Does Rear Hydration System Effect Aerodynamics?
This came up in the "Race Day Setup" thread http://members.endurancenation.us/T...fault.aspx and I figured this was worthy of it's own thread.
I recently added a single bottle holder to the back of my bike to race IMNYC. I am targeting to race at ~210W (301 ftp), I weigh ~185lbs and am looking for around a ~5:35-5:50 bike split. I figured I would use my Speedfil A2 torpedo mount for re-fillable water. I will carry a 3hr bottle of Infinit on my downtube and I thought I'd carry my 2nd bottle of Infinit and tire stuff in the back (my spare and tire levers fit nicely inside the hole you can see in the bracket). At ~3hrs, I'll toss my first Infinit bottle and replace it with the one from the back and not have to stop at BSN (leaving an empthy rear cage for the 2nd half of the bike). BSN, will liklely take me 1-2 mins, so if the bottle slows me down much at all, probably worth leaving it in BSN and getting a quick stretch and a cold bottle. I got some negative feedback in the Race Day Setup thread to the likes of the following and it got me thinking...
1) Doesn't look "Pro", so Coach Rich will think it is Tri-dorky.
2) Why carry the extra ~1.5 lbs of weight (Infinit/bottle/cage/bracket) for ~3hrs just to have a warm bottle of Infinit as the reward
3) Why carry a round bottle behind your seat creating all that extra drag?
My thoughts on the above:
1) I don't really care about looking "Pro". I simply want to be able to go absolutely as fast as my training and ability will allow.
2) I figured the extra weight for ~2.5-3 hours wouldn't change my speed by all that much. On a super hilly course, you would definitely not want all the extra weight. But I'm doing IMNYC with gradual ups followed by gradual downs. I figured the extra weight would hurt me a bit on the ups but help a bit on the downs leading to a mostly negligible total time effect. Am I crazy? I did my race rehearsal fully unsupported without stopping 1at all by carrying all my extra water and spare Infinit. When I freeze the 2nd bottle of Infinit solid a day ahead of time and keep it in an insulated bottle, it is not too terrible hot at ~11:15AM when I am likely to start drinking it.
3) To me, this was undoubtedly the most important. So I asked my friend Google and he told me that the results are conflicting, but most pointed to no aero drag penalty for a behind the seat bottle setup. This one seemed the most interesting:
http://lavamagazine.com/gear/free-s...z22I138GBQ
Tests on behind the saddle systems 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. 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. (The meter 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’s 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 eventually made foam Dave—our stable and repeatable windtunnel dummy—though he’d 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 so that the air comes back together quickly after leaving your body. There wasn’t a huge variation in drag depending on the position of the bottles behind the saddle; whether high or low, near or far. Many people use these systems for storage of tires, tubes, and cartridges. 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, 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.
Here are a bunch more articles:
http://www.triradar.com/training-advice/the-best-place-for-bike-bottles-in-triathlon/
http://www.slowtwitch.com/mainheadings/techctr/waterbottles.html
http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=1956&printer=1
http://www.sportzhub.com/newsgroups/index.php?topic=3174.5;wap2
http://www.xlab-usa.com/aerodynamics.html (this one might be suspect since it's from a company that makes rear systems)
Here's are some negative articles about behind the seat bottles:
http://www.slowtwitch.com/mainheadi...ttles.html Back in 2003, John Cobb looked at this and showed a penalty of ~1min over 112 miles for behind the seat bottles (based on an avg rider putting out 150W).
http://www.powertri-blog.com/biking...stems.html If you're only going to read one article, this is the most thorough... They built on John Cobb's 2003 work and generally didn't like the rear systems. Commented near the end of article that The Xlab Carbon wing without bags and with CO2 performed pretty well, but condiser these drawbacks: 1) hard to do flying mount 2) risk of ejecting a bottle 3) expensive
http://www.neverreach.com/page/page/281090.htm This is from a company that makes a "competing" rear system... Our testing showed that having bottles behind the your saddle is akin to the flap being up on an airplane wing, which you probably know is the technique used to slow down planes. We found that simply having bottles behind the saddle slowed the athlete down by approximately 1 second per kilometer, or anbout three minutes over an Ironman!
So... What Say you ENers? Will having a bottle on my rear slow me down?
Comments
If you need (or want) that bottle on the back for your nutrition plan, then stick with the setup you have. I think you did a nice job with it.
Long Answer - The problem with any wind tunnel testing is that it is very specific to a certain bike/rider and it is hard to draw conclusion that really apply to every one. With enough runs and other variables just about anything can be proven in the wind tunnel depending on what data is shown. There are even reports out there that try to shoe aero helmets are slower if you look down at your computer too much.
The only ways you will truly know are:
1. Go to a wind tunnel
2. Do a field test
The field testing is actually quite easy and you do some searching on the wattage group. You basically find a loop course where you never have to hit the breaks and there is some elevation change. Then you ride the same loop over and over and over and change your setup in between. Golden Cheetah has an aerolab function that does the hard stuff for you. Then you compare the data and see which setup had the lowest drag.
My advice would be two fold:
1. In IM, I've learned nutrition ALWAYS trumps aero PERIOD. You could make up 5 minutes on the bike with aero goodness, but if can't stick to your nutrition because of it and you cramp, bonk, etc... you will lose way more then 5 minutes.
2. Instead of driving yourself crazy trying to draw conclusions on other peoples data, positions, setups, etc, just go do some of your own testing. Even if your testing isn't perfect, it should give you some indication of what was faster for YOU.
What Matt said. It is very dependent on the rider/equipment/position/etc (esp position). For instance if you are more upright it might shield that area and create 0 drag whereas if you are in an aggressive position the air flow might come right off your back and into the cylindrical shape of the bottle and cause 5-10 watts drag. Hard to know without testing. Just keep it simple. Figure out what you need to bring with you for your plan and strip anything else that you dont need. Aerodynamics are strange and not completely understood by even the best experts so when you get down to the small things there can be contradicting data out there. Minimal setups are almost always best. When you have your plan/gear set just trust it and execute. 1 or 2 watts will not ruin your day but worrying too much might.
Also, if you are really depending on the rear bottle for your race plan it might be risky to keep it in the rear holder as they tend to launch bottles. Just make sure you have a backup plan.
You've certainly done your due diligence, thanks for posting the links. My thoughts:
At the end of the day, the most important consideration is to examine your plan and eliminate as many items and moving parts as possible. In my opinion, the availability of aid stations and BSN allows us to greatly simplify our bike setups, reducing friction and the number of things that can go wrong. I can tell you countless of stories of very important bottles, tools, tubes, food, etc being launch, dropped, lost, etc. Whenever possible I prefer to use what's on the course or, if I'm doing something special (ie, concentrated Infinit) it's friggin' padlocked to the downtube and I plan to stop at BSN to get my reload.
Thanks everybody.
@Matt-- I kind of agree with that statement and I kind of disagree with it. Take the fancy schmansy new P5 for example. If you start with the TT version and then add the front faring that covers the exposed front brake you basically have the tri version and the bike will be faster.
Adding a disc cover onto your rear training wheel will not slow you down, but speed you up.
Those two are obvious, but there are other less obvious ways to fill air "voids" or vacums for dirty air with something that will fill that space and allow smoother air to flow out the back. Filling those voids, "could" make your bike faster. That's one of the reasons the tail on the backl of an aero helmet is so long and smooth. But if you tilt your head down and stick that tail straight into the "clean" air, it will slow you down (why the shorter tail aero helmets got popular).
So, if the rear bottle fills the void, it could make you "faster". If it sticks up and breaks the clean air coming down over your back then it will undoubtedly slow you down. I certainly do not have the time to do my own aero field test...
a) talk in generalities (stuff filling voids = good, adding stuff = bad)
b) speak from anecdotal n=1 experiences
c) take a wild-assed guess
Not sure how much that's going to be helpful to you with this specific question. My best SWAG from what I've read is that it doesn't slow you down at all. But am I willing to bet your bike split in a small number of weeks on it? Nope.
@John, I purposely used the word almost as you are correct, there are times when adding to fill avoid is good and you provided good examples. The other one I can think of is the bottle betweent he forearms.
Yes, P5 is designed to have certain things added in certain spots. I think your last statement sums it up well, and my point was that field testing is not that hard and doesn't take that long....so *IF* you really want to know, you could figure it out in an hour of testing. Otherwise it is all speculation and a lot of "coulds" like you said.
I've spend way to much time reading/thinking about this stuff and just wanted to share my experience/thoughts.
All evidence is that the thing between your arms is Good. The rest is anecdote and speculation. That said, I am quite sure my rear hydration system slows me down measurably just based on observations of my speed on reproducible runs. But it's a double and maybe not as tight as yours against your butt. So let's not worry about that.
But goodness, make sure any key bottles aren't vertical. Calories on the down tube in your own bike bottle is safe. Looks like you have a P3 there, and I have had good luck on mine keeping a bottle of Perform on the down tube bottle holder, even though it doesn't fit right. At least those bottles hold their shape ok, as opposed to the water.
I tend to have two race hydration plans... one for "hot" and one for normal. My "hot" plan involves living off the course because I can get more to drink...which means that I don't carry a concentrated calorie bottle. Just Perform (or whatever) and water except maybe the first bottle. Extra solids are a separate matter. Since I don't really care for Perform, my "regular weather" plan where I don't need tons of fluid is a concentrated bottle on the down tube (usually the 3-hour variety) and water only from the course.
Not going into the aero aspect, from what we've talked about and the potential for it to be a very hot day, I'd stick with your plan to have a frozen second bottle at BSN and ditch it in the back. I also think you can have a faster bike split if you take a minute to get off the bike at BSN and stretch for a bit. You'll be that much fresher than others at mile 90.... and I also agree with Matt A - your nutrition clearly trumps a small aero gain or taking a minute at BSN.... and mentally, picking up that cold bottle will be a nice boost at Mile 60....
My 2 cents....
Since then I have tested with adding and removing spaces. For me raising or lowering my bars 10mm had almost no effect as it was lost in the noise of my tests (and significantly less effect then aero helmet). Based on that I raised my bars, am riding the same bike splits I used to and having better runs off the bike. I also made nutrition and training changes as well, so I'll never know exactly what made the difference.
Really it is the chung protocol, it just does all the hard work for you. Litterally you ride the same loop at last 3 or 4 times which each setup and then adjust the sliders in GC aerolab until you you get the right profile on the graph. Then record what the drag is, and start adjusting for the next data set. I never found a good primer but the Chung Paper/Power Point deck had pretty much everything I needed. The key is just to read and understand how you have to do the test.... i.e exactly the same loop, never brake, weather conditions as stable as possible. Do you baseline tests before and after each other run so that you can make sure they are consistant and nothing else changed.
If I could figure it out, you definitely can.
http://bugs.goldencheetah.org/projects/goldencheetah/wiki/Aerolab
Final Verdict for me: I will be leaving my rack on the back, but will NOT have a bottle cage or spare Infinit Bottle on it. It actually makes a great holder for my spare kit (2 tubes, 2 CO2s and a tire lever) and I think weighs less than my under the seat bag and I'm sure creates less drag than my bag (even though both of these should be miniscule). I did notice that it was harder to get on/off my bike with a bottle back there which is different than I trained with. I think it was JT's post and part of what Coach P said at the 4 keys talk that pushed me over the edge. It should only cost me a minute or so, but I can look forward to the "reward" of a cold slushy bottle and get a quick stretch before I do the 2nd half of my bike. And I should get the added benefit of not carrying the extra 1.5 lbs around for the first 56 miles. This will be important since I have already added more than that to my waist line during my taper... Now, here's to hoping they don't lose my BSN bag... (just kidding and don't worry, I'm not actually stressing out over that.)