The Rail Prototype Wind Tunnel Data and Calculations

Our primary objective in sending 9 wheels to the wind tunnel was to see where the prototype of the Rail stacked up against our existing wheel offerings as well as the Zipp 404 Firecrest clincher. The challenge when you are sifting through the pretty substantial amount of data generated at the tunnel is how to present it in a way that it will be both meaningful and accurate. So we've developed a couple of charts which give you the executive summary, which we'll fill in with some context. 

The raw data is provided in grams of drag and aero watts, neither of which tell the complete story about how a wheel really performs. More commonly, you'll see wheels in the 40K TT standard, which shows how much time each theoretically saves over a benchmark wheel in a 40K TT at 30mph. That's what we show here:

Usually the benchmark used is a 32 spoke Mavic Open Pro, which is a ridiculous proxy for the "average" wheel on a TT bike but which makes it easy for a brand to claim they saved a big enough number of seconds to raise eyebrows. We compared everything to our FSW 23 wheelset, which is 21mm high and 23mm wide with 20 raidally-laced Sapim CX-Ray spokes. If you're concerned enough about aerodynamic performance to look at wind tunnel data, the FSW is much more likely to approximate the wheel you are currently using, or the one you would buy if you decide not to go with a deep carbon wheel. 

You will see from the data that under these test criteria, the Rail is faster than any of our current wheels, including the RFSC 85s which are 33mm deeper. It is also only 2 seconds slower than the Zipp 404 FC, despite having 8 more spokes in the prototype and being 6mm shallower. Our internal goal for deciding whether to proceed with the Rail was that it test faster than our current RFSC 58s and within a couple aero watts (about 6 seconds in this test) of the RFSC 85s. When comparing to the 404, we are not limiting the Rail to aerodynamics, as the improved road feel of the larger inside width is really the heart of the Rail's story. We also wanted to bring the depth down to 52mm to broaden its use range. So if we were within about 3-4 watts (9-12 seconds) of the Zipp at this stage we would have been satisfied. Coming in at faster than our 85s and within 2 seconds of the Zipp made our decision easy; we've already greenlighted the Rail for production.

The 40K TT at 30mph comparison is a standard, but an unfortunate one for consumers trying to fairly evaluate how different wheels would perform for them. Aerodynamics are amplified at higher speeds (as Dave points out here, particularly int he comments), but the average speed for most TTs and bike legs in triathlons is well under that. Wheel brands like the standard though because it shows a larger gap against benchmark wheels. To get a better sense of how each wheel performs at mortal speeds, have a look at the following chart:

The slower you are going, the less delta you see in the aero drag between each wheelset. If you want to do the math and figure out the 40K TT time savings at different speeds, use these calculations:

30mph: 11 grams of drag = 1 aero watt
25mph: 9 grams of drag = 1 aero watt
20mph: 7.5 grams of drag = 1 aero watt 

If 1 aero watt equals a difference 3 seconds in a 40K TT, the 39 second advantage of the 404s over the FSW alloys at 30mph drops down to 25 seconds at 25mph and 13 seconds at 20mph. 

Some people will ask about Angles of Attack (or you might ask about yaw in which case you mean Angles of Attack, as per Dave), knowing that they vary at different speeds. The faster you riding, the more likely you are to encounter lower angles of attack. At 30mph, over 80% of AOAs are at 10 degrees or below. At 20mph the sweet spot is between 10-20 degrees, comprising about 50% of all real-world conditions. The data above already reflects the distribution of AOAs at the different speeds.

We'll roll out more of our test data and findings over the next few days. You can sign up to receive future posts by email here if you don't want to keep hitting the refresh button. Until then, fire away with any questions in the comments. 

Back to blog


Nice!! Now I'm lusting over a production set of the Rail… Just what I need!! (the lusting, that is…) Without going back through several blog posts, which spokes turned out to be most impressive?

scott l booth

Looking at the results it seems that you guys have really put together quite a design in regards to aero performance. Congratulations on that.One of the things that struck me when looking at the graphs is how the 50 and the 58 were virtually identical in performance. Just based on this data alone it seems that the the extra 8mm doesn't buy you much at least in terms of aerodynamics. This, to me, seems to be one of the bigger surprises that has come out of your testing.

Joe Ajello

Shutup and take my money.No, seriously, when can I order a 20/24 with a Powertap build?

Mike E

Me like! :-)


Intresting datas – and good job on new rim!


Leave a comment

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