I have an aluminum seatpost in a steel seat tube and I have been noticing some slippage lately. The size of the seatpost and of the seat collar match perfectly the size of the frame. I inspected the seatpost inside and everything looks OK there as well: there is no grease, neither dirt. From a web search, I see that this problem is common for carbon seatposts/frames and there are solutions for that (fiber grip gel, for example). It's not as common for aluminium seat posts.
The seat collar manufacturer suggests a torque of 2.8 Nm on the bolt and I double-checked that as well. My last hypothesis of what can cause the slippage is the fact that I have a brake hanger for a cantilever brakes on the seat collar.
Is it possible that because of the seat hanger the seat collar doesn't clamp the post correctly? Should I try to increase the torque on the seat collar bolt because of the brake hanger?
Edit (includes solution)
I closed the seat collar on 3Nm without the hanger and realized that the gap is way too small for the hanger to fit there, see the picture. This would explain why the collar wouldn't clamp perfectly, although I haven't tried to ride it and verify that the seatpost is no longer slipping, but I am confident this is the solution.