I’m thinking about adding more hydration to my setup this year for gravel, except I dislike wearing a hydration pack (both feel/weight + I feel like I’m covering a large cooling surface).
Anyone studies out there on hip packs vs hydration packs? Seems like wearing it lower would benefit not only cooling, but bike handling too. 
Third alternative would be tri-cages or something, but I’m sure I’ll get run off the road at a gravel race if I show up with those.
FWIW I saw quite of few of these at the gravel race I did this past weekend.
Definitely personal preference here!
But for gravel specific I would personally go with a hydration pack 
The hip ones get a little heavy on my waist and move around, plus the pack is more aero and stays firm on you.
You can find pretty light ones from Camelback that don’t take over your back.
I’m not an expert but I’ve tried different versions of packs.
The regular packs like camelbak (I have a camelbak chase vest) and Uswe that sit up high are comfortable but in the heat it’s not my favorite thing covering part of my back.
Hip packs. The benefit it sits lower. Mine is evoc hip pack and it has a bladder but also an option for 2 bottles if you prefer. The downsides are you have this big strap across your waist. I don’t notice it too much but sometimes do. The pack doesn’t really bounce much but I do find on long rides it will slip lower since I don’t exactly have hips where the strap would rest.
I tried camelbak skyline Lr10 which is sort of a cross between the 2. It sits lower but has straps and a waist belt although that belt is not as thick as the evoc as it’s just secondary support. The problem with the pack is it’s huge. It fits nice. Holds 100oz of water and has big storage area. But it’s too big for the heat as it covers too much area.
I’m doing unbound 200 this year and am considering the following option. I can fit 3 bottles in cages on my bike. Then I bought 2 collapsable soft bottles by hydrapak. I can put 1 of this in my frame bag. And if I need another put it in my rear jersey pocket.
I’ve got quite a collection of bags from over the years of MTBing, commuting and bike packing so have quite a range of styles.
For a gravel race I couldn’t get the bum bag to sit in a good place on my hips to be comfy. Plus with a bottle and snacks it’s pretty high up and it feels like it would create a bunch of drag.
I think for racing I’d go for a light weight hydration bag/vest - vests will sit higher and thinner than a MTB bag so that would be where I would look.
Might be worth seeing what bags riding buddies have and trying out. If you were in the south west of the UK I’d offer but it’s quite a ride for you to get here!
Also, FWIW, I love my USWE pack. I’ve done 9+ hour races and it’s never been an issue for me. They have that thing perfected.
You forgot aero…see Dylan Johnson’s video below.
Plenty of people use them…just make sure you have tested the system thoroughly and you know they won’t jettison your bottles. If you jettison your bottles in a race, there is a good chance someone will try and run you off the road. 
I really really really like my Solomon running best for hydration. I think it is the ADV twelve or something. It spreads the weight out super evenly making it feel weightless, even when stuffed to capacity. Tons of easily accessible pockets while maintaining a sleek and presumable aero shape. Also doesn’t seem to hold heat in much.
@Power13 already posted a link to Dylan Johnson’s Youtube video where tested a hydration pack in the wind tunnel.
To add to the discussion, coming from the mountain biking side means I have grown up wearing cycling-specific backpacks to support one-day trips. Modern cycling-specific backpacks come with cooling features, which work well. The big difference between hip packs and backpacks (for hydration or otherwise) is that the latter can accommodate hydration bladders of up to 3 l. To my knowledge, hip packs max out at 1.5 l, i. e. half. In many circumstances, 1.5 l is nowhere near enough.
There are tons of cycling-specific backpacks in various sizes. My favorite brand is Deuter, but there are others. I currently own three backpacks of different sizes. My small one has a capacity of 12 liters (extensible to, I think, 16 liters), the bigger one has a capacity of 30 or 32 liters. You can get smaller ones and even slightly bigger ones. The small ones is perfect when e. g. riding to a race with a change of clothes, some food, etc. The bigger one is for mountain bike tours where I might pack additional stuff like a first-aid kit and the like.
On a gravel bike especially, I would welcome the ability to drink more safely, which means I will likely drink more regularly. That’s a huge deal. On really big days, I will combine two bottles with lots of carbs with a hydration pack. Basically, I tage small swigs of sugar water to take in carbs and use my hydration pack to stay, well, hydrated.
I recently fought the same battle, here’s how it played out so far (it’s never over, perfection can not be achieved).
I personally hate having anything on my back. Not just while cycling, but even in a daily life. And I’ve been always striving to stick to my bottles and nothing else (and there were few occasions when I regretted that). But I was doing LT100 few weeks ago, and my bottles-exclusive fueling strategy wasn’t coming together. Well, now when I look back I see that I could totally get away with two bottles on a frame and one in the jersey pocket, but back then I had some doubts. Given that it was 3 or 4 days before the race, I went for some old-school local shopping. And I thought I’ve found a holy grail, something I’ve never seen before in any reviews. It was a Fox Lumbar 5L Hip Pack. I had experience riding with Rapha hip pack (meh) and Dakine Hot Laps (meh either). This one immediately hit home for me. It literally hugs(!) your hips and doesn’t bounce on them, while being soft and comfy. As a test I used it on my last indoor ride, dialed the belt length/tension and was good to go. Or so I thought.
Only during the race I realized something. The hose was too short for me (I’m 193cm / 6’4’’ tall). So I had to put my head down to use it (and that’s where indoor ride wasn’t a good enough test, because obviously who cares then). Basically, you can forget about drinking from it on a descent. No big deal for one ride, but you would expect more from a gear that costs over a hundred USD. After the race I was hoping that there will be a poor man’s solution to the problem (simply a longer hose), but surprisingly enough hydrapak doesn’t make those. It’s standard size only. Bummer, because if not for the hose length - I really loved it while it lasted.
Trying USWE Outlander seemed inevitable (still kinda does), but I had one more thing to try. I have Salomon Adv Skin 5 hydration vest from the days when I was running on trails, so I bought Hydrapak 2L bladder to see if I can ride with that. And then maybe upgrade to uswe if the whole concept of a hydration pack even works out.
So far (one race, few long weekend rides) I really like it. I’m quite surprised, tbh. I think the main reason why it doesn’t feel bad is because it sits tight (literally like a second skin layer) and never bounces. Even though the vest spec says it’s compatible with 1.5L bladders, 2L fits into it nicely. It’s even 50g lighter that uswe (210g versus 260g)! The weight weenie inside me is very pleased. One potential disadvantage might be that it takes a few extra seconds to put it back on after a refill at the aid station, because it uses straps instead of a buckle. If that will ever start bugging me - I’ll go for uswe. But for now I’m settled.
One thing I use is a top tube frame bag for carrying an extra 1L bottle. I’ve been going back and forth with what to bring for a gravel ride/ race coming up. I’m going back to the trusty old top tube frame bag.
My personal pros and cons of a hydration pack:
Pro: less stops, easier to fuel/ hydrate while on singletrack or in a group.
Cons: don’t know how much fuel/ hydration I’m getting, harder to refill than a bottle, can be heaviest option.
Frame pack with 1L bottle.
Pros: nothing on my back, easier to refill all bottles, I can see how much sugar mix I’m taking in and drinking overall.
Cons: more weight on my bike and doesn’t look cool, have to stop once or twice for a 5+ hour event.
Hydration bags work don’t get me wrong but being a one man team. I don’t have extra hydration packs to leave at aid stations. I usually try to get to the finish as fast as I can and the ease of refilling 3 bottles at an aid station just makes more sense. It’s a PIA having to refill a hydration pack at a busy aid station. Plus I can tell you how many times I finish a long ride with a bunch of sugar mix still in my hydration pack. Maybe I haven’t mastered how to use it. I personally prefer the bottles because I can easily see how much sugar mix I’ve taken in.
My own opinion 
I pretty much wear a uswe pack for any ride/race over a couple hours. During the cooler months, I could manage many rides with just bottles, but I barely notice it’s on and I’m a big proponent of training with the gear you’ll be racing with. I’ll even wear it on the trainer inside sometimes. I’m not a fan of frequent stops when training, so being able to run a pack + a couple frame bottles gives me decent range if it’s not too hot. For racing, I have 2 packs and extra bladders, so any race that allows a crew gives me a very quick stop with just a pack swap (Unbound, leadville, etc.). Filling at aid stations can be a little trickier vs. bottles, but I’ve got it down pretty good.
I’ve thought of getting a hydration vest for my longer training rides where planning water stops is difficult.
Only issue - as a road cyclist, aren’t these against the rules?
I love my uswe zulo 6 - has all my tools, spares and 1.5L. most of my rides are no less than 3 hour MTB rides on natural trails with lots of tech riding. I just live having 1 bag that has everything in it that I can take with me.
Only downside with a hip pack is you can’t utilise jersey pockets - This is a big downfall IMO. I had an outlander 2 but it was too small to have water and all my spares. if I did it all again maybe i’d try to bigger outlander hydration pack.
USWE pack.
Hot races put in freezer (its liquid by the time you use it…if your bringing a pack, you are bringing bottles. Drink bottles then pack. Pack will keep you cool.)
Cold races put in hot. I keep mine unstrapped and it doesnt flop at all. I dont like having something strapped across my chest.
I’m an outlier here, but I use a Nathan running pack. It sits up nice and high, very firm (designed not to move around when running) and it has pockets up front that you can LOAD up with nutrition, etc. Probably not as aero as the USWE, but I have two of them and so far they are fantastic!
In addition to not being able to use jersey pockets I don’t like the big strap around my waist. To me it’s more of a pain than a pack on your back. Especially if your hip pack has a bladder in it making it heavier. For me it slowly moves lower hips the longer I ride. It’s annoying.
I’m thinking of trying the Uswe outlander pro 2l for unbound this year. But thinking putting just water in it. And have my bottles have my liquid nutrition