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  1. #1
    Senior Member BeckyW's Avatar
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    Setback Seatpost Strength

    I'm thinking of getting a Thomson Elite Setback seatpost, and have a question for you math/physics wizards. This product description says:

    The Thomson seat post has a clamp, head, and upper tube that are strong enough to withstand 350 foot-lbs of torque. The tube will start to yield and bend at the seat tube clamp at about 250 foot-lbs of torque. All other brand posts we tested flew apart at less than 150 foot-lbs of torque.
    I weigh 380. Should I worry? I know they make them stronger, but I also know the setback will make it more likely to bend.

    Thanks!
    "You must do the thing you think you cannot do." - Eleanor Roosevelt

  2. #2
    Extra Medium Member redtires's Avatar
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    errr......if I'm not mistaken, torque is a measurement of twisting force, so it really would not technically apply to your particular question. Perhaps Thompson is not using the term in it's true meaning. But, it is one of the strongest posts on the market as far as I know and I wouldn't think you'll have a problem. But, what I would do is periodically inspect the post for any type of deviation from anything but straight. Out of curiosity, does Thompson show any force tests that use "vertical" stresses as a term?

  3. #3
    Senior Member BeckyW's Avatar
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    I wish I could find that info... no luck, though.
    "You must do the thing you think you cannot do." - Eleanor Roosevelt

  4. #4
    Sensible shoes. CastIron's Avatar
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    If I had to pick a setback post for you, that would be it. Heck I just bought one myself (no setback).
    Mike
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  5. #5
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    Quote Originally Posted by redtires View Post
    errr......if I'm not mistaken, torque is a measurement of twisting force, so it really would not technically apply to your particular question. Perhaps Thompson is not using the term in it's true meaning. But, it is one of the strongest posts on the market as far as I know and I wouldn't think you'll have a problem. But, what I would do is periodically inspect the post for any type of deviation from anything but straight. Out of curiosity, does Thompson show any force tests that use "vertical" stresses as a term?
    Redtires
    No you are generating a torque on the post. The seatclamp will be a couple inches in front of the load, so a rotational force or torque is being applied.

    Becky
    Assuming something on the order of 4 inches of setback or 1/3 foot, you will be generating 125 ft lbs of torque at a static load. Even with a very big bump, I have trouble believing you would double that, much less triple it.

  6. #6
    Senior Member jaxgtr's Avatar
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    you should have no problems with this post.
    Brian | 2013 Cannondale SuperSix 5 | 2014 Trek CrossRip Comp | 2003 Trek 7300
    Quote Originally Posted by AEO View Post
    you should learn to embrace change, and mock it's failings every step of the way.

  7. #7
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    Why don't you e mail them and ask.

  8. #8
    The Site Administrator: Currently at home recovering from a couple of strokes,please contact my assistnt admins for forum issues Tom Stormcrowe's Avatar
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    Take the bumps by unloading the saddle slightly with your leg muscles. You should do that anyway. That significantlyreduces any lateral or torque stress. I'd say if it bends, just don't mention weight, just try for the warranty.
    on light duty due to illness; please contact my assistants for forum issues. They are Siu Blue Wind, or CbadRider or the other 3 star folk. I am currently at home recovering from a couple of strokes. I am making good progress, happily.


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  9. #9
    Extra Medium Member redtires's Avatar
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    Quote Originally Posted by Halthane View Post
    Redtires
    No you are generating a torque on the post. The seatclamp will be a couple inches in front of the load, so a rotational force or torque is being applied.

    Becky
    Assuming something on the order of 4 inches of setback or 1/3 foot, you will be generating 125 ft lbs of torque at a static load. Even with a very big bump, I have trouble believing you would double that, much less triple it.
    True...I realize that you can use the moment arm formula and it is technically called torque. And yes, I had to look that up

  10. #10
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    Quote Originally Posted by redtires View Post
    True...I realize that you can use the moment arm formula and it is technically called torque. And yes, I had to look that up
    Worry not... I spent a long time intending to be, and as a physics/engineering major...

    Then I dropped out of college and became a cook.

  11. #11
    Perma-Clyde (51)'s Avatar
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    Quote Originally Posted by BeckyW View Post
    I'm thinking of getting a Thomson Elite Setback seatpost...


    ... Should I worry?
    I don't think so. I've been riding one for almost a year (3,000+ miles) hovering between 330-350. No bend yet, and I have 8" of post exposed!

    How much post do you have exposed? Why don't you get one without the setback?

    I'd be more concerned riding whatever seatpost you have now.
    http://www.trailerparkboys.org/forum...fault/beer.gif In wine there is wisdom, in beer there is freedom, in water there is bacteria. -Ben Franklin

  12. #12
    Senior Member BeckyW's Avatar
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    I'm not sure how much will be exposed - the one I have now is about 14 cm exposed from clamp to rails, but it's suspension, so it gets mashed down a bit (also keeps slipping! grrrr... how awful would it be to wash all the grease off it and rub some dirt on it to make it stickier?? )

    I want to try setback because I keep finding myself trying to scoot back in the seat, and sitting like that, I'm getting all chafed! Ouch! My knees are way out in front of my toes with the pedals @ 3:00, and it's hard to keep the balls of my feet on the pedals. The height's about right (leg extended with heel on the pedal just lifts me off the seat)... so... setback makes sense, right?
    "You must do the thing you think you cannot do." - Eleanor Roosevelt

  13. #13
    Senior Member barba's Avatar
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    Your bike will bend first, I would bet. The Thomson post is a really well made piece of equipment.

  14. #14
    Gorntastic! v1k1ng1001's Avatar
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    I can't imagine a better seat post overall.

  15. #15
    Evil Genius oopfoo's Avatar
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    Quote Originally Posted by barba View Post
    Your bike will bend first, I would bet. The Thomson post is a really well made piece of equipment.
    +1 I concur. The Thomson Elite is CNC-made from solid aluminum billet. There are few seatposts more significantly overengineered than this particular post. It's extremely well made.
    -- Michael

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