# November in the wind tunnel: is wider faster?

What a question!  It might be simpler to ask "how long is a rope?" as there simply is no one answer to this question.

In the simplest terms we can look at, aerodynamic performance of every wheel we tested suffered when the wider tire went on.  There has been much speculation over this one recently, but the results of the tests we ran conclusively show that, in terms of measured aerodynamics, narrow tires are faster.

The question we were perhaps more intrigued to have answered was whether one rim or another tolerated wider tires better than others.  Unexcitingly, the answer to that is also no; all rims suffered a similar drop off in speed when outfitted with 25mm versus 23mm tires.

Now, back to my "how long is a rope" question - how wide is a 23mm or 25mm tire?  For that matter, how tall is either tire?  As the chart below shows, that answer varies widely (I slay me) based on the rim to which it's mounted.  The biggest determinant of inflated tire width and height (and thus inflated volume) is the interior width of the rim - the distance between the brake tracks.  The relationship between interior width variance and tire inflated volume is steady in direction (wider interior rim reliably equals more inflated tire volume), but the magnitude of the change is not as perfectly predictable.  For example, despite both rims having 18mm between the brake tracks, the tires we measured inflated bigger on Rails than on Pacenti SL23s.  But a basic rough rule of thumb is that for every 2mm gain in width between the brake tracks, you will gain 1mm in inflated width.  So if a tire of a stated size runs true to size on an Open Pro that is 14mm between the brake tracks, it will measure 2mm wider (which is equal to the most common size increment jump) on a rim with 18mm between the brake tracks.  Which means that if you prefer a 23mm tire on a traditional-width rim, you can use a 21 on a Rail and get the same volume (more explanation of that to follow).  And that, I promise, is the absolute last time I will mention an Open Pro in any discussion of aerodynamics!

The interesting part that follows on from this is that, when you measure two rims with the same tire, you aren't necessarily measuring the same tire on them.  The 23mm Conti 4000s II that we used measured 24.3mm wide on the 404, but was a full 1.5mm wider on the Rail (and .4mm taller on the Rail, but to keep things simpler we'll focus on width).  Similarly, the 25mm Conti 4000s II that measured 26.7mm wide on the 3.4 front measured 27.3mm wide on the Rail.  Tires also set up relatively lower on the Enve rim compared to the width increase - the 23mm tire was .1mm taller on the 404 than it was on the Enve, despite the tire being .6mm wider on the Enve than the 404.

Given the negative relationship between width and speed, and given that tires measure bigger on our rims than on any others tested (which we knew they would - those who've followed the Rail story know that design parameter #1 was an 18mm interior width), we had to peel the onion back a little bit on that one.  Interpolating the difference between 23mm and 25mm tires on the 404 creates a line that predicts where tires of widths between those two would fall.  Create the same line with the Rail 52, and you see that for any given actual inflated tire width, the 52's "seconds saved" line is above the 404's.  Of course we wouldn't be us if we didn't point out with equal emphasis that the 34's "seconds saved" line is below the 3.4's, so by using the same metric, a 3.4 is a little bit faster than a 34 for any given inflated tire width.

The current trend is absolutely for wider tires.  Note that when we decided to test two tire sizes, we chose a 23 and a 25, not a 21 and a 23.  Wider tires have been shown to have lower rolling resistance at equal pressure (don't worry, we're building a better mousetrap to measure that), and as many people have learned, offer advantages in both comfort and handling.  Inflated volume also has serious ramifications for what tire pressure to use, which we will discuss in much more detail later, but the strange looks I've gotten for the past two years when I tell people what psi I use now make perfect sense.

There is a terrific amount of interrelated data that comes out of this, all of which will come out over the next several installments, but for now the myth (if there really was one) that wider tires are aerodynamically faster is busted.

great job Dave, nice work. Zipp recommends 23mm tires with their 404 wheels and a 25mm tire with their super9 carbon clincher disc as they say that those wheels were designed around those size tires. So its not surprising that you got the same data that they probably got for the 404. Also, good point in that there is no such thing as the "eyeball wind tunnel"Ian

ian spivack

Look again Brian. The size of the dots represents the depth if the rim not the width of the tire. We use color to make it a little easier to see how 23s and 25s set up on each. Zipps for example at both green dots, Enves both yellow. The darker hue in each instance is the 23mm tire and the lighter hue is always the 25mm tire. I'll add a line of copy to the graphic that explains this more clearly than the legend alone, which doesn't quite get us there.

Mike May

Dave K

Bill – Great question with a somewhat complicated answer. We started with an 18mm inside width because of Mike. He got a set of wheels built with WI H2/H3 hubs and Velocity A23 rims (18mm inside), by way of testing other interesting stuff in the market place. After riding them for a little while, he was sold on the wide rims. I tried them and loved them. After using them more and more in situations where cornering was at a premium, our mutual thought sort of became "aero schmaero – these wide rims make you FASTER." So the Rail was born out of the thought of marrying that ride that we loved on the A23 with as aero a section as we could do. Even the name speaks to this – it took about .04 seconds to come up with the name, and it's all to do with cornering. We were after the best wheel around the entire course, factoring everything in. The other thing we noticed as we switched to riding wider rims was that we went back to preferring 23mm tires, from the 25s that we'd both been riding. Having studied the whole thing from a lot of different angles, that intuitive move back to smaller tires made perfect sense. On a Rail, you're getting the effect of having a tire somewhere up to 2 full sizes bigger, depending on the rim you're coming from. If you like 25s on your 14mm inside width rims, go down to a 23, lower the pressure about 5 psi, have a smoother ride, and better cornering. And yes, given equal tire dimensions, it sure does look like the Rail is the go. Something like a 22mm Continental Attack front could be the ultimate slayer tire on a Rail. Steven F – You aren't in the market for carbons, that's fine. We aren't offended. It's okay. Notice that our baseline alloys are wheels that perform well in these metrics, not the slowest dog of a wheel we could possibly find to show off a big yawning gap. We build a lot of nice alloy wheels. A huge part of this is giving our alloy customers great info too. This entire story isn't just about carbons, the info here is directly applicable to alloys too.

Dave K

Dave K and Jay M, thanks for sharing your thoughts. Having carbon wheels is important for many people, some of whom you pointed out in your posts. Whether you're placing second in national championships or a former cat 1 racer, seconds do make a difference. I might be repeating myself, but for the other 95% of us, a carbon wheel might save seconds over a long ride or a strava time but won't provide any other meaningful benefits other than feeling good with such wheels.All that being said, the Rails are probably the best deal for new carbon wheels in the market today. It's the only thing pertaining to cycling that I haven't upgraded to yet, for the reasons listed above and the cost associated with it. But maybe sometime in the future..

Steven F