top-speed chasing (11) at lightning speed depending on the situation - not spectacular, but hardly slower than the daring ride on the top tube. The aggressive downhill position is strenuous and can only be sustained in the long term by athletes with good joints. A little yoga could therefore be the key (and not only in this respect) to leaving your fellow cyclists standing on the downhill in future ...

CONCLUSION

- On a 1O % gradient, the body position determines the maximum speed
- 88 km/h are possible rolling with an optimised posture
- Our test rider reached 75 km/h in the normal lower link position

* cwA = drag area. ** Vmax = maximum speed in free rolling for 7 or 10 % gradient.