Aerodynamic gravel bikes in the wind tunnel: Are aero gravel bikes the future?

Overview of aerodynamic gravel bikes

The images are still unfamiliar, but fascinating: the best road pros hammering over paths with fist-sized boulders, agonising up crazy climbs with spinning rear wheels and balancing down the steepest trails. The 2023 UCI Gravel World Championship in Veneto, Italy, was undoubtedly a spectacle and, at first glance, had little in common with the fairly predictable road races. However, anyone who watched not just the edited highlights, but the entire broadcast, will have noticed that the competition was run like a classic road race for most of the time - only on much worse roads. The arrival at the finish line may also have seemed strangely familiar to cycling fans: Several kilometres of wide, well-paved roads through Treviso. The fact that the riders were on their racing bikes with comparatively wide tyres was hardly noticeable to outsiders.

This means that, with the exception of very difficult off-road sections, the conditions for success in such races are comparable to those on the road. This also means that the most aerodynamic set-up possible increases the chances of success in certain racing situations, especially for breakaways and in sprints. Compared to road bikes, however, most gravel bikes are not very aerodynamic. Is this wasted potential? TOUR tried to find out.

Anyone who studies the pros' equipment in detail will realise: aero matters - aerodynamics is an issue, even off-road. Not only tight-fitting racing suits and aero helmets are the order of the day, but also high-profile rims, narrow handlebars and aerodynamic chainrings. The first manufacturers are already turning speed into a concept, with streamlined framesets, aero rims developed for wide tyres and narrow integrated handlebars being marketed specifically for off-road bikes, and there are even aero gravel socks. The products are above all exclusive and expensive, but is it really worth it or is it just for show? What influence does the choice of wheels and tyres have? And what do the results mean for the leisurely travelling pace at which most gravel bikes are used? TOUR carried out exemplary tests in the wind tunnel.

Few reliable findings

To date, there have only been a few serious publications on the subject, and we are still feeling our way around the topic. Our data is therefore partly derived or based on estimates. The few reliable tests come almost exclusively from wheel suppliers who use scientific methods to approach the optimum rim shape for wide tyres and lower speeds. One of these is Jean-Paul Ballard, whose company Swiss Side not only builds aerodynamic wheels, but also offers aerodynamic advice as a service.

He uses the same wind tunnel as TOUR, the channel of the Gesellschaft für Strömungstechnik (GST) in Immenstaad on Lake Constance, and has also produced some fundamental findings on the subject of gravel bike aerodynamics there as a kind of by-product. In his measurements, Ballard found that a typical gravel bike at a measured speed of 30 km/h requires almost 40 watts more pedalling power than a very fast aero road bike: around half of this is overcome by air resistance, the other half by the rolling resistance of the tyres. We can confirm the order of magnitude with our measurements, but of course the comparison lags behind and we are driven by further questions: How far can the aerodynamic disadvantage of a gravel bike be reduced? Are the differences between gravel bikes comparable to those of road bikes, where the difference between a good and a bad set-up can be up to 20 watts, even though the speeds travelled are lower?

Into the wind tunnel

To investigate this question, we also took three gravel bikes with us to the latest wind tunnel test. Included is the brand-new race gravel bike Grail from Canyon, which has at most subtle aero ambitions. There is also an older but beguiling aero icon from Ridley, the Kanzo Fast, which is intended to mark the fast end of the gravel spectrum. The 3T Exploro is one of the most extreme gravel bikes on the market, with space for mountain bike tyres and matching aero profiles on the frame. The trio should answer the question: What savings potential do aerodynamically designed frames offer if wide knobby tyres disrupt the airflow beforehand?

As a rule of thumb: from around 20 km/h, air resistance becomes the dominant factor slowing down the ride on a gravel bike, despite the significantly higher rolling resistance of the tyres. This is a speed that even amateur cyclists can easily reach off-road. Above this speed, the aerodynamic effect becomes measurable - if there is one. In our analysis, we assume a speed of 35 km/h, which is realistic as an average for professional races and under favourable conditions for amateur riders. Compared to road racing bikes, which we normally consider at a speed of 45 km/h, the aerodynamic drag values and differences are roughly halved.

Our experiment shows that the differences can still be significant. First, the less aerodynamic Canyon and the streamlined 3T duel with the same wheels and tyres: fast DT Swiss carbon rims and 40 millimetre wide knobbly tyres. In fact, the aerodynamic 3T is slightly faster and the difference is quite significant at around five watts. The sleek-looking Ridley, which is not only equipped with pronounced aero profiles, but also with an aerofoil-shaped handlebar set, can even outperform the 3T: this bike can save a further seven watts if the same wheels are used. That shows: Even behind thicker tyres, the differences in the frames are similar to those on a road bike. They just don't have such a dramatic effect because of the lower speeds.

Reduced effect of the impellers

Interesting in this context: According to previous findings, wheel tuning is aerodynamically less efficient on a gravel bike than on a road bike. It has been proven that high rims roll faster even with thick tyres. However, because the rim is covered more by the wide tyre and can also "sail" less well in crosswinds than a well-tuned combination with narrower road tyres, the advantage of an aerodynamically good wheel over an aerodynamically moderate wheel is reduced. According to measurements by SwissSide, this is around five watts with 40-millimetre tyres.

If you want to get more out of your wheels, you should first consider your choice of tyres: Is a narrower tyre or less tread justifiable? The tyres have a measurable effect: the fast Ridley saves even more power with narrower tyres. Switching to studless tyres of comparable width reduces pedalling power by three watts. The effect of narrower tyres is surprisingly small, switching to 34-millimetre tyres only saves a further two watts. Depending on the terrain, it is important to weigh up whether the compromises in comfort and grip still make sense. However, with the right choice of wheel and tyres, you can still save a good 15 watts of pedalling power at a speed of 35 km/h.

This is relevant for professionals at the Gravel World Championships, as it can make the difference between victory and defeat. Amateur riders can be three to four minutes faster on a 100-kilometre route with 1000 metres of elevation gain at an average speed of around 25 km/h. For long-distance events, where you ride alone most of the time, this is an interesting order of magnitude.

The influence of seating position and clothing has not yet been taken into account - as we know, there is still a lot more to be gained here. TOUR has determined the approximate values for this in various tests on the track and in the wind tunnel.

Do aero gravel frames make sense?

The diagram shows the aero potential of different gravel bikes and tyres. The values apply to a speed of 45 km/h, so they can be compared directly with the data for road bikes. A speed of 35 km/h reduces the absolute values by about half. The slowest set-up is the Canyon Grail with a round tube frame and 40-millimetre off-road tyres. Interesting: The aero frame of the 3T saves as much as switching to significantly narrower and treadless tyres. With fast gravel tyres, the Ridley Kanzo Fast comes close to being a good road racing bike.

Fast on gravel

If you want to lose as little time as possible off-road, you should tune in the right places. These parts help the gravel bike to gain more speed - the specifications apply approximately to a speed of 35 km/h.

Aero dress instead of flapping clothes: up to 30 watts

Often underestimated: The aerodynamically undesirable influence of loose-fitting clothing is enormous. Anyone taking part in a race should therefore make sure they wear tight-fitting clothing. Compared to competitors who are wearing baggy shorts and a mountain bike shirt, this saves up to 30 watts.

Clip-on trailer: approx. 10 watts

It's particularly useful when the going gets tough: on tarmac sections and less rough dirt tracks, it's worth switching to the recumbent position - even if this is significantly higher on the clip-on trailer than on a time trial bike. Up to 10 watts can be saved. The minimalist attachments are otherwise hardly disturbing, the Pro model shown places the computer clearly visible in front of the handlebars. Forbidden in competitions with a mass start, but a game changer in long distance races.

Narrow handlebars: 5-10 watts

The advantage always rides with you: Narrow handlebars reduce the frontal area of the bike and rider in all situations. Lambda Racing offers the XX model, which is 35 centimetres narrow at the brake levers, but flared at the lower handlebars. In the brake lever position, there is a similarly large aero advantage as with a fast frame set. At 42 centimetres in the lower handlebar grip, you retain the usual control on more difficult terrain.

Aero wheels: approx. 5 watts

High aero wheels on gravel bikes are less effective than generally assumed - at least that's what the measurements taken by TOUR and the wheel manufacturers Hunt and SwissSide suggest. One explanation besides the lower speed is the overall profile of the rim and tyre: The thick tyre hides the rim profile more than on the road bike. For aerodynamic reasons, the bulbous rim would also have to be wider than the tyre - which is almost impossible to achieve in terms of production technology. Depending on the tyre width and track profile, flat but significantly lighter rims may therefore be the more sensible tuning option.

Smooth tyres: 2-3 watts

A direct comparison is difficult, but our measurements suggest that studs also cause air resistance. If you only ride on hard and dry surfaces, you can do without coarse tread - and save a few more watts. Switching to the narrowest possible tyres has surprisingly little effect: You can roughly estimate one watt for every two millimetres of tyre width. However, the effects on handling are enormous off-road.

Single drive: 2 watts

Surprising, but true: the gearing can also contribute to aero performance. If you only use one chainring and dispense with the front derailleur, you can save a few more watts. A flat aero chainring is also a visual statement - available from Alugear for various shifting systems for 80 euros.