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.
16 comments
one interesting observation is the slope of the various lines between the same rim and different tire width. It seems to me that also has some info in it. The less negative the slope the less the impact of the change in width, so for instance the change between the tires on the Rail 34 is more than on the Rail 52s for instance. This seems to have some aerodynamic info on the way the air travels around the tire and rim combo? Could it suggest that it could have an impact on the yaw effect as well? Is this something you can comment on?
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.
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?
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.
Steven F – One man's infinitesimal is another man's world championship – be that an actual championship of the rainbow stripe variety, of the Tuesday night variety, or of the beating an hour for a 40k TT variety. While we were the first to present time gains versus actual legitimate options (which probably pissed/pisses off a lot of people who prefer to show their wheels against the market's slowest possible wheel), and to test against a benchmark that we weren't guaranteed and rigged to beat, our point is neither to show that the gains are dismissively small or a mandatory benefit. Your drafting comment is off the mark by a ways. Your jersey drafts way more than your wheels, as does your helmet. When I check my power files of races or group rides when I use "fast" wheels versus "normal" wheels, the one thing that jumps out is the amount of time I spend coasting when I'm on fast wheels. To me, who's been paying close attention to this on many hundreds of rides over tens of thousands of miles for the last few years, the story of aero wheels is told in the gap that closes in front of me but opens behind me on a balls to the wall fast corner, and the shocking number of times I have to drag a hand out of the draft to check my speed in a pace line – far more dynamic stuff than a 40k TT, one of which I've never done and have no plans to do so. The 40k TT is actually just as crappy a yardstick of aero as the robotically-steady-state-incline-plane calculations that are used to express weight effects on climbs (nonetheless, aero is still more important than weight).The jersey and helmet examples are rider-specific examples that are maximized. While at the wind tunnel, I asked the guy who's seen more of it than anyone (who we will incidentally have a fascinating interview with posting in a couple of weeks) specifically about the helmet one. His advice? Unless you test it on yourself, get the one you think looks pretty or comes in the color that matches your team jersey. One helmet could save you 20 seconds and cost me 20 versus the helmet that we both used to wear. A faster helmet isn't always faster, but a faster wheel is always faster. Nothing you as the rider can do to lose the gain from a faster wheel except use your brakes. The price spread in our wheels that we measured is is $750: the Kinlin-based set costs $775, the Rail set costs $1525. I believe a set of the Enves would have been the most expensive wheels we tested, at something like $2750, with the Zipps not far below that, if at all. So in the case of our wheels, no, it's not a premium of thousands, though there is a premium. But the point regarding that is that we're doing the testing, we're showing people as accurate a relative ordering of the wheels we offer as possible. Even the manufacturers of the alloy rims we tested haven't tested them, it was left to a relatively small company to give people that info. We are not in the top ten or 20 or whatever vendors of Pacenti rims in the country, I'll tell you that. Part of it is that we have a broad range of alloy wheel products that we use to try and tailor to the rider's situation as possible, the other part of it is that people have been swinging that bat longer than we have. Our alloy choices even six months ago were limited to one option. We're gathering steam in that venue, and trying to offer a unique service to people evaluating what carbon OR alloy wheels they want to be. If someone's in the market for alloys, we've got the options and hopefully our having done this stuff puts us high up in the decision set. If it's a paradigm where people are creating their decisions based on info we've provided but buying from someone else to save $10 or $20, that's fine (he says sarcastically) but we won't be spending s-piles of money on projects with no ROI.Another point from which we won't be dissuaded and which we're constantly seeking ways to show is that Rails are the best riding wheels you're going to find. As discussed at the top of the comments, one thing this takes is lowering your pressures. There are a lot of people who've been unimpressed with the ride initially, the first question of whom I ask is "what psi did you use" and they tell me some variation of "110, which is what I always like!" Well, effectively what they just did is ride the wheels they like at 110 at 120 instead. The lightbulb moment usually comes about 4 minutes later, when they've had a chance to dump some pressure and learn "holy crap, that IS nice!"So yes, there is a benefit to anyone. You with fast wheels is faster than you without them. There are guys I race against who train harder and got better tickets in the genetic lottery. Equipment can't erase the gap, but if I'm going to make the break or have my best finish, hell yeah I'm giving myself what help I can in doing that. Okay, tank on empty. Brian – Mike will have to address that, but if Weird Al wants to make a song about us, that's cool. I'd rather Criminal-era Fiona Apple wrote a song and shot a video about us, but sometimes you take what you get.