Airburst

While stripping down an old Diamondback MTB to repaint it, I pulled the seatpost out and noticed it hadn't been very deep in the frame. Being slightly paranoid that the frame might have been damaged by the marginal seatpost insertion and me not being the lightest young lady around, I felt around the inside of the seat tube with my finger to see if I could feel any cracks. A few centimetres in, the tube diameter seems to increase by a few millimetres - I can feel a step inside the tube. I did some digging online, and apparently this could be a sleeve inside the seat tube, which was something Peugeot used on some of their old frames. Those frames, like this one, take a 24mm diameter seatpost, which lends weight to the idea.

My question is this: As the seat tube diameter increases after about 30mm of depth, is a longer seatpost going to be any use, given that it won't actually be in contact with any part of the inside of the tube other than the sleeve?

FBinNY 

You answered your own question, no it's not going to make any difference. It also wouldn't make any difference even if it did touch the wall. The general guideline for a seatpost is that it needs to be 2 to 2.5 times it's diameter into the frame. This ensures transfer of bending forces from on to the other, and also ensures that the post can't cam out.

Like with everything else, after a point more doesn't make a difference. It's like digging holes for fence posts. If not deep enough the post will rip out. But once you reach the point where the post will snap before it rips out, there's n o point in going deeper.

Airburst

The trouble is, 24 times 2 is 48, 24 times 2.5 is 60. So I'd be aiming for 50-60mm of depth by your standard, which I've just about got in terms of actual length of seatpost inside the frame, but only the first 30mm of the post is actually touching anything. I suppose a more important question is actually "will 30mm of engagement with the seatpost be enough to prevent damage to the frame?", and there's not really much I can do about that if the answer's "no".

FBinNY 

The 30mm is probably OK since the frame maker determined that it would work for the tubing. Understand that the support length needed in the middle, is different that at the end. I'd prefer more engagement length because IME posts with short sleeves tend to creak due to flex, which more engagement would prevent. But that's not a question of safety or structure.

Airburst

Yeah, I figured it wouldn't have been designed like that if it was gonna break. On reflection, it makes sense that 30mm would be OK if one of the concerns is the post camming out, obviously it needs to have the end clamped in order to do that.

I haven't noticed it creaking at all, so I think I've gotten lucky in that regard...