Mike described a huge part of the design brief for the Rail in the last entry, and the part that he described really can't be minimized. When I started racing my mountain bike more, I quickly realized that my biggest gains weren't to be made in going faster, they were to be made in not slowing down. Guys were killing me in the turns, which was a double bonus to them - they could gap me in the turn, and then since they were holding more speed out of the turn they'd extend the gap after the turn, while I burnt matches to accelerate to their speed. Handling counts for a lot, on every surface.
We also thought it was critical to optimize around a 23mm tire. A lot of wheels out there (Zipp 404, for example) are optimized around 21mm tires, and have a narrower inside and outside dimension at the tire hook. All of their, and seemingly everyone's, aero data is presented based on testing with a 21mm tire. Everyone we know rides on 23mm tires (many even ride 25s, which becomes somewhat superfluous when you've got a wide enough inside dimension), 23mm tires are most widely available (both variety of offerings and availability for purchase), and several studies have shown that wider tires (up to a point) have lower rolling resistance. We knew how well the 18mm inside dimension with a 23mm tire handled, and a whole lot of other people agreed with us, so optimizing around the 18mm inside width and 23mm tire combination made absolute sense to us. Of course our baseline raw drag data from the wind tunnel will probably appear a bit higher than other manufacturers show, because they all test with 21s. To help you (and us) make more accurate comparisons, we're testing a benchmark wheel with 23s instead of the 21s around which they're optimized, but which no one uses.
Once we knew that we wanted to wind up with 18mm for the inside width, the next parameter to nail down was depth. We chose 52mm as a start point for a couple of reasons. First, there is a significant aerodynamic threshold right around there. Ever thought why the UCI is such sticklers about shapes that are more than 3:1 deep? Well, given an 18mm inside width, we knew we'd wind up at 25mm for the outside width. 25 times 3 is 75. A 23mm tire plus a 52mm rim is 75. Makes sense?
If 52 gets you right to that threshold, why didn't we go to 60 or 70? First, weight. Added depth costs weight, and we didn't want to wind up with good aerodynamics at the expense of practicality on different courses. Many of the new rims are showing that there are only very small gains to be made with deeper rims, enough so that rims much beyond the 50mm range are really being pushed into the "special use" category (pretty much tt only). Second, even though the shapes we were investigating promised a lot of gains in crosswind stability (more on this in a bit), at some point surface area is surface area, and that turns into a liability in windy conditions. So we figured 52 would get us to that magical nexus of three inflection points.
Now we had width and depth, but what about shape? Fortunately, I'd worked with a lot of foil shapes before, and there's a ton of transferability between sailing shapes and rim shapes. Step 1 was to tap into the (extrememly public) database of NACA shapes. So much of innovation seems to be finding new ways to apply old knowledge, and NACA is nothing if not old knowledge - the agency opened its doors in 1915. It is also, in fact, rocket science. I skipped straight to the 4 digit, 00XX series sections, as I knew that the 4 digit series sections would put the maximum thickness in about the right place along the chord, and that I wanted a symmetrical foil (side to side) which is what the 00XX sections have. From there, it was a game of figuring out ratios. Since both the tire and the rim are both leading and trailing edges, I wanted to try and get them roughly symmetrical. That was pretty easy to do with the 0024 section. Oddly enough, just days after we'd really committed to that route, Mavic came out with their new 80mm rim, which also relied on the 0024 section. A classic "good news, bad news" situation - clearly our choice had a lot of validity, but we'd also be accused of being serious copycats. Fortunately (at least we hope), we slice and diced it such that our setup would replicate an 0024 with both tire and rim, where their tire side goes 0024 but their rim side goes 0011. Their rim is less blunt than ours - more on this in a bit.
NACA 0024 foil plot
Speaking of rim bluntness, I don't think it's necessarily possible to accurately model steering response as a function of rim shape, but again I was fortunate to bring some sailing experience to bear. I've trimmed A LOT of sails, and part of what makes the good guys good at it is the ability to match the conditions with the needed shape. The illustration below, looking down from above at cross sections of sails, helps make the point. When conditions are more turbulent or challenging (for example, when the waves are steep), or when the person driving the boat maybe isn't quite world class, you give a deeper, more forgiving entry shape. On the other hand, in more ideal conditions, you set up a narrower entry angle.
image courtesy UK Sails
The sharper entry angle will be faster in ideal conditions, but it is far more prone to stall. In the case of a rim with a narrow entry, the flow can switch from side to side, which would be a jarring input. As I explored in a post about TdF TT equipment this summer, a narrow rim like the HED 3 spoke does very well in low apparent wind angles. Mavic choosing the 0011 shape for their 80mm rim makes sense, as it's more of a dedicated TT wheel, which will operate most often at the low yaw angles where sharper, narrower rims are going to be relatively strongest. With a sail, you have the basic shape and then a bunch of controls that help you modify and manipulate that shape to suit the conditions. Obviously you can't do that with a rim, so a forgiving all purpose shape works well for an all purpose wheel.
This is a seriously condensed (and yet challengingly detailed - thanks for following along this far) version of the design process, but it gives a good sense of how setting an initial parameter made the rest fall in line. Now we have a yellow plastic manifestation of that figuring, which we are about to take to North Carolina to have wind blasted at it so that we can know how fast it is or isn't. We'll write all about that experience in a couple of weeks when we've got the results and have had time to digest them.