When Mavic rolled out the CXR80 last year, they introduced an entirely new take on aerodynamic wheels with the CX01 design package. It used a NACA airfoil profile created by pairing specifically shaped tubular tires, deep section carbon rims and the “Blade” outer rim strips to cover the gap between the two and create a perfectly smooth surface.
Now, they’ve added two 60mm deep variants called CXR60, available as both tubular and clincher. They’re designed to combine the lowest possible drag in any weather condition and improve stability in heavy, varying crosswinds.
Tall, wide rims have been proven to be good aerodynamically, but Mavic says the tire represents 85% of the frontal area of the wheel, so it has to be part of the formula if you really want optimal aerodynamics. Not only is the tire is designed into the overall profile as part of the NACA tear drop shape, even its tread pattern and profile shape were designed to create tiny sections of turbulence to create a boundary layer of air that would then hold the airflow as close to the tire and wheel as possible. The net result is a more laminar airflow and less total turbulence, which means less drag and more speed.
And now it’s lighter, cheaper and gives you the option of running a clincher…
AERODYNAMICS TECH TALK
The concept is simple, but it’s taken them a long time and plenty of testing to get there. But the story of why they got there is an important part of it:
First they looked at all of the factors that can slow you down. Rolling resistance depends on many factors, only some of which are controlled by the wheel and tire. That means Mavic can only control some elements when designing the parts – outside their control is rider weight and air pressure, both inside the tire and outside environmental conditions. What they can control is rubber compound and shape. Fortunately, rolling resistance is a comparatively small fraction of the equation, and it’s constant, so its impact becomes less of a factor as speed increases.
According to Mavic’s research engineer Brieuc Cretoux, Drag = speed². Power = speed³. So, if you ride at 20kph and you want to reach 40kph, drag is 4x higher and the power required to get there is 8x higher.
They can’t do much about your power, so it came down to decreasing drag…which is affected by quite a few things. Air density is one, and that depends on altitude, temperature and humidity. Lower air density means less “fluid” to push through. Unfortunately, they can’t control the conditions you ride in, so they kept looking.
Air flow patterns are also dependent on the speed, too, with flow around objects being predictable up to a point (A), then becoming unpredictable (B) before becoming somewhat predictable again (C). As Murphy’s law would predict, the speed at which we usually ride a bike falls smack in the middle of that crappy unpredictable range (B, red in the graph above, or roughly 30-50kmh). CFD can help, to a point, but CFD software is typically developed around high speed (car, plane) applications, and it’s not easy to implement with so many variables like bike frame and human anatomy variances.
So, the answer is to bring it to the wind tunnel. Test time is expensive, so Mavic’s fortunate in that they have a good relationship and close proximity with the University of Geneva, Switzerland. This lets them spend more time than most testing early stage prototypes in many iterations. It also let them test an empty frame (without wheels) and develop their own balance to measure the strain on the system when the fans are blowing. Strain otherwise known as drag.
They spent about 400 hours in the wind tunnel in 2011, and at least 150 more in 2012, testing more than 160 designs and various wheel, tire and bike configurations.
Mavic is the only cycling brand to do product testing here, and they created their own balance (fancy name for the platform with the strain gauges) to record drag in multiple axis by putting the sensors on the rotating part. This provides more accurate measurements (+/- 2.5 grams of drag!) than sensors that remain linear to wind flow even when the bike is positioned at an angle to simulate crosswinds. It also measures spinning forces, and considering wheels can account for 20-30% of the bike’s drag (about 8% of the rider/bike combo), improving it at all angles and when spinning is quite important. The wheels spin on a belt, which puts minimal upward force on the wheel and absorbs any irregularities in the tire’s outside diameter, keeping the bike stable. It’s speed adjustable to match wheel speed to wind speed, too. They are developing an inverse power meter style hub that would simulate resistance for testing with a rider on the bike.
Part of the reason for the 60mm versions were to offer something closer to an everyday performance wheel. An 80mm deep wheel isn’t likely to appeal to most riders, but Mavic’s saying the CXR60 is for “triathletes, time trials and hilly races.” Yes, a deep section aero wheel for hilly races.
Perhaps counterintuitively, they say shallow wheels will have reasonably good aerodynamic properties when facing directly into the wind, but quickly add a bit of drag when hit by a crosswind. In contrast, deeper wheels will have a slightly lower amount of drag straight on, with much, much less drag up to about 15º to 18º yaw angle, then creeping back up but still taking a while to equal the drag of a shallow wheel. So, for anything other than all-day climbing, an aero wheel could make sense…so long as the system weight doesn’t offset the reduced rotational mass of a lightweight, standard wheel.
How does this happen? As the yaw angle increases, the frontal surface area of an aero wheel increases, which you’d think would create more drag.
There are a lot of things that affect drag reduction at increasing yaw angles. The leading and trailing edges shape the airflow, and the smoothness or roughness of those surfaces affect how laminar that airflow is around the object, which is why they made the tread patterns the way they did. But those two factors only explain how they keep the airflow smooth. To wrap your head around why an angled airfoil has less drag than one facing straight forward, the easiest way to think of it is imagining there’s a finite amount of resistance allowed. If the wheel is directly faced directly into the wind, all of the resistance is pushing backward against the wheel. As the angle changes, some of that resistance starts pushing to the side, so there’s less to push back. To a point. Once the angle becomes too severe, usually around 14-18°, turbulence starts adding drag to the equation and throws that example out the window. (If you’re an aerodynamicist, don’t balk…this is a Sesame Street mental illustration only)
Like the CXR80’s, the drag curve follows a “W” pattern, which shows that drag is less at a slight yaw angle. To determine the range of yaw angles they wanted to optimize the wheels for, they created a small wind vane and tested it on riders around the world to see what range of average crosswinds people were riding in.
Another important tidbit from their research is that the front wheel has the largest impact in overall bike performance, in terms of aerodynamics. The drag curves of a standalone front wheel largely mimics the drag curve of the complete bike, including the stall angle, showing that it has a huge influence on overall bike aerodynamics.
The last bit to consider is stability. Once the effective yaw angle of the wind reaches the wheel’s stall angle (the angle at which turbulence starts increasing drag and the curve above starts heading back upward), it not only starts creating more drag, but can affect the stability of the wheel depending on how abruptly drag increases. Mavic made a big point of noting how round and subtle the curve is at their stall point compared to other wheels they’ve tested. In the real world, it means sudden gusts or changes in wind direction aren’t as likely to throw you off your line.
WHEEL DESIGN & FEATURES
NACA Airofoil shapes are the gold standard for aerodynamic shapes, providing the best ratio of width by length by curves. With the CXR80, they used 0024 and 0011, For the CXR60 T (tubular), they used OO29 for the leading edge and 0017 for the trailing half of the wheel. The CXR60 C (clincher) uses 0027 / 0012 shapes. The front half of the wheel is responsible for 60% of the wheel’s drag, the back half is the rest, so the front gets the more ideal aero profile shaping.
At the hub, they tested different flange heights and widths, and the hubs are shaped to provide the best blend of lateral stiffness, low drag and light weight. The CXR 60’s use the same rims and spokes (other than length) as the CXR80.
Compared to the CXR80, the CXR60 tubular requires just 0.4 watts more effort, and the clincher is only about 1 watt more.
But, in real life, crosswinds require us to fight against the wind and lean the bike into the wind or wrestle with the bar to keep a straight line. With the 60’s, lateral wind resistance is much lower, and the effort needed to mitigate the side winds grows more disparate when all things are considered.
Note that the forces acting against them create a relatively flat curve for all CXR wheels. This is the aforementioned stability – performance and handling is predictable.
From there, comparative testing moved to combining the wheels with tires. With both the 60 and 80, Mavic’s wheels showed much lower drag (in watts), thanks in part to a tire and wheel that were developed together. Where this comes into practice, in triathlon at least, was proven last year on the windy Kona Ironman course with sponsored rider Fred Van Lierde. He said he was able to run the 80’s front and rear and stay tucked in the drops and still control the bike where others with deep section wheels had to sit up and hold the bull horns to keep their bike straight. That kept him in a better aero position, and thus faster.
The CXR60 T uses a carbon braking surface like many of their clinchers. It’s not the new surface treatment developed for the CC40 C because these wheels are made on a different factory, not Mavic’s own Romanian facility (yet), so they’re not willing to share the process with foreign, third party factories.
The CXR60 C uses an alloy tire bed with the fantastic Exalith brake track treatment.
The clinchers use an entirely new rim bed extrusion to provide the Blade’s mounting channel. So, while the outside rim measurement is wider than, say, a Ksyrium, the inside bead width is still just 13mm. Nowadays, that will make most people balk at first. But, keep in mind, the key goal with this wheel/tire system is aerodynamics, and Mavic’s global PR manager Maxime Brunand says a narrower tire on a slightly wider rim makes for better aerodynamics. It’s also lighter, and, again, all of these parts were designed together as a system to maximize performance.
Interestingly, the tubular (right) is actually wider than the clincher, a design cue necessary to get the proper NACA profile based on the different shapes between the two types of tires.
Yksion CXR tires use Griplink / Powerlink compounds for more grip in the front and decreased rolling resistance/better power transfer in the rear. Same tread pattern front and rear, and they get a new reflective sidewall treatment that also creates a quieter interaction between tire and Blade for acoustically friendlier performance.
They tested different flange heights and widths, and the hubs are shaped to provide the best blend of lateral stiffness, low drag and light weight. The CXR60’s use the same rims and spokes (other than length) as the CXR80’s.
ACTUAL WEIGHTS & SPECS
The CXR60 T comes in at 1024g front and 1187g rear (including tires and glue).
The CXR60 C is 1208g front and 1389g rear (including tires and tubes).
Yep, we weighed the Blade strip, too. 22g per side (44g per wheel).
CXR60 SPECS:
- 60mm deep x 27mm wide rim (tubular max width, clincher is slightly narrower)
- 16/20 stainless steel double butted spokes
- Alloy axles and oversized bearings
- Clincher: 820g front / 1005g rear (claimed, without tires)
- Tubular: 730g front / 915g rear (claimed, without tires)
- Price: TBD, but will be less than CXR80
- Available: TBD
- Includes skewers, wheel bags, Yksion CXR 700x23c tires and tubes
OTHER INTERESTING AERO NOTES FROM THE LAUNCH
- When humidity is higher, the air density is actually lower (fact), which means you’d go faster. It’s not significant, but if you we’re trying to beat a record (or Strava segment), a hot, humid day might be the best time to go for it.
- Altitude is a bigger factor, and higher is better because the air is less dense. Assuming you can breathe.
- Rolling resistance is lower when it’s cold because the tire will have less deformation, which means less loss of energy.
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The wheels are developed to make their pros faster, but their pros are able to hold a much faster average speed. So do these benefits apply for those of us averaging 17-19mph instead of 25mph? Yes. Perhaps more so. Slower speeds mean we’re on the bike for a longer total time, so the energy and time savings is even greater as your time on the bike gets longer.
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Another thing to consider: as your speed drops, the effective yaw angle of any crosswind increases since your frontal speed is proportionally less. This make wheels with good crosswind stability and aerodynamics even more important.
Related: Check out our factory tour of Mavic’s Annecy, France, world headquarters here.
