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.  

Back to blog


Well maybe I am confused. I thought an earlier version of the 404 is used as the reference standard (control). To me that would mean the chart indicating the 404 (23/25mm) is a more recent version of the 404 measured against the reference standard in order to show a time gain. If the are the same why would there be a time gain


Great work gentlemen but as someone who lives for graphs I was repulsed (not really) that you used big dots for the 23 mm tires and little dots for the 25 mm tires. This is inverted and must be corrected before Weird Al finds out and writes a song about your graphing errors to go along with his hit song about grammar errors.


Thanks for putting all this effort to do this research. These results are truly eye opening—that a Zipp 404 is only about 24seconds faster at 30mph over 40k than a SL23-based wheel. That means that instead of finishing the 40k in 49min:42sec on a 404, the same person would theoretically finish in 50min:6sec on a SL23. So what does that translate to in terms of speed differential? 30mph on a 404 vs. 29.76mph on a SL23. That's infinitesimal.And that's assuming that the person is facing winds of 30mph. These differences are exponentially smaller at 25mph and 20mph winds. And other than a time trial/triathlon, these differences don't matter due to drafting.Finally, to add another dimension to this, Specialized has been doing a lot of aero testing in their new wind tunnel (check out their Youtube channel, or search for "Win Tunnel" without a "d'). Some of the results they found: a tight fitting jersey saves about 45 seconds faster over a 40k TT than a loose fitting jersey. A semi-aero helmet saves 42 seconds over a traditional helmet. Et cetera.What this means in my mind, considering the thousands of dollars it costs to buy a deep carbon wheel over a good aluminum clincher, is that a carbon wheel is probably the LAST upgrade I would consider. You would need to be a pro racer to really benefit from these wheels, in which case the wheels are (often) free. So the question is, is there any benefit for the 99.5% of us who are not pro racers, other than looks and the placebo effect of feeling faster?

Steven F

Tim – I get it. The control in this instance is the SL23 rim, which was the slowest of the rims we tested, therefore it became the baseline. The 404 that we used was exactly the same wheel that we used last time. We keep it in a safe place. Even though they have changed the 404, it has 18 spokes instead of the 16 that ours has, we continue to use that specific wheel as a valuable baseline for us. Ron – Yes, the slopes are a little different. In reality, the differences are small. It can have to do with the longer span being better equipped to deal with it, but the reality is that you just don't know. Rim width as a function of rim depth, and consequently tire width as a function of rim depth, is obviously greater with the shallower rims. Had we solely wanted to maximize the aerodynamic speed of the 34, to the exclusion of other things that we know are valuable, we might have shaped it a little differently. As it is, one thing you will consistently hear from everyone who's ever been on the 34 is that it is the most stable wheel made in crosswinds (And more, and more surprising, info on that later-the wind tunnel is now able to quantify side pressure, but it's so new as a measurement and a concept that we are quite honestly struggling with how to present it). We also wanted it to share the 18 mm interior with that the 52 has, because neither Mike nor I have any interest in riding a rim that sets tires up differently then Rails do. So that's kind of a long way of saying "I'm not positive one way or another" in answer to your question. But I think it's got to do with width versus depth, And as I said, the differences are actually quite small.


"30mph on a 404 vs. 29.76mph on a SL23. That's infinitesimal."Tell that to my road teammate, who has finished second at elite nationals TT on two occasions in the last 3 years, once because a schmuck official erroneously banned his TT helmet on the start line.It's all a matter of perspective, right? Many of the intended buyers of these wheels are not pro racers, but are looking for performance improvements of exactly the magnitude offered in a package that allows privateer racing (or hard riding) to fit into their budget.Me, for example: an aging, formerly decent cat 1 (downgraded to a 2) now with a family, still trying to keep it fast in a way that allows me to save for my son's college education. Saving 10 seconds in a TT, going two minutes faster up Mount Washington, not flatting in a MTB race: all of these are still important to me, balanced against cost, day to day rideability, and versatility. It's pretty sweet that the guys at November "get it" in terms of what their target consumer wants, and are not full of ka-ka marketing hype.

Jay M

Leave a comment

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