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Wind tunnel test: What is the fastest descending position?

Peter Stuart
6 Dec 2018

Cyclist visits the Boardman Performance Centre in Evesham to test whether Frooming on the top tube is faster than a hanging Pantani

It was in the Tour de France in 2016 when Chris Froome stunned audiences not with his moped-like accelerations or his infamous oscillating riding style, but his incredible new descending technique.

For those who haven't seen it, it consisted of sitting on the top tube, with his body hugging his stem, yet still somehow pedalling. It makes a stark contrast to the positions of yesteryear – for instance Marco Pantani’s iconic over-the-rear-wheel cougar-like descending style.

We wondered exactly how much difference time has made, and whether with 'marginal gains' wisdom Froome is indeed a step ahead of Il Pirata. So we decided there was only one way to find out (without pitting ourselves against a steep incline and a death wish).

The kind folks at the Boardman Performance Centre were up for the challenge, and so we headed to Evesham, Worcestershire to put all variety of positions to the test, and see which was fastest.

The Baseline

We begin by measuring drag on a very basic position on the drops. For this, I sit relatively upright, in a position that I would ride on a normal flat stretch while pedalling, rather than one where I’m at a speed where I would be aiming to cut as much drag as I possibly can and stop pedalling.


For every run we measure my coefficient of drag area (CdA), which determines how much drag my position will generate at different speeds.

For this first one I score 0.295, which means that to reach 35kmh I need to generate 208 watts of power, on the flat.

On the drops

Our first true descending position is a simple tuck on the drops, with my hands over the brakes. 


In this position I scored a CdA of 0.1993, a huge drop in drag. As the wind tunnel doesn’t generate downhill force, we still use a calculation of the wattage needed to reach 35kmh as a indicator of the efficiency of a position, based on CdA.

At this drag, I would only need 154 watts of power to travel the same speed. But this is far from the fastest position.

The Froome

Sitting on the top tube is not a comfortable position, and I struggled to hold this for more than 30 seconds. Froome manages to pedal while in this posture, which would certainly take some practice.


The Froome position is obviously a well-calculated one, as I saw my CdA drop down to 0.1718. That saw my 35kmh power drop down to 139 watts - making this position a full 69 watts faster than my baseline position.

At a very rough calculation, if I added that wattage to my watts at a speed of 35kmh, that would see me travelling around 5kmh faster with no extra effort. The gain would be proportionally more at at a higher speed.

There are some disadvantages, though. While Froome’s position lowers the centre of gravity, it also puts the weight largely over the front wheel, which can change handling characteristics and potentially make the bike a little less stable. He’s obviously nailed it, but us lesser mortals had better be cautious.

The Pantani 

Pantani’s position has truly been consigned to history, with only the most obscure of current pros ever attempting anything like it. Looking at the data, it may be clear why.


The Pantani position gave me a CdA 0.1947, which was only very slightly faster than my tuck on the hoods. That’s a gain of only 1 watt (although 55w faster than a normal cycling position), yet gives up the ability to pedal while descending. 

Perhaps, though, Pantani knew very well that weight over the back wheel can help overall stability, and his position was more about getting the right position on the road rather than the right position against the wind.

The Obree

We were unable to really capture the spirit of Graham Obree’s Old Faithful riding style, but it seemed a fitting name for a position that tries to put the back as flat as possible.


As it happens, this is my regular descending position when the road is straight and there's no chance of obstacles that require sudden braking.

When it comes to approaching a corner, or any uncertain territory, the hands must return to cover the brakes.


So, what are the gains? Well, pretty good. My CdA was recorded at 0.1679, a very slippery score and the fastest of all of our runs.

The power needed to travel at 35kmh was just 137 watts.

The position offers a low centre of gravity, and nice stability with weight nicely distributed front-to-back, but the lack of control over the drops is a big sacrifice, and anyone attempting it needs to be very confident in handling.

Conclusions

So what can we conclude that the 'Obree' is the fastest position in all circumstances? Well no. 

Everyone is a different shape, and so what's quickest for me may not be quick at all for someone with narrower shoulders, narrower handlebars, a longer body, a shorter neck, more or less weight, a lower riding position...

Then there's the fact that we only tested a single yaw angle, the angle at which the wind hits the rider. That angle was 0°, whereas more commonly a rider will experience a range of 0-15° yaw.

As with much of aerodynamics, the complexities of the science mean that it's hard to extrapolate much, or to make any hard and fast rules.

If you really want to know what's fastest for you, nothing beats a trip to the wind tunnel.

Many thanks to the Boardman Perfomance Centre for conducting our test. Click here for more info on the aerodynamic testing at the centre