is the Zipp cage actually that much different to other cages that bottles won't come loose?
I've not used enough other cages in BTA config to make a comparison. Here's what I say about how we designed the Zipp cage, and hopefully that's helpful.
It was designed explicitly
for BTA use. That's why it's not a "split" design and instead has a band that spans across the whole bottle. That's to keep the bottle from ejecting vertically if you hit a big bump. When I tested some of the super-light Zipp cages on the BTA mount (which we finished the design of first), they would eject bottles because of the split; the cockpit of the bike deflects more than the main triangle when you hit a bump, and the SL cages were designed for use within the main triangle.
Now, you can go with a tighter cage with a split design - like the XLab cage - but in the same way that those cages hold the bottle really, really tight, I also find that it's harder to pull a bottle out. I love the XLab cages; that's what I use behind the saddle, because they are designed to have a bottle pulled straight up. When you're pulling a bottle out of a BTA cage, you are pulling it what would be more laterally if the cage were mounted in traditional up/down configuration.
So basically, the Zipp cage is designed is to be secure in the BTA orientation while also having it be pretty easy to slide the bottle in and out. Does that make sense and is that helpful?
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