I feel my first introduction to biking aerodynamics got here in round 2015. I would not too long ago picked up highway biking after racing mountain bikes in my youth, and I would determined to enter the native ‘Exeter Wheelers Wednesday evening Membership 10’ time trial.
As I reached right down to take my bottles out of my cages on the race HQ (learn: random village corridor), a pal stopped me and repeated a hand-me-down axiom that claimed empty bottle cages had been slower than merely carrying your bottles.
I bear in mind being fascinated by the concept aero trumps weight, however took his recommendation as gospel, slid my bottles again into their cages, and set off (carrying a long-sleeve Castelli Gabba. Clearly, a lot was to be discovered).
In 2015, the entire marginal positive factors philosophy had been in motion for just a few years at Staff Sky, however it was removed from gaining widespread acceptance. The last decade since has seen discovering these small financial savings turn into ever prevalent, and in lots of circumstances, the important thing to success, each on the high stage and in grassroots biking, too.
Right here at Cyclingnews, we have spent the previous 12 months build up the ‘CN Labs’ content material sequence, utilizing wind tunnels, rolling resistance rigs and extra to empirically check as many biking parts as we are able to, to assist our readers perceive the place to spend their cash within the pursuit of going sooner.
We have discovered the fastest aero helmets, examined budget vs aero cycling kit, in contrast 11 of the best superbikes available on the market, put 18 road wheelsets head to head, and we even rolling resistance tested road tyres.
Immediately’s check focuses on bottles, and we had loads of inquiries to reply: Must you use one, two and even three? If one, the place must you mount it? Are aero bottles all they’re cracked as much as be? And with all of the limelight being on reducing core temperature and optimising cooling methods, is there a penalty for operating massive 750ml bottles?
And, most significantly, to take me again to 2015 and full the circle of my journey into aerodynamics, was my pal proper? Are empty cages slower than full ones?
Let’s get caught in and discover out.
The check
To seek out out, we headed to the wind tunnel at Silverstone Sports activities Engineering Hub. This can be a cycling-specific wind tunnel, and it permits us to measure the CdA (Coefficient of Drag x Space) of a given merchandise.
CdA is basically a measure of how simply one thing strikes by means of the air. The ‘Cd’ portion (Coefficient of Drag) pertains to how simply air passes over an merchandise’s floor, whereas the ‘A’ is the merchandise’s Space. Put merely, the larger an merchandise is, the extra air it would hit, however should you change the merchandise’s form and floor materials, you’ll be able to cut back the Drag Coefficient. Because of this a bullet is extra aerodynamic than a coin of equal measurement, for instance.
We examined utilizing a Dice NuRoad SLT gravel bike, and ensured every thing aside from the check variable remained fully unchanged between checks, together with the wheels, tyres, crank place and so forth. This bike provided us the possibility to check a 3rd bottle on the underside of the downtube, however the primary purpose we used it was for its tyre clearance, as a result of on the identical day we additionally examined the aero efficiency of assorted highway tyre widths. The outcomes of that had been fairly surprising, concluding that 40mm tyres are likely faster for most people.
We examined the next setups:
- No bottles
- One bottle, mounted to the down tube
- One bottle, mounted to the seat tube
- Two bottles, on the seat tube and down tube
- Two tall 750ml bottles, on the seat tube and down tube
- Two empty bottle cages, on the seat tube and down tube
- Three bottles, on the down tube, seat tube, and the underside of the down tube
- A single Elite Aero time trial bottle, mounted onto the down tube
- A single Elite Aero time trial bottle, mounted onto the seat tube
- A pair of Elite Aero time trial bottles, on the seat tube and down tube
We examined the setups with out a rider. I admit it will be extra lifelike to check with a pedalling rider, however the accuracy of the end result would have been misplaced given the small variations we had been attempting to measure. We opted for clear knowledge with assured conclusions.
We examined every setup at 40km/h – my considering right here was that anybody questioning what number of bottles to make use of and their splendid place might be racing, and sure doing shorter, more durable races. Novice riders doing longer races or sportives, the place the speeds are extra seemingly slower, would prioritise carrying the additional gasoline, and should you’re driving over 40km/h, you are in all probability sitting in a bunch the place aerodynamic influence is decreased.
We examined at 5 yaw angles, in five-degree increments from -10° to +10°, to simulate the wind hitting the bike at numerous angles.
Every seize – basically the interval throughout which the CdA measurement is taken – was held for 15 seconds, and if any sudden turbulence was discovered within the CdA readings (ie, if it randomly jumped up or down mid seize) we might repeat the check. Fortunately, every check recording was steady.
Confidence margin
We carried out a repeat of the primary configuration (no bottles or cages) on the finish of the day to grasp what kind of variance we may see in our outcomes, even earlier than we modified something.
We calculated the margin as 0.0385 watts or a CdA distinction of 0.00004 m². This can be a very small margin, even for a check with out a rider, however one which’s reflective of the minimal interference we had been having. We did not must take away the bike from the stanchions, flip the cranks, or change wheels. Other than the addition and elimination of bottle cages, every thing else remained untouched. We additionally managed to check every thing inside about an hour, so there was minimal temperature swing all through the check, and the machines had been given loads of time to heat up beforehand.
The caveats
As with all of our CN Labs tasks, Cyclingnews doesn’t declare that this knowledge is the ultimate phrase on the aerodynamic efficiency of the gadgets included, however quite a further stream of unbiased, unbiased testing and data for our members.
The outcomes are merely consultant of our day of testing. We hope that being clear about our methodology and our protocol permits readers to understand the info whereas additionally understanding the larger image.
We additionally perceive {that a} rider’s pedalling legs might have an effect on the airflow across the bottles and have an effect on any of the financial savings discovered. Nonetheless, we expect the bike-only knowledge is efficacious and worthy of sharing.
The outcomes
When displaying the outcomes, I’ll begin by displaying the CdA of every configuration. I’ll then use these figures to quantify the ability required to pedal at a given velocity.
Usually, we’d use this components, which assumes every thing else (weight, drivetrain friction and rolling resistance) stays fixed:
Energy (watts) = 0.5 x AirDensity x CdA x Velocity^3
Nonetheless, given there may be fairly a sizeable weight distinction in including (full) water bottles to a motorcycle, I will even issue within the further energy required because of this weight.
For that, I’ll use this equation, which is a simplified model of the complete equation however one that’s typically accepted as being correct at decrease gradients.
Energy (watts) = Weight x Velocity x Gravity x (Gradient ÷ 100)
Added collectively, it will give us the ability required to beat air resistance and gravity. In a real-world situation, you’d additionally think about rolling resistance and drivetrain friction, however since we’re solely within the variations between every bottle setup right here, we’ll assume these components stay fixed and set them at zero.
Since we’re working with a motorcycle solely, the CdA and weight are each small, and due to this fact the ability required will probably be small. That is for comparative functions solely, we all know bikes cannot pedal themselves.
Nonetheless, for the sake of displaying the impact on a full rider, I will even apply the variations to a baseline rider with an arbitrary CdA of 0.3 and weight of 83kg. For this, I’ll set the no-bottle-cages setup because the ‘zero’, and regulate the CdA and weight from there.
Within the graph above, I’ve ordered by CdA from lowest to highest, and added an approximate weight determine for every configuration (bottle, cage and liquid) to the facet.
From a purely aerodynamic perspective, there are just a few attention-grabbing factors we are able to infer from the info.
As we anticipated, the aero time trial bottles from Elite are extra aerodynamic than customary spherical bottles.
Operating a bottle on the seat tube got here out on the similar CdA as operating one on the extra conventionally used down tube, so everytime you need to run a single bottle, there isn’t any proper place to place it. Use whichever feels extra comfy for you.
If all-out velocity is what you are after, operating the Elite bottles is the answer, but when you do not have a set, then you definitely’re higher off operating nothing in any respect. After all, that solely applies if you can also make it by means of your chosen occasion with out water.
Maybe most attention-grabbing is that operating three bottles, with the third on the underside of the down tube, was sooner than operating two, and even sooner than operating two 750ml bottles.
That will help you quantify the variations, this is a graph that reveals how these CdA figures translate into watts required at 40km/h.
This reveals that from the quickest possibility, the Elite Chrono aero bottles require two fewer watts than the worst possibility, the 2 empty bottle cages.
Nonetheless, all the above is beneficial when driving on the flat, however they ignore the influence of weight when driving on hills. After all, most programs embrace a mixture of uphill and downhill, however to maintain this from changing into a 10000-word essay, I will concentrate on driving uphill.
Utilizing the approximate weights for every configuration, I’ve calculated the ability required at 0%, 2.5%, 5%, 7.5% and 10% gradients.
The rationale I’ve completed it at 5 totally different gradients is that I needed to point out how a lot the significance of weight grows as hills get steeper. Within the above graph, I’ve proven the lightest and heaviest choices in our check, no bottles vs three bottles; a swing of 1.8kg. At a gradient of 0%, the ability required to beat gravity is zero for each choices.
At 2.5% the baseline takes a further 21.8 watts, rising to 26.7 watts while you add the three bottles, a distinction of 4.9 watts. At 10%, that very same 1.8kg weight acquire will value you 19.6 watts.
Right here you’ll be able to see that on flat terrain, the Elite cages save 0.9 watts in comparison with the no-bottles-no-cages baseline configuration. That saving shrinks to 0.4 watts while you begin climbing, and while you hit gradients of 5%, the extra weight begins to make itself recognized, making you 0.2 watts worse off.
For the heavier choices, akin to two or three bottles, the variety of watts required grows considerably at greater gradients, with the heaviest possibility costing you a chunky 20.4 watts in comparison with the baseline. Keep in mind that is for the bike with out the rider although, so this determine is considerably exaggerated.
Right here, with our figures adjusted to imitate an actual rider, I’ve calculated at a barely extra attainable velocity of 30km/h. The entire purpose for including a rider into the equation is to make it extra real-world relevant, however no one can journey uphill at 40km/h, so it made sense to regulate.
Right here, on the slower velocity, the accessible financial savings are extra modest. There’s simply 0.4 watts on the desk between the baseline and the quickest possibility on flat terrain. In reality, all setups are inside simply half a watt. As quickly as you introduce any gradient, the accessible aero financial savings are offset by elevated weight, even when the bottles are empty.
At 10% gradients, there is a 15-watt distinction between greatest and worst.
The takeaways
On flat roads, on the subject of carrying water bottles in your bike, your quickest possibility is to make use of time trial bottles. In reality, even should you do not want water, the bottles act as fairings that make you quicker. That might be the Elite Chrono bottles as we have used right here, however there are many options, together with the Tacx TT or the aero bottles that got here on the Trek Madone.
The financial savings are small, however not insignificant; within the area of half a watt at 30km/h, round one watt at 40km/h, or extra as velocity will increase. With regards carrying one bottle vs two or three bottles three bottles, and their optimum place, the variations are even smaller, albeit nonetheless important should you’re trying to eke out each final second out of your efficiency.
Nonetheless, as quickly as you begin driving uphill, these small financial savings are rapidly offset by the rise in weight, that means should you’re driving primarily uphill, your quickest strategy is to not carry any bottles or cages in any respect. Nonetheless, that is solely relevant should you can end your journey safely, with out dehydration or overheating. You may lose much more than the 15-watt deficit we noticed right here should you fail to hydrate correctly otherwise you overheat.
Assuming you have labored out how a lot fluid you need to carry, the above knowledge ought to hopefully be a helpful information on one of the simplest ways to take action.
Conversely, it must also function a reminder of how marginal these positive factors are. Having completed all of the maths right here, I not too long ago selected to run two spherical 750ml bottles in a gravel race. I knew I would be racing for round 4 hours, in temperatures round 25C/75F, and my common velocity can be within the area of 30km/h. With all that in thoughts, the aero penalty would value me 0.9 watts, however since I had plans to shelter in teams, that will be decreased additional. The burden penalty would value not more than 4 watts on the steepest kicks, however much less on common. The advantages, nonetheless, can be an additional 500ml of water that I may use to hydrate and funky myself (on high of the vitality drink I had in a hydration pack), avoiding the necessity to cease at feed stops, and saving me much more time ultimately.
Importantly, we additionally did some testing on hydration packs on the identical day, together with a check carrying the hydration pack inside a skinsuit vs on high as regular, in addition to checks with the Rule 28 Gravel Swimsuit, which holds the hydration bladder in a sleeve on the rear. We’ll share the info for that quickly.
However to return to the very starting, my pal was proper. Empty bottle cages are certainly slower. On flat terrain, they’re half a watt slower than no bottle cages in any respect, though that saving shrinks to only a tenth of a watt when in comparison with leaving your bottles in place.
