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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.

enter image description here.

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    If all surfaces are reasonably clean and smooth the presence of the hanger should make no difference in the relationship between bolt torque and the pressure applied to the seat post. – Daniel R Hicks Jun 11 '17 at 1:40
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    @DanielRHicks Well, if the hanger doesn't allow the clamp to close properly, then it does make a difference. And I believe it's what is happening in my case. – Alessandro Cosentino Jun 11 '17 at 20:46
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    I accepted @mikebaranczak's answer as it gave me the direction to the solution. – Alessandro Cosentino Jun 11 '17 at 22:07
  • @Criggie Thanks a lot for your suggestions too! I upvoted your answer. – Alessandro Cosentino Jun 11 '17 at 22:08
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When the bolt is tightened, is the cable hanger free to move around? If it is, then it can't be causing the problem. But if it's held in place rigidly by the clamp, it probably means that it's preventing the clamp from closing all the way. In other words, the clamp is clamping the hanger, not the seatpost like it's supposed to.

Another thing to investigate: remove the seatpost and the bolt, and check for debris in the seat tube slot. You should be able to slide a credit card in there easily. I had a problem like that once, caused by a sloppy paint job that left some overspray in the slot, which prevented the two parts of the clamp from coming together. I used a hacksaw blade to clean it out.

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    Excellent point. I'd put the hanger directly under the nut and not between the "blades" of the seat post clamp. Not under the bolt side because they tend to have a lug to stop them spinning. You'd also route the brake cable on the same side of the seatpost as the seatpost clamp nut. – Criggie Jun 11 '17 at 6:26
  • The cable hanger is held rigidly by the clamp and it is not free to move around. That's why I suspected that could be the issue. – Alessandro Cosentino Jun 11 '17 at 9:56
  • @Criggie Can you please elaborate on the solution you propose? I am not sure I understand it. I would love to see a picture of where you would put the hanger. – Alessandro Cosentino Jun 11 '17 at 10:02
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2.8Nm for a collar on a steel/aluminum post is incredibly small. My carbon frame bike (with aluminum seatpost) specifies 5nm and 5-7 is a common value (Refer park tool website). I probably run around 2Nm, as I use carbon paste and have it no tighter than needed to stay put, but would happily go higher if needed.

First have you cleaned and greased the threads of the clamp bolt. This makes a big difference to the clamping force at a given torque. If you want to stay at 2.8nm, try carbon paste. Its designed to increase friction and reduce required clamping forces on carbon, but works well on metals.

Without suggesting you become "A gorilla with a spanner" - You could "over torque" the clamp (or replace the clamp that has a larger number printed on it). I can't imagine crushing a steel frame/aluminum post at any reasonable torque you can get onto the bolt size a seat post clamp has. I would have a guess many seat posts on steel bikes are clamped at 10 times or more that value with the only problem being getting them undone when needed.

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    +1, especially the carbon paste. Only other suggestion is to make sure the bolt thread and shoulder are greased, because if you're actually sticking to 2.8N then that amount of torque doesn't have a lot to lose to thread friction. – Nathan Knutson Jun 11 '17 at 4:57
  • Although this answer is useful, it doesn't add much to other threads like this one: bicycles.stackexchange.com/q/37108/1621 My question was specific about the brake hanger problem. Thanks anyway. – Alessandro Cosentino Jun 11 '17 at 10:43
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@mikebaranczak I'm putting this as a separate answer, but feel free to incorporate this in yours.

enter image description here

Alessandro, Mike's suggestion is that the thickness of the hanger in the BLACK position may be enough to stop the clamp closing around the seatpost.

In theory you should be able to move/wiggle the hanger with the seat post clamp done up fully. If you can't, then all the pressure is on the hanger. It may be possible to file away some metal of the seat clamp and/or hanger, but that may create other problems later.

My suggestion is to put the hanger in the RED part so the seat clamp can still do up. Downside is it means the cable-line is not centered, and may mess up the geometry of your brakes.

If you disconnect the hanger from the seatclamp, does the seatpost still slip?

Your other option is to replace the whole seat clamp with something like this:

https://www.universalcycles.com/images//attributes/large/136755.jpg

  • Thanks for making this into an answer, @Criggie. > If you disconnect the hanger from the seatclamp, does the seatpost still slip? Hard to answer, the slippage is very slow: 1cm every 3 hours or riding or so. And I don't feel like riding for hours without rear brake :) – Alessandro Cosentino Jun 11 '17 at 10:35
  • I thought about the Paul hanger, but it's not really a clamp, it's something extra that you put around the seatpost. I see two downsides with that: 1) price, I can't find it anywhere for less than 40$, 2) aesthetic: it would mean having two clamps basically..kind of ugly. – Alessandro Cosentino Jun 11 '17 at 10:38
  • It's not a seat clamp really, see here: paulcomp.com/wp-content/uploads/2015/08/… It's something that clamps around the seatpost. – Alessandro Cosentino Jun 11 '17 at 10:42
  • @AlessandroCosentino OK its a bad example, but a quick google image search shows there are many seat clamps with integrated hangers. Or explore filing down as a first option, if its the hanger obstructing the clamp from closing. – Criggie Jun 11 '17 at 19:53
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    I forgot to mention that the solution of your first picture is not possible in my case, because the bolt goes inside an inlet, see this for reference: dbyvw4eroffpi.cloudfront.net/product-media/196U/532/532/… – Alessandro Cosentino Jun 11 '17 at 20:49

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