AERODYNAMICS

The ideal method would not increase the aerodynamic drag or be affected by side winds. It would also be lightweight, and easily accessible. The most important aspects relative to a race are minimum aerodynamic drag and good accessibility.
As aerodynamic drag consumes around 90% of your power output and therefore energy, it is extremely important to carefully choose a hydration system that will not increase the drag even more. Unfortunately there are companies making aerodynamic statements that are not exactly accurate relative to their products. Often the reason for this is their lack of knowledge and experience when conducting wind tunnel tests.
It is very difficult for a cyclist to maintain precisely the same position for an extended time during a wind tunnel test. Often the cyclist’s small change in position has more effect on aerodynamic drag than the new product being tested. Wind tunnel tests are good for positioning the rider but not always accurate for product testing. Cervelo has moved more towards Computational Fluid Dynamics (CFD) as this can provide details not visible or measureable in a wind tunnel. They also use a ‘dummy’ in order to eliminate rider variations. Velodrome testing is also often inaccurate due to rider variability. Very few in the cycling industry, including magazine editors are experienced in aerodynamics, many people are ‘self taught’. So we need to be truthful and factual in order to make correct statements.

1.AERODYNAMIC DRAG.

The calculation for aerodynamic drag, the main force working against a cyclist is ½ rCdAV 2. Where ‘r’ is air density, ‘Cd’ is the coefficient of drag due to the shape of the object, ‘A’ is the frontal area of the object, and ‘V’ is air velocity.
In order to understand drag, we need to understand how air flows over an object.The first layer of air next to the object is the Boundary layer and the molecules basically stick to the object. The second layer rolls over the first layer and finally breaks away and becomes Turbulent. The reason is that air has mass and it cannot change direction quickly, the same as driving into a 50 mph corner at 70 mph, you are likely to not end up going round the corner but probably going off at a tangent. If air does this it becomes Turbulent. This is also why cyclists with a gradual rounded back usually have lower drag than a cyclist with a flat back, because the air can hang on longer on a gentle curved back.
As the air finally breaks away from the back of the rider it becomes Separated in large scale vortexes and eddies. This is a low pressure area. One can reduce the drag by adding items in this low pressure area so that the air has more time to reconnect and once again be streamlined. In general the longer you can make the faired tail the better. A triathlete’s body in the aero position is similar to an oval shape. An oval shape can have up to 4 times the drag of an airfoil shape like an aircraft wing, so the more you can fill in the low pressure area behind the rider’s posterior the better. The following link to the Cervelo website is very educational. http://www.cervelo.com/en_us/engineering/tech-presentations/
When you see a Team Time Trial you will notice that each rider tries to cycle close to the rider in front in the rider’s draft. They can reduce their drag by up to 20% in this way. So it is pretty clear that placing items in the right place behind the rider can definitely help.

2.COEFFICIENT OF DRAG.

The lower the coefficient the less power is required. A rider on an upright commuting bike will have a CD or 1.15, a touring road bike and rider CD of 1.0 and a road rider on the drops with tight clothing a CD of 0.88. Faired in recumbent bikes can have CDs as low as 0.11. A smooth ball will have a CD of around 0.47, but an oval shape can be as low as 0.1 or less and the reason for this is that the longer and more slender a shape is, the easier it is for the air to stay laminar and reduce the CD. As a rider changes their torso from sitting vertically to more of an aero position, a cross section of their torso changes from round to oval and the CD lowers. It is often easier to reduce the CD than reduce the frontal area. Long tailed helmets, aero framesets, aero and disc wheels, and skin suits also reduce the coefficient of drag.

3.FRONTAL AREA.

Minimalizing the frontal area is vital. Increasing the frontal area by just 5% would mean you would need to put out another 5% more power for the whole bike split. Using a great aero position will greatly reduce frontal area.
During a race the wind will hit you at different angles from straight ahead to sideways, and if you are lucky, from behind. If the wind is coming from the side, the wind will see more of the bike than if it was coming straight towards you. So even adding gear that you cannot see from the front of the bike, can add to the apparent frontal area and increase the drag. Examples would be frame mounted hydration systems with straws leading up to the front of the bike.
4.AERODYNAMIC STABILITY.
Aerodynamic stability is essential with any moving vehicle whether it be a car, boat, or a bicycle in windy conditions. It is critical as to where the Center of Pressure is relative to the Center of Gravity. Imagine a silhouette of a triathlete racing in the aero position, then the Center of Pressure is the center of the area of the silhouette. For predictable steering and handling, the Center of Pressure should be behind the Center of Gravity. If you were to lay the rider and bike on their side and found one point where you could support them, then this would be the Center of Gravity. Therefore as you add more side area to the front of the bike, such as with vertical ‘aero’ bottles with straws, or large drinking systems on the frame with tubing coming up to the front of the bike, you will move the Center of Pressure forward and make the bike less stable in side winds. The less stable, the more you have to take small steering corrections and each time you do that it causes increased tire rolling resistance which slows you down. On the contrary, if you add more side area behind the rider, you will move the Center of Pressure rearwards and you will have less steering corrections than if you had added side area to the front of the bike.

FLUID STORAGE

The question is front, frame or rear, or a combination? The answer depends on the race or training session length.

1.FRONT MOUNTED BOTTLES.

There are two types of front bottle, vertical or horizontal.
Vertical bottles have been mounted between the aero bars for nearly 20 years. They offer easy access to 20 to 30 ounces of fluid. Most of these types of bottles can be refilled while riding. Costs from $20-$100. One of the problems with vertical bottles is that you never know how much you have drunk until you empty the bottle. The main issues with them are that they increase the frontal area of the bike which slows you down, and have considerable side area of 40-45 square inches, which also makes the bike affected by a side wind. Drag is Frontal Area multiplied by the Coefficient of Drag, or the aero shape of the item. Frontal Area is the dominant factor. A 3/8 inch diameter straw alone generates around 50 grams of drag over 25 mph. To see how this feels just hold the end of a ¼ inch diameter pencil out of a car window at 25 mph and you can see how bad just the straw is. Now imagine the bottle can have 10-15 times as much drag as the straw, and you are holding that out the window for 5 hours. So how can this be solved?
Horizontal bottles have become very popular in the past few years due to XLAB introducing several versions of their Torpedo Mount into the marketplace. In fact horizontal bottles were used by both the male and female winners of the Ironman World Championship in 2010. They have the advantage that they are shielded from the airflow by the rider’s forearms from the side, and shielded by the hands from air directly in front. Horizontal bottles have been tested by Cervelo in the wind tunnel and they have stated that they are the front bottle type of choice for the lowest drag. Costs around $30 plus cage. Horizontal bottles definitely have less drag than any current vertical bottle.
The disadvantage is that you cannot just suck on a straw, but the advantage is that there is no messy refilling and when using a calibrated clear bottle you can accurately control your hydration. Many cyclists will set an alarm for every 15 minutes and drink the amount that they calculated. The XLAB Aqua-shot bottle is often used, as it is both clear and calibrated and even has a Squeeze and Squirt nozzle for faster drinking.
So is a vertical bottle faster than a horizontal bottle considering the drag and all other aspects?
Assuming a bike split is 5 hours and the athlete drinks 24 ounces per hour, then 120 ounces of fluid will be consumed. This means a typical vertical bottle will need to be refilled with a 24 ounce bottle 5 times. Measurements indicate it will take around 19-20 seconds at best to refill each time for a total of 95-100 seconds (1 ¾ minutes). Cycling with one hand and holding a standard bottle over the vertical bottle with the other can be very challenging in windy conditions.

Horizontal Aqua-Shot bottle.

The initial thought people have is that they will lose time due to having to take out the bottle from the cage and drinking from a bottle with a standard pull-push valve.
The reality is as follows- it takes 9 seconds to drink 4 ounces from an XLAB Aqua-shot bottle including the time to take the bottle out of the cage and replace it back in the cage. So drinking 120 ounces will take 4 ½ minutes. Are you out of the aero position?...not really because the Aqua-Shot does not have a valve to open and close, with your teeth, but instead it has a patented silicone valve that releases when it feels pressure. So you can keep your head in the aero position and just move your arm to reach for the bottle. So with a horizontal bottle your arm is out of the aero position only 2 ¾ minutes more than with the vertical bottle, and with the vertical bottle you are really out of the aero position with one bottle on top of the other refilling.
So is the extra 2 ¾ minutes in 5 hours worth it? The answer is definitely yes, for the following reasons. For the extra 2 ¾ minutes spent drinking you get;

1. Your whole bike split with no drag due to a front vertical bottle and straw that will save you substantial time.
2. You can monitor your hydration input precisely.
3. No messy and tricky refilling holding one bottle over another while riding with one hand.

This is why World Champions and hundreds of age groupers are switching to horizontal front bottles.

2.FRAME MOUNTED BOTTLES

Standard water bottles.
Standard water bottles are nearly 3 inches in diameter and aero frameset’s down tube’s are often only 1 ¼ inches wide so adding standard bottles in the frame increases the frontal area of the bike. You can use slimmer aerodynamic bottles on the down tube that are as thin as 1 5/8 inches wide, thereby reducing the drag considerably. The XLAB Aero TT has been used by an Ironman World Champion to win numerous 70.3 and other distance races. Cost around $60 for bottle and carbon cage.
Large frame mounted bottles.
The ones we are aware of are not only wider than the slim aerodynamic down tubes found on modern racing bikes but they have a considerable side area. With the increased frontal area and large side area, users have complained about being affected by sidewinds. The reason is often that the Center of Pressure has moved forward making the bike more difficult to handle. The other issue is again the straw’s aero drag. Cleaning complex bottle shapes and long tubing lengths can be time consuming for the athlete already struggling to find enough time to train. These units sometimes weigh as much as 300 grams. Costs around $100 plus a $10 cleaning brush and maybe a $20 frame mount if you do not have bottle mounts on your down tube.
Small frame mounted bottles.
The Cervelo P4 has a small 750 ml bottle that actually improves the aerodynamic drag of the frame. This would be good in short time trials as a hydration source but it is useful in any length race to improve the aerodynamics over competitive bikes.
We feel that frame mounted bottles are useful if they are kept small and the drag kept to a minimum.

4.REAR MOUNTED BOTTLES.

The idea behind these systems is that you store your fluids on the rear and bring a bottle forward when needed to refill a front vertical bottle or to replace a Torpedo style horizontal bottle. This design has been around for nearly 20 years and has been available in many different forms and with varying levels of engineering and quality. The basic principle is good as you are placing bottles and other storage items in the rider’s draft in turbulent air. However many designs cannot hold extra gear and some are located in positions that add aerodynamic drag to the bike. The rear position moves the Center of Pressure rearwards improving aerodynamic stability.

IDEAL LOCATION. Over the past 17 years we have been refining the precise zero drag position for rear hydration carriers. To do this we needed more than a wind tunnel for the reasons indicated above. Having had over 30 years of sailing experience we came up with the idea in 2006 of applying wool tufts as used on racing sails to determine the exact flow pattern of air over a rider. In 2010 Lance Armstrong used the same principle in evaluating his airflow.
Cervelo also uses wool tufts on their wind tunnel dummies. With the method we developed, using the XLAB Slipstream Vest, we were able to determine the precise position. We do not believe that any other manufacturer making rear systems has done the extensive testing with a tufted vest to find the optimum position for rear systems. XLAB is the specialist when it comes to rear systems.

We also found that it was important to allow the air that flows between the legs to travel cleanly around the aero seat post. Designs that placed the bottles below the saddle had much higher drag. So how high can the bottles be behind the rider? If you study photos of triathlon champions in their aero position, you can often see their race number lifting off of their back, an indication that the airflow has already separated. This is often the upper limit of the separated airflow, and as high as you should have bottles. However you do not need to go this high.
Basically you want the air to flow cleanly under the saddle, so the bottom of the rear system should ideally be no lower than the bottom of the saddle. We have found that 24 ounce bottles are a good height for rear systems, but you can also use smaller or taller bottles.
HIGH-GRIP CAGES. Rear systems have been criticized for launching water bottles. In fact, it is not the system that launches the bottles it is the ineffective bottle cages that are being used by some riders. XLAB developed the first and only bottle cages specifically designed with the high-grip required on rear systems. The cages called XLAB GORILLA cages have a grip force that is very closely controlled and due to the unique design makes them ideal for fitting on rear systems from any company. There is also available an XLAB GORILLA XT cage with twice the grip force for Mountain bikers or really rough roads. The GORILLA cages have been chosen by more World Champions, Pros and Age Groupers than any other cage for rear systems.

Summary on rear systems

Hides in the rider's wind shadow for the fastest way to carry fluids.
World champion's tunnel testing repeatedly proves XLAB rear systems are the fastest.
Unlike frame mounted and front vertical bottle hydration systems, rear systems less affected by side winds.
No tubing and valves eliminates possibility of bacteria.
Stiff aerospace design and carbon fiber or aircraft alloys eliminates side sway. Be wary of glass filled nylon mounts as they are not as strong.
Unique cages can help prevent bottle launching.
Prices start at under $40 and go up to $260 for a Pro level system with Gorilla carbon cages.
Carry your very own energy drink mix not whatever the race organizers chose.
Adjustable for bottle angle and height for easy access by your arm.
Refill or replace front bottle when convenient to you, not in the aid station mayhem.
Extra fluids on the rear, prevents running out before next aid station.
Carrying your special needs eliminates stress over someone else taking your special needs bag on long races.