What's with the hole in the new Trek Madone? Aerodynamics analysed on radical 2023 superbike

It’s no secret that drag is a big deal; in fact, at a typical racing speed of 40kph, drag makes up about 90% of the total resistance. It’s therefore little surprise that manufacturers are going to great lengths in order to tame it, and when we spotted what is rumoured to be the new Madone at the Dauphine, Trek became the latest brand to chuck the metaphorical rulebook out the window.

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Of course, we’ve seen some pretty radical solutions in the past. Ribble, for example, recently decided that chunky handlebars that would purportedly channel the airflow around the rider was the way forwards. Other brands have been dropping seat stays lower and lower and since the slight relaxing of the UCI rules, we’ve also seen the bottom bracket area getting chunkier like on the BMC Timemachine.

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However, this is most definitely the first time that we’ve seen a split like this at the seat tube/top tube junction, and the etching on the inside means we can safely assume that Trek plans on calling it ‘Isoflow’. So why is it there? Well, the name suggests both comfort and aerodynamics, with this being the natural successor to ‘IsoSpeed’.

Now, without a wind tunnel or independent testing, it’s going to be very hard to say whether this will actually work... but I did do an aerospace degree and plenty of aerodynamics at university, so I might as well put it to good use with a bit of speculation! 

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Let's start with some aerodynamic basics. Drag can broadly be split into three categories:

Pressure drag
This is caused by your frontal area and the wake you leave behind

Skin friction drag
This is sort of what it sounds like, essentially how much resistance surfaces such as your clothing, and freshly shaved legs create.

Shock drag
As not even Wout Van Aert is going at the speed of sound, we can discount that one in this case...

So, we’re now left with pressure drag and skin friction drag; which one should you be more worried about? Well at most speeds, pressure drag is the big one. Even the best cyclists in the world are still fairly un-aerodynamic objects in the grand scheme of things, so are referred to as 'bluff bodies'.

As mentioned earlier this pressure drag is influenced by both your frontal area and the wake behind you. It’s easy to think that your frontal area is the main cause and to an extent this is true, but only because that’s making the wake behind you bigger.

As you ride along, the flow of air separates around the body and bike, and forms a turbulent wake and low-pressure vortices behind the rider. This is what slows the rider down the most, in an aerodynamic sense at least.

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From CFD (Computational Fluid Dynamics) software it’s possible to see that a large ‘pool’ of this stagnant air sits directly behind the rider, and this is suspiciously close to where Trek has been doing its fettling. Typically, the movement of the legs causes a large amount of turbulence to the air around the seat post area. Much of this spills out around the legs, increasing the size of that all-important wake.

It would appear that Trek’s plan is that this hole will channel some of the faster-moving air from in front of the rider into the area of low pressure and slow-moving air just behind. If this is the case, then we can expect the usual claims of the bike being much faster. Well actually, we can probably almost guarantee those claims will be made…

Whether it actually works or not, unfortunately, we don’t know for sure. The science is promising but even with Trek’s large computational capability, CFD isn’t always 100% accurate. Just take this year's 'porpoising' controversy in Formula 1 as an example, where the cars have started to bounce aggressively at high speeds because they now generate a sizeable amount of their downforce from the underside of the floor following a radical overhaul of F1 rules. 

The fact that it’s turbulent flow in this area makes the job even harder but as a general rule, even turbulent flow is preferable to air that isn’t moving.

On a more positive note, it is reassuring to see that more and more manufacturers are considering the rider and bike as a complete system. After all, a bike needs someone riding it to work. The seat tube looks set to be an area for continued development, and you can see that Hope’s track bike was perhaps ahead of its time in focussing on this area. Going forwards, I personally think that we’ll see plenty of other pretty funky seat stay/seat tube contraptions as brands try to eke out every last watt.

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Is aerodynamics your priority when buying a new race bike? Or are weight and comfort more important to you? Let us know in the comments below…