November in the wind tunnel: Steady as she goes

One of the chief aims of the Rail series was to provide a stable and manageable ride.  This was a major consideration in the 52 being the 52, and not the 58 or the 60 or the 62.  The 34 was in large part born from the market demand for a wheel that would be nigh on invisible to crosswinds, even though early 52 reviews were near unanimously positive in regard to manageability.  

The cross sectional design of Rails attempts to create a near symmetry between rim side and tire side.   Obviously, tire choice affects this.  We had a 23 in mind, aware of the size to which most 23s would inflate on our chosen 18mm bead seat width.  Different tires have different shapes, and many people use different sizes. Nonetheless, the general gist stands.

To date, crosswind stability has been measured subjectively and anecdotally, never directly measured and quantified.  Thankfully, a wheel company from Indiana had been pestering A2 for an actual measured match to their CFD predictions.  Recently, A2 completed the measurement apparatus and algorithms to provide these measures. When you test a wheel now, your data sets include two new columns -  coefficient of drag, and center of pressure.  

Ceofficient of drag, simply stated, is how much pressure is pushing against your wheel - how strong is the push.  Units of measure are non-specific, but linear - meaning that .20 is twice as hard a push as .10, and 2/3 as big a push as .30.  Center of pressure describes the position of the push, relative to the hub, measured in centimeters.  Center of pressure of 2.35 would describe a push centered 2.35cm in front of the hub.  -3 would describe a push 3cm behind the hub.  

To date, we've never seen any graphical presentation of this data.  It's too new a concept, so we've taken a stab at it, which we think provides a clear picture of the relative power and placement of the crosswind's push on each wheel.  We have once again used the Tour Magazine angle of attack weighting in creating this chart, but we have used the 25mph weighting.  Our reasoning for this is that as wind speeds increase relative to bike speed, the likelihood of wider angles of attack increases.  We expect and welcome questions about this information and presentation, simply because we want it to be easily understood.  

Here is a link to a page that allows you to calculate apparent wind speeds and angles for any given combination (be sure and use the second box, the first one calculates to true wind speed).  Be aware as you do this that a windy city will have an average windspeed of somewhere around 10mph (the calculator uses knots - 10mph equals 8.7 knots), as measured at that city's airport.  Airport windspeed is measured high off the ground in an unobstructed place, and will overstate what your wheels are riding in by quite a bit - like 50% or more.


To say the results pleased us would be an understatement.  As the initial test of the 34 was underway, I was busily prepping the next wheel to test in the work room and poked my head into the control room to ask if I was in a good mood.  Dave, A2's engineer and a man not given to subjective statements or value judgments, said the aero drag measurements were going right along, but that the pressure measurements should put me in a very good mood indeed.  As the 52 ran and the data came up, my mood improved even more.  


As you can imagine, we're excited to see that our consideration of crosswind stability in the design of the Rail has been confirmed with such excellent results.  

Back to blog


So I looked up coefficient of drag on Wikipedia, and it appears to be defined in roughly in terms of drag force for a given fluid density, speed, and reference area. This means that the same drag force would correspond to a lower coefficient of drag over a larger reference area. (This makes sense: if something encounters the same force at the same velocity in the same fluid, but is bigger, we can say that its shape is more aerodynamic.) If I'm understanding this correctly, the numbers that you're referencing are thus basically "per square cm." But what matters to someone riding the bike is not the "pressure," so to speak, on the rim of the crosswinds, but rather the force (pressure x area). A geometrically more aerodynamic wheel might well be more susceptible to crosswinds than a less aerodynamic wheel because of a larger cross-section. The push (force, not pressure) is greater.Does this make sense?


Dave – I really do appreciate the transparency – Its a huge reason why I bought the 52s.I will also take a moment to comment that the 52's are damned stable. But they definitely do have a bigger sail effect with gusts than a lower profile wheel. No real way around that that I can envision :)


Yet another affirmation of why we're as transparent as we are with this stuff, and an example of the challenges of presenting new data. Thanks to Tom Anholt for pointing out the error. Basically, the wind tunnel creates the best point for CoP as 0, and doesn't correct for where the steering axis falls relative to the hub.We're actually going to add a table modified for a very representative bike's trail geometry. The effects of this will be to improve the relative position of the alloys (preliminarily, I think they go to 1 and 2 or 1 and 3 in our sweep), and makes the 34 better relative to the other carbons. The carbons seem to be 34-52-404-3.4 in rank. That will all be confirmed in the modified post. Erik – Even though several people had commented that the 34s are easy to handle relative to SL23s, we struggled with the sniff test too. "You can't argue with data" but we struggled to consider that the alloys were the worst of the lot. The tunnel has things right as hell, it's just a question of us putting "0" in the correct place. This all speaks to why we laced this post with invitation to comment, and again, why we believe in being so transparent. JP – The 52s still do well, and they're still fast. This has been a cool back and forth.

Dave K

With regards to the shallower alloy rims performing worse than the Rails…I spent a year on the the FSW23s w/ Kinlins and then switched to the Rail34s right when they came out…a Rail34 on the front is invisible in a crosswind as compared to the Kinlin wheel. Very noticeable difference.


This does not surprise me. I ordered my Rail 52s in the pre-order. I'm not a small guy (+/-84 kilos), but I've raced the crap out of them as my go-to wheels for crits and road races on my Focus Izalco Pro. I've never had a single issue with crosswinds with this setup. The 34s may work for others, but I freaking love these wheels. I also haven't had any issues with my FSW23s, but that is entirely another use case as they are used exclusively on a Moots touring rig.

Mark D

Leave a comment

Please note, comments need to be approved before they are published.