The aerodynamics of aero socks and fabrics - Part 1

 

Aero socks, like the Rule 28 socks shown in the picture to the left, are a popular clothing choice for time triallists and racers seeking and advantage.

In fact, aero socks are often mentioned as being one of the the best value bang-for-your-buck upgrades, considering the performance advantage they provide for their relatively modest price.

In this blog post, I'll explain why aero socks work.

During my 30-year career as a professional aerodynamicist, I've worked on many aircraft R&D projects that involve the same aerodynamic phenomena that apply to sock and leg aerodynamics.  I'm conscious that the majority of readers won't be familiar with many of the aerodynamics concepts I'll talk about, so I'll start with a basic explanation.  More knowledgeable readers might want to skip the early paragraphs.

Bluff body aerodynamics

The legs of cyclists, and cyclist's bodies in general, are what an aerodynamicist would call bluff bodies.  A bluff body is an object that will typically have a wide or irregular shape, and the nature of that shape means that the air cannot flow smoothly around it.  A streamlined body, on the other hand, is shaped so that the air can flow smoothly around it from the front all the way to the back.  An aeroplane wing or a dolphin are examples of streamlined bodies.  Unlike a bluff body, a streamlined body will have much lower drag.

The flow over a bluff body like a cyclist's leg will tend to be smooth only at the front of it, as shown in the right-hand picture above.  Towards the back, often at or close to the widest part, the air is unable to continue flowing smoothly, and it 'detaches' or 'separates' from the surface, as shown above.  In the region of separated flow there tends to be large eddies and a low pressure region, which 'sucks' the object backwards, contributing to the majority of the object's drag.  A cyclist's leg is similar to a cylinder, or a tapered cylinder to be more precise, where the thigh has a larger diameter than the calf and the ankle.  Clearly, the cross section of a leg is not exactly circular, as it is for a cylinder, but for the purposes of explaining leg aerodynamics and aero socks, the cylinder analogy works well.  There have been plenty of studies concerning the flow around cylinders, so we can use cylinder aerodynamic data to understand how aero socks work.

Cylinder aerodynamics

Before getting into the aerodynamics of cylinders, it's important to first explain that the drag coefficient for an object, denoted by the abbreviation "Cd", is in general not a fixed value.  Instead Cd is dependent on the flow conditions like the speed, air temperature and the size of the object.

For cyclists, whose frontal area can be easily adjusted by changing the torso angle and arm position, it's often more convenient to use the drag area parameter, "CdA", which is the drag coefficient multiplied by the frontal area.  Cyclists and time triallists often talk about their CdA as if the CdA value is a constant value for a given setup, but it's not really true.  To be fair, over the range of relevant cycling speeds, the changes in CdA are likely to be fairly small, so for practical purposes, considering CdA to be a fixed value is a reasonable simplification.

Changes in CdA occur because the change of a parameter called the Reynolds Number (Re). The Mach number will also affect CdA, but because we cycle at a small fraction of the speed of sound (which is 1230 kph) we can ignore that dependency of CdA on Mach number and focus only on the Reynolds number dependency.  Reynolds number describes the ratio between the inertial properties of the flow and the viscous properties of the flow.  This won't mean much to many people, so it's more helpful to explain what things change the Reynolds number:

• Doubling the speed will double the Reynolds number.  Riding at 40 kph means your Re number is twice as large as if you are cycling at 20 kph. 
• Doubling the size of the object, even if it has the same shape, will double the Reynolds number.  If an ankle has half the diameter of a thigh, the flow around the ankle will have a Reynolds number that's half the Reynolds number of the flow around the thigh.
• The air density and temperature will also affect the Reynolds number.  Increasing altitude will result in a lower Reynolds number, although there isn't a linear relationship like there is with the first two dependencies, speed and size.

The reason for explaining Reynolds number is because the drag of a cylinder-like object, such as a leg, is highly dependent on the Reynolds number.  The plot to the left shows the drag coefficient of a smooth cylinder as a function of Reynolds number.  Note that the Reynolds number dependency is plotted on the x-axis using a logarithmic scale, so it covers a very wide range of flow conditions.

I've annotated the plot to show the region (in blue) that's relevant for cyclist's legs, covering the 10-60 kph speed range.  Across this speed range, you can see that the drag coefficient is very similar for a smooth cylinder, and the Cd is typically a value around 1.2 for the whole blue range.  However, you will notice that at Reynolds numbers that are slightly higher than the blue region, at about 300,000-400,000, the drag coefficient curve reduces significantly.  This point, where the drag coefficient drops substantially is called the 'critical Reynolds number', and it describes a point where the flow around the cylinder behaves very differently.

At Reynolds numbers below the critical Re number, the flow around a cylinder looks like the flow shown in the top sketch on the left, having a wide wake, often with regular vortex shedding occurring from the cylinder and those vortices are transported downstream in wake.  This is where the drag coefficient is around 1.2.

Once the Reynolds number is larger than the critical Reynolds number, at about 300,000-400,000, the wake becomes much smaller, as shown by the bottom sketch on the left.  A narrower wake causes a smaller low-pressure region at the back, hence less drag.

So what is it about the increase in Reynolds number that causes this difference in the pattern of the separated flow and the size of the wake?  Well, the Reynolds number determines whether the air moving right next to the cylinder surface, called the boundary layer, is a laminar boundary layer or a turbulent boundary layer.  At higher Reynolds numbers, the boundary layer naturally becomes turbulent before the point where the flow separates.  This is important because turbulent boundary layers are much more resistant to flow separation than laminar boundary layers.  Therefore, at higher Reynolds numbers, the turbulent boundary layer resists flow separation at the widest point of the cylinder and instead the flow separates only at the very back of the cylinder, causing a narrow wake and a low drag coefficient.

So, to summarise:

• The drag of a cylinder depends of the size of its wake.
• The size of the wake depends on whether the boundary layer is laminar or turbulent at the widest part of the cylinder.
• The Reynolds number of the flow determines whether the boundary layer is laminar or turbulent.
• Hence the Reynolds number determines the drag of the cylinder.

However, the Reynolds number is not the only thing that determines whether the boundary layer is laminar or turbulent, as I'll explain in the next section.

Boundary layer transition tripping

As explained in the previous section, at higher Reynolds numbers the boundary layer will naturally transition from a laminar boundary layer to a turbulent boundary layer before the point of flow separation, and it's the turbulent boundary layer that enables the flow to resist separation at the widest part of the cylinder.

However, the boundary layer can also be 'forced' to transition from laminar to turbulent at Reynolds numbers below the critical Reynolds number.  This intervention to force the boundary layer to become turbulent is often called 'tripping' the boundary layer.  There are various ways to trip a boundary layer, but most methods involve some kind of protuberance, like a bump, a wedge or a band of roughness, that disturbs the laminar boundary layer and causes transition to turbulent boundary layer.

Hence, at low Reynolds numbers, below the critical Reynolds number, the only way to reduce the drag coefficient of a cylinder is to trip the boundary layer.  This is what the ridges and surface texture of aero socks do, and how they are able to reduce the drag of a cyclist's lower leg.

The plot above is similar to the one shown earlier, in the Cylinder Aerodynamics section, except that instead of showing just a single curve for a perfectly smooth cylinder, the plot shows several curves for cylinders with different levels of surface roughness.

The level of roughness is defined as "k/d", which is the roughness height divided by the cylinder diameter.  The perfectly smooth cylinder is the one with k/d=0, which you can see has a critical Reynolds number of about 300,000, as discussed earlier, above which the drag coefficient drops abruptly.  The other curves are for progressively rougher cylinders.  For example, the curve with triangular symbols is for a k/d of 4/10^3 (=0.004), which is equivalent to 0.4 mm roughness  on a 10 cm diameter cylinder.  0.4 mm roughness is about the same roughness as 40-grit sandpaper, which is a coarse sandpaper you'd use for DIY jobs.

For this k/d=0.004 example, you can see that the critical Reynold number is much lower, because the roughness is tripping the boundary layer at lower Reynolds numbers.  As a results, at a Reynolds number of 100,000, this rough cylinder has a lower drag coefficient, about 0.7, than the smooth cylinder has (which ahs a Cd of 1.2 at Re=100,000).  To non-aerodynamicists this might seems counter-intuitive, that adding surface roughness reduces the drag of the cylinder, but it's true, and it's all related to the state of the boundary layer.

This is how aero socks work.  The ridges in the fabric of an aero sock act like the roughness elements in this example, reducing the critical Reynolds number and therefore the leg drag at the Reynolds numbers that cyclists are operating at.

What trip height for what speed?

As a final word, it's worth mentioning that the transition trip height that's required, to give the lowest drag, depends on the Reynolds number.  Hence the trip height (which means the height of the ridges in the fabric), depends on the rider speed and also the size of the body part it's applied to.

The plot above shows the Reynolds numbers for a 50 kph speed.  This is the kind of speed that a high level time trialist would achieve and is approximately equivalent to a 20 minute time for a 10-mile time trial.  I've annotated the plot to show what the Reynolds numbers would be for an ankle, calf and thigh.  This is rather approximate and is based on my own ankle calf and thigh circumference values (26, 38 and 55 cm) to get an approximate equivalent cylinder diameters.  This is admittedly rather crude, because as mentioned previously, the leg doesn't have a circular cross-section. However, it's just to illustrate a point.

You can see that for the ankle, where (UCI-legal) aero socks are working, the best drag is  achieved with roughness height of about k/d=0.005.  For an 83 mm diameter cylinder, k/d=0.005 is a roughness height of 0.42 mm.  This is consistent with the fabric patterns used for aero socks, which have ridges and grooves that are about half a millimetre to one millimetre in depth.  There isn't a direct equivalence here, however, because aero sock fabrics use grooves and ridges, rather than distributed roughness, so it's likely that larger ridge height would be needed to trip the boundary layer in a way that's similar to how roughness behaves.  

Nevertheless, it's reassuring to see that plots of drag data for roughened cylinders is consistent with the fabrics that have been selected by manufacturers of aero socks.

In my next blog post, which I'll write in the coming weeks, I'll discuss this plot further and what other things it may reveal and imply. 

Aerodynamic Garmin Edge 840 out front mount

This is another 3D-printed bike part that I've designed recently.

During the spring this yea I had the idea to make a number of aerodynamic improvements to my road bike, in time for the summer time trial season.  Sadly (and as usual) I've had less spare time than I wanted.  Also, the CAD design work has taken me longer than I had anticipated.

Anyway, this mount for my Garmin Edge 840 computer is the first of several minor aerodynamic improvements that I'll create for my road bike.

What aero improvements are possible? 

I have always been intrigued by the claims made several years ago by Wahoo about the aerodynamic efficiency of their Wahoo Elemnt Bolt.  Those claims are summarised nicely on DC Rainmaker's site.  Wahoo claimed that the Elemnt Bolt had 50% less drag than the leading competitor (i.e. Garmin) with those drag savings equating to a 1.5 Watt saving (although they didn't quote what speed that was for) but apparently that corresponds 12.6 second savings over a 40km time trial.  Those savings are fairly small but not negligible.  To put that in context, 12.6 seconds is about half the penalty of having a round bottle on the down tube, according to Specialized's wind tunnel testing (see here).

DC Rainmaker also performed some wind tunnel tests of his own though, which showed that the savings for a Bolt are actually much smaller than Wahoo's claims, more like a 1 second saving, instead of 12.6 seconds, when the computers are mounted horizontally.  That's very small, a truly marginal gain.

Still, despite this very small saving, it's something I wanted to do.  I felt that the integration with my stem and handlebar could be improved too, which I felt could yield some additional drag savings.  Therefore, I pressed ahead and designed the mount.

Mount design

What I wanted from the mount was to something that:
    1) Had a more aerodynamic profile at the leading edge.

    2) Covers the Garmin's side buttons, which disturb the flow and aren't needed during a ride.

    3) Was blended into my stem and the circular section of my handlebar.

The design consists of a 'sleeve' into which my Garmin 840 easily slips into, and separately a mount that bolts onto the handlebar.  Once the Garmin is inside the sleeve, it can be fitted to the mount so that it's perfectly flush.  The front and back of the sleeve are shaped to help keep the sleeve perfectly flush with the mount, in addition to a central Garmin quarter turn mount (also designed and 3D-printed) that ensures it won't fall out.  All of this is quite difficult to describe with words, so I have uploaded a video to YouTube (see below) that shows it in action:

The sleeve and the mount have two cut outs at the bottom left and bottom right corners. This allows me to press the start/stop button on the right and the lap button on the left, as shown in the second video below.  There's also a small C-shaped cut-out in the left hand side of the sleeve's thin sidewall, that allows the on/off button to be pressed.  I felt that these three buttons were the only three that really needed to be pressed during a ride, with the other computer functions being available via the touchscreen.

Apart from hiding the protruding buttons via the sleeve, what makes this mount aerodynamic is the shape of (1) the leading edge of the fairing and also (2) the blending of the mount around the stem and handle bar.

The mount smoothly curves into the stem face plate and into the round profile of handlebar at the centre.  A lot of trial and error was required to get a shape that fits closely to double curvature shape of my 3T stem.  This is undoubtedly the most fiddly part of creating 3D designs - getting them to fit with existing parts and geometries that I don't have the CAD surfaces for.

For the leading edge of the mount, I chose to use a NACA 0024 aerofoil profile.  This is a general purpose aerofoil with a 24% thickness to chord ratio.  NACA's double-0 series aerofoil profiles are used for all sorts of things and I judged it to be a good choice for this kind of application.

The Garmin quarter turn mount and the handlebar mount are connected using M3 machine screws and nuts.  I used these dome-headed stainless bolts from eBay, which have dome-shaped heads that have a 6 mm diameter and a depth of 1.8mm.  

A few more photos 

I'm pleased with how it turned out.  I've attached a few more photos below.

The design at the moment is customised to the shape of my 3T Apto stem, so it won't fit to many other stems at the moment.  However, if you are interested in printing one of these for yourself, for your bike, then leave a comment below.  If I get enough interest, I'll create a generic version that will work with most alternative bar and stem set-ups and will upload it to Makerworld.

Cycliq Fly12 3D printed camera mount

Since buying a 3D printer at the start of the year, I've spent lots of time playing around with it, producing various things for my bikes and home.  A lot of my other bike projects have been put on the back burning temporarily while I'm enjoying the world of 3D printing and creating my designs in CAD.

I've already designed and printed a flow diffuser for my indoor cycling winter setup (see blog post here) and a fan mounting bracket (see here), and I have number of other bike-related things I want to create using the 3D printer in the coming months.

My latest creation, shown in the photo above, is a handlebar mounting bracket for my Cycliq Fly12 bike camera.

The Cycliq camera came supplied with a handlebar mount, also shown in the photo above, but one of them.  Since I often want to put the camera on several different bikes, I need to have several camera mounts, to avoid the need to unscrew and move the mount from bike to bike. Unfortunately, the spare handlebar mounts that Cycliq sell (see here) are quite pricey, at £22.99 each.

I'm not prepared to spend almost £100 to buy several mounts for my spare bikes.  Instead, I’ve created a replica which can be 3D printed.

It works well and only uses 16g of plastic filament to produce (equivalent to about 20 pence).  It requires a couple of M3 machine bolts and nuts, but that's it.  It works well, and I've even created a version with a slightly ovalized radius that fits onto some of my stems.

It's free to download in case anybody has a 3D printer and wants to use it:

https://makerworld.com/en/models/1333580-cycliq-fly12-camera-handlebar-mount#profileId-1372448

Custom made fan diffuser

This trumpet-shaped object is the diffuser that I've designed and 3D printed for my indoor cycling setup.  This blog post describes my reasons for doing this, how I designed it, and how the diffuser alters the airflow characteristics of the fan.

In my previous blog post I explained how my new improved indoor cycling setup now includes two Cleva Vacmaster fans.  The fans are excellent, providing powerful jets of air to help keep me cool during hard workouts.

The fans have three speed settings, with the fastest #3 setting delivering a airspeed of 31 kph at the centre of the jet, at a distance of 1 metre from the nozzle.  Directly in front the nozzle, the airspeed is obviously faster, 55 kph, but the measurements at 1 metre are more appropriate to how I use the fan.  On its lowest #1 setting, the speeds at 1 metre are approximately 60-70% of the airspeeds at the highest setting.

Why a Diffuser?

The problem I've found is that during the colder winter months, when it's around 10 degrees Celsius in my garage, the airspeed at even the lowest fan setting is a bit too fast.  This is especially true when doing easier Zone 2 endurance rides.  I find that need some airflow to avoid getting sweaty, but I only need a very light breeze.  With the more powerful Cleva fans I found myself often getting too cold, even with the fans on their lowest setting.   I would get too sweaty when the fans were off, though, so found myself cycling them on and off.

I decided that I would try to make a diffuser.  A diffuser should slow the flow down by causing it to 'spread out'.  This would, if it works, also cause the jet to become broader, having the additional benefit that the flow would cover more of my body, which should be helpful at times when I wanted the fan to be on it's full setting, to keep me cool.  To do that though, the diffuser would have to work properly, meaning no flow separation within the diffuser, in order to maintain full aerodynamic efficiency.  If flow separations occur inside the diffuser, then aerodynamic losses occur (total pressure losses), the result of which is that the air would slow down somewhat, but the jet would not widen correctly.  In that case, the same result could be achieved simply just by restricting the flow with an object that partially blocks the nozzle (e.g. a grid, or a gauze).

Diffusers are also used in a couple of other applications that people may be familiar with:

• On the rear underside of Formula 1 cars, and other race cars (see here).  Race car diffusers achieve the same result, slowing down the air flow by causing it to spread out, to expand.  The objective is slightly different though - when the flow slows down, it increases in pressure, returning to the ambient pressure as it leaves the back of the car.  This in turn allows the flow under that car to be faster, operating like a venturi, which reduces the pressure below the floor of the car to a pressure below the ambient air pressure, 'sucking' the car downwards, thereby creating aerodynamic downforce.
• Diffusers are also used in wind tunnels, behind the working section, to slow down the flow, allowing slower moving airflow in the return loop of the wind tunnel, reducing losses in the wind tunnel.

Fan Diffuser Mk.1

My first diffuser design was a straight tapered device, shown in the screen shots on the left.  I used the Autodesk Fusion CAD package to create the diffuser in two parts.  The first part is permanently fixed to the fan and replaces the standard black nozzle on the fan, having a similar shape and fittings.

The photo below shows how the two parts fit together. The fixed part has a forward facing slot around the edge of the nozzle.  The diffuser has a tapered flange which then fits into that slot.

The slot and flange connection is tight enough to stay there by itself, but for extra security I drilled a couple of small holes to secure it with 3 mm wood screws.

The next step was to see whether the diffuser worked correctly.  I taped down a number of small ~30mm lengths of wool onto the inside of the diffuser to help determine the flow quality.

Wool tufts are a common type flow visualisation technique used by aerodynamicists to determine where the flow is attached to the surface or separated.  Attached flow means the air is moving in the intended direction, flowing smoothly across the surface.  Separated flow means the flow has broken away from the surface, causing regions of  recirculation where the flow can be moving in the opposite direction, creating eddies that restrict the flow and reduce the efficiency of the diffuser.

The video below shows the that some of the wool tufts are quite stable.  However, other tufts are quite active, showing that the flow is separated, or nearly separated.  One tuft on the lower surface is being blown backwards, indicating it's in a region of re-circulating air caused by flow separation further upstream inside the diffuser.

Generally, the result was a bit disappointing.  I measured the characteristics of the airflow downstream, just to check it (see plot on the left).

This plot confirms that the airflow speed was being slowed down, but the jet wasn't getting much wider.  This is another indication that the diffuser was lacking efficiency because the flow separation in the diffuser causes losses that slow the flow down but do not cause it to spread out, meaning the mass flow rate was not being conserved because the flow through the fan was being constricted.  My feeling is that the angle of divergence I chose for the nozzle was too severe, causing the adverse pressure gradients in the diffuser to be too high, prompting the flow separation.

Fan Diffuser Mk.2

As a second attempt, I decided to try a diffuser with gradually increasing divergence, combined with a lot more internal vanes to help guide the flow.

In my mind, these multiple guide vanes would operate in a similar way to how corner vane cascades work in the corners of closed return wind tunnels to efficiently turn the flow through 90 degrees (see diagram on the right).

The CAD screenshot to the left shows the external shape of the diffuser (left), along with the fixed nozzle that permanently attached to the fan.  The screenshot on the right shows a cut-away cross-section, showing the shape of internal guide vanes.  The diffuser required about 400-500 grams of plastic filament (PETG) to make, and it took about 12 hours to print on a Bambu Lab P1S 3D printer.The number of guide vanes made it impossible to attach fixed wool tufts inside the diffuser, so instead I used a wool tuft on the end of an old wheel spoke to inspect the flow quality.

It can be seen from the video above that the flow quality looks good, with some clear changes in flow angularity across the exit of the diffuser, showing that the flow is being turned.  This was encouraging.  The next step was to measure the airspeed profile.
Airspeed profile measurements

I used the anemometer that I described and calibrated in a previous blog post (here) to measure the airspeed at 1 metre from the fan.  I measured the airspeed at 50 mm intervals up and down the centreline of the fan jet, as shown in the photo to the left.  I also checked the velocity variation to the left and right of the centreline, to ensure I was measuring the centre of the jet where the peak velocities were occurring.  The airflow speeds for the three different nozzles, with the fan on the highest speed setting, are shown in the plot below.The airspeed profile below shows that the Mk.2 diffuser is doing a good job, not only in slowing the flow down, but also spreading it out too.

For the lowest fan speed setting, the airspeed profiles are progressively lower, as shown in the plot below, but the differences are otherwise more or less the same.

Conclusion

Overall, I'm really please with the diffuser.  It makes a noticeable difference to the airflow on colder days.  On milder winter days, I turn the fan up to setting 2 or 3 to get the necessary cooling.  When the weather warms up even more, in the spring, I'll remove the diffusers altogether to maximise the airspeed.  For now though, in the winter temperatures, they're working great with the fans.

Added 11/10/25:  Noise Measurements

Somebody added a comment on when I published the part on MakerWorld, asking about the effect on the noise of the fan.  I was curious about the noise, so made some noise measurements.

I used just a simple iphone app to measure the noise.  With the phone right in front of the fan, in the middle of the jet, but 1 metre away from the fan, the noise is lower with the diffuser.  Without the diffuser (just the replacement nozzle) it was 76.3 dB on the lowest setting, and 87.4 dB on the highest setting.  With the diffuser, it was 61.5 dB and 76.3 dB respectively, so 10-15 dB quieter.  I the decibel reduction was due to diffuser slowing down the jet, so there's less of the wind noise.

I also measured the noise outside of the jet, still 1 metre downstream from the fan, but 0.8 metre off to the side.  In that case, adding the diffuser made it slightly noisier, 5.5 dB noisier on the low setting (57.5 vs 52.0 dB) and 6 dB noisier of the high setting (72.0 vs 66.0 dB).

Overall then, it not a big change, a bit quieter inside the jet due to reduced wind noise, but slightly noisier outside.

Indoor cycling set-up v3

In a couple of previous blog posts I showed my indoor training setups, the first one in 2016 here, and then my improved one built in 2020 here.  In the last few days I've improved it further.

With hindsight, my cooling fan setup wasn't as good as it could be.  The cheap pedestal fan that I've used for the last four years couldn't provide enough flow and cooling for the warmest days or the hard workouts, although it was okay most of the time.

That has changed recently though, after I bought two Cleva centrifugal fans, after seeing some good reviews from people on the TrainerRoad Forum, who discussed UK alternatives to the Lasko fan that was highly recommended by tghe guys on the TrainerRoad podcast.

Initially, I bought a single Cleva Air Vacmaster Air Mover fan, which about  £50 at the time.  This was great, but it has a manual switch, so I also bought a remoted controlled plug set from Amazon for £18.99, to allow me to turn the fan on, after I warm up, about 5 minutes into the cycle.  The fan speed was excellent, but the fast moving air stream was quite narrow, not broad enough to cool my whole body.  Therefore I decided to buy a second fan to give me more complete body coverage.

The second Cleva fan I bought was their more expensive Vacmaster Cardio54, which I bought for £79.99.  It's identical to the cheaper Air Mover, except that it has a built in remote control which is slightly better because it also allows the speed to be selected from the remote, rather than just on/off.

As shown in the photo at the top, I have mounted these two fans in two positions in front of my bike and trainer, one directed at my head/torso from above and the other directed at the lower half of my body.  After some experimenting, I found this to be the best setup.

The lower fan was easy to position, and required only a small block to get the angle just right.  Also, it could stay in that position and wouldn't get in the way when I'm not cycling.

The upper fan was a bit more tricky though.  The fan is capable of being mounted to a standard tripod threaded attachment, or alternatively mounted onto a TV wall mount that has a VESA mount.  However I didn't really want to spend more money on a TV mount, so I re-purposed the fan swing arm from my old pedestal fan, as shown in the photo on the left, using a cylindrical wooden rod as the arm.  To attach the fan, I made a 3D-printed backet, using the 3D printer that I brought late last year.

I designed the bracket in Autodesk Fusion 360, which is free CAD software. I incorporated a quick release mechanism into the bracket, re-using an old QR seat post clamp that I had spare.

This allows the angle to be tilted to the best angle to direct air on my head and upper body.  The swing arm allows the upper fan to fold out of the way, against the wall, when not in use.  I've made the bracket available to download here on MakerWorld.

I'm really pleased with the new setup.  The amount of cooling is excellent, and in the colder winter months it is occasionally even too much, even on the lowest setting.  On those colder days, I only turn the lower fan on.  The photos below show the setup with everything in place (left), and on the right it's how it looks when everything is folded away to make more space in my garage.

 

Indoor cycling fan upgrade and speed check

I recently bought a Cleva Air Mover centrifugal fan as an upgrade to my indoor cycling setup. 

It came highly recommended on several TrainerRoad forum posts (see here).  Compared with my old fan, it seems to be a lot better.  I wanted to measure exactly how much better it is, and this blog post describes my attempts to measure the air speed that the fan generates.  As usual, it's all a bit nerdy and over-the-top, so sorry about that.

Airspeed measurement methods

I bought a cheap anemometer (an anemometer is air speed measurement device) from AliExpress for an incredibly cheap £13.05.  It seemed good, but given that it was so cheap, I wanted to check its accuracy with a second source of airspeed measurement.

Having recently bought a 3D printer, I decide to make a pitot tube that I could use as a 2nd instrument to measure the air speed generated by the fan.  It was quick to create in CAD (<1 hour) and printed in less than an hour.

A pitot tube is a device that measures the total pressure of the flow, by having tube that is orientated in the flow direction.  When the flow enters the pitot tube is comes to rest within the tube, and when it does so the pressure will be the total pressure. Total air pressure is the ambient (static) pressure of the air plus the dynamic pressure of the moving air.

Hence, the difference between the static pressure and the total pressure is the dynamic pressure, which is a function of the air speed and the air density.  Hence, but knowing the air density and the by measuring the dynamic pressure (via the difference of total and static pressures) the air speed can be obtained.

I measured the difference between static and total pressure using a device called a manometer, which is the U-shaped device shown in the sketch on the left.  The manometer tube is filled with a liquid, shown in purple in the sketch, and could be any liquid such as water or mercury.  I used water because it was easy and its relatively low density helped with measurement precision.

I bought a manometer from Amazon for about £20.  A quick calculation in Excel allowed me to calculate the airspeed that corresponds to various manometer water height differences.  I also checked how sensitive the results would be to small changes on pressure and temperature, which affects the air density.

Results

I measured the airspeed at 0.5 metres from the fan nozzle and also at 1.0 metres from the nozzle.  These are distances that are representative of how far away the fan would be located from me when I'm cycling.

0.5 metre distance

The AliExpress anemometer gave a maximum airspeed, in the centre of the air stream, of 11.2 m/s.  The reading fluctuated by approximately +/-0.8 m/s.

My pitot tube gave a water height difference of 8 mm in the manometer, with a manometer measurement precision of about +/-1 mm.  This corresponds to an airspeed of 11.32 m/s +/- 0.8 m/s.  The nature of the pitot tube and manometer meant that any air speed fluctuations are damped, although the +/-1mm manometer measurement precision results in a +/-0.8 m/s uncertainty. The two measurement sources, the pitot and the AliExpress anemometer, therefore agree very well for this 0.5 metre case.

1.0 metre distance

At a distance of 1 metre, the AliExpress anemometer gave a maximum airspeed of 8.8 m/s, with +/-1.0 m/s fluctuations.

My pitot tube gave a water height difference of 4 mm in the manometer, +/-1 mm.  This corresponds to an airspeed of 8.0 m/s +/- 1.0 m/s.  Again, the two measurement sources therefore agree very well at this distance of 1m, and well within their measurement precision.

Speed directly at the nozzle

The Cleva website says that the maximum airspeed of their Cardio 54 fan, which is mechanically identical to their Air Mover Fan, is 54.0 kph, 45.0 and 34.2 kph for the three speed settings.  These speeds correspond to airspeeds of 15.0 m/s, 12.5 m/s and 9.5 m/s.  It's highly likely that these speed quoted by Cleva are the maximum speeds, directly in front of the nozzle.

I checked the speed using the AliExpress anemometer, and that device gave speed recordings at the fan nozzle of 15.4 m/s, 12.9 m/s and 10.7 m/s for the three settings, all with a variability of about +/-0.4 m/s.  These values are therefore consistent with Cleva's quoted airspeeds, and actually are slightly faster.

Back-to-back rolling resistance testing: MTB versus Cyclocross tyres

Last year, in October 2023, I published the results from some tyre rolling resistance tests (see blog post here) in which I tested my cyclocross bike with 33mm cyclocross tyres.

Those tests showed that my cyclocross (CX) tyres were much slower than my Schwalbe Thunder Burt mountain bike (MTB) tyres. The CX tyres had a CRR value nearly double the CRR of the MTB tyres, which was a huge difference.

However, the two tyres were tested on different days, with different bikes and different power meters, so there were a number of factors that cast doubt upon that comparison.  Since doing that test, and especially as the results were so surprising, I've wanted to repeat the test by testing both tyres on the same day, with same power meter, thereby improving the quality of the test.  This blog post describes my latest test results, and thereby allows me to conclude, with much more certainty, which of my two bikes is faster for dry cyclocross races.

Recap of previous test results

The previous results are shown in the plot to the left.

The data points shown in red are the CRR values obtained from my hardtail mountain bike tested in June 2022.  The CRR values reach impressively low values (~0.015), much lower than the two sets of cyclocross tyres plotted in blue and green.

As noted in the plot, the Challenge Grifo/Baby Limus tyre test was done a day when the ground was softer.  This will have certainly caused the CRR values to be higher, although it's not clear how much the softer ground accounted for the differences.  Note also that the softness of the ground was only determined qualitatively by me, with no measurements etc of the firmness.

New test setup

The new test is better in several ways:

• Three different sets of tyres, all tested back-to-back on the same day.
• Same power meter for all tests: Favero Assioma Pro MX-2 power meter pedals.
• The power meter pedals are a dual-sided power meter, rather than single sided power meter that I've used in the past for most of the previous testing.

The use of a dual-sided power meter ensures total power is measured, whereas single-sided power meters measure only one side (one leg) and then assume the other leg is producing the same power in order to infer total power. Dual-sided power meters are therefore superior, especially since I have discovered that my left leg/right leg balance is not 50/50 (it's more like 53% left, 47% right).  Also, and more importantly, I've also discovered that my left/right leg imbalance is not fixed, and it varies with power, so there is inconsistency and inaccuracy whenever I use my single-sided powered meter. 

Test order:

1. Drop Bar MTB with 2.35" (60 mm) Schwalbe Thunder Burt Super Race tyres @16 psi
2. Cyclocross bike with 33 mm Challenge Baby Limus HTLR tyres @20 psi
3. Drop Bar MTB with 2.35" (60 mm) Schwalbe Rocket Ron Super Race tyres @16 psi

The test was performed in my usual way (see Chung method description here), with the exception that I tested only one pressure per tyre/bike this time, using pressures that I know from previous testing give good rolling resistance results but without negative drawbacks like pinch flat risks.

Speeds were measured using a hub-based speed sensor, with the diameter in my Garmin head unit set to a fixed value of 2115 mm.  Measured speeds were then scaled in the analysis based on measured diameters to give the correct speeds:

  - Schwalbe Thunder Burt:  Speed scaling factor = 2228 / 2115
  - Challenge Baby Limus:  Speed scaling factor = 2110 / 2115
  - Schwalbe Rocket Ron:  Speed scaling factor = 2245 / 2115

Results

I performed the Chung method (virtual elevation) analysis using a fixed CdA or 0.40 m^2.  It's obviously wrong to use the same value for both bikes, because it's likely that the CX bike with a lower frontal area (narrower tyres, etc) will have a slightly lower CdA.  However, in the same way that I've done it previously (see here), by using this fixed value of CdA, any performance differences between the three setups, whether those performance differences comes from aero, rolling resistance or something else, will be attributed to CRR.  Hence, whichever setup has the lowest CRR using this style of analysis will be the fastest bike.

Results with Fixed CRR

Using a fixed CRR value of 0.0261, the virtual elevation plots show that the three setups are reasonably similar.

Note that the variations in the measured elevation (the green line), should be ignored because this is simply a drift caused by changes to ambient pressure during the test (note that a one metre elevation difference measured by the Garmin barometer is equivalent to just a 0.12 millibar change in ambient pressure).  The tests were done riding laps of a grass field, so the actual elevation always returned to the same value, in reality.

The variation in the orange virtual elevation profiles show which setup is fastest.  The CX bike with 33 mm Baby Limus tyres is fractionally slower than the drop bar MTB with Thunder Burts, because it's VE profile rises slightly.  The fastest setup is actually the third one tested, the MTB with Rocket Rons, since the VE profile falls slightly for that third test.

Results with adjusted CRR values

The CRR values can be adjusted to 'flatten' the VE profiles, as shown below.

This plot shows that the three setups have the following CRR values:

• Drop Bar MTB with 2.35" (60 mm) Schwalbe Thunder Burt tyres:  CRR = 0.0261
• Cyclocross bike with 33 mm Challenge Baby Limus HTLR tyres:  CRR = 0.0263
• Drop Bar MTB with 2.35" (60 mm) Schwalbe Rocket Ron tyres:    CRR = 0.0251

Note that difference between the Rocket Ron CRR of 0.0251 and the Baby Limus CRR of 0.0263 is equivalent to 6.7 Watts of rolling resistance for a 85 kg bike+rider travelling at 15 mph.  These differences aren't significant, but they aren't particularly large differences either.

It's also interesting to note the average speeds, although it's more difficult to conclude which setup is faster form these speed, because the average power are not exactly the same.

• Drop Bar MTB + Schwalbe Thunder Burt tyres:  Avg speed = 13.79 mph @203W avg
• Cyclocross bike + Challenge Baby Limus tyres:  Avg speed = 13.33 mph @193W avg
• Drop Bar MTB + Schwalbe Rocket Ron tyres:    Avg speed = 13.60 mph @195W avg

Incidentally, one of the benefits of the virtual elevation method is that it's not necessary, as a tester, to hold a fixed power.  That's a really inconvenient constraint that many testers put upon themselves if they don't use a virtual elevation method and try to hold a fixed power.

Finally, it's interesting to note that the final two laps for the Rocket Ron have a significantly rising profile, even though the first four laps were flat.  Those last two laps would have a measured CRR of 0.0271, which is strange.  I have no explanation for these weird results during the last couple of laps.

Conclusions

I set out to see repeat my previous tests but with a better standard of testing, to see whether my previous conclusion that MTB tyres are faster for cyclocross still holds true.

The conclusions from this test aren't quite so clear.  The MTB tyres are still slightly faster, but not significantly so this time.  The MTB tyres are only about 1-7 Watts faster.  Therefore, I'll likely use my cyclocross bike if there is any chance of mud, since my CX bike has much better mud clearance and is less likely to get jammed up from sticky mud.

Perhaps the most surprising result of this test is that the Rocket Ron tyres were actually faster than the Thunder Burt tyres, despite being how knobbly they are compared with the Burts, and considering the tyres are similar in every other respect.  This goes against what I'd expect and is contrary to Bicycle Rolling Resistance tests (although the Rocket Ron performs quite well on BRR).  This is a test result that would be worth trying to reproduce at some point.