Two days ago I talked about systems so that you can drink while down in the aero position. Sometimes for long rides and races it is also helpful to carry more than just one or two bottles. For that purpose, you can install a two bottle holder that comes out the back of your bike – right behind/under your seat. (I think this is the version I have, except it looks like they added an extra spot to store a CO2 cartridge which sounds awesome).
Some companies also make containers that fit in a bottle cage and hold standard bike repair items if you do not have another bag to carry necessary tools to fix/replace a tire if needed. I personally use a little bag that fits between the bottles on the back of my bike behind my seat for a spare tire and tools.
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