I have an aluminum MTB hardtail frame with a pair of standard bottle cage holes on the seat tube. I want to insert a seatpost fairly deep into the frame sometimes, so that the saddle stays low. The limitation of my frame's maximum insertion depth is the top hole position. The bottle cage boss has a part of its fixture protruding into the tube. Thus, while theoretically there is about 20 or more centimeters of the seat tube left down to the bottom bracket, the post can only go so far.
With my current seatpost, I solved this problem by sawing off several centimeters of it. This way, I could have the saddle as low as I wanted it to be. I had to account for the change of minimum insertion depth of course, as the part of the post was not there anymore; I was OK with that. Yet, I want to keep the currently achieved minimum saddle height, or even decrease it a bit.
I am now waiting for a new seatpost to be delivered, and it is several centimeters longer than my current (sawed-off) one. Unfortunately, I cannot shorten it, because it is a dropper; apparently, the trick with a saw will not work on it.
I am thinking about approaching this issue from a different angle. I do not use that particular bottle cage mount. If I manage to remove it, or at least its internal protruding part, I will have enough additional centimeters of clear straight tube (until the second, lower bottle hole) to stick my new dropper post in.
This idea spawns a number of questions.
- Is it possible to achieve at all?
- Straightforward drilling of the down tube from the outside of the mount hole and making a bigger hole to get rid of the fixture sounds like an obvious but very dangerous option. The frame strength will suffer.
- Instead, a careful filing of the mount point inside the tube sounds safer. But it can only be done by reaching from the top of the down tube (with a post removed) with a very long file, or a similar instrument. I do not know if such tools are available to bike shops.
- Is it reasonable at all? Is that internal part of a bottle mount supposed to bear significant mechanical load?
- I cannot imagine I am the first one to face this problem and to imagine such a solution, but has anyone ever tried it? Are there any documented attempts of such (un)successful operation? Maybe on a steel frame (but even I cannot dream up trying that on carbon stuff).