Hydration Pack or Feed Zones, Which is More Aerodynamic?
Anybody who has done a 100+ mile gravel race knows that how you are going to get enough water is a big discussion topic. There are so many options, picking up bottles along the way, hydration pack, hip pack, frame bag, bottles in the pockets, and every combination you could imagine. At last year’s edition of SBT GRVL there was quite the controversy caused by a few of the top pros opting for a hydration pack rather than stopping at aid stations to refill bottles.
SBT GRVL 2022
The riders that opted for a pack almost exclusively seemed to use the USWE Outlander Pro. There were complaints that since they didn’t stop at the aid stations to get water, there was an unfair advantage. The riders with the pack seemed to rebut in unison that they had to carry extra weight all day to be able to do that so it wasn’t an advantage. STB is such a heavy climbing course that it is hard to tell who was right. We had some fun internal debates here at SILCA which was the faster option. We decided that the only logical way to decide it was to head over to the wind tunnel and put it to the test. How much of a penalty is the pack and is it enough of an aero/weight penalty to make it worth skipping the aid stations?
The Test for Aerodynamics
While aero is certainly not the only factor at play here, it is the one that is the most difficult to quantify since we can just weigh a pack and see how much of a difference a couple pounds makes. We took Dylan Johnson with us to the tunnel and brought a whole host of packs, bottles, etc. Here we wanted to isolate what the aero penalty might be for running a hydration pack.
The three hydration packs we brought were an Osprey Raptor with a 2.5L bladder, a USWE Outlander Pro w/2L bladder, and a USWE Hip Pack w/1L bladder. First up was the Osprey bag and we got just 2.1w of a penalty at 35km an hour. Not a bad first result and it was looking promising that the bag might be worth running.
Next up was the USWE Outlander Pro. After we got through the test, we were a little blown away. It was 1.3w faster than not running a pack at all at 35kph. The hip pack was up next at a 1.8w advantage, but it is only 1L of water so that might not be enough water to be skipping more aid stations. It was also not the fastest way to carry an extra liter of water.
These packs got us thinking, “what about when you get in an aero position on the bike, does it expose the bag to the wind more?” We put Dylan in a tuck position with the pack on his back and it was actually 3w faster than just being in the tuck position. When we looked at the photos of the position the answer became pretty clear that it was filling in the gap between his helmet and back. It was acting a bit like an hold school aero helmet with the long tail.
The fastest test we saw for hydration was adding a liter bottle to the middle jersey pocket which was a 3.6w gain. If you are going to run just one extra liter, the bottle in the pocket was certainly the way to go. Along these lines we also looked at running liter bottles instead of the 750ml bottles which was a 1.4w gain.
There are a lot of numbers here but we have a full graph to help walk through how each tested.
*The baseline position was two 750ml bottles on the frame with nothing else
**Positive numbers are an aero penalty over the baseline, negative numbers are watts of aero savings compared to the baseline
***The "aero tuck" position with the pack was compared to just the aero hoods position with no pack. The goal was to identify if once in an aero position the pack would be a positive or negative and it was more helpful with the aggressive position.
If the course was flat, running the USWE pack would be a no brainer. It is more aero and allows you to carry enough water to not stop at the aid stations. That obviously forces a tough decision for the other racers to go without water or give you a gap. With SBT, there is enough climbing that a few pounds can make a big difference.
We decided to take Keegan’s Strava file and input the course into Best Bike Split. While it isn’t going to take into account drafting, it will give us a good comparison if the entire thing was a time trial so while it isn’t perfect, it is about the closest we have. Keegan was showing a 268w normalized power for the day and we are guessing around 150lbs with the helmet, shoes, kit, etc. We plugged these into best bike split with the CdA we recorded without the pack and this came out to 6:25:05, just about 10 minutes slower than his actual time. Considering he wasn’t on the front the entire time, the group was able to go faster so this looks like a great baseline. Since Best Bike Split gives you two CdA options we could enter the aero hoods position and hoods to simulate a difference between different positions.
The next step of the simulation was to change the CdA by the amount we saw in the wind tunnel when adding the pack in both the hoods and aero hoods position. This was a little bit faster, but when we added the 5lbs that 2L of water plus the pack would be, the result came out to 6:26:33 or 1:18 slower to run the pack. Now it really gets into how much time are you at the aid stations, how much do you get from working in the group, etc. There was one piece that we didn’t consider for this simulation and that was that the pack wouldn’t be full the entire day. We assumed that the average weight of the pack throughout the day would be 3lbs.
When we re-ran the simulation, the estimated finish time was 6:25:05. The exact same time as no pack at all, to the second. 6 plus hours in the saddle and run the pack or not in this simulation was an absolute wash.
Who Wore it Better?
Running the USWE Outlander Pro is absolutely an advantage for a top racer at SBT GRVL. Not only was it a wash on course, you got to skip the minute or two that you would have been stopped at aid stations. Finally the biggest part of the equation is that when you are choosing equipment for race day you need to focus on where the race is going to be won. SBT GRVL isn’t going to be won in the first 20 miles, the winning move is likely made in the last 20 or so miles. If you are running a pack, its probably empty or close to it, meaning you get the advantage with almost none of the weight penalty. The last 11 miles of the SBT GRVL Black course are also mostly downhill or flat which is how Keegan averaged just over 29mph for the last 11 miles.
Other Potential Gains For you
Looking at the simulation we ran, it is safe to say that it was the right call to go with the USWE pack, but doesn’t that mean everybody should go out and buy that bag for this year’s race? Maybe not. If the goal is to get 2 more liters on the bike, our test would show there might be better options. Certainly, starting with a liter bottle in the back pocket is a no brainer for performance. That was 2.5x the gain that the USWE Outlander Pro was. That still leaves you another liter short of water though.
If you carried a third bottle, you could certainly stop at one of the early aid stations and make it back to the group after filling just one bottle. That could be a great way of going about it since after all the race winning move isn’t going to go away 80 miles from the finish. The other thing that was a good option was the USWE hip pack with a liter bladder. It saved 1.8w at 35kph. The debate here is, are they cumulative? That was a test that we didn’t run, but if you got the same weight and a 5.4w advantage instead of the 1.3 from the pack alone, surely that would be faster. Does the combination of the bottle in the pocket and the hip pack equal the sum of its parts, is it 1+1=3, or does it somehow negate the benefits of both?
When you look at the test of all hydration options, one thing you will see is that the bigger the bottles were, the faster the bike got and removing them all together was a noticeable penalty. This result comes with a disclaimer that our test was done with an aero optimized frame with a really wide down tube. The combination of a wide tire and wide down tube likely means that there was no flow attachment on the down tube. Giving a bottle there would give something for the air to reattach to and clean up the back end just a bit. This is not likely to be repeated on most frames and really is a testament to how well designed the Ostro Gravel really is.
There is also a very real possibility that the fastest possible option is something that we didn’t touch on at all. There is the frame pack option, two bottles in the jersey pocket, or you could go full aero nerd and put an aero water bottle on the top tube, etc. Feel free to let us know what your fastest set up is going to be and luckily this is why they ride the race. At the end of the day somebody has to put the power through the pedals to come across the line first.
I wonder how much better the airflow over the pack would be if you ran it under the jersey.
I typically race with some pickle juice in a low profile flask in my rear pocket. I specifically got the low profile flask to “not mess up my areodynamics”. Sounds like i need to put the 5 Ozs of pickle juice into a large bottle. A nearly empty bottle provides the aero benefits without the weight.
@Zach that is the brand of the frame bag that we tested
What does the outer shell part mean when you mention the frame bag?
What if you sew an attachment on bib shorts to hang a hydration bladder? One could imagine sewing a thin neoprene sleeve to bib shorts to house a 2L bladder. You are then saving the weight of the pack and the aero-disruption of the straps, etc. Having all of that inside your jersey has got to be more aero.
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