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.  

Older Post Newer Post

  • JP on

    So 52 should be easier to handle than 34! Faster to boot!

  • Erik on

    Looking at this, it doesn't seem to pass the smell test for me, having ridden the wheels on the top and bottom of the list pretty extensively with the tested 23mm tire. Also, the Rail 34 scoring worse than the 52 seems odd in a metric designed to reflect stability. Granted, I've not ridden a Rail 34, but I recall reading from November blogs that they are decidedly stable.CDA x COP doesn't seem to be a valid form of comparison to me. There's two issues in my experience handling a deeper wheel in the wind: there's the twisting effect on the front wheel, and the overall sail effect of being pushed sideways.A consistent amount of force sideways, or twisting on the front wheel isn't a huge deal, though it can be tiresome. The real bear to handling is how the wheel handles itself when the wind isn't consistent: call it the semi trailer effect – the change in pressure that occurs at a large 18 wheeler passes you. Some of the energy just pushes you sideways, and some of it translates into the front wheel wanting to twist on you. For me, the twisting is scariest and most uncontrollable, but a large gust can move you sideways a considerable amount. Combine the two, and it gets really unpleasant.For me, it would seem that the amount of difficulty handling the wheel would be the product of the sideways force, and the twisting. The overall cda would matter less in this case, because the pressures fore and aft on the bike aren't contributing to the twisting effect on the front wheel. Sure its slowing you down, but its not contributing to instability. The SL23's and other shallow wheels just don't buffet and twist with the amplitude that the deeper wheels I've ridden do.To me the instability of the front wheel is probably a product of the lateral force of the wind on the wheel at a given yaw, and how far off center the center of pressure is, and this is a large factor in how unstable the wheelset feels as a whole. The rear wheel is probably almost entirely due to the lateral force, though perhaps there's some effect of the center of pressure relative to the distance to the steering axis. This would give some sort of total wind instability number composed of the side force plus side force times center of pressure.

  • Dave on

    Yup, and the net is that it has the least susceptibility to crosswinds of any wheel we measured within the sweep from 0 to 20* angle of attack (yaw angle, if you must). We have a few thoughts on why it does so well in this range, which we'll discuss soon, but we wanted to get this data out and get the conversation started. We know there will be a lot of 'say what?' since the shallowest wheels scored not so great. One thing in particular that we don't know is whether the wheels we tested all happen to be relatively good. Just like we tried to show aerodynamics only among wheels that are legitimately fairly fast, as opposed to Showing aerodynamics versus absolute dogs, we don't know how bad the worst wheels in the world are in this measurement, nor do we know the absolute best wheel in the world. Anecdotally, most people who have used the 34 and compare it to shallower alloy wheels they have used remark that it is much easier to handle even than shallow alloy wheels. We also know that Zipp put a lot of effort into designing stability into the 404. From those points, it would be fair to assume that this group is at least "a good group."

  • JP on

    Wow! Am I reading this right? Rail 52 has the 2nd lowest CDA (behind 404) and 2nd lowest COP (behind Rail 34).

  • Will on

    If I'm reading this correctly. Based on the second chart you posted, in comparison of the 34 and 52, the 34 takes crosswind closer to the hub and have a higher drag. Meaning it would have less push towards one side, but you'll feel more drag than the 52? Also, based off the first chart, theoretically the 52 handles crosswind better than the 34?P.S. Not too knowledgeable on the technical terminology, so trying to use plain English. Hope I'm not confusing because of that.

Leave a comment