4

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.

  1. 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.
  2. Is it reasonable at all? Is that internal part of a bottle mount supposed to bear significant mechanical load?
  3. 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).
  • 2
    With a regular post I'd suggest that you machine a slot in the post, to clear the bottle boss, but I guess that would be iffy for a dropper. I would (on an Al frame) be a little leery of drilling out the boss, given that the post end will be exerting stress near there. – Daniel R Hicks Jul 4 '16 at 0:23
  • I'd suggest you consult a local framebuilder for a price to remove both bottle mounts, and close up the holes. Compare this with the cost to do it yourself. This will void any frame maker's warranty of course. Framebuilder might even give you an opinion about whether its worth doing at all. – Criggie Jul 4 '16 at 0:47
  • Its not common at all, sounds like the bike may be too big. The shorter Rock Shock Reverb at its lowest is only 255mm total length (and some other brands are shorter), and hard tails I have used will take at least 150mm of post, typically over 200mm. – mattnz Jul 4 '16 at 1:24
  • 1
    @Criggie aluminium frame - sensible framebuilders won't touch it. – Móż Jul 4 '16 at 3:11
  • Sounds like you got the wrong dropper post, or the frame is too big for your riding style. – Criggie Jul 4 '16 at 4:00
5

Drilling the seat tube is a bad idea, for the reasons you've indicated. Very rarely a bike shop will have a tool for reaming a seat tube (more usually framebuilders have those) that can to the task in a slightly safer way.

BUT from my experience with a welded steel frame where I didn't bother to back-purge (lesson learned!), even once you weld an extension onto the end of a half-round file, it's a tedious and difficult job to file down irregularities inside the seat tube. You're trying to apply pressure half a metre away from the part you're holding, and that's hard to do. You need a long, strong extension on that file. This is why framebuilders have seat tube reaming tools (which are expensive).

A reaming tool will probably not help you, because that boss is a single, hard obstacle and will likely damage the tool if you can even get it into position to cut the boss. If you do get it in there it's as likely to cut the opposite side of the seat tube away as cut the boss.

A better approach would be to drill out the boss. It's likely that the boss is actually a separate assemble riveted into the frame - they drill a hole, insert the boss, then compress it so it clamps into place. If you can drill out enough of the boss it will fall out, leaving you the original hole in your frame.

Which is the real problem with this approach - at best you'll end up with a hole in your seat tube, and the BB will fill up with water. You can out tape over the hole, and replace the tape when it starts to fail, but it's not ideal. Welding a cover plate on will reverse the heat treatment of the aluminium, leaving a soft spot in the tube. Soldering a patch on would be better, but soldering aluminium is hard (it's an excellent heat conductor!)

The extra stress from the end of the post should be minimal, most seat tubes are actually butted so that minor bends or dents to the tube don't lock the seatpost in place. You'll find the top 100mm or so is tight on the post, after that you have 0.1mm or so space around the post.

  • So the procedure I was thinking of is called "reaming" - thank you! Yes, and it also makes sense that reaming won't be efficient in my case. Basically, this leaves me with some sort of drilling as the only option. – Grigory Rechistov Jul 4 '16 at 6:41
0

I guess there are no dropper posts with shorter total length available? Seems like there is a need for one, especially with dual suspension bikes that have shorter seat tubes.

You could drill out the top lug and thread of the bottle holder. Much easier to access this way then filing from inside. But you would be left with a slightly larger hole in your seat tube.

  • > I guess there are no dropper posts with shorter total length available? The seatpost in question is Magura Vyron and it only comes in 446 mm length. There are not many (if any) alternatives on the market at the moment. There are either dropper posts with stealth routing, and my frame is not designed for them (hello drilling again), or external routing, which is also undesirable given how many cables are there already. – Grigory Rechistov Jul 4 '16 at 6:28
  • [Failed to finish the previous comment in time, continuing] The Magura's post instead has electronic wireless controls. But it comes in only two sizes, and both of them are very long. I wish they made shorter versions, as current 446 mm turns out to be a tad too long even for L-sized frames. – Grigory Rechistov Jul 4 '16 at 6:37
  • As a tall rider - 446 mm is still too short for me. Horses for courses. – Criggie Jul 4 '16 at 11:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.