Wednesday, October 29, 2014

An Oldie, but a Goodie...Field Testing Frame Differences with a Power Meter

I originally posted this back in 2008 on the forum and is an example of the types of equipment differences that can be determined with careful field testing.  Here's the link (click on the title below) to the full thread on's long but informative, and there's some pretty good "back and forth" with a few skeptics:

Something Borrowed...Something Fast!

So take this guy and his position:

...and put him on this guy's bike and adjust it so that they fit identical.

Then, let the first guy test both of these "back to back" using the same 404 wheelset with PT SL hub and cover....Any guesses on what the aerodynamic drag differences (if any) one would see?

Using RChung's most excellent methodology described here: Method to the madness

Here's the results for the P2K with the 404 wheels (the wheels on the P3C above):

And here's the results for the P3C:

So...what's the bottom line?

Well...taking the same rider, the same wheels, the same basebars and brake levers, and with the seat and extensions adjusted to deliver the same basically zero yaw conditions I apparently measured a drag difference of ~.023 m^2 of CxA (or Cda, whichever you prefer - .228 m^2 for the P2K and .205 for the P3C). Using Doc C's "rule of thumb", that basically equates to ~2.5s per km of time savings.

With only an extremely small amount of crosswind however (I could just barely detect it on my skin, not enough to even move leaves on trees), the drag on both setups drops significantly, with the P3C setup dropping more at .190 m^2 vs. the P2K's .220 m^2 for a growing difference of ~.03 m^2. That translates to a difference of ~3 seconds per km...or a full 2 minutes over 40K.

Can you imagine comparing the difference between a P3C (or even the P2K) against a frame that actually increases in CxA with increasing yaw, which is actually fairly common?

Sunday, September 7, 2014

New Zipp Tangentes - Speed, Course, and SLSpeed - the Crr results

A bit over a week ago, Zipp announced at Eurobike the release of their entirely new tire line; specifically, the Tangente Speed and Course clincher models (in 23C and 25C versions for both) along with the Tangente SLSpeed Tubular models (a 24C and a whopping 27C version).  Below you'll find my roller testing results for these new tires. The Speed models are based on a 220tpi nylon casing and forgo an under the tread puncture belt, while the Course models use a 127tpi nylon casing and use puncture belt.  The SLSpeed tubular models use a similar tread that is glued to a 320tpi cotton tubular casing with a latex tube inside and they also feature a puncture belt.

The previous Tangente models from Zipp were apparently very good from an aero standpoint.  From a rolling resistance standpoint however, they were "less than stellar"...especially in this day of tire companies understanding the value of low Crr tires and their effect on performance and "comfort".  So, one of the main drivers of this new tire development was in lowering the Crr of their tire offerings.  On this point, I'd have to say that they've succeeded, in that Course models are basically tied with the current "industry standard" Continental GP4000S 23C from a Crr standpoint, with the Speed models being slightly faster. Their new tubular models, the Tangente SLSpeed, are also very low rolling resistance, with the 24C model basically tied with the benchmark Schwalbe IronMan Tubular, and the 27C (with the helping of its extra wide casing) taking over the current top spot for a "brand new" tire on the overall list of tires I've tested.

So, here's the nitty-gritty data for these tires, including data for the Schwalbe IM tubular, my "benchmark" GP4000S 23C tire, and the old model Tangente tubular.  All of the clincher data for this round was performed on a Zipp Super 9 clincher disc, with the tubulars all tested on Zipp 900 tubular discs for consistency. In rank order from lowest Crr to highest:

Zipp Tangente SLSpeed 27C Tubular = .0028,  26W for pair @ 40kph, width = 26.8mm
Zipp Tangente Speed 25C Clincher = .0030,  28W for pair @ 40kph, width = 24.8mm
Zipp Tangente SLSpeed 24C Tubular = .0032,  29W for pair @ 40kph, width = 23.5mm
Schwalbe IronMan Tubular 22C = .0032,  30W for pair @ 40kph, width = 21.7mm
Zipp Tangente Speed 23C Clincher = .0033,  31W for pair @ 40kph, width = 23.8mm
Continental GP4000S 23C = .0034,  32W for pair @ 40kph, width = 24.8mm
Zipp Tangente Course 25C Clincher = .0035,  32W for pair @ 40kph, width = 24.7mm
Zipp Tangente Course 23C Clincher = .0035,  33W for pair @ 40kph, width = 23.8mm
Old Zipp Tangente 23C Tubular = .0045,  41W for pair @ 40kph, width = 22.4mm

So...what are the takeaways here?  Well, I think it's fair to say that Zipp accomplished their goal of significantly improving the Crr of their tires, which can be easily seen by the comparison to the old Tangente tubular. Even the slowest of the new tires would save ~7W for a pair at 40kph, with the faster tires requiring more than 10W less at 40kph.  That's significant.  Also, in comparison to the GP4000S, the 25C Course model is basically tied with it, both in Crr and in actual tire width (at ~24.7mm) with the 23C Course model being only slightly slower (within the error margin of the testing).  The 23C version of the Speed models tested out slightly faster than the GP4000S (again within the margin of error) but measures a full 1mm narrower when mounted on the same rim.  That should help its aero properties. The 25C Speed model, however, is significantly lower Crr than the GP4000S saving a predicted 4W for a pair at 40kph, while measuring out at the identical width.

On the tubular front, the new 24C Tangente SLSpeed tire rolls just slightly better (within .0001 Crr) of the Schwalbe IronMan tubular, which is not surprising considering their similar construction (tread glued to a 320tpi cotton casing).  The 27C Tangente SLSpeed tire rolls VERY fast, but its extra wide 26.8mm mounted width is going to result in an aero hit.  I'd call that one a "rear use only"...but only as long as it can be shown that the width doesn't "give back" aerodynamically the gains that are made in Crr.

Well, that's the Crr results. What remains to be seen is how these tires perform aerodynamically.  But, as I've said before, and we've seen recently with tires like the Specialized Turbo Cotton, low Crr can "make up for a lot of aerodynamic sins".

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 ;-)