This blog is a repository for all my the bike-related studies over the past 20 odd years. The content ranges from modelling and analytical studies, to hacks and bodges made to my bike equipment. It's all been done in an attempt to make myself faster on a bike, to make the most of my mediocre fitness.
Blog posts have been written from 2020 onwards. The dates of the posts reflect when the work was originally done.
It’s almost complete now, with just a few older posts still to write up.
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.
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:
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.
I've worked in the aerodynamics department of a large aircraft manufacturer for over 20 years. In my day job, I'm responsible for the prediction, modelling and measurement of the drag and lift of our aircraft. Much of my interest and knowledge of engineering and aerodynamics spills over into my main hobby, which is cycling (Road, MTB, cyclocross and time trialing).
