The cost of water bottles
7.7.03 by John Cobb
(www.slowtwitch.com)

The pursuit of better technology is a humbling experience. For more than a decade I’ve traveled to wind tunnels to perform tests on bikes and race cars and the athletes that ride and drive them, all in an attempt to demystify the conundrum of air in motion. The aerodynamic cost or benefit of water bottles has been a popular topic on internet forums in recent months, and my experience in testing them—the subject of this article—was humbling because my results differed from my preconceptions, and from my public comments on the subject prior to doing the testing.

Back in '86 or '87 I tested water bottles in various locations on bikes and we learned some good things. Almost all the bikes at that time, however, had round tubing. Most of the bikes were made of steel. That tubing was not very large and the down and seat tubes were only just over an inch in diameter. There were a few aero tubes out of steel, but they were small—just under a half inch wide by one and a quarter inches tall. This is half the size of today’s aero aluminum and carbon shapes, in both the X and Y axis. These smaller tubes didn't test well with standard round bottles attached to them. We also didn't know anything about the relation of side winds to overall drag.

As the interest in the knowledge of aerodynamics picked up we were testing all kinds of things: equipment, body shapes, wheels, frames, helmets, and just an ongoing list of cycling-related products. As part of all this testing, a few—most notably Jim Martin—developed computer programs that helped us translate the effect of drag to one’s bike time. I believe these time/drag calculaters are getting pretty good, and I’ll apply some of them further in this article.

This most recent testing on several water bottle configurations took place at the Texas A&M wind tunnel. I chose a Quintana Roo Tiphoon in 55cm, which in terms of shape and style is a pretty representative bike. It had shaped aero tubes and was set up for triathlon racing, with Mavic Kysrium wheels front and rear, Syntace C2 clip-ons, pursuit bars, bar-end shifters and a round seat post with about 6cm showing. My test pilot was Bryan Cowan, who runs our Shreveport Bicycle Sports store. He's 5'10" and weighs about 150 lbs., races at the Ironman distance and occasionally joins me for a Krispy Kreme donut.

This was a pretty typical race setup, so as a reference point I started by testing the “base line” bike, with no bottles or cages anywhere, just rider and bike. I had decided that for this and all the water bottle tests I would use a "bell curve” to try to estimate a real-world effort. I estimated, in other words, the percentage of time that I guessed each wind angle would be felt on the bike during a representative ride, so as to approximate the winds faced in a typical race. I used "0" yaw (meaning the rider would face no side winds), all the way up to 30 degrees of yaw, at 5-degree increments. I decided that 0 = 15%, meaning that no side winds would be experienced by the rider during 15% of his ride. He’d experience a 5-degree yaw 20% of the time (5 = 20%), and the balance of this hypothetical ride broke down this way: 10 = 20%, 15 = 15%, 20 = 15%, 25 = 10%, and 30 = 5%.

As further explanation, the bottles referenced in the testing below were only the standard, round bottles you're most used to, in the smaller size (not the tall, 25 oz. bottles). The only exception to this was when we tested a front bottle, and the one we chose was Profile Design's between the handlebar model. Our reference to "high" and "low" behind-the-seat bottles denotes the two different styles of bottle carriers today. By "low" we mean those in which the tops of the bottles are at about the same level as the top of the saddle, and "high" means the bottom of these bottles is almost at the height of the top of the saddle. In both cases we tested two bottles side-by-side in their carriers.

One of the more impactful things we have learned over the years is that small changes in one area of the bike affects other areas, e.g., a rider’s legs might change the results of how wind affects a particular bike tube. To be as real-life as possible, then, I placed Brian aboard for these tests, and on a bike set-up that was not tuned for minimal drag, but in a good, comfortable and powerful Ironman set-up. The baseline run for the bike and rider was 7.537 lbs. of drag at 30mph, and I felt this was pretty typical of the drag a good bike and rider would generate for that distance (except for the difference various aerodynamic wheels would represent). This means that every exposed square inch of surface, bike and rider, has a force of 7.537 lb. to overcome at this speed (I realize that 30mph is not a typical Ironman speed. It is the speed used in wind tunnel tests performed by most of the industry, and we use it to be able to compare with the studies we and others are doing).

There were several things I wanted to test: standard water bottles, front bottles, Camelbacs/Hydropacs, rear-bottle setups and aero bottles. I didn't quite get it all done—the economics of $500 per wind tunnel hour impinges on the degree of comprehension in our tests—but I did test the things I felt were most important. We realize that people make important equipment decisions based on testing like ours, so we spent a good bit of time trying to be sure the results we did achieve were pretty accurate.

I'm going to publish two sets of numbers below, one is for a 40k distance using a 225-watt average and the other is for an Ironman distance using a 150-watt average. I think these are pretty typical numbers for a decent age-group athlete over these two distances. The drum roll please…

Ave. Drag
40 km
112 mi
base bike, no bottles
7.537
1:07:26
5:56:48
down tube bottle only
7.370
1:06:58
5:54:29
seat tube bottle only
7.433
1:07:09
5:55:22
bottles on both tubes
7.598
1:07:36
5:57:38
Profile bottle only
7.337
1:06:53
5:54:01
Hydropac w/40oz. only
7.556
1:07:29
5:57:01
Never Reach only
7.561
1:07:30
5:57:07
behind-seat low bottles
7.658
1:07:45
5:58:27
behind-seat high bottles
7.578
1:07:32
5:57:27

What does all this mean? Well, you need fluids to finish triathlons, and it turns out that having a down tube bottle on an aero frame isn't all that bad. It also means that I, once again, must eat humble pie, because I have been preaching that frame bottles are bad. Interestingly enough, having both down and seat tube bottles is in fact bad, so I was half right. I was surprised how effective the down tube bottle was. My guess is that it breaks the air around the seat tube, so the bike acts more like a frame with no seat tube, so, less drag (Softrides are examples of this phenomenon).

I was initially skeptical about these results, so we got out the smoke wand and put some wool tufts on the frame in various places. If you videotape the smoke and slow the replay you can see how all this might work. I could not see it by just watching the smoke, hence our need to slow everything down. Then you could see the interaction of the tubes, the bottles and the rider’s legs. I went back through some of my early years testing and realized we never tested frames and bottles with riders—only the bare bikes—so I'm pretty sure it's the rider that makes for a lot of these changes, combined with the much larger aero tubing.

What about all the different front- and rear-mounted water systems? We’ve got more testing to do in these areas, but the benefits of carrying enough fluid so as to keep properly hydrated far outweigh the aero differences. I think the handiness of the "Never Reach" system, as an example, makes these something to consider—whatever system makes fluid easy for you to drink from, and doesn't eject your bottles, would be a wise choice.

As always, I plan to do some more testing and try to continue to unravel some of these mysteries. I didn't get the aero shaped frame bottles tested, for example. I had done a good bit of that a few years earlier when I was designing a new frame for Lance, and found that an aero bottle worked very well on his seat tube. But aero frame-tube bottles aren't readily available at this time, so I've deferred that testing for now.