Just before a recent event — 34 hilly miles (54.7 km) in summer heat — I impulsively bought a double water bottle cage and had it installed on Basil’s saddle rails.
This is a Profile Design system; one of its selling points is that having the bottles in back is supposed to be aero-dynamic. This is not a consideration for the pace at which I normally ride; however, having three water bottles on hand for long rides is excellent.
(Also, another cyclist told me that my metal water bottles look like rockets when they’re riding in these cages. That may be justification enough.)
I don’t notice the cages or the water bottles while riding, and the bottles remained well-seated on the test run, which covered 34 miles of hilly terrain and speeds of up to 31.2 mph/50.2 kmph (gulp). An elastic “wire” around each holder grips the bottles so that they are unlikely to pop out while the bike vibrates down the road.
I can’t vouch for how easy (or difficult) it is to remove and replace the bottles while actually cycling; I’ve just barely mastered drinking from the bottle in front of me. I swapped empty bottles for full ones at rest stops, and drank from my front cage while on the fly.
The cages extend Basil’s length a bit, but not enough to be an issue most of the time. The angle is adjustable, so the bottles can be set whatever way is most convenient. Mine are quite upright, but the bottles could be tipped in toward the rider, if that makes the bottles easier to grab from the front.
The mount and bracket are metal and look well-designed. I expect them to prove quite durable.
The cages are completely out of the way when my Brompton is folded; they aren’t wide enough to rest on the supporting surface when Basil is folded and lying on his side, so there’s no obvious vulnerability there. Because the mount is attached to the saddle rails, there’s no interference with the Brompton fold, either.
For easier access, I dropped Basil’s saddle bag a bit. Although it’s not quite as easy to use the bag as it was before the installation, it’s still no problem to get to the gear inside; I got used to the change quickly.
For travel to or in a place like New York City — anywhere space may be at a premium, and the need to carry so much water less essential — I will disconnect the cages, and leave them home. Removing the double bolts (you can see the heads in profile, above) leaves just the nose of the mount, which curves above the saddle bag, and is surprisingly unobtrusive.
This was a terrific set-up for my first real summer weather ride; I’m eager to see how well it serves as the season continues.
Update 4 August 2013: I installed this cage on the original Brompton saddle, which has narrow rails; a commenter has noted that the version he purchased does not fit on his Brooks B17 saddle. If considering this bottle cage, it might be worth contacting Profile Design to see if the model you are buying will work with your saddle rails.