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When I first got this bike, I got the saddle adjusted almost perfectly. Recently it slipped back (so that the nose sticks up). I loosened the bolt under the saddle, tilted the saddle back into the proper position, tightened the bolt up, and within a week the nose had tilted back up. Did the same thing again, but cranked the bolt even tighter; still happened.

I noticed that there's visible "scarring" or scraping on the top of the seat tube where the clamp that holds the saddle (not sure the proper term for those pieces) grabs onto the tube.

This is a 2009 Surly Long Haul Trucker (touring style bike; equipped like a commuter) with the stock seatpost (pretty sure it's 27.2mm. No real branding on the seat post). I'm a fairly heavy rider (near 250 pounds) and I do sometimes ride over some pretty rough stuff, so I suspect that might be part of the problem.

So far I'm seeing maybe 3 options:

  1. Get a hex tool with more leverage and tighten that thing down even more
  2. Some sort of super glue or epoxy (or maybe just a bit of old innertube?) to keep the seatpost from sliding back
  3. Replace the seatpost (and clamp assembly, of course). Except I don't know what to replace it with that wouldn't just get the same problem again...

Update: Pics: (sorry, phone pics, but I think they're workable)

First: overall picture, very careful to take the picture close to level. Top tube has just a teensy amount of slant, almost level. Overall picture of saddle angle

Closer in (you can see some of the scarring on the seatpost): Closeup picture of saddle angle

Attempt at closer-in picture of scarring after disassembly, but can't focus close with this camera: seatpost scarring

Here's the saddle itself, which has no bending, no visible damage to the rails, etc.: seat is fine

Here's the actual pieces of the "clamp". On the right is the bottom piece that sits on top of the seatpost and under the seat rails, there's matching "scarring" at the points that touch the seatpost. On the left is the top piece that goes on top of the rails. On top is the bolt that goes through everything from the bottom (with a curved sort of washer thing that fits into a matching curved area on the seatpost). The opposite side of the "bottom piece" basically looks just like the visible side of the "top piece", and the opposite side of the "top piece" is boring. clamp pieces

Update 2: Used a hex tool with a longer arm (more leverage), removed the bolt, greased it up and tightened it as much as possible, not worrying about damaging the already damaged seatpost further. The grease helps to allow a bit more tightening. This held up for a couple months but eventually seat started sliding back again.

Final solution: Thomson Elite seatpost, specifically the shorter model with a setback in silver ordered through my LBS. (they had black in stock but decided I was willing to wait an extra week to get silver) It's a two-bolt design that seems to be the top-end for seatposts that don't slip. I considered a Salsa two-bolt seatpost that was a less expensive, but it didn't seem to be as heavy-duty. I also considered a Nitto S-83 which is a two-bolt seatpost about the same price as the Thomson and has more of the classic touring look to it, but the Thomson seemed to be a bit better engineered and I like the markings on the Thomson that help you keep the same adjustment.

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I wonder if you're riding too far back on the seat (ie, the seat might need to be moved straight back), and when you're hitting the bumpy area, you might have just enough leverage to tilt the seat up. Also, pictures are worth a 1000 words :) –  Jared Harley Aug 26 '10 at 18:13
    
@Jared: Good idea, but I would say the seat should be moved straight forward not backward. –  jdv-Jan de Vaan Aug 26 '10 at 18:20
    
@jdv dangit, you're right, and now it's too late to edit my comment :) –  Jared Harley Aug 26 '10 at 18:33
    
The saddle and its rails are fine. It's the clamp for the seatpost that slides back. I'll snap a couple pictures real quick. –  freiheit Aug 26 '10 at 20:02
    
But sliding the seat forward or backward will change my leg angle, distance to handles, etc... was pretty happy with the seat adjustment before it starting tilting back... And I should've said "tilts", not "slides" in that previous comment... –  freiheit Aug 26 '10 at 21:05
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4 Answers 4

up vote 7 down vote accepted

The problem is that there is only one bolt holding the seat at the angle you want. Put enough torque on the back of the saddle and can overcome the friction that's holding the seat in place.

A few options:

  1. Tighten up that bolt as much as you dare (but you've already tried that).
  2. Increase the friction between the seat post and the bottom of the seat clamp -- either scuff it up or use some carbon grease (ask your LBS, I've used it to keep my seat post from sliding down).
  3. Switch to a seat post that uses a two-bolt design to hold the seat in place. This is what my current and previous bikes used, one bolt pulls the nose down and the other tilt the seat back. Get enough tension on the two and your seat stays right where you want it to.
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I think I'm gonna have to cave in and get a two-bolt seatpost. Any specific suggestions? Like any of these: velo-orange.com/vogrcrusepol.html rivbike.com/products/show/nitto-s-83-seatpost/11-078 ? salsacycles.com/components/shaft_seatpost rei.com/product/770147 –  freiheit Aug 29 '10 at 18:25
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I don't know the workings of the Salsa post, but the other three are the two-bolt type I was thinking of. What you'll want to watch out for is the post diameter (must match your current post) and the amount of setback -- more might be better in your case since it'll reduce the amount of torque applied to the back of the saddle. –  darkcanuck Aug 30 '10 at 1:31
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I also have a 2009 Surly Long Haul Trucker with the same seat post and the same problem. I was able to tighten the single bolt enough to prevent the seat from sliding back. I'm also 100lbs lighter. I think the seat post is a terrible design and one of the cheaper parts that came with the bike.

I use a Thompson Elite seat post on my mountain bike which has two bolts that prevent the slipping problem. It also has markings on the seat clamp so that you can put the seat back to it's known angle after taking it off and reinstalling it.

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I talked to the folks at my LBS and the Thompson Elite seatpost was the one they recommended. –  freiheit Sep 7 '10 at 16:26
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As Jared suggested you should check the seat hasn't shifted. Also it could be an idea to check that the "grating" that causes the friction has good grip before you start turning it. Loosen the bolt, adjust saddle, tighten it a little and try turn it a little bit. If it wasn't getting good grip, it may shift a few degrees. Then tighten. If the seatpost is aluminum don't force it.

Otherwise it is probably time for a new saddle...

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It doesn't seem like the seat has slid back or shifted any, it's that the clamping mechanism that holds the saddle to the post has slid back on the post ... There's a pic of the underside of the saddle itself now: the saddle is totally fine except for some of the shiny having rubbed off the logos on top... –  freiheit Aug 26 '10 at 21:08
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Found this helpful idea in another forum, it helped me so I'm passing it along. Get some sandpaper, glue two strips back to back so both sides are rough. Wrap around the rails and tighten down. Seems to work for me ! Good Luck

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I was thinking I would have used about #80 emery cloth. –  Daniel R Hicks May 21 at 23:13
    
Maybe I wouldn't use this as a permanent solution but to fix the issue until you can get a more permanent fix, e.g. while on a journey. –  Benedikt Bauer May 22 at 8:16
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