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Ask the Engineers: Hydration and Aerodynamics

September 29, 2011 - Filed under: Ask the Engineers

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With Kona just around the corner, we thought it would be interesting to share some wind tunnel tidbits. Since Cervélo engineers have spent so much time in the wind tunnel, we’ve checked just about every combination of stuff we can think of. Of course we keep thinking of more stuff, so we keep going back to the tunnel! Since we are committed first and foremost to helping athletes go faster, here are some ideas to help you find free speed.

Figure 1 Cervélo engineers have tested a few bikes in the wind tunnel over the years.

Figure 1 shows some of the bikes we tested at one of our wind tunnel trips in 2007. Since then we’ve made many more trips and tested many more bikes.
Why should you listen to us? Because we’ve spent a lot of time in the wind tunnel, and what we’ve learned about aerodynamics could fill a library. One of the things we’ve learned is that you have to include the rider to truly capture the aero interactions between legs and bike, body and bottle. Over the years we’ve also learned the value of using foam Dave-an exact replica of Dave Z in his aero position machined from foam- which provides more repeatable data- especially when moving between different bikes.

There are many places to carry water on a triathlon bike. We are interested in the impact each location has on performance. Cervélo has visited this line of inquiry a number of times over the years, as it pertains to bike design as much as it pertains to athlete needs. We tested the now ubiquitous bottle mounted vertically in the aero-bar, and we also looked at mounting a traditional round bottle horizontally between the arms. We looked at various behind the saddle mounting locations, and of course we measured drag with bottles mounted on the frame, varying location and shape. Basically: bottles in front of the rider, bottles behind the rider, and bottles on the frame.

Bottles in front of the rider
We should start with the hydration in front of the rider. Having the most aero solution for bottle storage is no good if you don’t drink. As one athlete put it, “weaving down the road with advanced dehydration is not very aero either.” The big advantage of hydration systems mounted to the aerobars is that you always see it. It’s always reminding you to drink, and you drink more because it’s easily accessible.

We compared two options for aerobar mounted drinking systems- a system that hangs down vertically in front of the head tube (see figure 2) and one that mounts a standard round bottle horizontally on the extensions between the arms (see figure 3). What the testing indicated was that the vertical bottle added some drag depending on the system and shape of the head tube- but not as much as a standard bottle mounted on the frame. The really bad part was the straw sticking up.

Figure 2 A vertically mounted aero drink system between the aero bars increased drag, but less than adding a bottle on the frame.

However, a standard round bottle mounted horizontally between the rider’s arms was a revelation! It filled in the turbulent area behind the rider’s hands and actually reduced the drag. So this set up was not only faster than the vertical bottle, but faster than no bottle at all!

Figure 3 That’s right, adding the horizontal bottle between the rider’s arms reduced aerodynamic drag by a huge 56 grams on average. That’s a power savings of about 5.6 Watts when riding at 40 km/h.

Bottles behind the rider
Behind the saddle bottle mounts have some issues- it’s not so easy to elegantly mount and dismount the bike and you have to get a good strong cage and mount to avoid the bottles being ejected over bumps like vertical missile launchers. However, the number of people using these systems and the mini industry it has spawned testifies to their popularity. The limitations mentioned above and the fact that the bottles are difficult to grab- especially from an aero position- means that these systems are not generally the first choice for hydration, and bottles are probably transferred to one of the other positions for regular use.

There are a variety of tests on behind the saddle systems that show a wide range of results. Some have shown a slight drag penalty and some a significant decrease. Rather than test every product in every condition, let’s look at some general conclusions that can be applied to most cases.

We planned a series of runs in which we varied the bottle height and distance from the rider. To speed up expensive wind tunnel testing time, we fabricated a quick-adjust bracket to mount bottles behind the seat in various heights and extensions (see figure 4). It wouldn’t survive real riding, but did the job marvelously for our testing purposes.

Figure 4 Three positions for behind the seat bottle locations. Of course the metre stick was removed before the wind was turned on.

The data for this particular design showed a significant drag reduction compared to no bottles or cages behind the seat. Of course when we measured without a rider drag went up by an astounding 350 grams . This is an obvious demonstration of why it is really important to test with a rider on the bike. It might seem less obvious to some, but it’s still true that similar aerodynamic interactions occur everywhere between body and bike, albeit often on a smaller scale. That’s why we made foam Dave- our stable and repeatable windtunnel dummy- but he had not been conceived at that time.

You do want to keep the bottles (and whatever else you have stored with them) as narrow as possible to allow the air to come back together as fast as possible after it leaves your body. There was not a huge variation in drag depending on the position of the bottles behind the saddle, high or low, near or far. Many people use these systems for storage of tires, tubes, inflators etc. This looks to be a good location as long as they don’t stick out and grab air.

This data was mostly collected in 2007 with a live rider at 0 degrees of yaw, but if you keep the bottles close to the rider and not too wide the results extend out to higher yaw angles. At any rate, while test results can vary somewhat, the lesson is that bottles behind your saddle do not have a large penalty and, depending on the design, may have a significant advantage.

This series of tests is illustrated by CFD simulations we’ve run subsequently, one of which is shown in Figure 5, which show that the athlete’s body has a low pressure recirculation zone behind the seat that the bottles appear to fill nicely.

Figure 5 CFD (Computational Fluid Dynamics) simulation showing recirculating air in the low pressure wake zone behind the rider’s body, a good place to carry bottles. Interestingly, the streamlines you see here represent just some of the air. Of course you’re completely surrounded by air.

Bottles on the frame
Regardless of the options mentioned above, bikes still have bosses and many athletes opt to carry hydration in the traditional manner. Let’s face it: you might want to carry a bottle on the frame.

Fast forward to January 2011 and LSWT test 1101. Now we have DZ, our “stable and easy to work with” wind tunnel mannequin. As mentioned above, we test a lot. We test our own bikes and prototypes, and we also benchmark our competitors’ bikes as well. But we go farther than that: in January 2011 we tested a batch of tri bikes three ways: with no bottle, aero bottle or round bottle, once in each position, seat or down tube. Because frame shape affects drag differently depending on bottle location, one position is sometimes faster than the other. The images below show bikes set up with the bottle in the most aerodynamic configuration for each. Whether or not you ride a Cervélo, find your bike and bottle (aero or round), look at the photo and install your cage where it’s shown there: it is the fastest location for your bike.

The Cervélo P4 with its integrated P4 bottle is the only case where the water bottle does not add drag and slow you down seen below in Figure 6.

Figure 6 Fastest configuration for every bike. Note that every bike has “no bottle” as the fastest configuration, except the P4, which is fastest with the P4 bottle. If you add a bottle to the other bikes you add drag.

If you use a round bottle on these bikes it is always fastest on the down tube but still adds typically 40-50 grams of drag. (See figure 7.) Note the Giant has no bottle bosses so technically you won’t be installing one unless you’re willing to improvise. We used wind tunnel tape.

Figure 7 Fastest configuration with a round bottle. From left to right: Cervélo P4, Scott Plasma III, Trek Speed Concept, S-Works Shiv, Giant Trinity Advanced.

Next we tested an aero water bottle. We used an Arundel aero bottle for all of these tests but we would not expect the best position to change with other similarly shaped bottles. When the P4 bottle was replaced with the Arundel the drag increased by just 17 grams¹, about half the penalty it added to the other bikes. Some of the lessons from the shielded water bottle on the P4 were applied to the new Cervélo S5 which is built for bottles, and it is the only other bike where the aero water bottle has zero drag and effectively “disappears”².

The aero water bottles added some drag to all of the other bikes but it was roughly half of that of a regular bottle. The best position for the aero bottle on each frame is shown below, and you will note that the best position differs depending on the frame. As a general guideline, it is always best to eliminate as many gaps around the bottle as possible. If you custom fit the Arundel on a P4 to eliminate the gaps the added drag drops to zero³.

Figure 8 Fastest location for an aero bottle. From left to right: Cervélo P4, Scott Plasma III, Trek Speed Concept, S-Works Shiv, Giant Trinity Advanced.

Conclusion
So what did we learn from all of these tests? The big lesson was that a bottle mounted horizontally between your arms (but it must be between your arms, not too high or low) makes you faster- faster than no bottle at all! That should probably be your first choice for regular drinking. The aerodynamics of this position combined with the fact that it is always in front of you reminding you to drink makes it a great choice. The Cervélo P4 integrated bottle or a behind the saddle system is the next most aero choice and both of those are excellent options for storage of spares and other equipment as well. If you have a P4, please come by the Cervélo booth at Kona for free P4 bottle. Cut it open for storage, or continue to drink from it, but either way keep taking advantage of this design!

One final nugget is that we found that a Bento Box behind the stem was a solution that had no drag penalty. However, this may depend on stem & spacer details and how many gels and bars you stuff into it.

We all need hydration and we all have preferences, so we hope that regardless of which set up you favour, this article will help you maximize your triathlon performance. Good luck and Mahalo.

¹ Average drag of LSWT 1101 Run 217 vs. 219
²⁾ Average drag of UWAL 2008 Run 31 vs. 35
³⁾ Average drag of LSWT 1011 Run 509 vs. 513 & 514

26 responses for this Entry

Damon Rinard, Cervelo Engineer says:

April 03, 2012

Hi Michael, Interesting idea. I don't have an answer, and I don't know anyone who's investigated your question. My experience in the wind tunnel suggests it might be close and could go either way! This is exactly the kind of question a smart athlete brings with them when they visit a wind tunnel. Possibly the result could vary with different riders' body shapes. Cheers, -Damon

Michael says:

April 02, 2012

Hi Damon, I have an S2 and am wondering if there is much aerodynamic benefit to be gained by using just one down tube mounted bottle cage and bottle, while keeping the second bottle in my back jersey pocket (not currently using skin suits). I do mostly hard group training rides and Pro I/II crits. Thanks, Michael

Damon Rinard, Cervelo Engineer says:

March 07, 2012

Hi Don, Good questions. Needs a wind tunnel or CFD project to answer them. Cheers, -Damon

Don Lee says:

March 06, 2012

Thanks for this awesome research article. After reading this, I ditched my frame mounted bottle (Profile Design Razor) on the seat tube and rigged up an aerobar hydration setup using zipties and a carbon bottle cage. I also custom rigged a carbon cage right underneath my Fizik saddle by drilling a couple of holes underneath the saddle so that the bottle now points back and slightly up. I figured this would be a "faster" setup than just the typical 2-bottle saddle setup since it's much narrower. Question...do you think this setup might actually helps the low pressure wake reattach better behind the rider since the air looks to be quite turbulant without any type of hydration bottles? Would creating more length (by using bottles and whatnot length-wise) behind the rider help fills the low pressure area? Also, being a detailed oriented person, I bought a stainless Klean Kanteen bottle with a smaller cap and smoother profile for the front aerobar setup. Looking at some of the water bottles I have, some of them have a very large frontal profile and I can only imagine that those would be like a mini-parachute even if they are between your hands/forearms. The 18oz stainless bottle I got has a smoother and more seamless neck to body dimension. Maybe that accounts for something?

Damon Rinard, Cervelo Engineer says:

December 22, 2011

Hi Johannes, We've tested a lot of different shapes for aero bar extensions, but never in a head-to-head comparison. In gerneral though, most riders have the lowest drag and most comfort when the forearm is level and the top of the thumb is also in line. Usually this means ski-tip or J-bend extensions. One tip we like is to place the shift lever in the hand, with the pivot axis aligned roughly with the second knuckle. This often means cutting the extension to bring the shifter into the hand. This takes the lever out of the wind and means you canshift without moving the arm. Cheers, -Damon Rinard, Cervelo Engineer.

Johannes says:

December 21, 2011

Hey guys! Thanks for the super informative article. I really love your scientific approach to look at things in cycling :) I am waiting for my new P3 and I am not sure which extension to use. Any advice? Have you any information weather ski bend extensions have an aerodynamic disadvantage? Thanks in advance, Johannes

Damon Rinard, Cervelo Engineer says:

November 03, 2011

Hi Ron, as you probably know, at Cervelo we develop and test in the wind tunnel lots of full- and truncated airfoil shapes many years ago. While a truncated airfoil is tempting from a structural standpoint, it almost always has higher aerodynamic drag than a full airfoil; it takes distinct airflow conditions and cross section profiles to combine to make a faster shape overall. In practice these conditions are quite rare. However, since Cervelo engineers engineer bikes as a system, including the aero effects of all the other objects like water bottles, spinning wheels and the rider, we have developed a design method that produces different airfoil shapes according to the specific conditions at each area of the frame, including both full- and truncated foils. Basically, we design bikes, not a shape. Because of this system engineering approach, you can see different shapes on the same Cervelo: full airfoils (symmetric and asymmetric) where they make sense (usually head tube, down tube, seat stays), as well as truncated airfoils where they make sense (often the fork crown near the down tube, down tube near the bottle, seat tube near the rear tire). The fastest bike doesn't have just one airfoil shape, but the best shape for the specific flow conditions at each specific location. Cheers, -Damon

Ron says:

November 03, 2011

I realise Cervelo trys many things in the search for the optimum aero shapes. What about the truncated air foil design as per Trek and Scott. Is this real? What have Cervelo found when comparing traditional air foils vs a truncated air foil shape?

Damon Rinard, Cervelo Engineer says:

October 15, 2011

Hi Justin, The fastest bike without a bottle is the Cervelo P4. If you add a frame bottle to any other bike you've added drag. We are working with Honda Racing on a new data acquisition project, but much of the technology that F1 teams can offer bike companies, we already have in-house, developed as part of our Project California. That makes a difference: rather than a hand-off of tech from the F1 engineers to a bike company, Cervelo has those engineers working for us, actually designing all our bikes - so every Cervelo benefits from that know-how, not just a special edition model. Since our engineers are already doing the serious aerodynamic and composite analysis F1 engineers might bring to ordinary bike companies, we already know the engineering design details that can help even the less expensive Cervelos perform better. Once you know how, why wouldn't you? That said, when you spend more for a higher priced Cervelo, you really do get better performance. The S5 is a perfect example: the world-leading low drag and high BB stiffness of the all the S5s is included in even the S5 base model, with more and more weight saved as you go up the thte S5 Team and S5 VWD.

Justin Huang says:

October 15, 2011

Another question, does cervelo work with some formula1 racing team? Like McLaren and Specialized, they have lots of experience with aerodynamics technology, it might helps you to develop a bike.

Justin Huang says:

October 15, 2011

Without bottle, which bike is the fastest?(lowest drag)

Damon Rinard, Cervelo Engineer says:

October 11, 2011

Hi Jason, We only tested with arms horizontal. Reults might be different with arms up.

valdir dos santos says:

October 11, 2011

ola bom dia gostaria de comprar um bike cervélo a p2 ou p3 tenho 3000 mil reais da pra comprar um bike boa tenho 177cm e 85 kg

Jason Hudson says:

October 10, 2011

Does the regular bottle between the forearms stay faster as the hands come up into the "mantis" position? Or does the result only hold true if the forearms are horizontal?

Damon Rinard, Cervelo Engineer says:

October 04, 2011

Hi Alejandro, The Torhans Aero 20 looks like a well designed system. They even gave the straw an aero fairing, which seems like it should reduce the biggest drag source (we've measured up to 50 grams aero penalty for a straw alone!). We'll certainly add this to our wind tunnel list. Thanks, -Damon

Damon Rinard, Cervelo Engineer says:

October 04, 2011

Hi Rex, In University we made a fast HPV (still holds the course record at Fiesta Island, I think). It was a nice low racer recumbent, but without the fairing it was slooooow. We never figured out why, but adding the fairing made it into an absolute rocket ship. What a blast to fly past other racers in a time trial! Cheers, -Damon

Damon Rinard, Cervelo Engineer says:

October 04, 2011

Hi Jason, the Profile Design Aerodrink is a good option - added only about 2 to 8 grams of drag, much less than some other options. If you don't drink regularly from your frame bottle, you could consider replacing it with an aero bottle to save about 20-25 grams of drag. But as you mention for longer events where you might take a hand up you'd probably still want your round bottle.

Alejandro says:

October 03, 2011

Can you test the new Torhans Aero 20 with DZ? There's so much hype about this new system and a lot of triathletes would really appreciate it.

Rex says:

October 03, 2011

Recumbents? Why not? seems such a waste of energy when a better solution clearly already exists?

Jason says:

October 03, 2011

I normally race with a Profile Design Aerodrink style bottle between the aerobars. I carry a second bottle on the downtube (the only mount on the P2). When the Profile is empty, I reach down and refill it with the downtube bottle. If it's a longer race where they hand out bottled water, I can use that to refill as well. This keeps me in the aero position longer and it's also safer as I don't need to reach down and compromise stability. Another trick to do is try to drink on the uphill/slower sections of the course where aero isn't as big a factor as climbing strength.

Damon Rinard, Cervelo Engineer says:

October 02, 2011

Good thinking Jason, drinking is the main trade off. I don't remember who did it now, but someone did the measurements a few years ago. If I recall correctly, even with a frequent "high arm" drinking style, you can't cancel the negative drag of the horizontally mounted bottle.

Jason says:

October 02, 2011

Great analysis. Question about the practical drag in a racing situation. If you have the "torpedo" style mount, you have to get out of the aero position in order to drink. Wouldn't this cause a lot of drag over the course of a long ride?

Damon Rinard says:

October 01, 2011

Hi Ken, The different bottle positions we were able to reach with our adjustable rear system were not significantly different in our tests. However the two bottles were laterally close to each other so we recommend don't let them stick out to the sides too much and you should be good!

Damon Rinard, Cervelo Engineer says:

October 01, 2011

Hi James, We've not tested DZ witha Camel"Front." It's on the list of next time though!

Ken Morris says:

October 01, 2011

Can you elaborate on which rear designs seem to reduce drag? Thanks, Ken

James Liu says:

September 30, 2011

How aero is DZ with a CamelFront?