Do aerodynamics matter off-road? Yes, more than you might think...

"Aerodynamics doesn't matter below 20 kph"  is something I often hear on cycling podcasts or internet forums.

Sometimes the "20 kph" gets substituted with 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 situations, ranging from recreational riders 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, with those % values having a fairly narrow range of 78-87% even over that very large 150-450W power range.  This is already an important point to note: Although the number of Watts lost to aerodynamic losses varies significantly across the three 150/300/450W rider power cases, but the percentage of the aerodynamic losses is fairly similar for all three cases.

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 proportion.

It's clear then that yes, aerodynamics do matter 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 improving our aerodynamics when riding off-road.

I did a similar calculation to before, modelling road, gravel and MTB cases at those three different powers (150/300/450 Watts).  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 improvement (or % time improvement) from a certain aero improvement are fairly similar whether you're riding on the road or off-road.  Also, the % speed improvements are only slightly dependent on rider power and speed.  That aero helmet would improve Filippo Ganna's speed @450W by 1.24%.  However, it would also improve the speed of a 150W MTBer by 0.70%, which isn't much different.  This, surprised me and I think most people would also find the similarity unexpected.

Remember though, that the percentage of the rider's power that is lost to aerodynamic losses is fairly similar (88% for the 450W road bike case, versus 78% for 150W), even though the number of Watts lost (395W vs 117W respectively) varies significantly.

Still, I think there is a conventional wisdom that says the Pros, who ride faster, are the people that need to - and benefit most from - making aerodynamic improvements.  In fact, that's not really true. 

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 10 mph cyclocross (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 seconds 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 savings are almost as significant at slower off-road speeds as they for a road bike's 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?

If you don't believe me, I urge you to do the calculation yourself and leave a comment below.  The maths needed to calculate power losses due to aerodynamics and rolling resistance isn't too complicated.  The calculations that I did in Microsoft Excel only took about an hour or two to do.  If you need helps with the equations for the various power losses, refer to my old blog post here.

One final example: Unbound 500 + Keegan Swenson

As a bit of fun, let's consider an example that's loosely based on 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, despite the slower off-road speeds.  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 an important factor.

If we consider time saving in seconds, instead of % time saving, slower riders will actually improve their time to cover a certain distance by more seconds than a faster rider does.  This is counter-intuitive, but true.

The aerodynamics of aero socks and fabrics - Part 2

In my previous post about aero socks, I explained how the ribs and grooves on the fabric of aero socks can induce boundary layer 'transition', from laminar to turbulent, which helps to keep the airflow around the ankle and calf 'attached' for longer, which then leads to a narrower wake of separated flow and therefore less drag.

I also explained an important aerodynamic parameter called the Reynolds number of the flow, which depends on the speed of the airflow and also the size of the object, and how that affects the behaviour of the flow.  

I showed the plot above to illustrate the factors that determine whether the fabric of the aero socks will cause boundary layer transition to occur at a lower Reynolds number, compared with what would happen naturally on a smooth surface.  This plot shows how the drag coefficient of a cylinder varies as a function of Reynolds number for different surface roughness heights.  "k/d" in the plot is the surface roughness height divided by cylinder diameter.  Surface roughness has a similar effect to aerodynamic fabric ribs in that it causes a the boundary layer to transition from laminar to turbulent earlier (i.e. at lower Reynolds numbers) than what would occur naturally on a smooth surface.

In this post, I'll dive deeper into these effects and discuss what else we can infer from plots like the one above.

Why size is important

The Reynolds number depends on the size of an object, in addition to the speed of the flow.  For a given object shape, for example a cylinder, a larger object will have a higher Reynolds number than a smaller object having the same shape, even if the airflow speed (i.e. the riding speed) is the same.  This is shown in the plot above; the ankle, calf and thigh lines (the red, green and blue lines respectively) are at different Reynolds numbers, different positions on the x-axis, despite the airflow speed being the same 50kph for all three.

This difference in Reynolds number for different body parts is, I think, one of the reasons why we see clothing manufacturers use different fabric textures on different parts of skinsuits.  Looking at the plot above, it's clear that a cylinder with a smaller diameter, such as an ankle, will have a lower Reynolds number, and therefore needs to have a larger surface roughness (or height of the fabric ribs), in order to trigger the boundary layer transition in the optimum way, to achieve the lowest drag coefficient (Cd) at that Reynolds number.  The red ankle line on the plot above would achieve it's lowest Cd at 50 kph by having a roughness of ~k/d=0.005.  For an 82 mm diameter cylinder, which is my ankle diameter, a k/d value of 0.005 corresponds to a roughness height of 0.41 mm.

On the other hand, for a thigh, the plot above shows that the lowest drag coefficient at 50 kph would instead be achieved with a much smaller roughness of about k/d=0.0003.  For an 175 mm diameter cylinder, which is my thigh diameter, a k/d value of 0.0003 corresponds to a roughness height of just 0.05 mm.  To put that in context, the width of a human hair is 0.05-0.10 mm.

So for the ankle, the optimal roughness is 0.41mm and for the thigh it's 0.05 mm. That's an order of magnitude difference in optimal roughness (fabric texture height) values for the ankle and thigh!  This wouldn't be obvious or intuitive to most people I think, and would probably be surprising even for most aerodynamicists too.  This could be why we don't see ribbed aero fabrics being used on the thigh area of shorts and tights, like the offerings shown above from Rule28, despite ribbed fabrics being used on the lower leg.  The natural texture of a standard lycra clothing fabric probably provides the optimal amount of roughness (0.05 mm) for the thigh area at speeds around 50kph.

Interestingly, I remember an Q&A reply provided by Rule28's founder, Sam Calder, in a TrainerRoad forum post. In that post he explained that aero fabrics don't work on the thigh because of the rotating nature of the thigh when pedalling, which makes it difficult to find a fabric texture that reduces drag.  The rotating element might indeed be an additional factor that complicates things, but I believe the difference in Reynolds number that the thigh experiences, compared with the ankle, is another big reason why aero fabrics don't 'work' on thighs.

Why speed is important too

The Reynolds number also depends on the airflow speed, however, in addition to the size.  This is important, because very few of us (sadly) cycle at 45-50 kph, which is the speed that cycling wind tunnel tests are often performed at.  45-50 kph is a suitable speed for professional and high level amateur time triallists.

As a quick aside, another reason why wind tunnel operators prefer to test at the higher end of the speed spectrum is because the drag measuring equipment in the wind tunnel, called the balance, will be more precise at higher speeds.  Higher speeds produce significantly higher forces.   The drag at 50 kph is approximately 2.8 times more than the drag at 30 kph, for a given drag coefficient.  The wind tunnel balance will normally have a certain force precision, in terms of Newtons of force.  Therefore, if testing at 30 kph instead of 50 kph, then the drag force is 2.8 times less, so the precision of the data collection will be 2.8 times worse.  The guys from Specialized made a similar comment in an interview in this BikeRumour interview.

If the riding speed is slower, the Reynolds number will be proportionately lower. The plot below shows the Reynolds numbers for the ankle, calf and thigh for 40 kph, instead of 50 kph shown previously. 40 kph is closer to the speed that I would average for a 10 mile time trial.

Note that at 40 kph, the optimal roughness for the ankle is now a k/c of 0.007, which corresponds to a roughness height of 0.57 mm.  For 50 kph the optimal roughness was a smaller k/c of 0.005 (0.41mm).  Therefore, slower speeds need a larger surface roughness, or more prominent aero fabrics ribs and grooves, to achieve the lowest drag.  If the optimal aero fabric for 50 kph is used at 40 kph, there is a chance that the fabric texture is insufficient to cause boundary layer transition.

At speeds even than 40 kph, the effect is even more significant.  A speed of 25 kph is too slow for anybody riding a time trial or triathlon, no matter how unfit they are, but it's an appropriate average speed for many off-road racing scenarios.  The plot below shows how the Reynolds numbers for the ankle, calf and thigh move to the left on the x-axis when considering 25 kph.

There are a couple of really interesting observations to make from this 25 kph plot, compared withe previous ones:

1) Aero socks probably won't work at 25 kph: The k/d value that gave the lowest ankle drag at 50 kph (i.e. k/d=0.005) is completely ineffective at reducing the drag at 25 kph, according to the plot.  In fact, even the optimal k/d for 40 kph (0.007), is ineffective too.  At 25 kph, the drag coefficient for those k/d lines is at the upper 1.2 value, which is the drag coefficient for cylinder when it experiences fully laminar flow and laminar separation.  Those 0.005 and 0.007 k/d roughness values are not able to 'trip' the boundary layer, to cause the early transition to turbulent flow.  At 25 kph, a k/c of 0.02 is needed instead, which is 1.64 mm, which is larger than what the UCI now allows.

2) Aero fabric might work elsewhere though: The k/d value that provides the optimum drag for the thigh, however (the blue line below), is now 0.004.  For a thigh diameter of 175 mm, this requires a roughness height (k) of 0.7 mm.  Without that roughness, for k/d=0, the plot below shows that the thigh would experience laminar separation and has a Cd of 1.2.  For an optimum roughness height of 0.7mm (k/d=0.004), the drag coefficient would be half that, 0.6.  

What this shows it that the effectiveness of roughness, and therefore the effectiveness of aero fabrics, depends on the size of the object and the speed.

At 50 kph or 40 kph an aero fabric is not needed on the thigh area, and may be counter-productive, whereas at 25 kph it might actually be needed to improve the flow around the thigh and reduce the drag.  This is something that so far, I haven't seen any clothing manufacturers investigate or utilise.

There are a few assumptions behind what I've explained in this post, particularly the use of the roughened cylinder analogy to explain how textured aero fabrics work on the legs and arms.  Still, I think there is a good chance that aero fabrics used in unconventional places, like the thigh, could produce clothing that performs very well at lower speeds associated with off-road events like gravel, mountain bike and cyclocross races.  There is a big market there, and a potential opportunity for a clothing manufacturer to produce something that performs well for those events.

As a final remark, there may be people that read these last few paragraphs and think to themselves that aerodynamic improvements have little benefit at slow speeds.  This is a common misperception, and a wrote a blog post recently (see here) that shows that aerodynamic improvements have a surprisingly similar benefit at slower off-road speeds compared with their benefits at faster road and time trial speeds.