Should you ditch the extra bottle to save weight, or carry it to avoid stopping? It's a hot topic on the road.cc forum, we've crunched the numbers to find out!

Bechdan posed an interesting question on the road.cc forum last week:

"I find it hard to bring myself to add the weight of an extra water bottle (0.75kg) on long rides, having spent plenty of time and money on getting my bike light and efficient. But I also know the engine won't function without water.

"What is more efficient, stopping to refill or carrying the extra weight?"

This got us having a chat among the road.cc team, and we thought we'd share what we think. It's just a bit of fun but let us know in the comments down below if you reckon we've missed anything important.

As Bechdan says, your engine won't function without water. You'll obviously grind to a halt sooner or later if you don't drink – that's life! If you struggle to refill your bottles when you need to, your performance could suffer, in which case you've clearly pushed things too far and you'd have been better off carrying the extra water in the first place. We'll take that as read.

For the purposes of this, we'll assume that you can refill your water bottle more or less when you need to.

Right, will the time gains you make from reducing weight by carrying just one water bottle instead of two be greater than the time spent stopping to refill more frequently?

Working out the answer is complicated because a whole load of different factors are involved. Adding mass by carrying extra water affects rolling resistance and speed, and speed in turn affects aerodynamic drag...

Luckily, other people have done the heavy lifting by creating online bicycle performance prediction calculators. Phew! Bike Calculator is one of our favourites.

Essentially, you can punch in figures covering to a whole load of different variables (rider and bike weight, distance, gradient, and so on), and get a predicted time out of the other end. This is exactly what we need to answer Bechdan's question.

## Assumptions

• We'll assume that the aerodynamics of the bike and rider are the same whether they have one water bottle on the bike or two. Clearly, there will be a small difference, but we'll ignore it here.

• We need to pick a weight for our calculations: let's go for rider (including clothing and anything they're carrying) + bike + one full water bottle = 85kg.

• Bechdan has allowed 0.75kg (or 750g) for an extra bottle of water. We'll assume that's for 750ml of water rather than for a 600ml bottle, plus the of water in it, plus the bottle cage.

We'll add another 90g for the bottle and a further 30g for the bottle cage. This means that carrying a second 750ml bottle of water would add 750g + 90g + 30g = 870g (or 0.87kg) to the overall system weight.

• We'll assume that a rider gets through 500ml of water per hour, so a rider with a single 750ml bottle of water will need a refill every 90mins, and it'll be 3hrs between refills for a rider with two 750ml bottles (in reality, the amount of water required will depend on temperature, humidity, riding intensity and the individual rider).

• We'll also assume that the rider carries the whole of the 750g of the water (whether in the bottle or having drunk it) for the whole time, and that there are no losses caused by sweating or taking a leak.

Let's compare two riders who weigh the same, riding the same bike over the same course in the same conditions. The only difference is that one rider has a single 750ml water bottle on their bike and the other rider has two, so...

• Bill, carrying one 750ml water bottle, total system weight: 85kg

• Ben, carrying two 750ml water bottles, total system weight: 85.87kg

## Flat ride

Let's take a 100km flat ride at 220 watts (25°C, elevation 100m, clincher tyres, riding on the hoods) and put those figures into Bike Calculator. It gives us these ride times:

Bill: 183.12 minutes, which is 3:03:07

Ben: 183.25 minutes, which is 3:03:15

So Bill has saved just 8 seconds of ride time by doing without a second water bottle on this totally flat course.

Plus, Bill will need to stop to refill his single water bottle because he's riding for over 90mins whereas Ben won't need to stop (1,500ml is enough for 3hrs; we'll let both riders run on empty for the last 3mins or so).

And, if we're assuming no loses to sweat, refilling his water bottle will add overall system weight for the second half of the ride, so Bill's ride time saving will actually be even lower (he'll do 49.15km @ 85kg in 01:30:00, and 50.85km @ 85.75kg in 01:33:10 = 03:03:10).

We can safely say that it would take Bill much longer than this to stop and refill his bottle, so Ben will get to the finish first in this scenario.

## Uphill ride

What about if the riders are in the Alps taking on a 35km climb with a 6% gradient (Galibier is close to these figures when ridden from the north), all other variables remaining the same? Bike Calculator gives us these times:

Bill: 159.58 minutes, which is 2:39:35

Ben: 160.98 minutes, which is 2:40:59

So Bill rides for 1min 24secs less than Ben, but Bill will need to stop to refill his water bottle whereas Ben can keep going.

Plus, if we're assuming no loses to sweat, Bill's refilling of his water bottle will add weight for the second half of the ride, so his ride time saving will actually be even lower than that (he'll do 19.74km @ 85kg in 01:30:00 and 15.26km @ 85.75kg in 01:10:07 = 02:40:07).

If Bill wants to get to the top first he needs to find a source of water, refill his bottle and get going again very quickly. If he comes across a tap by the side of the road at just the right time he might have a chance... but it's a long shot. If he has to head into a shop or cafe to buy a bottle of water, no way. Even in this extreme case, it doesn't seem worth riding without a second bottle on the bike.

## Conclusion

The bottom line is this: if you're ever going to drink more than a single bottle's worth of water on a ride, you're better off taking a second bottle with you from the start.

Mat has worked for loads of bike magazines over 20+ years, and been editor of 220 Triathlon and Cycling Plus. He's been road.cc technical editor for eight years, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a past winner of the Cycling Media Award for Specialist Online Writer.

Augsburg [31 posts] 3 weeks ago
3 likes

Haha,  we live in the Sonoran Desert in souther Arizona USA.  There is no alternative to carrying enough liquid this time of year.  Heat exhaustion and dehydration are no joke here, where hikers die every year from a lack of proper preparation.

My wife and I EACH carry 2 to 3 liters of liquid for 2 to 3 hour rides.  We carry mostly water, but one sport drink to replace electrolytes.  Not only must we carry the weight, but we must keep it in insulated bottles (e.g. hydroflask), to keep the liquids cool and keep our core temps from elevating during the ride.

IanEdward [326 posts] 3 weeks ago
3 likes

Ha! This is the sort of research we need more of, is there an 'ignobel' prize for cycling journalism?

I carried two bottles at all times when we were in Girona a couple of weeks ago as I was terrified about the forecast temperatures (they never got above 33C thankfully but we were prepared for 36C...). All that matters is that my bike didn't *feel* any heavier, it's not like I was ever going to be troubling any Strava leader-boards.

Sriracha [170 posts] 3 weeks ago
4 likes

Truth is, when it's blistering hot and you don't know where the next potable tap is, you eke out what you have and are reluctant to drink the last drop. So you feel the effects even without actually running dry, plus you suffer "range anxiety". That was my experience last year cycling during "la canicule" in Provence.

So this year I took two bottles. So the weather was cooler, naturally!

Vorix [2 posts] 3 weeks ago
1 like

I think there is a massive psychological effect here as well. If you only have one bottle and you aren't sure you will be able to refill it easily then you will drink less as you look to conserve it. Best case scenario is that you are slightly dehydrated and this will impact your performance a bit. Worst case could be much more serious, as the first poster alludes to. That second bottle as insurance will mean you are more likely to drink at optimum times and maintain your overall performance, despite the minor weight penalty.

mylesrants [501 posts] 3 weeks ago
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Dear Bill.

You are not riding your bike enough if you have time for this daft quandry.

yours

earth

slappop [79 posts] 3 weeks ago
3 likes

Whenever I go cycling, I have my wife drive behind me in the car. If I need water, I make my way back through 'the peloton' and stock up (while getting some quality 'sticky bottle' time).

When I've finished with the bottles, I toss them by the side of the road and she picks them up.

It's all excellent training if I ever decide to turn pro.

nortonp [16 posts] 3 weeks ago
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You've missed the point. When it's hot you need one bottle with isotonic drink and one with water. You can use the one with water to drink and to pour on yourself to cool you. Just don't mix them up.

Simon E [3812 posts] 3 weeks ago
2 likes
nortonp wrote:

When it's hot you need one bottle with isotonic drink and one with water. You can use the one with water to drink and to pour on yourself to cool you. Just don't mix them up.

"Need"?

I don't think so.

Judge dreadful [382 posts] 3 weeks ago
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I value my health over my ‘times’. If it’s hot, it’s 2 bottles every time.

Sriracha [170 posts] 3 weeks ago
2 likes
Tones0000 wrote:

You forgot that the water weight reduces through the ride so the 2 system weights are almost equal once rider 1 has to refill. This swings the result even more towards the 2 bottle method.

They did not forget. They made a simplifying assumption that no water was lost, that drinking it simply moved it from bottle to body.

You could instead make the assumption water drunk = water transpired (i.e lost). Assume also that it is drunk at a constant rate and that both consume two bottles over the ride. Graph water weight over time.

So the one bottle graph will be a saw-tooth, two triangles. The two bottle graph will be one triangle, double the height but the same baseline as the saw-tooth. Hence double the area, which is the integral of weight over time.

So whichever assumption you make, the two bottle rider is carrying twice the water weight of the single bottle rider, however both riders average water weights are halved if you assume water drunk is water lost.

Tl;dr the absolute difference halves, the relative difference remains constant at a factor of two.

Zebra [50 posts] 3 weeks ago
1 like

I live in the tropics.  Anything over 40km, you need two or more bottles.  For me, the usual option is (prepare for condascending sneers now...) a hydration pack, simply because I can then carry 2 litres or so and don't need to rely on spotting a water source on route (of which there are very few anyway).

Woldsman [333 posts] 3 weeks ago
0 likes

Big bottle in one cage and a tool caddy stuffed with fruit bars, flapjacks, gels etc in the other.

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JohnnyEnglish [35 posts] 3 weeks ago
0 likes

Or just take one big bottle, and one small one...

Plasterer's Radio [546 posts] 3 weeks ago
2 likes

If you think you need 2 bottles, take 2 bottles.

This is simply daft roadie weight-phobia.

ShaunC [4 posts] 3 weeks ago
1 like

I carry 2 so I have variety in what I drink and its not always a sweet drink, and can switch to plain water occasionally.

Tones0000 [16 posts] 3 weeks ago
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Sriracha wrote:

They did not forget.

I respectfully disagree. As you mentioned, the article and calculator used assumes a fixed total system weight of 85kg for Rider 1 (Bill) and 85.87kg (Ben) for the entire duration of the ride. The article states that there are no losses due to sweat etc.

Under the article's assumptions, Rider 2 (Ben) does have a total system weight of 85.87kg for the entire ride but the article incorrectly states that Rider 1 (Bill) has a system weight of 85kg for the whole ride when in actual fact he has a system weight of 85kg for the first half of the ride only, then a system weight of 85+0.75kg = 80.75kg for the second half of the ride after he refills his bottle.

This means that Rider 1's (Bill's) time calculation needs to be done in 2 weight steps for before and after the bottle was refilled. It's clear from the article's last 2 assumptions (and the calculator inputs) that this is not the case. This means that if calculated correctly, Rider 1 (Bill) should get a slightly slower time compared to what is reported in the article.

Further - I don't believe the integral of your water volume over time has any physical application. The water consumed is already measured in the vertical axis. If your chart plotted water consumption per second over time then the integral would give the total water consumed but this is not what you are proposing. The fact is that the weight differential between Ben and Bill of the 750g extra water only occurs during the first half of the ride and is equal after that. This is the same whether there is water loss or not taken into account for both riders.

FluffyKittenofT... [2692 posts] 3 weeks ago
0 likes

Get a faster friend (or a friend with an e-bike) to accompany you while carrying _four_ bottles of water.

Sriracha [170 posts] 3 weeks ago
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Tones, yes, you are correct about the implication of the article's no-losses assumptions in respect of the single bottle rider. When he refills he still holds the contents of the first bottle in his stomach, so indeed with the bottle refilled he now carries two bottles worth of water, the same as the two bottle rider. I overlooked that.
But I'll hold to my assertion that under my assumption that they drink to replace lost sweat (so the rider weight remains constant whilst the bottle weight steadily decreases) then the 2-bottle rider carries on average 1 bottle over the length of the ride whilst the 1-bottle rider averages 1/2 a bottle over each half of the ride, hence half a bottle over the whole ride, hence half the other rider's burden. Or did I miss something there also?

Tones0000 [16 posts] 3 weeks ago
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Sriracha wrote:

Or did I miss something there also?

A generic model with a fixed linear rate of water loss for both riders (other than when a bottle is refilled) does give an average water weight difference between the riders of half a bottle over the ride. This also holds in the article example with a 0 rate of water loss. The problem is that the article uses a full bottle weight difference between the riders over the ride and not half. This was the point of my original comment.

Of course for the half bottle average weight to be correctly applied, the course route would need to have the same average geographical characteristics in the first half of the route as in the second half (and both must be equal to the average characteristics of the complete route in total).
The piecemeal approach proposed in my second comment allows the calculation to be performed without this requirement.

Mat Brett [679 posts] 3 weeks ago
2 likes
Tones0000 wrote:

Under the article's assumptions, Rider 2 (Ben) does have a total system weight of 85.87kg for the entire ride but the article incorrectly states that Rider 1 (Bill) has a system weight of 85kg for the whole ride when in actual fact he has a system weight of 85kg for the first half of the ride only, then a system weight of 85+0.75kg = 80.75kg for the second half of the ride after he refills his bottle.

That's true. Amended. Glad to see someone is taking this seriously!

Tones0000 [16 posts] 3 weeks ago
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Mat Brett wrote:

That's true. Amended. Glad to see someone is taking this seriously!

As a professional nerd I've always pedantically tried to calculate things correctly - even if the difference in the end is much smaller than the margin of error from all the other assumptions

Tones0000 [16 posts] 3 weeks ago
2 likes

Article is now updated to correct this issue.

You forgot that the water weight reduces through the ride so the 2 system weights are almost equal once rider 1 has to refill. This swings the result even more towards the 2 bottle method.