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Multiple seatpost clamp bolts breaking

pirniepirnie Posts: 242
edited August 2018 in Workshop
Hi All,

I have a winter/turbo Aluminium Trek bike that's a few years old now. I'm having an issue that basically every time I adjust the saddle height I end up snapping the bolt on the seatpost clamp. This has happened now with 4 different designs of clamp, every time using a torque wrench to less than the torque specced on the clamp.

I can't get my head around what the issue is. The bolt just keeps going in with little resistance then snap before the seat post is clamped tight (can be wiggled round left and right by hand). The last clamp I had lasted a good while (it was a Hope MTB clamp) before the same thing happened again tonight.

Has anyone come across this before? The seatpost is the correct size for the frame according to the spec on Trek's website.

All advice appreciated.

Posts

  • MatthewfalleMatthewfalle Posts: 17,380
    sounds like you have the incorrect seat post size. too small so matter how much you tighten the vlamp the frame has noyhing to clamp onto

    #backtobasics
    Postby team47b » Sun Jun 28, 2015 11:53 am

    De Sisti wrote:
    This is one of the silliest threads I've come across. :lol:

    Recognition at last Matthew, well done!, a justified honour :D
    smithy21 wrote:

    He's right you know.
  • pirniepirnie Posts: 242
    sounds like you have the incorrect seat post size. too small so matter how much you tighten the vlamp the frame has noyhing to clamp onto

    #backtobasics

    That was my initial thought too... The post is a 27.2mm Thompson Elite, and everything I can find online suggests the original seatpost was also 27.2mm. I'm sure I matched the sizes when I bought the new post too (sadly I don't think I have the original any more). There's not a common size very close to 27.2 is there?
  • pirniepirnie Posts: 242
    Hmm, having got the tape measure out (not the most precise I know but sadly I don't have any calipers) I get the feeling the Thompson Elite is measuring slightly less than 27mm, I wonder if that's the issue.

    Not good quality control considering it's laser etched 27.2
  • cooldadcooldad Posts: 32,601
    Firstly, throw away the torque wrench. Totally unnecessary for a seat clamp on an aluminium frame.

    Then measure the seatpost circumference and use pi (basically divide by 3.14) to work out the diameter. Much easier than trying to measure the diameter with a tape measure.
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  • pirniepirnie Posts: 242
    cooldad wrote:
    Firstly, throw away the torque wrench. Totally unnecessary for a seat clamp on an aluminium frame.


    In general I'd agree and generally its for my carbon bike... However when I'd sheared 2 bolts I thought it might be worth checking what torque I was going up to
  • crankycrankcrankycrank Posts: 1,830
    cooldad wrote:
    Firstly, throw away the torque wrench. Totally unnecessary for a seat clamp on an aluminium frame.

    This. OP didn't mention what brand/type of torque wrench but if it's a clicker style these can often go out of calibration and at the light torque measurement required for a seatpost clamp a little out of calibration can be more than enough to snap bolts. I've run into this building engines where torque accuracy is a must for some bolts and have had various clicker types going out of calibration so far that it ended up taking almost twice the marked force to get it to click. Something like this beam type wrench is cheap and stays accurate. https://www.ebay.co.uk/itm/Neiko-03727A ... SwpbVbYyPV
  • imposter2.0imposter2.0 Posts: 11,386
    Something like a Ritchey torque key, preset to 5nm is pretty much all that is needed for seat clamps (and bars). Either way, sounds like either the post itself is too small, or the clamp is too big. It would be useful to know where the bolts are snapping and whether they are bending at all...
  • pirniepirnie Posts: 242
    All good points bout the torque wrench. However, I have enough experience to know I'm not over torquing the bolts.

    Given it's been several different designs of clamps I'm definitely leaning towards the post being too small. A quick google seems to show Thompson posts do measure up small compared to others. I might try a coke-can shim tomorrow once I can get to the LBS for a new seatpost clamp
  • cyclecliniccycleclinic Posts: 6,865
    Torque wrenches are useful if they are accurate but they are not always. Also feel is important. You should be able to feel how tight you can go before rounding or snapping.

    If the post is the right size, carbon paste can help.vi have had this numerous times with the correct size post for the frame but it sinks when riding. Also if there is a 0.2mm mismatch get a drinks can and cut up and make a shim. This trick works a treat. Some frames have seat tubes slightly bigger than they should be.

    All this however is part of basic mechanics 101. If your breaking seat post clamp bolts I would suggrst you stop working on bikes and learn some skills because what else are you getting wrong. Don't be a torque wrench slave.
    http://www.thecycleclinic.co.uk -wheel building and other stuff.
  • pirniepirnie Posts: 242
    Don't worry, I'm definitely not a torque wrench slave. I've worked on my own bike for years and on an ali bike will normally tighten things by feel and am pretty confident on it. I'd just never met this before so thought I'd ask advice. Like I said earlier, the bolts were shearing well before they were getting anywhere close to a "normal" tightness.

    I'll try the drinks can shim later and see if that makes a difference.

    Cheers for all the advice!
  • imposter2.0imposter2.0 Posts: 11,386
    In case you missed the question earlier - where are the bolts breaking? At the end? in the middle?
  • pirniepirnie Posts: 242
    Imposter wrote:
    In case you missed the question earlier - where are the bolts breaking? At the end? in the middle?

    It's varied, but usually pretty close to the head, or a few mm down the threads
  • pirniepirnie Posts: 242
    Okay, so I have it figured out I think.

    Because of the way the setback on the Thompson Elite works (it's a bend in the seatpost) and the fact that my saddle height puts it close to the bend the seatpost (about an inch below) it's slightly smaller at the point I'm clamping it than on the main straight portion of the post. You get a lot of extra play in the seat tube as you approach that point.

    ThompsonSetbackRetouched2.jpg?w=430&h=430&a=7

    I've shimmed it with a coke can and it seems to tighten up a lot better now
  • Ben6899Ben6899 Posts: 8,983
    Is it not worth fitting a seatpost that doesn't need a shim?
    Ben

    Bikes: Donhou DSS4 Custom | Condor Italia RC | Gios Megalite | Dolan Preffisio | Giant Bowery '76
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  • pirniepirnie Posts: 242
    Ben6899 wrote:
    Is it not worth fitting a seatpost that doesn't need a shim?

    Yeah, I will do, but this should at least allow me to ride this weekend while I get my hands on one
  • Ben6899Ben6899 Posts: 8,983
    Nice one.
    Ben

    Bikes: Donhou DSS4 Custom | Condor Italia RC | Gios Megalite | Dolan Preffisio | Giant Bowery '76
    Instagram: https://www.instagram.com/ben_h_ppcc/
    Flickr: https://www.flickr.com/photos/[email protected]/
  • Vino'sGhostVino'sGhost Posts: 4,129
    the thompson stuff is good but if you have that little seat post showing surely you'd be better off with a straight post with a set back clamp?

    irrespective of everything else, if youre shearing / snapping bolts as you tighten them then unless theyre made of cheese you're using more than 5nm of force.
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