Digging Deeper Into the Hunt SUB50 Limitless Aero Disc

Along with launching the new SUB50 Limitless Aero Disc wheelset, Hunt also published a white paper to detail the testing and development that lead to the creation of the new wheelset. I know not everyone is a hardcore tech geek when it comes to these things, so consider this a middle ground between reading the 34 page paper, and our release article. If you’d like to read the whole thing though, here is the link. If you have any interest in aerodynamics and product development, it’s a pretty interesting read.

For wheels, one of the biggest developments for aerodynamics, has been the disc brake. That might sound counter intuitive, because you now have this block sitting on the fork and chain stays that isn’t the most aerodynamic part on the bike and hubs have been redesigned to handle the disc. But what it did is remove the shackles of using caliper brakes that limited the width and also required a brake track to be designed into the rim. This means more material (more weight) and limitations on having a flat, parallel section where the rim meets the tire. Add in the extra heat generated during braking and the affect it has on air pressure, and you can see how rim brakes limit what you can do.

With disc brakes, wheel manufacturers are now open to all kinds of new design parameters. They can go wider, experiment with various shapes, and rethink the layups to help create a more aerodynamic profile and a lighter and stronger wheel. It also allows for wider tires, without the lightbulb affect, providing better handling, more comfort, reduce rolling resistance, and additional improvements to aerodynamics. Now add in the rise or tubeless tires which help to reduce rolling resistance even further.

What a time to be riding a bike.

Fast forward to 2021 and the Hunt 48 Limitless Aero Disc. At the time, it was a unique design that was pushing the boundaries of the width of rims. At 48mm tall, 35mm wide, and an internal width of 22.5mm, it was perfect for the 25-28mm wide that were becoming commonplace. Even the 30mm tires being used at Classics races were a great for the 48 Limitless.

Other brands began to take notice, and started experimenting with widening their rim design as well. Today, it is not uncommon to find widths of 32mm from other top brands on the market.

SUB50 Project

The SUB50 project was created as the evolution of the 48 Limitless. Hunt left the design parameters open, other than keeping the rim height at or below 50mm to create an all-around wheel. Keepingin mind current trends, they wanted to keep the wheel optimized for 28-30mm tires, use a hookless rim design (we’ll get into that more later), and a tire bed design complaint with the latest ETRTO tubeless standards. The key metrics they were tracking were aerodynamic efficiency, aerodynamic stability, system weight, and tire shape, all of which have major impacts to how the wheel and wheel/tire combo perform.

Using the 48 Limitless as their starting point, Hunt started with Computational Fluid Design, before heading to the wind tunnel. This allowed them to rapidly test prototypes much faster and easier, and leaves the wind tunnel for more proof of concept than testing finer variables. Hunt used a Mavic developed Ponderation Law called Wind Averaged Drag, which is widely accepted in the cycling industry as a means test across a range of yaw angles, and how to weight each. Essentially, it shows that riders spend most of their time at 0 degrees to +/-2.5 degrees. From there, as you increase by 2.5 degrees, the percentage of time a cyclist faces air flow at these yaw angles decreases down to less than 1% at 25 degrees. For CFD testing, Hunt went out to 16 degrees, which passes the stall point on the wheel. For wind tunnel testing, they went down to 20%.

CFD Background

CFD is something we all talk about, and generally understand the concept, but it is very interesting how it actually works. Essentially, the object is created in a digital envrionment and comprised of a mesh network of cells, similar pixels in a display. Each cell acts like a sensor and tracks how digital fluid, or in this case, air, flows over each cell. All of the data from each of these tiny cells is then combined to get a bigger picture about how the object will act with air flowing over it.

Hunt used two scenarios in the SUB50 project, wheel and tire only, and wheel on bike. For the wheel and tire only (based on the Schwalbe Pro One 700×28), they tested just the rim shape, as adding in spokes, nipples, and hubs would add considerable amounts of data points that would have significantly slowed testing down and add uncertainty. All of that, along with the fine details tire like texture and tread pattern, can be more efficiently reviewed in the wind tunnel. For the full bike testing, they used a set up based on the Argon18 E119+ from previous testing, and focusing on the non-drive side to help simplify testing. While the wheel and tire only test focuses more on the front wheel, the full bike gives a perspective of how the rear interacts with air flowing past the front wheel and frame, which can allow for unique designs for each wheel.

Hunt’s in-house engineering team created a total of 42 different rim profiles. Each design typically underwent 30 simulation runs at about 12,000 hours per rim. The profiles consisted of different variations of shoulder width (the widest point of the rim to the point where the rim starts to curve at the sidewalls where the tire sits inside), nose radius, and rim depth. These variations are minute, but it still plays a combined part in the wheel design.

Shoulder Width

Starting at the shoulder width, Hunt’s engineers knew this would be an important area. The 48 Limitless sits at 30.9mm, but for SUB50, they bumped it up to 32.0, 32.2, 32.4, and 32.8mm on a 34.2mm wide rim. During testing, drag reduction was it’s lowest at about 15 degrees yaw angle. From the original 48 Limitless, version 2.12, which uses a 32.4mm shoulder, tested the fastest, besting the previous design by almost 50% at the stall point.

Nose Radius

Next was the nose radius. This is actually a very important part of the design, because it is the last point of contact of the air flowing over the leading side of the wheel, and the first point of contact on the trailing side. In a perfect world, you’d have different profiles for each, but since a wheel rotates, the best you can do is to find the best compromise. For this, they tested a radius of 12, 13, 13.5, and 14mm.

In testing, all were generally an improvement over the previous design. The smaller radius designs had higher drag down to the stall point, while the larger radius has less drag, but a sooner stall point due to flow separating sooner and the more abrupt transition from the nose to side walls. To find a happy medium, the 13mm radius was selected, trade-off between low drag at lower yaw angles and a later stall over the 14mm radius.

Rim Depth

Lastly, the rim depth was the last parameter that was looked at. This is another area where a trade-off was needed. Deeper wheels have less disruption of airflow, but also face increased forces from crosswinds. All-around wheels typically sit at about the 50mm range, while riders looking for increased speed, usually at the expense of additional weight and crosswind stability. For this round, Hunt is looking at that all-around role the 48 Limitless currently sits at. Their deeper wheel, the 60 Limitless, could be one of the next wheels they begin to look at, but it still has a few years left in the product cycle.

Hunt started with the 48 Limitless’ 48mm depth, and reviewed two versions at 49.5 and one at 51mm. One of the 49.5s, V2.19, kept the same shoulder width and nose shape, while V2.23 was reduced to a shoulder width of 32mm. Performance decreased with the larger variance between the external width and the shoulder, so they stuck with the wider shoulder. The 1.5mm increase over the 48 Limitless yielded a solid improvement, with the 51mm even better. The 51mm profile was deemed a incremental improvement that was not worth the increase in cross wind stability.

Going back to the 49.5mm profiles, it is interesting to look at the difference in performance for the internal widths and how it affects the shape of the tire and overall aerodynamics. Hunt tested two versions of the 49.5mm tall, 32.4mm wide shoulder, and 13mm nose radius, one with a 23mm internal width and one with 25mm internal width. The 25mm internal width was considerably faster from 5-15 degrees due to the shape of the tire created, smoothing out the transition between the tire and the rim.

All of this testing lead to three designs being prototyped for the wind tunnel. The two variations discussed in the previous paragraph, along with a version with internal nipples to see if any improvement in drag could be made.

Rear Wheel Design

So now that we are done with the front wheel, let’s go to the back. While initially tested as wheel/tire only, it was quickly moved to full bike testing due to the the importance of the interaction between the frame and wheel. With the frame sitting in front of the wheel and creating the initial flow of air around the tire and wheel, different shapes were explored from the front wheel design. This means the nose radius on the trailing side of the wheel is more important than the front, leading the design to focus more on that area of the tire’s rotation.

While the front wheel has a greater benefit from a wider design, the rear wheel saw improvements with a narrower design. Hunt test options mimicking the front (49.5mmx34.2mm) with a 13 degree radius and a 10 degree radius. They also looked at a 30mm wide option with a 9mm radius and a 7.5mm radius. In testing, the 30mm wide wheels tested nearly identical, with a significant increase in performance over the wider wheels at the higher yaw angles.

Next, they tested rim depth. Many brands over the years have gone with deeper rear wheels, trying to gain additional aerodynamic efficiencies with the reduced affects of crosswinds on steering stability at the cost of higher weight. To test this, Hunt went with 49.5, 54.5, and 59.5mm depths, with a 30mm width, and 9mm radius. Testing as a complete bike with the 49.5mmx34.2mm front profile, the rear saw the greatest improvement at the higher yaw angles. This lead to the decision to take the lowest and tallest versions to the wind tunnel for the rear, a version of the 49.5mm rear with a 25mm internal width, along with a version of the front rim laced to a rear hub.

Wind Tunnel Testing

3D printed models of each rim design were created with the help of Formlabs, built into full wheelsets. They were created in sections that could be replaced as needed, with one unique section for the valve stem. These wheelsets weren’t strong enough to be ridden, but strong enough to hold a mounted tire at pressure with little to no deformation. They were then sent to GST Winkanal in Immenstaad, Germany. GST was create in 1986 for Airbus Space and Defense, but is now independently operated and used as a low-speed tunnel, well suited for cycling.

For the 3D printed prototype testing in August 2022, Hunt opted for a 2022 Scott Foil and Schwalbe Pro One TL in 28 and 30mm widths. Their prototype wheelsets were inflated to 30psi, while competitor wheels were inflated to 60psi. Roller and wind speed were set at 45 kph (~28mph). Yaw angles were tested from -20 to +20 degrees. Testing was done without a rider to help pick up the smallest important details, which wouldn’t be detectable with a rider.

Along with testing drag, cross wind forces (steering force) were also tested across the yaw angles. The Hunt engineers had two goals here: create a profile that keeps forces as predictable and delaying the stall point as long as possible, and lower overall side forces as much as possible to reduce the work the rider needs to exert to compensate.

Hunt also brought competitors’ wheels to the GST to compare to the SUB50 prototypes. The criteria for their selection included disc brake wheels, approximately 50mm in depth, and an internal rim width of +/-2.5mm to the SUB50 prototypes, keeping measured tire sizes similar to the SUB50.

Before testing against the competition, Hunt had a few variables to test.

First, internal versus external nipples. In testing, Hunt found a small savings in drag across most yaw angles, but when applying the Wind Averaged Drag (WAD) calculations, the savings were only 0.2W for both 28mm and 30mm. This wasn’t enough to bring them in, which also happens to make truing wheels easier as well.

When testing the two versions of the rims, one with a 23mm internal and the other with a 25mm internal, Hunt didn’t expect much of a difference from the rim itself, but it does show the importance of matching tire sizes. The 23mm internal width tested marginally better with the 28mm tire, while the 25mm internal tested marginally better with the 30mm tire. In the end, with the release of the new ETRTO guidance that 28mm tires should be mounted on rims with a minimum internal width of 23mm, Hunt went with the 23mm internal option for final production testing.

For the rear wheel, there were some interesting results between the 28mm and 30mm tires in testing. For the 30mm tires, the 25mm internal width tire actually tested slowest of the bunch, as the tire was 0.7mm wider than the 23mm internal rim. Even the 59.5mm deep rim didn’t show a considerable improvement, even with a 10mm increase in depth.

On the other hand, the 28mm tires tested better on the 59.5mm deep wheel, with just a slight increase in drag for the 49.5mm wheelset with the 23mm internal width.

Final Production Testing

In the end, the 49.5mmx34.2mmx13mm radius and 23mm internal width front and 49.5mmx30mm widex9mm radius with 23mm internal width rear was chosen for production. The front, V2.19, out performed the wider internal width with the 30mm tire and showed similar performance with the 28mm tire. The rear, vR4, was the fastest of the 49.5mm rims, while also helping to reduce the overall weight of the wheelset compared to the taller rim profile. Hunt feels this offers the best combination of aero performance, cross wind stability, weight, and tire compatibility for an all-around wheelset.

Final production testing was done in June 2023 using a Cannondale System 6. All of the conditions were the same as testing during the prototype phase, with an increase of 60psi for the production Hunt wheels. Due to temperature changes throughout the day during testing, this caused variations in windspeed in the tunnel by 0.7m/s. For the purpose of the white paper, Hunt focused on morning results, though they did include all data throughout the day in the appendix.

Competitors Hunt tested included Zipp 454 NSW, Zipp 303s, Enve 4.5 SES, Princeton Carbon Works Peak 4550, and Roval Rapide CLX II. Others, including the 48 Limitless were included in testing, but not referenced in some of the final charts. All were tested with the same Schwalbe Pro One TL in 28 and 30mm sizes. These wheels averaged a depth of a range of 45mm up to 58mm, and average widths from 27.58mm to 29.79mm. Weights ranged from 1378g to 1540g.

For the 28mm runs, the taller, narrower wheels showed lower drag numbers in the central yaw angles. This includes the Roval, Enve, SUB50, and the 454 NSW. As yaw angles spread wider, that is where greater separation is seen, with the Roval having the lowest drag and the SUB50 and Enve swapping places on either side. Interestingly, between the two, the SUB50 had the more consistent drag and stall points compared to the Enve. The Roval’s performance likely stems from a wide front rim profile, and narrow 21mm internal width, though this did make for the most narrow tire width measured in the group. While rolling resistance was not part of this study, it could be something interesting to look at as we have seen data showing wider can be more efficient.

Other important considerations are weight and cross wind. While the Roval and Enve wheelsets might have slightly better drag numbers, they are also 130-140g heavier. When you think of climbing or how many times you accelerate out of corners, that weight difference adds up over time with.

For cross winds, the SUB50, Roval, and Enve all performed similarly. This translates into a more stable feeling as yaw angles increase, with the stall point happening further along.

For the 30mm wide tire testing, the Shimano Dura-Ace C50 was included. It was included in the 28mm testing, but only in the afternoon sessions. Across the range of wheels, there were not any significant changes to the results seen on the 28mm.

Final Conclusions

Differences in drag between the top three wheelsets were less than 1W difference with either tire. The Roval, Enve and SUB50 wheelsets performed similar, with the SUB50 offering the best combination of aerodynamics, weight, and stability between all wheels tested.

Beyond aerodynamic gains, the stability shown in the steering moment data creates a wheel that allows riders to hold aerodynamic positions longer, helping improve overall aerodynamics of the bike and rider as a complete package. Adding in the wider tire profile from the 23mm internal channel, riders can take advantage of the rolling resistance gains of wider tires mentioned earlier, along with the comfort that comes with it. This creates a wheel that is made to be fast on smooth or rough roads.

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