Friday, July 26, 2013

Mavic CXR60
So aero they made a custom tire

by Jack Mott


For a little over a year, the new breed of Mavic wheels has been on the market in the form of the CXR 80, a deep wheel with a bunch of unique features that has proved to be an excellent performer. The CXR 80 was the TT wheel of choice for the Garmin-Sharp pro cycling team, and has also been ridden with success in the local bike racing scene, even in crits. However, it's available only as a tubular, and is deeper than some people are comfortable with. Now a medium-depth version of the CXR line is here, the CXR 60, and it comes in clincher too! The key features of the CXR line of wheels include:

New Aerodynamic Shape



Mavic's CXR line takes a unique approach to wheel shape. Unlike the rest of the industry (which somehow simultaneously adopted "U" shaped rims), Mavic uses a combination of custom tires and a "blade" that fills in the space between tire and wheel to have complete control of the entire wheel-tire shape. If Mavic's wind tunnel results are accurate, this gives them a sizeable aero advantage over other wheels.


Downsides to this approach include a reliance on Mavic's custom tire, which may or may not have great rolling resistance to go with its excellent shape. If the rolling resistance is not good, it can more than offset the aerodynamic advantage. This is of particular concern in the tubular versions of the wheel, as the rolling resistance is not very good with that tire. Tom Anhalt has measured the rolling resistance of the Mavic clincher tire and thankfully found it has excellent rolling resistance, details here.

The other downside is dealing with the blades. The CX01 blade snaps into the rim after you install the tire. The process is quite simple, but would be an extra step to deal with when changing flats on training rides. On the other hand, you don't have to use the blades on training rides; you can save them for race day. Additionally, you need to take care that the brake pads are adjusted properly, or they could grab the blade when you apply the brakes. There have been a few reports of blades coming undone during bike races, possibly due to contact with other bikes. They can then get wrapped up in your hub and end your race. The blades might best be saved for time trials and triathlons, where bike-to-bike contact is rare.

While Mavic literature insists that the blades are only compatible with the Mavic tire, during our testing we used it along with the Continental GP4000S and experienced no issues or wear on the sidewall. The aerodynamic fit seems to be excellent as well (click to zoom):

CX01 Blade Fit with Mavic Tire
CX01 Blade Fit with GP4000S Tire

However, given the good rolling resistance of the Mavic clincher tire you may want to stick with it, especially up front.

Exalith Braking Surface

The braking surface on the clincher version of these wheels is a special metal surface that Mavic calls Exalith 2. It has a directional texture, and is designed to work with a specific Mavic Exalith brake pad. The braking power and modulation on this is excellent, especially in the rain due to the texture feature. Our resident Cat 1 cyclist tester said she thought it felt better than her standard aluminum rims. While other racers get stressed out if there is rain in the forecast, you can remain calm and focus properly on your race, a nice advantage. The downside is the necessity of using the Mavic Exalith brake pad, which you may have to swap on and off if you use other wheels for training. In a pinch, other pads will work fine, but the Exalith surface seems to tear them up pretty quickly. You also must take care to orient your front wheel in the proper direction for optimum braking. We did test the braking with the wheel installed the wrong way around, however, and it still worked well.

The tubular version of the wheel uses a more traditional carbon braking surface.

Aero Tuned Hubs

The hubs on these wheels clearly have some aerodynamic thought put into them. The central section is as small as possible, and the flanges are also kept small and covered with smooth aero-shaped caps:



Real World Testing

Road testing revealed what we expected. Mavic wheels are very well built! There were no issues or drama under hard cornering or acceleration, and no flexing to cause the rear wheel to rub when pushing sickwatts up steep hills. We tested the wheels in a Cervelo S5 and a Litespeed C1 and found no clearance issues on either. Our resident Cat 1 cyclist Kat Hunter gave them a thumbs up, noting the exceptional braking performance. Meanwhile, our Cat 3/triathlete was able to decimate the local Strava scene in his neighborhood. Testing during windy days showed the wheels to be stable and predictable.



We noticed that the outer width of these wheels is quite wide, a bit wider that our HED Jets, in keeping with the current trend of sizing the wheel to match the width of 23mm tires. However, much of that width is used by the channel that you snap the CX01 blades into. The actual width of the rim from the tire's point of view is actually quite narrow. Rather than shaping the tire to fit the rim via extra wide bead hooks, Mavic uses the blade to provide a seamless tire to wheel transition. This design does make it a bit harder to work your tire onto the wheel. Below you can see the a blade, still snapped in place on the left, and the groove that the blades snap into on the right:



The construction of the clincher wheels is similar to HED Jets, with a carbon fairing attached to an aluminum rim. Mavic also applies a carbon fiber "cap" on top of the metal rim, however. The only exposed metal is the black Exalith braking surface and the black spokes.

Pricing and Availability

The CXR 60 will begin hitting Austin Tri-Cyclist (and other stores probably) in mid August.  Pricing info will be in soon. Each wheelset includes a bunch of nice extras:
  • BR601 quick releases
  • Rim tape
  • Wheelbags
  • Exalith brake pads
  • Valve extenders
  • Multifunction adjustment wrench


Friday, July 12, 2013

2013 Tour De France Stage 17
Light or Aero?

Coming up this Wednesday is my favorite kind of time trial. One that goes up big mountains and back down again. Inevitably we see a wide array of differing equipment choices. TT bikes or road bike? Light wheels, or aero wheels? The decision actually ends up quite complex and some teams have better options than others. This stage features two category 2 climbs with grades of around 6% for a total of 32 kilometers, almost none of it on the flat:




Modeling the Course


We can turn to our trusty equations of motion for a cyclist to get some insight into the complexity of the equipment choices faced on a day like this. We can model the course with a decent approximation on the  website at Analytic Cycling. By inputting the average gradient and distance of the two climbs and two descents we have a pretty good model of the course.  The power and mass of the cyclists will vary but we can use some average cases to see how a TT setup vs road bike setup might compare for a hypothetical pro badass. If you want to play at home you can use the code below in the free-form data entry box (click advanced) to use my course model, and tweak it to your own liking:

{ {0, 5500, 6.3 ,0,.004,0,0,0}, {5500, 6500, 0.0 ,0,.004,0,0,0}, {6500, 13000,-6.0,0,.004,0,0,0}, {13000,22000,6.3 ,0,.004,0,0,0}, {22000,31000,-6.3,0,.004,0,0,0}, {31000,32000,0.0 ,0,.004,0,0,0},}

TT BikeRoad BikeRoad Bike w/ Clip Ons
Mass74kg73kg73.2kg
Watts395400400
CdA.25.32.29
Predicted Time50:2852:1251:16
Uphills Only34:0534:1633:59
Uphills Only +10% Power31:0331:2331:04

We assume a 5 watt power less on the TT bike. Some riders suffer no power loss at all, while others suffer a bit more. We also assume around a 1kg weight penalty which is typical, though some teams may be able to hit the UCI minimum with their TT setups for some riders.  We then use average CdA values for TT vs "Drops" positions.  With these inputs you get a predicted time savings of 1:43 for the TT bike over the road bike, a huge advantage.  But things are not so simple. The descents are not straight lines, the turns and switchbacks will discount aerodynamic savings on the way down the climbs.  Athletes will likely pace themselves with higher power outputs on the climbs and less on the descents.


Since the downhills introduce a lot of complexity, we can also look at just the uphill sections of the course. In this case the road bike with clip-ons has a slight advantage.  But if those uphills are paced with 10% more power than the downhills, the gap narrows to almost a tie. In fact a slight tweak to any of these inputs could change the order around completely.  So  you can see there is a delicate balance at play.  An athlete who is not comfortable descending on their TT bike, and who makes more power on the road bike, might want to opt for the road bike with clip-ons.  An athlete who has TT expertise and has prepared and ridden the course ahead of time may be confident enough to tackle it on a time trial frame for the best overall time. You can also expect various hybrid options to be employed, such as road bikes with clip-ons and road frames with full aero cockpits.

#SecondGuessingPros


One of the best parts about being a fan is you can second guess the pros decisions. Whether it is tactics or equipment we can use our imperfect knowledge to have some fun and play armchair Directeur Sportif! What equipment will teams use on Wednesday, and what equipment should they use? Expect many pros who have no GC ambitions to be on road bikes. They find them more comfortable, and safer, and time is not a big concern for them.  For the GC contenders I think we will see a mixture of approaches.

Team Sky

Team Sky has no aerodynamic road bikes at their disposal, and their road frames are also the heaviest road frames in the tour.  I also expect that Froome has spent plenty of time preparing for this particular course and will be prepared to ride the TT bike on it. That should be the fastest option for him, though he may be the favorite on this course no matter which bike he chooses if he stays on his current form. 

Movistar

Movistar faces the same bike choice as team Sky, a heavy road bike or the new Pinarello TT bike. Their only GC contender now is Nario Quintana, who may not have enough experience with time trial bikes to make that choice on this stage. We may seem him on a road bike trying to make up the time on the uphills.  I expect this to cost him time, but his excellent climbing should net him a decent placing nonetheless.

Saxo / Astana / Omega-Pharma

These teams have two excellent choices from the Specialized stable of bikes, the Shiv TT, and the Venge.  Depending on the weight of various builds and the fit needs of their riders, we may see Venges with clip-ons, or Venges with aero cockpits, or Shivs in action on these teams.  Smart choices will be critical for Contador, Fuglsang, and Kwiaykowski. Anyone opting for bullhorn bars and STI shifters will likely give up time. 

BMC

Cadel Evans at BMC will face the same choice as the specialized teams, an excellent aero road bike or an excellent TT bike. Cadel has very...unique... TT fit needs, so there is no telling what will go down here. This course should suit him though, and the choices and results will be interesting. 

Radioshack-Leopard

Andy Schleck will have a choice between an excellent time trial frame in the Speed Concept, and the new semi-aero Trek Madone.  Look for him to be on a road bike given his complete lack of interest in riding TT bikes, ever. Perhaps a good battle between him and Nairo Quintana!

Garmin-Sharp

Garmin has three choices here which will no doubt have team tech gurus and riders scratching their heads. They could ride the super aero but slightly heavy Cervelo P5.  Or, as they have done in past uphill TTs, they could equip the S5 road bike with a TT cockpit to get very nearly the same aerodynamic prowess at a lower weight.  Lastly, they could use the new Rca frame, which is among the lightest frames in the tour, and semi-aero as well.  This might give them a weight budget to use deeper wheels and still hit the UCI minimum weight limit.  Dan Martin and Andrew Talansky will be the guys getting the best setups, look for them to be on either the S5 or the Rca, possibly with an all out TT cockpit.





Sunday, July 7, 2013

Watts/kg Tour de France Math

Recent claims on the internet have put Froome's first mountain stage at having done between 6.37 and 6.5 (depending on who you ask) watts/kg for a 23 minute effort, and have labeled this as suspicious. How accurate is this?  What is the range of error on that calculation?  If you want to play along at home you can head over to the Aeroweenie calculator and play with the variables yourself.

I can replicate the 6.37 watts/kilo claim with the following inputs on analytic cycling:

  • CdA:.43
  • Air Density: 1.1kg/m^3
  • Mass (rider + bike): 77.8kg
  • CRR: .004
  • Road Incline: .083
  • Speed: 21.7kph
Let's look at each variable individually and see what kind of certainty we can have about them:

CdA

This is the rider's coefficient of drag times their frontal area. It is affected by the shape of their body, their position, their equipment, and even the angle of the wind at each moment as they ride.  We have absolutely no way of knowing Froome's average CdA as he climbed. The best we can do is look up that around .40 is typical for riding on the hoods, then add a little bit because he is tall. A variance in his CdA of just .04 would vary his watts/kg calculation by +/- .07   Variations in bike frame aerodynamics alone can vary CdA by .02, let alone particulars of his body shape and position which could cause it to vary much more than that.

Air Density

Air density depends on temperature,  pressure and how much water vapor is in the air. Without taking direct measurements on location at the time, we are once again guessing. A 0.1kg/m^3 variation in air density alters the W/kg result by +/- .03

Mass

This one is especially amusing. If we currently consider Froome's performance suspicious, wouldn't it be even more suspicious if he actually weighed 72kg instead of 71kg? Well if we  add 1kg to his mass in the model it actually drops his calculated watts/kg by .02!  Think about that for a minute. What does that say about using watts/kg as a metric of doping suspicion in the first place?

CRR

Coefficient of rolling resistance reflects the force resisting forward motion caused by the tires as they roll along the road. This is affected by the tire used, how well the tire is glued, the tire pressures chosen, the weigh distribution of the rider, and the quality of the road surface. A plausible value can range from .004 to .002 depending on how smooth the road is.  How smooth was the road yesterday? My model above assumes .004, if it was actually .003 because the roads were smooth, or because Froome had excellent tires glued really well, or both, then the watt/kg drops from 6.37 to 6.2! So a plausible variance in CRR will cause a variance in watts/kg of .16

Road Incline

The stated values have been 8.3%.  That of course is not perfectly constant (another source of error). What if that value is wrong by just .1%?  Another .16 variance in watts/kg

Drafting

Drafting still helps even on a climb, at the speeds they were going as many as 10 watts could have been saved in the draft, which would vary the watts/kg another .08 assuming Froome drafted about halfway up.

Other Error

Wind could have a huge effect on the result dwarfing all the other inputs combined. Other variables with smaller but still very real effects include pacing, the actual meter by meter variation in road incline, lines taken around the turns and the quality of lubrication of the drive train and bearings. For now we will ignore all of these and pretend we got lucky on the wind.

Total it Up

If we add up the errors above we get .52, which means Froome could be anywhere in the range of 5.85 to 6.89 watts/kg.  Remember that we are ignoring wind, and that a heavier Froome would produce less suspicious values than a lighter Froome! 

A Solution

Quit doing these watts per kilogram calculations. They make people who are not fully informed think real science is happening when it isn't. Just time the riders up the climbs and account for the degree of headwind vs tailwind. The watts/kilogram math is a silly distraction. All of the same comparisons to historical performances can be made with the stop watch alone. Or, at the very least, when you tweet or blog your watt/kg calculation, tweet two numbers. The low guess and the high guess. Not much to ask.