Friday, November 22, 2013

Aero Tuneup Take 2
Cheapass Aero!

Previously we detailed a slick aero tuneup on a Cervelo P2, which was very nice but also a bit expensive with a high priced aerobar, and $300 worth of aero brakes and stems. So here is another take on the aero tuneup from Scott Morgan.


  • Used Vision base bar: $40
  • Used Tektro Center Pull Brake: $45
  • Cable Stop: $5

Here are the before and after shots, nice!





Tuesday, November 5, 2013

The Sum of Marginal Gains

Is it worth it, how much does it matter? Is paying attention to skewer alignment to save a tenth of a second really a useful way to spend your time? Do you really need to pace intelligently or can you just bike "all out"? Do these theoretical rolling resistance and aero time savings really occur in the real world?

I recently had the opportunity to compare two cases, from the same race, on the same day. I had access to the average power, height, weight, and equipment info of two riders who competed in the 2013 Austin 70.3 bike leg. The bike course at this race includes rolling hills, plenty of turns, and lots of imperfect pavement. It is very much a typical real world scenario. The difference was rather startling.

Rider 1
217 watts average
6'0"
170 lbs
3 hours 10 minutes
17.6 mph

Rider 2
221 watts average
5'10"
142 lbs
2 hours 21 minutes
23.8 mph

This is a massive 49 minute difference for the two athletes, on the same day, producing similar power.  How much of that can be explained by the difference in mass, size, and power alone?  We can plug this data into the equations of motion of a cyclist along with a reasonable approximation of the Austin 70.3 course and see that size, mass, and power account for only about 15 minutes of the difference at most. This leaves 34 minutes unaccounted for over the 56 mile course. Both riders were on entry level TT frames, and used their aerobars. 34 minutes.

Where does that time difference come from? A few minutes plus or minus could be attributed to power meter error perhaps. The rest comes from the sum of marginal gains, including some of the following, but not limited to, and in no particular order:

Rider 2 put a lot of effort into her bike setup, rode the course ahead of time, and had an intelligent pacing plan and this paid off.  Rider 1 is already well on his way to improving and I expect will surprise himself in the near future.










Friday, October 25, 2013

Aero Tuneup

by Jack Mott

Some of us get attached to our older bikes and, happy in our long-term relationship, see no need to spend thousands of dollars on a newer and more expensive model. One can get very envious, though, of contemporary super bikes like the Fuji Norcom Straight and Cervelo P5their sleek shapes make our tried-and-true race steed look like it's ready to be put out to pasture. The Norcom Straight and P5 can offer up to a second per kilometer of aero savings, boasting beautiful front ends that hide the cables and brakes from the wind. Fortunately, with a bit of cleverness and careful part selection, you can update the bike you know and love to bridge that aerodynamic and aesthetic gap, giving it a new lease on life.

For our test case we used a Cervelo P2 ridden by Kat Hunter, editor of this blog and ATC Racing TT specialist. The P2 is a great bike, with real aerodynamic engineering, good handling, and a good fit for Kat. However, as you can see in this photo below, compared to a modern super bike, the front end presents all kinds of bolts, cables, and surface area to the wind.




Aerobar
The most important thing to address here is the aerobar. Aerobars must, first and foremost, support your ideal position. After that, pick one that presents the least frontal area to the wind and that keeps cables internal and tidy. Newer versions of the 3T Aura, pictured above to the left, have improved their cable routing so they stay in the bar all the way to the stem, exiting out the back. Look for bars that keep the mounting hardware as minimal and out of the wind as possible. A great budget option is the older aluminum Vision base bar and clip-ons. They use a very aero shape and a smaller stem clamp diameter for reduced surface area. Fancier options with integrated stems include the 3T Ventus II and the Zipp Vuka Stealth.

For Kat's bike we had to stick with UCI-legal options and went with the HED Corsair, which offers a nice integrated brake lever with built-in return spring. We paired it with Vision clip-ons, which are comfortable for her and present minimal mounting hardware to the wind.

Cables
Hiding the cables from the wind offers a fairly small aero advantage, but a huge aesthetic one, and is often easy to do. You can do a pretty thorough job just by putting some thought into your cable routing. Experiment with different routes and find one that keeps the cables hidden from view. Often a zip tie or some electrical tape can work wonders to keep the cables tidy.

We went a step further with Kat's P2 and got out a drill. In standard form the P2 shifter cables enter at the down tube, while most newer bikes have them enter at the top tube. We found this handy tutorial from TriRig on how to modify your P2 to accept top tube cables. The procedure is relatively simple, but be warned that this could void your warranty, and this is in no way officially sanctioned by Cervelo or ATC. The same procedure works on both the older P3 and P2, and may work on other bikes as well.

Stem
Another neat trick in lieu of zip ties to keep the cables tidy is the TriRig Sigma stem, which offers some great aero features. It helps route the cables cleanly, exposes no bolts to the wind, and has a small, smooth frontal area. It has an optional bottle cage mount, so you can throw away a few more zip ties if you use a between-the-arms bottle, and, lastly, it offers a cable stop for center pull brakes. The catch is that it is only available in 90mm length and two different rises, and you have to cut your steerer tube to the exact height. You can't put any spacers above the stem, so you can always go lower, but never higher.

Installation is not difficult. You cut the steerer tube of your fork to a few millimeters below the top of the stem, mount it with the included top cap, and run your cables over the top of it. If you have a center pull brake, you run the front brake cable into the cable stop in the middle of the stem, as shown below.

Sigma stem, with cover off

Once the cables are routed, you then squeeze the cables together and bolt the cover on. If you have Di2, you can mount the control box inside the cover, facing up through the slot so you can see and operate it.

Sigma stem, with cover on

On the left we also mounted an additional piece that allows you to bolt a bottle cage directly to the stem. If you won't be doing that, you can leave that piece off. The finished product with bottle cage mounted looks like this:




Brakes
Tektro Center Pull
Retrofitting the integrated brakes of bikes like the Fuji Norcom Straight onto an older P2 or Slice isn't quite possible, but you can get very nearly the same aero advantage with careful part selection.

Magura Hydraulic
A normal brake up front is only about an 8 second per 40k disadvantage compared to no brake at all, and some of the center pull options get very close to eliminating all of that drag. One option is a standard Tektro or Campy center pull caliper. These mount easily, brake well, and are affordable. You will need to add a cable stop or use the Sigma stem to get them working since you can't run cable housing to them.

Another great option is the Magura hydraulic brake. They have top-notch aerodynamics and better braking power than standard calipers. You may be able to find good deals on these at your local bike shop from people who didn't want to go hydraulic on their P5s and new P3s. If you don't want to go hydraulic either, you can use the TriRig Omega brake. It can accept either cable housing or bare cable, so you don't have to mount a cable stop if you don't want to. The Omega has a wind-tunnel-tested shape that, along with the Magura, makes it one of the most aero brakes you can buy today. TriRig was a sponsor for ATC Racing this past year, so of course we went with the Omega.

Fork
A bike's fork is one of the most critical aero parts of the bike. Like the aerobar, it is up front hitting clean air, and it affects how air flows around the bike and front wheel. Over time, many bike companies have tweaked and improved their forks. If you have an older model year P2 or P3, an easy upgrade is the latest Cervelo fork. Cervelo claims this fork is about a 1.5 watt, or 6 seconds per 40k, advantage over the best previous generation forks. In fact, any bike with a standard 1 1/8" head tube could upgrade to this fork. Kat's bike had the previous generation 3T fork, so we swapped it out for the new model. If you sell the old fork, this upgrade isn't even very expensive overall.

Final Result
With modified cable routing and brakes, a trick stem, and the latest fork, we have managed to achieve many of the aesthetic and aerodynamic features of much more expensive bikes with integrated front ends. Kat will put the new setup to the test this weekend at the Austin 70.3 triathlon as she competes in the relay category hoping to set a screaming fast bike split. UPDATE: 56 miles in 2:21:21 on 221 normalized watts. Fastest relay split by 6 minutes. Congrats Kat!




Friday, September 6, 2013

LG Course Aero Road Helmet
Stealth Aero!

by Jack Mott

Aero road helmets are all the rage, and previously we covered the slick Giro Air Attack. Some riders, however, are unwilling to defy fashion trends, or are worried about the Texas heat in a helmet with so little ventilation. The LG Course aero road helmet comes to the rescue!

Based on appearance alone, it's hard to believe the Course could be as aero as Louis Garneau claims in their white paper, given that it looks very much like a normal road helmet. But some very good aero testing results have been coming in from ERO Sports. The helmet has tested fast on multiple riders; they report it consistently tested about 2% less drag (over the entire rider-bike system) than the Air Attack, and never slower. This included both aero positions and road bike positions. So we decided to check the helmet out.

We picked up a size Medium from ATC, and on close inspection you can see the trick with this helmet seems to be that it is basically letting the air pass straight through over your head and out the back again. Interestingly, this seems to make ventilation better than most normal road helmets, as both my wife and I noticed we could feel the breeze through our hair more with this helmet as compared to our usual ones. This, combined with carefully controlled frontal area, probably explains the surprising aero goodness. At 250g, the weight is also low, and it fits us both very comfortably. A really neat bonus feature is an included rear light that velcros onto the back of the helmet.

Integrated Rear Light Included
So we have a helmet that is light, aero, ventilated, and includes integrated safety features, but is there any catch? It is a bit more expensive than the Giro Air Attack and doesn't have the cool shield feature as an option. Also, both the white paper from LG and the testing at ERO Sports are lacking any high yaw data, so it is possible that the Course doesn't fare as well in crosswind situations as the Attack's smooth shape does. Overall, though, the Course is an amazing helmet that we recommend highly. ATC has them in stock now, so try it out yourself. The helmet is available in three sizes and in red, white, or black.

CFD Designed, Wind Tunnel Tested

Thursday, September 5, 2013

Pacing on the Bike

by Jack Mott
Missy Ruthven of ATC Racing finishing the Mopac TT

Position, power, and pacing are three keys to a solid time trial or triathlon bike leg performance. Obviously, you want to be capable of producing as much power as possible by training hard. We previously talked about position and equipment setup. Now let's talk about pacing. Optimal pacing gets you the most speed for your available energy. To a very close approximation, the best pacing strategy on a flat course is to hold constant power for the whole course. The most common mistake is to start out too hard because you feel good, which you pay for later.

40k TT - a little too hard the first half, but not bad
To illustrate the kind of time that can be wasted by bad pacing, we can run some numbers through the website Analytic Cycling. Suppose a 75kg rider is capable of producing 250 watts for an hour, and completes a flat 40k time trial with perfectly even pacing. This rider would complete the 40k in about 59 minutes and 24 seconds.  Now suppose that this rider makes the classic mistake of going out too hard and does 275 watts for the first 30 minutes, then starts to blow up and has to average 225 watts for the rest of the ride. This would result in a finishing time of 59 minutes and 32 seconds, about 8 seconds slower for the same average power.  That doesn't sound too bad, but in fact it would be impossible for the rider to maintain that power output of 225 watts after going out too hard.  The harder you push, the bigger the physiological price you pay, and if you start out too hard you will not be able to maintain the same average power.  A more realistic scenario is that the rider would have to drop down to something like 200 watts after starting out that hard, which would bump the time to 1 hour and 53 seconds. Almost a minute and a half slower.

So you don't have to have the new Cervelo P5 to see significant gains. You can potentially save more time by pacing properly than you can by upgrading your bike (of course, you could do both!). All you have to do is practice and ride smarter!  The most sure-fire way to nail your pacing perfectly is to use a power meter, but with practice you can do quite well without one. We will discuss how to approach both situations.

Pacing with Power

The first step to pacing an upcoming TT or triathlon bike leg is to pick a power goal.  For time trials, you should estimate about how long the event will take you to finish, and then look at your past power data to figure out what power you are likely to be able to do for that duration.  The Mean Maximal Power chart (or MMP) is very useful for this.  At a glance you can see what your best ever power production is for a given duration. For example, suppose you have an upcoming event that will take about 20 minutes.  Load up your MMP chart with some of your recent training history. In Golden Cheetah it will look something like this (WKO+ has a similar chart):

Click to Zoom
At a glance I can see that the best 20-minute power I've done is around 280 watts, but I can also see that there is a little bump around 17 minutes, where my best power is 300 watts.  This implies that I've never done an all-out 20-minute effort, and am probably capable of doing a little more than 280.  So a reasonable power goal might be 290 watts for the upcoming TT.  Be sure to take into account weather conditions and your fitness and fatigue levels when setting your goal. Hot weather will often lead to significantly less power, for instance.

Triathletes will usually want to set their power goals by doing practice bricks or by extrapolating from past races.  A great way to set a power goal is to do a bike ride that simulates race conditions as closely as possible, and then do a short run afterwards to be sure the pace left you fresh enough to run well.

Once you have your goal power, you don't want to follow it blindly on race day.  You might be capable of more, or you might have aimed to high. You need to listen to your body to some extent, but you also need to try to defy it sometimes when it tells you to slow down!  Since the most common mistakes are to start out too hard and to give up too soon, I like to use the following protocol:

1. For the first half of the event do not ever go above your goal power, but if you feel terrible, you may go under it.  This is especially important in the first few minutes.  It is okay to surge for 4 or 5 seconds to get up to speed, but then settle down, no matter how amazing you feel. It will pass, I promise.

2. For the second half, never go below your goal power, but if you feel great, start trying to raise it up gradually.  This ensures you won't totally miss out on any unexpected fitness or heroics.  We are all naturally capable of more than we think, so no matter how bad it hurts, never drop below that goal power in the second half, you can do it.

That is all there is to it. With this general approach you can almost guarantee you nail your bike legs and time trials every time. However, don't skip the section below where we talk about pacing without a power meter, because sometimes mechanical problems will leave you without your power meter, and you should be ready to perform well, and without stress, when that happens.

Pacing by Feel

The challenge with pacing by feel is the incredibly strong tendency to start out too hard.  The adrenaline of race day and your fresh anaerobic stores will leave you ready to go 100 watts or more too hard for the first few minutes, for which you will pay dearly later on.  Do not go out too hard. Do not go out too hard!

The most important thing you can do is practice. If you have a 40k event coming up, practice 40k TTs a few times. If you have a half iron race coming up, practice a 56-mile bike ride, evenly paced with as few stops as possible, and then run afterward.  Pay attention to the wind, your speed, and how you feel to get an idea if you have paced it well.  An evenly paced time trial will generally feel very easy for the first few minutes.  As you get near the halfway point, things will start to be very hard; you will not believe that you can keep up the effort the whole time.  After halfway begins the ultimate suffering that you must fight through and never give up. A well-paced triathlon bike leg will be quite different. Ultimate suffering should be avoided and used on the run instead!

With practice you will get to know what it feels like to pace evenly.  You can even borrow a power meter or use the CompuTrainers upstairs at ATC to practice.  If you do own a power meter, occasionally practice with the display covered by tape, and then review how you did after the fact. This will leave you capable and confident on race day even if something goes wrong with your power meter.

Pacing the Hills

Hills will disrupt the simple plan of holding even power. The proper approach to maximize speed on hills is to go a little harder on the uphills and a little easier on the downhills. The most common mistake is for people to launch out of the saddle and throw an extra 100 or 200 watts on the uphills. This is too much; instead, raise power by 20 to 50 watts depending on how steep the hill is, and lower it by about that much on the downhills.  For longer distance triathlons, consider putting a cap on your power output about equal to your threshold or one-hour power to ensure you don't dip into anaerobic reserves.  If you don't have a power meter, just remember to raise your power on the uphills, but don't hammer like a mad man. On the downhills don't give up and coast; keep a little bit of effort on the pedals and shift as necessary to keep moving well.  Again, longer distance triathletes may want to consider coasting on any decent downhill to conserve energy for the run to come. Time trialists and short distance triathletes should keep pedaling whenever possible!

Pacing the Wind

Wind follows the same pattern as hills, but to a much lesser degree.  The harder the headwind, the higher you should raise the power, and the stronger the tailwind, the more you should reduce your effort. However, the optimum change is very small, only 2 to 4 watts in either direction.  This is so small of a difference that it can be hard to put into practice even with a power meter, so you can feel free to just ignore it and pace evenly for the most part.  Exceptions include cases where the tailwind on one part of the course will be so strong that you run out of gear. In extreme cases like that, you will want to push much harder into the headwind since you will get a forced rest with the tailwind.








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.