Friday, August 30, 2013

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

Update Note: Since the testing of these tires and the publishing of this post, it's come to my attention that Schwalbe has apparently discontinued using latex tubes within their tubular tires. As such, one should expect the tubular version of the IM tire to roll significantly slower than tested below; in the range of 2-4W slower per tire. The tire tested will now be listed in my spreadsheet summary as "Out of Print" - 14 Nov 2015

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.