Behind-the-saddle bottle mounts: are they aero? A 25-rider test
Are behind-the-saddle bottles aero?
Yes, when they sit in the right place. In AeroGain's 25-rider velodrome test at 45 km/h, two bottles held wide and high behind the saddle made riders faster than riding with no bottles at all: a median of 5.3 W with the Aero BTS PRO and 5.0 W with the Aero BTS.
Two bottles mounted close together returned 3.3 W, one bottle high behind the saddle 2.0 W, and one bottle slung below the saddle was slower than carrying nothing, by a median of 1 W. Same bottles, same riders, same speed. The difference is where the bottles sit in the air behind you.
| Setup behind the saddle | Median W saved vs no bottles | Conditions | Riders |
|---|---|---|---|
| Aero BTS PRO (wide V) | 5.3 W (best rider 14 W) | 45 km/h (35 to 50 km/h normalised), 0° yaw, 1000 ml bottles, TT position, race suit | 25 (all riders) |
| Aero BTS (wide V) | 5.0 W | subset of the 25 | |
| Two bottles close together | 3.3 W | subset of the 25 | |
| One bottle, high behind the saddle | 2.0 W | subset of the 25 | |
| No bottles (baseline) | 0 W | 25 (all riders) | |
| One bottle below the saddle | −1 W (slower) | subset of the 25 |
Why can bottles behind you make you faster?
Because they fill the turbulent wake your body leaves behind. In AeroGain's 25-rider velodrome test, the setups that put the most bottle volume high, wide and close to the rider's back saved the most against no bottles at 45 km/h.
A rider in the aero position is not a clean shape. The air leaves your back and hips and breaks into a large, turbulent area behind you, and that wake pulls you backwards. Most of your drag is you, not the bike, so the wake behind your body is one of the biggest pieces of it. Filling part of it with something solid, close to your body, lets the air close behind you more gradually. It is the same idea as bottles between the arms, which fill the turbulent air under your chest.
Three things decide how much you get:
- Volume. Two bottles fill more of the wake than one: 3.3 W for two close together, 2.0 W for one high behind the saddle.
- Width. Spreading the two bottles into a wide V fills more of the wake's width instead of leaving its outer edges empty. Same two bottles, held wide: 5.0 to 5.3 W.
- Height and closeness. The wake starts at your back. A bottle hung low under the saddle sits in the air around the rear wheel instead, and was the only setup that measured slower than no bottles.
The gain grows with the size of the wake. The biggest gains we have measured were on larger riders, who leave more turbulent air behind them to fill.
What is the best rear hydration setup for triathlon?
Two bottles in a wide V, high behind the saddle and as close to your back as the saddle allows. In AeroGain's 25-rider velodrome test, that layout (the Aero BTS PRO) saved a median of 5.3 W at 45 km/h against no bottles, and measured 1 to 4 W faster than two bottles close together, with a median of about 2 W.
Every number is a median: half the riders gained more, half less. The best-matched rider gained 14 W with the Aero BTS PRO against no bottles. That is the ceiling, not the expectation. For a few riders another layout won.
Against two bottles close together, the layout most riders already use, the gain is real and not enormous. If your bottles are on the frame or slung below the saddle, the step is bigger. A behind-the-saddle bottle mount that holds the bottles wide and high is what turns rear hydration from a cost into a gain.
What is 5 W worth over an Ironman bike leg?
About 1.5 to 2 minutes over 180 km at the same power. That is AeroGain's conversion of the median 5.3 W from the 25-rider velodrome test (measured at 45 km/h, against no bottles) into time, for riders between 45 and 30 km/h.
Watts at 45 km/h are a way of expressing a change in drag (CdA). The same change saves fewer watts at lower speed, but you are on the course for longer, so it saves more time: about 1.5 minutes at 45 km/h and about 2 minutes at 30 to 35 km/h. The roughly 2 W over two bottles close together is worth about a minute over 180 km for slower age-groupers (about 60 seconds at 22 km/h), around 50 seconds at 30 km/h and about 35 seconds at 45 km/h.
How many bottles do you need behind the saddle for an Ironman?
Two. In AeroGain's 25-rider velodrome test at 45 km/h, every two-bottle layout behind the saddle beat both one-bottle layouts on the median, and one bottle low under the saddle was slower than none.
Fluid volume is a separate question from bottle count. If you need less fluid for a shorter race, keep both bottles in place and run them part-filled. A BTS hydration system also carries spares between the bottles: 900 ml of storage in the Aero BTS PRO, 800 ml in the Aero BTS.
BTS or BTA: where should the bottles go?
Both, if you carry more than one bottle; never on the frame. Between the arms, AeroGain's 35-rider velodrome test measured a median of 8 W saved against no bottles at 45 km/h; behind the saddle, the 25-rider test measured 5.0 to 5.3 W for a wide V.
- Clear the frame first. Frame bottles sit in clean air and add drag of their own. The front-bottle numbers are in the best bottle position on a triathlon bike.
- Put the bottle you drink from between the arms, on the Aero BTA system.
- Carry the rest behind the saddle, two bottles in a wide V, with your spares in the storage between them.
How do you stop bottles ejecting from a behind-the-saddle mount?
Use high-grip cages. With the Aero BTS and Aero BTS PRO mounts, we recommend TriRig and XLAB carbon bottle cages.
Aero BTS PRO or Aero BTS: which BTS system for triathlon?
The Aero BTS PRO measured a median of 5.3 W and the Aero BTS 5.0 W against no bottles in AeroGain's 25-rider velodrome test at 45 km/h. Both carry two bottles in the same wide V on the same saddle clamp; the difference is the shell.
- Aero BTS PRO hugs the bottles more closely, so it fills more of the volume behind you. 900 ml of storage. Its raised sidewall does not take side-loading cages.
- Aero BTS uses the standard cup. 800 ml of storage. Takes side-loading cages.
In AeroGain's testing, the standard Aero BTS is typically 1 to 2 W slower than the PRO when the same rider tests both. The group medians (5.0 and 5.3 W) sit closer together, and in a few tests the standard measured slightly faster. Both take a Garmin Varia radar mount, and both are approved by World Triathlon and Ironman. All rear systems are in the rear bottle mount collection.
How did we test?
AeroGain's 25-rider velodrome test ran at Ballerup Super Arena, Copenhagen, on the riders' own TT bikes, in their aero position and race suit, at 35 to 50 km/h, normalised to watts at 45 km/h and 0° yaw. Every result is compared with the same rider riding with no bottles.
Every setup used 1000 ml bottles, each rear system as close to the rider's back as possible. Every rider tested the no-bottle baseline and the Aero BTS PRO; not every rider tested all six setups. CdA is modelled from power, speed, air density and mass recorded once a second, with the rider holding the position through repeated lap groups. Results are medians across riders, with the best result named as the ceiling.
FAQ
Which bottle cages work on a behind-the-saddle bottle mount?
The Aero BTS takes side-loading cages. The Aero BTS PRO's raised sidewall blocks side-loading cages, so riders who want one take the standard Aero BTS cup. No bottles or cages are included with either.
Will a BTS bottle mount fit my saddle?
Fit is set by your saddle, not your frame. Each clamp is made for one saddle model, and the list is on each product page. If your saddle is not listed, we do not make a clamp for it yet.
Does the gain only matter at 45 km/h?
No. 45 km/h is the speed we report at so results can be compared. The same drag change saves fewer watts at 35 km/h, but you ride longer, so it saves more time: about 2 minutes over 180 km for the median Aero BTS PRO result at 30 to 35 km/h.
Is a BTS hydration system legal in Ironman races?
The Aero BTS and the Aero BTS PRO are both approved by World Triathlon and Ironman.
Keep reading
- Aero or weight: what saves more time on a flat course and at IRONMAN Nice?
- Which time trial helmet is fastest? Velodrome data from real riders
- The best bottle position on a triathlon bike
- How to hold your aero position for a full Ironman bike leg
- Behind-the-saddle bottle systems
- Book a velodrome aero test
