Why you shouldn’t race with a reservoir or bladder

Several reasons why running packs with reservoirs suck - soft flasks for the win!

Running back pack, hydration vest, whatever you call them, they are an essential part of every trail runner’s gear, especially when you start going longer and do ultra marathons.

Some have reservoirs (also called bladders) that allow you to carry a large amount of water on your back, while some have soft flasks stored on the front. But which is best?

Soft flasks.

Soft flasks are best, by a long, long way. I hate reservoirs and you should, too.

Difficult to know how much is left

Most reservoirs carry two litres, which is a decent amount of liquid.

Now imagine the following situation: you filled up at an aid station a few miles ago and have been taking sips along the way. Your pack is getting lighter, but how much water is left? Have you drank a litre? Maybe more?

Once you start doing trail marathons and ultras, you should have a hydration strategy and plan your consumption. With soft flasks, you can easily tell how much is left – if they are empty or nearing it – and there’s no guess work.

Difficult to fill and takes longer

One thing that I noticed while volunteering at my first ultra recently is how long it took people to fill their reservoirs.

You don’t want to be at an aid station for too long. Personally, I want to get into one and move through it as quickly as possible. With a reservoir, you need to take your pack off, remove the cap or seal, fill it up – two litres takes a while – then reverse the process.

Running hydration vest with two soft flasks
No need to remove your pack with flasks

With soft flasks, the pack stays on you at all times. You can enter an aid station with both flasks ready, fill them up quickly and reinsert them as you’re leaving.

This saves so much time, especially if you are doing a longer race with several aid stations. If you’re spending an extra 30 seconds faffing with a reservoir at each station, it soon adds up and creates a massive difference to make up in the race.

Extra weight

Extra weight slows you down and even when they are empty, packs with reservoirs are usually heavier than ones with soft flasks.

A reservoir with its long hose are around 200-300 grams heavier. That might not sound much, but pick up your mobile phone. Now imagine two of those in your hand. Would you want to carry that amount of weight given the choice?

Can only carry one type of drink

You can only have one drink in a reservoir. With a couple of soft flasks, you can obviously carry two different drinks at the same time.

Water in one flask, an electrolyte or carbohydrate-rich drink in the other, or maybe simply two of the same drink but different flavours for a bit of variety. It’s up to you.

Can’t use it to help cool you down

Perhaps not so relevant here in the UK, but if you are doing a race in hotter conditions and want to bring your core temperature down, you can always pour some of the contents of your soft flask over your head.

Cooling down with the aid of water from a soft flask
Try doing this with a reservoir and its hose

Also, if you know how far it is to the next aid station and you’re feeling OK, you can always tip some water away if you think you have too much on you.

You want to be smart about how much water you are carrying and not lugging unnecessary weight. Try and be as light as possible at all times.

Offloading water with a reservoir is cumbersome in comparison, that’s if you even know much you’ve got left.

Hinders running up and down hills

A reservoir on your back works against gravity. If you are running up a hill, you want the weight to be at the front so you can lean forward. This keeps your centre of gravity perpendicular to the hill, which is the most efficient way of getting to the top.

It’s easier to lean forward when you have the heaviest components of your vest – namely a couple of flasks – at the front. Whereas a reservoir on the back does the opposite and pulls in the wrong direction.

The same is true for descending, too. You want to lean into downhills as much as possible and let gravity do some of the work. Having a load of liquid on your back will shift your weight backwards, increasing braking forces and slowing you down.

Pain to clean

Cleaning a reservoir is frustrating, fiddly and time consuming.

The trickiest and most annoying bit is the tube, which can only be cleaned properly with a special brush. Even then, you can’t be sure it has been thoroughly cleaned, and good luck drying it when finished.

With a soft flask, you just unscrew the cap, give it a good scrub with washing up liquid and leave it to dry. Simple!


So that’s several reasons why I think they suck, especially in a race scenario. Maybe I am missing something? If you’re a reservoir evangelist, leave a comment below and persuade me otherwise.

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