Friday, August 29, 2014

Some more Conti Crr data

I often get asked about how some of the other Continental tires roll, such as the Attack/Force models, and the 23C version of the SuperSonic tire. Well, thanks to Eric Reid (who sent me a new Attack and a slightly used Force) along with Thomas Gerlach (who sent me a VERY well used 23C SuperSonic) I know have a couple of those data points.

To be clear on the SuperSonic, when I say very well used, I mean VERY well used...according to Thomas, this tire has ~600-1000 miles on it of rear tire used.  When mounted on my Mavic Open Pro test rim and inflated to 120 psi, I measured a flat section on the tread that was a full 7mm across. In other words, it was near "end of life" ;-)

So, without further ado, here are their results:

Continental Supersonic 23C (WELL worn) = .0027, 25W for pair @ 40kph
Continental Attack 22C = .0035, 32W for pair @ 40kph
Continental Force 24C = .0029, 27W for pair @ 40kph

By comparison, here's the results for the "benchmark" GP4000S:

Continental GP4000S 23C = .0034,  31W for pair @ 40kph

My suspicion is that a new 23C Supersonic would probably test out closer to .0030.  The worn Crr result puts it currently at the top of my Crr chart, but with a hefty asterisk.

The result for the Attack model shows it to roll about the same as (or just slightly worse than) the GP4000S, but with its narrower profile (22.7mm vs. 24.2mm) it should be slightly more aerodynamic.  The Force model rolled very well, although it too had a small amount of miles on it.  It basically put it in a tie, or slightly behind the top mark for a brand new tire I've tested, the Specialized Turbo Cotton. However, it also measured a fairly wide 25mm on my narrow test rim. On a wider rim, such as a Zipp Super9 disc, it would most likely measure as much as 26mm.

The spreadsheet linked to in the upper right of this blog has been updated to include these results.

Tuesday, July 15, 2014

There's a new Sheriff in town...

If you check the link to the overall Crr spreadsheet in the upper right of this blog, you'll see that there's a new top entry.  It's the tire pictured above, the Specialized Turbo Cotton 24C which was introduced today. It sits firmly at the top of my list of production tires I've personally tested on the rollers, with a predicted on-road Crr of .0029, as compared to the Crr of .0031 for a trio of tires that used to be in a virtual tie (the Vittoria EVO Open Corsa Triathlon 22C, the Vittoria EVO Open Corsa Slick 23C, and the Schwalbe IronMan 22C Tubular).

The fact that it's so well rolling is not surprising considering it's construction. Like those other 3 tires, it's based on a 320tpi cotton casing. The main difference from those other tires is, of course, the tread that's glued onto it, which in this case is made from Specialized's proprietary Gripton material that they introduced last year with the S-Works Turbo tire. I've found this compound to truly live up to it's name, as can probably be guessed by the amount of wear on the labeling in the picture above. it rolls really well (~4W less for a pair @40kph as compared to the "benchmark" Continental GP4000S) and rides/handles great, but is it aero?  Or, at least "aero enough"?  As I've written before, Crr can make up for a great deal of aerodynamic "sins" . Is the Turbo Cotton at least aero enough to take advantage of its ultra-low Crr to make it a good choice when aero is of a concern?  Apparently...yes.

In interactions with the folks at Specialized, I was given a spreadsheet that shows the drag data for a test run taken with Zipp 404FC clinchers in a Venge road bike.  The tires tested were the S-Works Turbo, the Turbo Cotton, and the Continental GP4000S 23C, which has been shown to have excellent aerodynamics on a variety of wheels. The plot of CdA vs. yaw angle is shown below:

As you can see, it shows that the Turbo Cotton is basically tied with the GP4000S at low yaw angles (up to 5deg), but then the GP4000S results in up to .010 m^2 lower CdA at 15deg of yaw. It's also interesting that the S-Works Turbo appears to be more aero than the Turbo Cotton. This could perhaps be because of the "lip" at the interface of the glued-on tread and the casing on the Turbo Cotton.

Another way of looking at this is by combining the Crr results with the aero results.  Here's what the combined power would be for 40kph (85kg load).  As can be seen, the low Crr certainly helps the Turbo Cotton beat the GP4000S all the way out to 10deg, while at 15deg of yaw the Conti actually beats it by ~3-5W. Above 15deg, the margin narrows again.'s pretty fairly obvious what I would do with this right?  Time to analyze the combined aero+rolling drag for a weighted average of yaw angle, like was done in the blog post I linked to above.  In short, here's how that turned out (using the Crr from my own roller measurements):

What does that mean?  Well, using the Mavic generated wind yaw angle weighting (as in the previous analyses), it shows that the excellent Crr of the Turbo Cotton tire overcomes the lesser aerodynamic performance at higher yaw angles (as compared to the GP4000S), at expected apparent wind speeds up to >50 kph.  And it stays faster than the S-Works Turbo all the way up to apparent wind speeds of 60kph.  Fairly impressive.

I've ridden a pair of these tires on my road bike for the past few weeks...and I have to admit that I've found my new favorite "all around" tire for racing.  It's a no-brainer selection for road races and crits due to its naturally (because of the construction) good ride quality and cornering grip (because of the tread compound). I've yet to TT on these tires...but it's pretty tempting. They seem to work fairly well for Tony Martin for that purpose. Then again, I have a feeling Tony Martin's average apparent wind yaw angle is pretty darned low :-)

Sunday, July 6, 2014

Crank Length? Whatever...(within reason)

 Discussions about bicycle crank length have been a fairly hot subject recently. There's a trend in TT/Tri circles to use shorter than "normal" crank lengths. This is done to allow folks to either open up the distance between their torso and leg at the top of the pedal stroke (to be better able to produce power) and/or allow them to lower the angle of their torso relative to the ground in an attempt to get more aerodynamic. Sometimes it's done for both. One thing that is commonly brought up is questions about how using shorter (or longer) than normal cranks affects sustainable power output. Does it...or not?

There's a lot of data out there showing that wide variations in crank length do not have significant effects on  power output, such as the work done by Jim Martin at the University of Utah, summarized here:

From that presentation, "170mm cranks would compromise the power output of the shortest and tallest riders by AT MOST 0.5%. For example 6 watts out of 1200"

Another takeaway: "Crank length and pedaling rate influence metabolic cost and efficiency only by influencing pedal speed."

How can that be? Doesn't it logically make sense that for an optimum setup, a person needs a crank length proportional to their leg length?  For example, there are websites out there that claim to identify the "ideal" crank length for a given person ( However, one thing that most folks miss on that site is that the lengths of cranks recommended with that method uses an underlying assumption with no apparent basis, i.e. "The standard crank length of 170mm is optimum for a cyclist with a 31-inch inseam." The entire method is anchored in that assumption...and yet, the Martin study summary I linked to above shows that power production is relatively unaffected over a wide range of crank lengths for a wide range of leg lengths. Is there an optimum?  Perhaps...but, even if there is, the Martin data shows that varying from that optimum (even by relatively large amounts) doesn't appreciably affect power output.

But, doesn't a shorter or longer crank affect your "leverage"? Sure...but that difference in leverage can be compensated for elsewhere in the drivetrain. The crank isn't the ONLY lever between your foot and the ground. The gears themselves, along with the rim and tire diameter are also "levers", which can be varied. One thing to keep in mind is that physiologically, our muscles have a preferred shortening speed, which results in a preferred foot speed at the pedal. I'm not talking about cadence...I'm talking about the tangential velocity that the foot travels at during the power production (i.e. downstroke) phase of the pedal cycle.

The Challenge:

A little over a month ago, in a discussion on the Wattage Google Group about the effects of differing crank lengths, the following was posted in response to the idea that changing crank lengths could be "compensated" by varying gearing selections:

"The gearing argument has been raised many times. It's usually raised and argued a lot by people not on the big side of the spectrum,  ie people that are already on somewhat ideal cranks.

My counter argument is this:

If you are riding say 170mm cranks and you firmly believe that gearing can wholly make up for cranks of the wrong size. I encourage you to take the "Pepsi Challenge" that challenge is this:

1: Go TT up your favorite 15 minute or longer hill at LT or greater power (by HILL I mean something with average grade 6% or greater) with your current setup and time it a few times.
2: Then put on crank arms that are only 87.5% in length of your current setup.  That would be 148.75mm if you are currently riding 170s. You now have my proportional experience setup of me riding 175mm cranks.
   Take your own medicine and re-gear appropriately. Now ride that setup for a few weeks and go TT up your favorite hill a few more times and time it.

If you can show me you can do the same time with your medicinal gearing going full tilt, then I'll happily eat some humble pie.

I've offered that challenge for a few years now, and suprisingly enough NO ONE HAS TRIED IT. Go figure.  News flash, I and many other long legged people that have actually taken that "pepsi challenge", have come to the very real conclusion the longer arms are faster. Now we can quibble over why they are, but that's kinda irrelevant, fact is they are.

So I really challenge you or anyone else who is of normal size to take that challenge and see what it's REALLY like to be riding something so far out of what for what your body needs. Been over 5 years since I first offered that challenge, and I'm still waiting for someone of "normal" size to try it out...patiently.


Not being one to shy away from a challenge like that...especially since the one doing the challenging is someone I know...I decided to take it up. Luckily, I have a friend (Greg Steele, of Beehive Bicycles in Salt Lake City who let me borrow a set SRM cranks with the adjustable length option.

These cranks allow for an adjustment range of 150-190mm. To do the testing, I decided to use the first 2 miles of a local ~8% average grade hill. From the Kirby Palm "method", I used as the first test a setting of 175mm since that's what his formula claims is the "optimum" for my 32" inseam (as measured per his instructions). In order to ensure that my lowest gearing was as equivalent as possible, I chose a 12-25 cassette for use the 175mm cranks.  Since the crank length was pretty close to my normally used 170mm, all I did for adjustments was lower the saddle slightly. After setting everything up, off I went to the hill and give it "full stick" for my baseline. Here are the results:

Time = 12:50, Power = 289W, Cadence = 73 rpm, HR = 170bpm

For the short crank length, I wanted to use something that was equivalent, or below, the "87.5% below optimum" in the challenge above. For that, I chose 150mm, which is actually only 85.7% of the "optimum" 175mm length. In order to assure nearly equivalent low gearing, I chose a 12-30 cassette to pair with my 39T small ring.

Now then, when making such a large change in crank length, it's important to make sure to maintain the relationships between the saddle, the bars, and the foot during the "power", or downstroke part of the pedal cycle. As such, when I adjusted the crank length from 175mm down to 150mm, I did 2 things with the saddle.  First, I raised the seatpost such that the distance between the portion of the saddle I rest my sitbones on and the pedal at max extension was the same distance, but I ALSO moved the saddle rearward on the rails so that in the end it was a full 25mm further rearward relative to the center of the BB than when the cranks were set at 175mm.  This was done to keep the relationship between my lower leg and the pedal equivalent at the "3 O'clock" portion of the pedal stroke.  Because of the movement in the saddle (both up and rearward), that also meant I need to move the bars the same directions to keep the bar to saddle relationship as close as possible. Luckily, I had a stem handy that was short enough, and with a high enough rise so that I could do this with a fairly simple stem swap.  It wasn't the exact dimensions required, but it was close enough. Shown below is an overlay of the 2 setups.

So...after making the setup change a few days after the baseline run, I then ventured out to the test hill and with the only "accomodation" being the ~15 minute ride over there, I did a run with the 150s.  Here are the results for that:

Time = 12:50, Power = 286W, Cadence = 84 rpm, HR = 171bpm

Or...basically the SAME time, power, and HR as the 175mm crank length run. As expected, the cadence increased so that my tangential pedal speed and average pedal force were the same.  This can be easily seen with a Quadrant Analysis plot of each run.

175mm Crank QA

150mm Crank QA

Pretty much identical, no? what does this all mean?  It means that determining an "optimal" crank length for bicycling isn't very important. Instead of worrying about it from that standpoint, just understand that a wide range of lengths are acceptable, and use it as a "lever" (pun intended) for other things, such as fitting issues. One thing you might want to be wary of is going too large, in that that actually can start causing problems at the top of the pedal stroke and/or prevent the most aero position for a given event. It's somewhat hard to go "too small" (within reason) on crank length...just make sure you're geared adequately for the course profiles.

Oh...and it also means Kieran needs to eat some "humble pie"....nom, nom, nom ;-)

Monday, December 9, 2013

A Compendium of Tubeless Crr Results (plus getting up to date with some Vittoria and Specialized results)'s been awhile since I posted.  Sorry about that...but, after setting up a wheel for road tubeless as a part of the last post on the Schwalbe Ironman tires, I decided to try to get my hands on as many road tubeless tires as I could to see if there were any "gems" in the bunch.  In the past, the road tubeless offerings all tended to have less than stellar Crr results (mostly due to the butyl air barrier layers applied), but the Schwalbe IM offering showed that there might be some road tubeless offerings finally getting their Crr down there.  So, to start, here's how they stacked up, with a Continental GP4000S (latex tube) in there for comparison:

 Now then, as you can see, some of those tires compare favorably to the "benchmark" Continental GP4000S, but I also think it's important to keep in mind the measured widths.  In this case, all were measured on a Zipp 101 rim (internal bead width = 16.25 mm):

IRC Roadlite Tubeless 25C = 26.8mm
Continental GP4000S 23C (latex tube) = 24.7mm
Schwalbe IM Tubeless 22C = 23mm
IRC Formula Pro Light Tubeless 23C = 24.6mm
Hutchinson Galactik Tubeless 23C = 22.5mm
Hutchinson Atom Tubeless 23C = 21.8mm

As you can see, the IRC Roadlite Tubeless 25C measures nearly 27mm across when mounted on the Zipp rim...that's HUGE.  It makes for a great road/training tire, especially on the rear, and in fact that's what I've been using for that purpose for the last few months. For front tire usage, especially due to it's narrower width and aerodynamic features, the Schwalbe appears to be the best of the bunch.  The Hutchinson tires are narrow, but their Crr values are not so, it was my experience that the Hutchinson tires were significantly more difficult to mount (tight beads) than the Schwalbe or IRC tires. As for the IRC Formula Pro Light...that one is a bit of an enigma for me.  It's Crr is in the "decent" range (not great, but not horrible either) but it seems to be the most fragile of the bunch.  I used it for a short time as a rear tire and quickly suffered punctures large enough to not allow the sealant to work...and I think there are better choices for front, I'm not sure where I would actually prefer using that tire.  That's a bit disappointing really, because I think that it's unique latex based air barrier layer is the way to go for tubeless applications.

Tubeless Thoughts

After having ridden and played around with road tubeless offerings over the last few months, I've come to the conclusion that the purported "advantages" of running tubeless tires (with sealant) in road applications are really only realized if the vast majority of your punctures are from relative small items (i.e. 1mm or less).  Anything larger than that, and the air volume is too small and the pressures too large, for the sealant to effectively seal AND let you continue riding...with cuts or punctures larger than 1mm, you will most likely end up having to pull over and swap in a tube anyway.  So, if most of your problems with flatting are due to things like goatheads or "michelin wires", then tubeless with sealant is a really good way to go.  If you instead have problems with things like "pinch flats" (from hitting sharp edges or objects) you can actually get a significant improvement in performance just from using latex tubes and/or larger width tires.  Sure, latex tubes take some unique setup considerations for reliable use, but they're really no more of a hassle than setting up a tubeless tire using sealant...and in some ways they're easier on a day to day basis.

Vittoria and Specialized Results

Over the past few months, I've been testing some tires for Greg Kopecky and for inclusion in some review articles he's written.  An example is seen here ( Listed below are some additional tire results that I'm adding to the overall Crr spreadsheet linked to in the upper right of the blog.

Friday, August 30, 2013

Schwalbe Ironman Tires - A Clincher, A Tubeless, and A Tubular

Earlier this year, Schwalbe announced a set of tires marketed towards the triathlon/TT fact, they're branded with the Ironman logo, so it's not too hard to figure out the target market ;-)

Anyway, the interesting thing about this announcement was that it wasn't just a single tire, but actually 3 tires: a clincher, a tubeless, AND a tubular model.  The design goal for this line of tires was to come up with the best combination of tire properties (i.e. Crr, aero, and durability) for going fast (and far) against the clock. For aero, the tires are sized at 22C and have a noticeable parabolic shape.  Additionally, there's a pattern molded into the sides of the tire that is intended to act much like the boundary layer trip features we've seen on tires like the Mavic CXR offerings.

Luckily, I was able to get my hands on a set of these tires and was able to put them on the rollers to see how they do.  Upon first inspection, the clincher and the tubeless tires appeared to be virtually identical, with the tubeless model appearing to have an extra layer molded to the inside (most likely an air barrier layer), so I expected the tubeless to roll slightly worse than the clincher model with a latex tube.  The tubular model is actually a traditional style "sew up" (i.e. a casing with glued on tread, not a 1 piece vulcanized model) with what appears to be a fairly high TPI casing with the a tread cap glued on that looks and feels just like the clincher and tubeless models. did they roll?  Here's the answers:

Schwalbe Ironman Tubular (22C)   = .0031
Schwalbe Ironman Tubeless (22C) = .0035
Schwalbe Ironman Clincher (22C)  = .0041

Interestingly enough, it appears that the tubeless version has LOWER Crr than the clincher, even with the clincher using a latex tube.  I find that very curious...that means there must be something different about the compounding or the casing with the tubeless, because there's no way an added butyl air barrier layer should be lower loss than a latex inner tube.  In fact, at the time of the testing, the Schwalbe Ironman tubeless model was the fastest rolling tubeless tire I had tested, or even as compared to the tubeless tires Al Morrison has tested in the past.

Curious about what some miles would do to the Crr on the tubeless model, I left it on my rear wheel for just over 300 miles and then retested with the result of:

Schwalbe Ironman Tubeless - w/335 miles = .0033

Now THAT is the fastest tubeless model tire I've tested to date (I've got a bunch of tubeless tires I've been testing and I'll post a "compendium" soon), and the only one close to it is significantly wider (25C vs. 22C).

But, the real eye-opener of the group was the tubular model.  Obviously, we know that the type of tire construction used (high TPI casing, latex tube, etc.) makes for a fast rolling tire.  But, to be able to pull that off with a relatively thick tread cap glued on means that there must be some "magic sauce" in the tread compound.  Of course, the performance of that tire also begs the question of why they just don't make an "open tubular" version of the tire for the clincher market instead of the current clincher...

So, it appears that they've done a good job on the Crr front.  The clincher is on par with tires like the Michelin Pro 4s, the tubeless is pretty fast (slightly faster or slower than a Conti GP4000S, depending on miles), and the tubular is smoking fast as well.  If the aerodynamics comes close to other tire models, these tires would definitely be an intriguing option for TTs and triathlons, especially for folks who plan on going pretty fast and/or in low yaw conditions (because of the relative narrowness) .

Also, as one extra data point on latex vs. butyl, I tested the clincher with a butyl tube instead and here's how it rolled:

Schwalbe Ironman Clincher (22C)  = .0046

Once again, this shows that a butyl tube "costs" ~3W per tire as compared to latex...just sayin'  :-)

The latest overall charts:

Saturday, August 10, 2013

Even more Crr results...and another example of why Crr matters, Mavic edition

I did a short bit of roller testing yesterday.  The main incentive for that was I was able to get my hands on a prototype set of the new Mavic CXR60C clincher wheels and I was itching to see how well the new CXR clincher tire rolls.  Back in May I attended the press introduction for the CXR60 wheel line on behalf of  You can see my review of the wheels at that time here: Mavic CXR60 Intro.

At the time of the press introduction, none of the attendees were able to ride the clincher version of the wheels, so a big question mark in my mind was how well the tires performed from a rolling resistance standpoint.  From the wind tunnel results, obviously the wheel+tire system performed excellent in regards to aero drag, but I already had experience with the tubular CXR tires and found them to be much so that they basically "wasted" the aerodynamics.  More on that later.

In any case, here's the results from yesterday's roller testing:

Mavic CXR clincher protoptype (23C) = .0036
Challenge Triathlon clincher (23C)        = .0034
Challenge Triathlon w/Panaracer R'Air =  .0042

Besides the Mavic tires (I tested 2 and they were nearly identical) I also tested a Challenge Triathlon clincher.  Both the Mavic and the first run of the Challenge Triathlon were run with latex tubes, and then I decided to run the Challenge tire again after swapping out the latex tube for a Panaracer R'Air tube.  This tube is a butyl based tube that is advertised to be compounded to be more flexible like a latex tube and I was curious to see if it made any difference in the rolling resistance.  It did...but just barely (~1W for a pair @ 40 kph)...and that improvement is definitely not worth the cost of the tubes, especially considering one can get a latex tube for the same price.

The Mavic tire's Crr of .0036 is a very respectable result...much better than I was anticipating based on what I had measured for the tubular and what the Mavic engineers had claimed the difference was between the tires.  By comparison, the average Crr I've measured for brand new Continental GP4000S tires is only slightly better at .0034, and is significantly better than the Michelin Pro4 Service Course Comps at .0041. Don't forget...for this testing (and the uncertainties involved) I consider anything within .0001 of Crr to be basically "tied".

At the end of the article I linked to above, I had created a chart showing the combined affects of Crr and aero drag like I outlined in a previous blog post (Why Crr Matters...) Shown below is how that chart looks with the measured Crr for the CXR60C prototype tires.

It's fairly obvious from that chart that the CXR60C is the fastest wheel+tire system that Mavic makes.  In fact, the difference for a single front wheel at an expected apparent wind velocity of 40 kph is on the order of 5-6W on average in favor of the CXR60C over both the CXR80 and the CXR60T tubular wheels.

The latest published version of the roller testing Crr spreadsheet can be found in the link at the upper right under "pages".

Sunday, August 4, 2013

Aero Field Testing using the "Chung Method" - How sensitive can it be?

As some of you may know, I've been field testing bike stuff and positioning with a power meter for 4 or 5 years now.  My method of choice is Robert Chung's "Virtual Elevation", or VE protocol, sometimes known as the "Chung Method".  He has a great presentation on it here: . When I first read Robert's info, I wrote up a spreadsheet that I've used since then to analyze everything from position changes to tire air pressure effects. Of course, since I wrote that spreadsheet for my own use, it's not exactly the most "user friendly" (Hey, I know what I'm supposed to do, I wrote it! ;-)...but, don't worry, everyone else is in luck since Andy Froncioni (the main tech guy behind Alphamantis and the ERO facility that recently opened at the indoor track in Carson) added a version of the same calculations (called "Aerolab") to the freeware power meter analysis software, Golden Cheetah.

So, the question with this type of testing usually comes down to just how sensitive can it really be...especially as compared to something like a wind tunnel?  Admittedly, there are some limitations to this type of testing, the main one being (at present time) that the results are mostly limited to zero yaw conditions, but as we saw in one of my previous blog posts, the most common yaw angles a TT'er or triathlete encounters are usually centered around zero yaw.  Using the tool to make evaluations at zero yaw still can hold a significant benefit for someone interested in improving/testing bicycle aerodynamics.

A couple years ago, Dr. Andrew Coggan published a blog post titled "A Challenge to Cycling Aerodynamicists" in which he described a field test he undertook to take up something he coined the "Tom Compton Challenge".  In short, it's an effort using known geometric shapes to try to determine the "sensitivity" of the aerodynamic field testing method.

Well, last year I discovered that my preferred field testing venue for VE runs had suffered some "traffic rerouting" that had made it much less appealing for the purpose (part of that "discovery" occurred when Andy sent his test setup to me to try and the results from my first course were very mixed due to excess vehicle interference after the nearby roads had been modified).  So, I started scouting around for an alternative course and luckily found one that is much closer to my home (I can ride there in just a few minutes) and that has laps that are significantly shorter than the old course (shorter laps = shorter test run time).  Both of these courses are best described as a sort of "extended halfpipe", an "out and back" course having a U-shaped elevation profile that allows for turnarounds to be taken at low speeds and thus avoid braking. Since identifying the new course, and having done just a few tests on it, one thing I wanted to do was to repeat the type of testing that Andy did and attempt to characterize the potential "sensitivity" of the course using the VE method.

Using Andy's setup as a guide, I set about figuring out what sorts of objects I could use for the test.  I took a quick trip to the nearby Michael's craft store and acquired some styrofoam spheres, 2", 3", and 4" in diameter.

In my garage I had an appropriate length of 1/2" diameter wooden dowel, and short work with a hand drill on the spheres and I had a setup that placed the spheres well out to the side where they should be in clear air while riding. Also shown in the pic above on the left is a small washer which I placed on the end of the dowel during the first run instead of a sphere.  I did that to act as a "cap" and make it more likely that the flow over the end of the cylinder stayed perpendicular.  Here's how the dowel and sphere setup looked after being zip-tied to the basebar of my TT bike.

The hole in each sphere ended up being a nice friction fit, so swapping between spheres was a very simple process. 

All runs were recorded with my trusty old yellow-cap PT Pro wheel mounted on the rear, with a cover in place to turn it into a de facto disc.  I prefer to use a PT for my aero field testing since it eliminates the uncertainty of variations in drivetrain resistance across the gearing, plus the PT's "coasting zero" feature allows me to have the power meter zero while soft-pedaling (i.e. turning the pedals slowly while freewheeling) down the descents of the course at least once per lap.  That helps to minimize any power meter drift during the runs.

So, with the test rig sorted out, it was time to head out to the test course and do some runs! To minimize wind and traffic effects, I prefer to head out to the course early on a Saturday or Sunday morning...before the small neighborhood that the course road services begins to wake up and starts moving around. A couple weekends ago, I headed out on a Sunday morning and rode over to my test course in 5 minutes, taking a small musette bag with the spheres, a notebook, and a couple of small tools I might need.  Starting at 6 am, I did the runs in the following order:
  1. Rod only (w/washer "endplate")
  2. 3" sphere
  3. 2" sphere
  4. 4" sphere
I mixed the runs up like that since I suspected that the "rod only" and 2" spheres may be close to the same measurement (part of the rod is covered up by the sphere) and I wanted to make sure there was a good separation between the cases.  I actually didn't sit down and calculate out the expected differences in CdA beforehand.  I wanted to first determine what the VE analysis showed as the differences from the baseline (run #1) and then see how close to the calculated values the VE runs were.  I did this because the method I use for determining the CdA using VE is a visual "leveling" procedure, and so there's a bit of "judgement" involved in determining what value best "fits" the overall plot to being level, and I didn't want that judgement being affected by any predetermined knowledge of what the expected differences should be.

Once I returned home, it was time to download the PT files into the computer and do the VE analysis.  As I described above, although it can be done in GC's Aerolab feature, I prefer to use my own home-brewed spreadsheet, mostly because I find it easier to expand the vertical scale to get a better handle on the leveling procedure, but also because I've recently added a feature that varies the on-road Crr as a function of the ambient temperature.  In order to use the spreadsheet, the following inputs are required:
  1.  Total Mass - Easy to get just by stepping on a scale with bike in hand
  2. Weather Conditions - this means air temp, dew point temp, and barometric pressure (to determine air density).  Luckily, there's a personal weather station listed on Weather Underground literally less than a block from my test course that has updates loaded every 5 minutes.  Using that station also allows for a cross-check on ambient wind conditions to make sure it stayed calm during the test runs.
  3. Assumed Crr - For this, I use a weighted average of the front and rear tire Crr that I've determined from roller testing.  The spreadsheet then compensates for the expected Crr due to the difference in the test ambient temperature and the 20C temperature to which my Crr results are normalized.
As an example of the spreadsheet and what the VE profile plot ends up looking like, shown below is a snapshot of the first run from the testing:

OK then, let's get to the results.  Using the procedure outlined above, my best determination for the measured CdA from the runs was as follows (in order that runs were performed):
  1. Rod only  = .2484 m^2
  2. 3" sphere = .2498 m^2
  3. 2" sphere = .2486 m^2
  4. 4" sphere = .2510 m^2
Now, how does that compare to what should be expected for those shapes?  To determine that, I made a spreadsheet that calculated the expected CdA changes based on the typical values of Cd (in the Re number range of interest) for a sphere and a cylinder (sphere = 0.47, cylinder = 1.17) and their respective cross-sectional areas based on my actual measurements. As mentioned above, when I compared the "rod only" run to the sphere runs, I had to subtract the portion of the rod that was covered by the cylinder from the CdA calculation.  I then took those expected changes in CdA and added them to the measured CdA from the "rod only", or baseline run to determine the expected CdAs for the runs with the spheres.  Here's how they compared:

Run # - Sphere      Measured CdA (m^2)     Calculated CdA (m^2)   Difference (m^2)

2. - 3" sphere                  .2498                              .2501                          .0003
3. - 2" sphere                  .2486                              .2488                          .0002
4. - 4" sphere                  .2510                              .2518                          .0008

Another way of looking at it is the expected and measured differences from the baseline:

Run # - Sphere      Meas. Diff. from Baseline(m^2)     Calc. Diff. from Baseline (m^2)

2. - 3" sphere                       .0014                                            .0017
3. - 2" sphere                       .0002                                            .0004
4. - 4" sphere                       .0026                                            .0033

One last way of looking at this is from the perspective of expected change from from the previous run.  Here's how that worked out:

Run # - Sphere      Meas. Diff. from Previous (m^2)     Calc. Diff. from Previous (m^2)

2. - 3" sphere                       .0014                                            .0017
3. - 2" sphere                     -.0012                                           -.0013
4. - 4" sphere                       .0024                                            .0029

Not bad, huh?  I've always said that when using this technique I consider measurements that are within +/-.001 m^2 to be basically "tied", and the above appears to bear that assumption out as being fairly conservative.  It also gives me confidence that with careful technique I should be able to easily detect CdA differences on the order of .003-.005 m^2 and greater.