Sometimes the "20 kph" gets substituted by 15 kph or 25 kph or some other arbitrary speed, but regardless, these kind of statements suggest incorrectly that there is some threshold speed below which the aerodynamic drag suddenly becomes zero, or negligible.
As an aerodynamicist, I tend to get irritated by these kind of statements. As the plot above shows, the power losses due to aerodynamic drag get progressively larger at faster speeds, but there is no speed 'threshold' at which aerodynamics doesn't matter. It's a continuum. A more appropriate question to ask would "How important is aerodynamics at xx kph?". This is the subject of this blog post: How much does aerodynamics matter off-road, at those slower speeds?
What % of power goes to overcoming aerodynamic losses?
To answer that question, I calculated the power losses for three power outputs, and three scenarios:
- Power outputs: 150 Watt, 300 Watts & 450 Watts
- Scenarios: Road Bike, Gravel Bike, Mountain Bike
The 150-450W power range covers a wide variety of rider abilities and situation, ranging from recreational rider doing an endurance event, to a professional rider doing a shorter effort.
The road, gravel and mountain bikes scenarios are represented through changes to the rolling resistance coefficient values (CRR) primarily, but also some small changes to the aerodynamic drag area (CdA) to reflect the more draggy set-ups for gravel and mountain bikes:
- Road Bike: CdA = 0.32, CRR = 0.0040- Gravel Bike: CdA = 0.34, CRR = 0.0133- Mountain Bike: CdA = 0.40, CRR = 0.0159
The road bike CRR values comes from my own testing. The off-road CRR values come from data gathered from the excellent testing performed by John Karrasch, using Cat 2 gravel CRR values for the gravel bike and Cat 3 gravel for the MTB case. I used values for the Specialized Pathfinder 700x45 mm tyre for gravel (CRR=0.0133) and the Maxxis Aspen 29x2.4" tyre for MTB (CRR=0.0159). Those are both popular and reasonably fast gravel and MTB tyres.
The plot below shows what percentage of the rider's power output goes into overcoming aerodynamic losses, and what percentages are lost elsewhere. To keep things simple I've assumed zero gradient, so gravitational losses are zero.
The aerodynamic losses are the largest percentage for most of the nine cases shown in the plot above. Not surprisingly, for the road bike case, the aerodynamic losses dominate, a fairly narrow range of 78-87% even over that very large 150-450W power range.
The off-road cases are interesting too though. The percentage of the power lost to aerodynamic losses is still significant, and accounts for over half the power losses in most of the off-road cases. It's only the two slowest cases, the 150W cases, where the rolling resistance losses slightly exceed the aerodynamics losses. Still, in those two cases, aerodynamics still accounts for about 40-50%, which is still a significant %.
It's clear then that yes, aerodynamics does mattes off-road, even across this wide range of scenarios which cover the vast majority of off-road riding abilities and conditions.
Out of interest, I calculated how much slower you'd need to go for aerodynamics to become insignificant. I modelled a very slow 16 kph (10 mph) case, which I think represents a low level amateur racing cyclocross in the most foul winter conditions, having a very high CRR of 0.06 and a power output of 250W (which by the way is fairly representative of my own cyclocross races). In that case, at such so slow speeds, the aerodynamic losses are only 7%, so far less significant than rolling resistance losses through thick mud. Even so, aerodynamics is still not negligible, even in this extreme case of a muddy cyclocross race.
Are aerodynamic improvements worth making?
This is a slightly more interesting question. While the percentage of aerodynamic losses, discussed above, show that aerodynamics is important, what most of us really want to know is whether it's worth the effort of making aerodynamic improvements when riding off-road.
I did a similar calculation to before, modelling road, gravel and MTB cases at those three different powers. However, I calculated how much faster the speeds would be if the CdA was reduced by 0.012. This 0.012 reduction to the drag coefficient is a 3.0-3.8% reduction. It represents the kind of aero benefit that you'd achieve by swapping a non-aero helmet for an aerodynamic road helmet, like the Specialized Evade. In fact, I calculated this 0.012 value from this video posted by Specialized, by reverse-engineering their quoted 40 km time trial time saving of 42 seconds.
The plot below shows how much the speed improves by, as a percentage, by making that same aerodynamic improvement for all nine cases. Note that the % time saving, to cover a certain distance, is exactly the same as these values, since % speed increase and % time saving are the same:
The plot above shows that the % speed (or % time) improvements forma certian aero imporment are fairly similar whether off-raod or on the road. The % speed improvements only slightly dependent ofn rider power and speed too. That aero helmet would improve Filippo Ganna's speed @450W by 1.24%. however, it woudl also improve a the speed of a 150W MTBer by a fairly simialr 0.7%. This, surprised me and I think most people would also find the similarity unexpected.
If you think that's counter-intuitive, it gets better...
The previous plot showed % time savings. However, if you plot the time saving in seconds instead, the results are truly mind-blowing:
Since faster riders cover a certain distance faster than slower riders, a certain % improvement is a smaller number of seconds-saved for a faster rider than for a slower rider. The plot above shows the number of seconds saved for a 40 km distance for these nine scenarios, plus the muddy 10mph CX case. As you can see, not only are off-road time savings still roughly similar to road bike savings, the slower 150W riders actually save more time through the same aerodynamic improvements! This is something that I've calculated in the past, but I still find it counter-intuitive.
I expect many people will find this result hard to believe. Aerodynamic saving are almost as significant at slower off-road speeds as they for road bike higher speeds. This is true for a wide range of riding abilities and scenarios. Not only that, the time savings for slower riders are actually higher than for faster riders.
Don't believe these results?
One final example: Unbound 500 + Keegan Swenson
As a bit of fun, let's consider Keegan Swenson's win at the Unbound 200-mile gravel race in 2023. He completed the 200 mile in 10 hour, 6 minutes, with an average power of 271W. That's an average speed of 19.8 mph or 31.9 kph.
If I make some simplifications by assuming he rode the whole distance solo and on flat terrain (both huge over-simplifications, admittedly), that speed and power is achieved with a CRR of 0.01635, which is not unreasonable. For that ride then, we have the following:
- CRR = 0.01635
- CdA = 0.34
- Rider + bike = 85 kg
- Air pressure = 101,250 Pa
- Air temperature = 20 degrees C
- Air density = 1.203 kg/m3
- Drivetrain efficiency = 3%
- Speed = 31.9 kph
- Power = 271W
Keegan, 271W, non-aero helmet (CdA=0.340)
-> Aerodynamic losses = 142.0 W (52.5%)
-> Rolling resistance losses = 120.8 W (44.6%)
-> Drivetrain losses = 7.9W (3%)
-> Time = 10 hours, 6 minutes, 0 seconds
If I consider that we make an improvement of 0.012 to Keegan's CdA, which is the aero helmet benefit that we considered previously, we now have:
Keegan, 271W, aero helmet (CdA=0.328)
-> Aerodynamic losses = 140.9 W (52.1%)
-> Rolling resistance losses = 121.9 W (45.0%)
-> Drivetrain losses = 7.9W (3%)
-> Time = 10 hours, 0 minutes, 23 seconds
So that 0.012 reduction in CdA (3.5% aero improvement) results in 5 minute, 37 second time saving (0.93%).
Now the interesting bit: If we take the same scenario, but change Keegan's 271W power to half that, 135W, we're now representing an identical rider, bike and course, but we're modelling a fairly low level amateur who just trying to complete the race. They would obviously be riding slower, due to their reduced power.
For the baseline case, with the non-aero helmet, they would complete the Unbound 200 course in 14 hours, 34 minutes:
Amateur, 135W, non-aero helmet (CdA=0.340)
-> Aerodynamic losses = 47.4 W (35.1%)
-> Rolling resistance losses = 83.7 W (62.0%)
-> Drivetrain losses = 3.9W (3%)
-> Time = 14 hours, 34 minutes, 0 seconds
Now, the same aero benefit gives:
Amateur, 135W, aero helmet (CdA=0.328)
-> Aerodynamic losses = 46.7 W (34.6%)
-> Rolling resistance losses = 84.4 W (62.5%)
-> Drivetrain losses = 7.9W (3%)
-> Time = 14 hours, 27 minutes, 27 seconds
So for the amateur, that same 0.012 reduction in CdA results in a time saving of 6 minutes 33 seconds, which is more minutes saved than Keegan!
Conclusion
To conclude, aerodynamics do matter off-road. The % time savings, and % speed increases, are broadly similar to the benefits on the road when travelling faster. They are the same order of magnitude as the % benefit on the road, because in the vast majority of off-road cases, the aerodynamic losses are still the largest power loss. Even at very slow speeds, aerodynamics are not negligible and remain a important factor.
If we consider time saving in seconds, instead of % time saving, slower riders will actually reduce their time to cover a a certain distance than a faster rider. This is counter-intuitive, but true.
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