“When a rider drafts behind another rider, the rider behind has a reduction in power required due to having less drag as they are riding in the low pressure wake of the rider in front. This can be as high as a 29% reduction. The other result is that the rider in front also goes faster as the air sees the two riders as virtually connected and the airstream jumps the gap between the riders, causing a lower drag on the front rider.
So the idea is to place the rear bottle in the draft of the rider, in other words in the low pressure area usually in the location of the back of the seat/seatpost. One is trying to basically fill in the low pressure void behind the rider. This has been validated in Tunnels all over the world for years but recently reaffirmed with Velodrome testing using the Alphamantis technology with world champions who are able to hold constant aero positions and power outputs.”
For me, I think no bottle at anywhere one the bike should be the fastest option. But from XLab article conclude that using XLab rear bottle cage with bottle is lower drag than no bottle.
It’s absolutely possible for an object such as a bottle, with an appropriate shape and size and appropriately positioned relative to the rider to reduce the drag of the system. It’s not a foregone conclusion however, just because you buy an XLab bottle, that you will reduce drag. In general you’ll want the bottle close to the rider and somewhat continuing the body shape into the region previously occupied only be the wake. Wake minimisation is the goal.
You don’t need an XLab cage or bottle, cable ties and a cheap cage with a good grip will work too, and may well be a tidier (and cheaper) solution.
With aerodynamics, shape and the resulting flow regime generally matters a lot more than volume.
The Specialized Transition was proven to be faster with the Virtue water bottle installed. That is why you still see them at races with the bottle in place but not being used. (I’m one of those guys)
I don’t know if 29% is reasonable for a bike but NASCAR talks about this effect all the time. The guy “pulling” always wants someone right on their ass because it makes both of them faster. It also saves fuel.
I spend a lot of time in pace lines in the 22 to 25 mph range and my power usually changes about 20% from pulling to moving into the last wheel spot.
There is no doubt that “filling” the area behind a rider can possibly in very specific circumstances very slightly reduce some of the aero drag generated by a rider’s body.
But the devil is always in the details.
The shape and dimensions of the object(s) that one uses to fill this space is critical. I myself would highly doubt that large vertical water bottle(s) as pictured would do the trick …
It’s been proven time and time again with all sorts of powered vehicles (aka race cars) that having someone behind your car makes both of you faster, that the tailing vehicle has SIGNIFICANT reductions in effort and drag and that airflow off the back of the leading vehicle is much smoother since it doesn’t have time to become turbulent and create drag.
You might see semis on the highway with the big flaps off the tail ends of their trailers, those are there to move the vortexes created further away from the trailer and reduce drag, same effect a water bottle could have on a bike.
Yes, that first picture is from a wind tunnel study Cervelo did years ago in which they compared drag without and with bottles behind the seat in different positions. They proved that a bottle (or bottles) behind the rider improves aero. Cervelo’s study used to be on their website, but it has been gone for a while. If you search hard enough, you can find it and read the original protocol and findings.
The 1st photo may have been with Cervelo, but from what my Dad has told me (who was there that day) it was a customer buying the Cervelo from our store Nytro and wanted to take it to the tunnel to test and dial his position. You can see the Nytro bottles. Also the person has our Slipstream vest which funnily enough (and very high tech I must say) was hand sewn by my Mom
Happy to see others have chimed in here vs the brand saying yes it is true and giving a hint of bias…or maybe a lot!
Representing the brand, we are definitely truthful in our aerodynamic data. We not only have wind tunnel data going back to the late 1990s/early 2000s from our Nytro store days, but also through our CFD software, which you see an image from on your post, and also athlete testing at the VELO Center in Carson. The founder/designer is also an aerodynamic engineer who has designed rockets/yachts and parts for military/commercial aircraft.
This very scenario you bring up came out at the VELO Center with a former athlete of ours. Coach was telling them to have minimal amount on the bike, as a good sponsored athlete they insisted they at least try the rear carrier, and sure enough they actually saved time by having the carrier on there!
Now %/time saved will vary based on your particular setup and also your positioning - thus why we said as high as a 29% reduction.
Some people will read this as “anything mounted behind my seat will make me faster” in reality, its possible but you’d have to actually test your different options to get an answer.
Wait…does leaving my bike rack on my car improve fuel economy? Score!!!
