Mavic unveils Cosmic CXR60
Written by: Tom Anhalt
Added: Sun May 19 2013
An additional part of the Wheel-Tire System is the CX01 blades which are used to "fill the gap" which is normally present at the junction of a tire and rim. As discussed above, any dramatic shape changes in the air flow field can cause flow separation and thus increase the drag of the object. The valley typically formed by the tire to wheel junction, whether it be a clincher or tubular design, presents one such of an area. The idea of the CX01 blades is to fill that valley in and thus add an additional drag reduction. As we saw last year, these blades are made from a flexible rubber flap which is bonded to a rigid plastic mount and the resulting structure clips into a mating groove on the rims. At the present time, the CX01 blades aren't allowed for use in UCI regulated competitions (Mavic is still in discussions about getting them approved for use with the UCI) but for the majority of the Mavic buying public (i.e. most triathletes world-wide and road riders in the US) that should not be an issue since their equipment isn't required to comply with the UCI regulations.
Lastly, the hubs for the CXR wheel line are all the same as the hubs that were introduced last year on the CXR 80 and have been specifically designed to be what they consider the best compromise between aerodynamics, flange height and spacing (and the resulting wheel stiffness), and mass. The thin, bladed steel spokes used on all wheels in the CXR line are identical except for the specific length required for the individual models.
One thing that the Mavic engineers wanted to make sure they were able to capture was not only the translation drag force on the items they were testing, but also the power required to rotate the wheels while being exposed to the translation air flow, or the “power to rotate”. To do so, they came up with a design where instead of the wheels being driven by a solid roller during testing, the tires instead lightly contact a belt below each wheel. Since the tire contacts the belt on an unsupported span between two pulleys and the object under test is supported by the balance mounting fixture, that means that there is only an extremely small amount of deflection in the tire, which helps to minimize the effects of tire rolling resistance on the power required rotate the wheels. There is just enough contact between the belt and the tire to prevent slipping, and the speed of each wheel is matched to the air speed of the test section. For rider testing under power, Mavic is in the process of developing a Comete disc that will have an electronic brake built into it so that the rider can pedal against a realistic load. The Mavic engineers claim that the balance will measure to a precision of +/- 2.5g for a wheel, and +/-5-10g for a pedaling cyclist.
Well...enough about the design and facilities, let's talk about the wheels. The new CXR60 wheel range is composed of a tubular (CXR60T) and a clincher version (CXR60C), both with a depth of 60mm. The tubular version is based on the NACA 0029 and 0017 foil profiles, while the clincher version, apparently due to design restrictions related to the clincher construction, is based on the NACA 0027 and 0012 profiles.
Here's a quick primer on NACA foil designations: the first 2 digits of the 4 digit number represent the curvature of the centerline of the air foil. In this case we're talking about air foils that are symmetric, and thus have a straight center line, so those numbers end up being zero. The final 2 numbers represent the ratio of the maximum width as compared to the overall length of the foil, as a percentage. For example, the 0029 designation mentioned above means that the cross section width is 29% of the overall length. For a wheel, this overall length means the total length of the rim including the tire. A 60mm depth rim, plus a 23mm tall tire results in a total length of 93mm. 29% of 93mm is 27mm, or what is claimed as the maximum width for the CXR60T. The CXR60C uses a 0027 designated section, so it has a slightly narrower overall width.
The tire used on the CXR60T is the same tubular tire as was introduced with the CXR80 wheel last year. Although technically the profiles of the leading edges of both wheel designs are slightly different, they're close enough that using the tire designed for the 80mm deep wheel doesn't appreciably affect the performance. Surely, the boundary layer trip features on the tire sidewalls help to minimize that as an issue.
For the clincher version, the tires are a brand new product and although appear outwardly the same as their tubular counterparts, they are in fact nearly completely different aside from the sidewall patterning. The clincher tires are supplied by a different source than the tubulars, and the casings and compounds are different as well. Mavic is fairly quiet on the details of the tire construction, but I was able to get out of them that the clincher version is based on a 127tpi nylon casing. They were silent on whether the compound was carbon or silica filled, though. Apparently, the rolling resistance of the clincher tires is approximately 3-4W lower than the tubular, but that figure is for a smooth surface and with butyl tubes, which means that for a typical road surface roughness, the difference would actually be more on the range of 5-6W. Replace the butyl tube with a latex one, and the Crr difference would be even greater, and based on measurements I've made of the tubular tire, perhaps as much as twice that amount. I'm anxious to get my hands on the clincher tires to do some roller testing to confirm these figures.
The basic structure of the CXR60T tubular setup is nearly identical to the CXR80 design, so I'll instead concentrate on the new features of the CXR60C clincher model. When I first got a look at the new clincher model, my first thought was that it's basically an updated Cosmic Carbone SLE with its aluminum rim with carbon cap construction. It has the same Exalith braking track, and unlike the tubular CXR wheels, even the weave of the carbon cap is the same as the older Carbone. The big difference though, aside from the deeper section of the cap as compared to the older wheel, is the fact that the aluminum rim has been completely reconfigured. The width between the braking surfaces has been increased, but that wasn't enough to make room for the CX01 blades. That means that Mavic needed to take the approach of actually decreasing the internal width between the bead hooks from a typical 15mm (on a 19mm external width rim) to 13mm. That's what I was talking about when I said that some of design decisions are bucking the current industry trends, where most wheel makers are currently increasing that dimension. Although many wheel manufacturers are selling that increased width as an advantage in handling or rolling resistance, in reality there is little, if any, evidence to back up those claims. In fact, as Slowtwitch's Greg Kopecky has pointed out, many folks are putting tires on clincher rims that are narrower than what is advised by the ISO standards for those particular rim widths. The only real advantage of running narrower tires than the standard allows on wider rims is better matching of the tire width to the rim width for better aerodynamics. Knowing this, the engineers at Mavic decided that it was overall better to reduce that internal bead seat dimension to allow the inclusion of the CX01 blades instead. The actual performance would be better overall and the tire retention would be improved as a bonus.
Additionally, as with the CXR80 introduction, they also discussed the data they took with their own wind angle device during outside riding and the resulting average percent time at yaw angle plot. They used this data to create a weighted averaging approach for the wheel data to more realistically compare power differences between wheel designs. The weighted average actually values low yaw angle performance to a greater degree, but the Mavic wheel designs tend to perform much better than their competition at higher yaw angles, where they don't stall as early, nor as dramatically. Even so, according to the weighted average, the Mavic wheels still come out ahead aerodynamically. Additionally, because the data used to form the weighted average was from a large number of rides and riders, that means that it's possible that the same plot for a slower than average rider might be flatter and thus more highly value the higher yaw angles. That would again favor the Mavic wheels.
Now, on to one of the performance factors of lesser importance: the mass. Both of the CXR60 wheels are not what most would consider light weight, but that really shouldn't be an issue. At claimed masses of 1645g and 1825g for the tubular and clincher versions of the wheel sets respectively, the wheels are not largely out of line with wheels of similar construction and performance. Despite popular opinions to the contrary, the physics of the cycling show that even when including rotational inertia, wheel mass effects (within the range of reasonable variances) are basically an order of magnitude less important than other properties such as aerodynamics, even for fairly steep climbing. Enough said.
Overall, these 2 new wheel systems from Mavic emphasize their renewed efforts at addressing the aerodynamics of their wheels and using a systems approach to their wheel designs. For this, they should be applauded. In taking that systems level approach though, it would be valuable if they were to include the effects of the rolling resistance properties of their tires, especially since the optimum aero properties of their products depends on the tires. In their presentation, they showed that using their weighted average of yaw angles, the clincher version is within 0.4W of the tubular version in aero drag power. In the testing that was observed during the presentations, the clincher version was actually slightly lower drag than the tubular between 0 and 10-12 degrees of yaw. Considering that the rolling resistance of the clincher tires results in a 10X greater difference at least (and most likely MUCH higher on “real roads” and with latex tubes) it's pretty clear to see that the clincher version of the CXR60 is the big standout in the lineup here and is the better performer of the two, and is arguably an even better performance choice than the CXR80. With its Exalith brake tracks, it has the potential to be the do all wheel set of choice. A rider could just remove the CX01 blades, put on some wider and more durable tires, and use them for training. Training on deep wheels is an important factor in becoming comfortable with them for racing. Then when race day comes, they could easily swap the rubber for the Yksion CXR clincher tires and install the CX01 blades and go race. It wouldn't even require a brake pad swap, and the braking would always be excellent.
Shown below is a plot of the CXR60 wheel performance if the rolling resistance is included. To create that plot, I used Mavic's own weighting law and aero drag data and combined it with rolling resistance measurements, see blog post here for more detail: http://bikeblather.blogspot.com/2013/04/why-tire-crr-matters.html. For the new CXR Yksion tires, I used an assumed Crr value based on the measurements of the tubular version, with the differences noted by Mavic subtracted out (real world values, not smooth surface). As you can see, the CXR80 and CXR60T have nearly identical performance, mostly due to the use of the same tires, but also due in part to the very close aerodynamic performance, with the CXR80 pulling slightly ahead at higher expected apparent wind velocities. The interesting thing is that even with just the Crr difference quoted by Mavic for the new clincher tires (adjusted by a factor of 1.5x to represent the performance on real world road roughness) the CXR60C handily beats the other wheels in the CXR line over the entire range of apparent wind speeds shown. Also included in that plot is a curve showing the expected overall aero+rolling resistance performance of the Bontrager D3 Aeolus 5 clincher wheel with Bontrager's R4 Aero tire. For the rolling resistance of the R4 Aero, I used the value I measured in roller testing with a latex tube installed. Despite the Mavic wheels' better aero performance, the lower rolling resistance of the Bontrager setup allows it to handily beat the Mavic wheels, and it only begins to be beaten by the CXR60C at apparent wind speeds greater than 55 kph, which is a VERY high wind speed. Of course, it's also possible that putting a latex tube inside the Yksion tires could drop their Crr to be close to that of the R4 Aero, at which time the advantage would go to the CXR60C due to it's better aero performance, as represented by the shallower slope of its combined aero+Crr drag curve. This potential outstanding performance for the CXR60C also points to the fact that a clincher version of the CXR80 would also be a stellar performer. When asked about if that was in the plans, the Mavic folks were a bit coy on the subject and said that they want to concentrate on the CXR60s this year, but that a CXR80C might be an option in the future.
The stated availability of the wheels was July for the CXR60T and September for the CXR60C. As of this writing, the US pricing was unavailable, but I would expect it to be similar to the CXR80 ($2799, including tires, CX01 blades, brake pads, valve extenders, and wheel bags) for the CXR60T tubular version, with a slightly lower pricing for the CXR60C clinchers.
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