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seat post failure analysis

Old 07-29-20, 04:14 AM
  #1  
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seat post failure analysis

my riding style must be too harsh...bent the 35 cm steel post.





i only weight 140 pounds.

Last edited by mtb_addict; 07-29-20 at 04:51 AM.
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Old 07-29-20, 04:23 AM
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Also looks like the clamp's been tightened too much. Was the tube dented by the seatpost clamp before it got bent?
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Old 07-29-20, 07:47 AM
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Cubewheels claim looks right. Also, the seat post looks rather long for this frame.

In the past, Iíve asked a metal fabricator to weld a 7/8Ē diameter (the size of the seat clamp) steel tube inside a 1Ē diameter one to build a (VERY) heavy, but tough, 450mm seat post. About 120mm of it was inside the frame, just for precaution.

Iím not using that anymore, but it held me at 345lb back then. Maybe thatís your solution.
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Old 07-29-20, 08:26 AM
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My guess is that it's just an inexpensive, weak seatpost. Not made to be extended that far out of the seat tube. Get any decent post with a clamp head for the seat on it and you should be OK. A few choices here for under $20. Measure how far the post you have extends from the frame clamp and add a minimum of 3"/75mm. Also make sure you know the correct diameter of the seatpost for your frame. https://www.google.com/search?ei=Y4U...sclient=psy-ab

Last edited by Crankycrank; 07-29-20 at 08:29 AM.
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Old 07-29-20, 08:44 AM
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How much of the seat post is below the top of the seat tube?

Might be the picture angle, but it looks as if the top part of the seat post is a bigger diameter than the internal diameter of the seat tube. Was there something funky going on with that model seat post where it narrowed up at the bottom.
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Old 07-29-20, 09:16 AM
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It's Mild steel , chosen for keeping cost low, bike cheap, not chrome-moly / 4130 that is stronger, but costs more..
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Old 07-29-20, 09:41 AM
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Originally Posted by dwsmartins View Post
Cubewheels claim looks right. Also, the seat post looks rather long for this frame.

In the past, Iíve asked a metal fabricator to weld a 7/8Ē diameter (the size of the seat clamp) steel tube inside a 1Ē diameter one to build a (VERY) heavy, but tough, 450mm seat post. About 120mm of it was inside the frame, just for precaution.

Iím not using that anymore, but it held me at 345lb back then. Maybe thatís your solution.
Definitely that....I'm not sure what the reccy is for a step-through frame is WRT max seatpost extension, but I'm guessing this is way passed that.

Also...the rusted Builder Bob bolt in place of a proper seatpost binder bolt is suspicious too...
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Old 07-29-20, 09:48 AM
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the post is well inside the tube. more than 8 cm inside.

Last edited by mtb_addict; 07-29-20 at 10:06 AM.
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Old 07-29-20, 10:28 AM
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but it is soft, common steel , like 1010..
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Old 07-29-20, 10:49 AM
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Looks to me like a case of poor quality steel. I doubt the pinching at the back would have happened on a good post. I've never seen that and I've owned some funky bikes over the years.

Now that bend looks just like the bend my Hupel Rider post took (except yours is bent considerably more) when I hit a frost heave going >50 mph (80 kph) racing a mountain descent. Hupel Rider - a cheap post of soft aluminum. Replaced it with a Zeus post of much higher quality and had no more issue.

There are lots of steel posts like yours that are strong enough to have as much showing as yours and suffer all kinds of abuse. I've never heard of a Raleigh 3-speed post bending and some of those have seen epic abuse. If they bent, my dad would have passed on that story to me. He an d his buddies were some of those abusers. Measure up the diameter of your post and look for a new one.

Ben
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Old 07-31-20, 12:14 PM
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the diameter is 25.4".

i noticed in reviews on Taobao that some little kiddie bikes very long seat post with 25.4.


so, this could explain it...this long seat post I got is actually for kids bikes...not designed to take the weight of an adult.

Last edited by mtb_addict; 07-31-20 at 12:24 PM.
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Old 07-31-20, 12:36 PM
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I bought a 1" ChroMoly seat post Imported thru Seattle Bike Supply under their Red Line BMX brand.
it worked fine with a foot extending out of the frame..

Wald is a purveyor of a similar diameter post but of mild steel..
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Old 08-03-20, 11:50 AM
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what about "stainless steel"?
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Old 08-03-20, 12:12 PM
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i assume aluminum 25.4 seat post is no good?
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Old 08-03-20, 12:20 PM
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what about "stainless steel"? & i assume aluminum 25.4 seat post is no good?


You have to be specific , as to alloys the ones like Kalloy are rather thick wall because its not a high strength alloy ..

6061- T 6 is stronger , so is adequate though the tube wall is thinner , so weighs less ..

some companies broach out the hole so tube wall thicker in alignment with the stresses applied. ..

likewise stainless steel what alloy in specific.. ?





Last edited by fietsbob; 08-03-20 at 12:24 PM.
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Old 08-04-20, 05:13 AM
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Originally Posted by mtb_addict View Post
the diameter is 25.4".
That would be the strongest seatpost ever!
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Old 08-04-20, 07:05 AM
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Do others see a bent seat tube, centered at the dropped "top tube"? It might be optical distortion from the camera but I've seen enough bent STs on ladies frames to not look for this too. This hinge point is why mixte frames are so much stiffer, their "top tubes" go past the ST and to the rear drop outs which move the compressive forces from a ST bending type to a drop out one. Andy
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Old 08-04-20, 09:17 AM
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Top picture shows the buckling deformation of the type seen on the underside of top & down-tubes

On 'totaled' bikes suffering front end crash damage..
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Old 08-04-20, 11:18 AM
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Probably caused by cheap steel seatpost, plus the clamp on the low-cost frame not being perfectly round.

THe other problem is that if you are resorting to an extra long seatpost, the frame is probably too small.
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Old 08-08-20, 03:50 AM
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Originally Posted by fietsbob View Post
You have to be specific , .. ?


AL 6061
thickness 2.0 - 2.5 mm

only $5 !



Last edited by mtb_addict; 08-08-20 at 03:59 AM.
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Old 08-08-20, 05:37 AM
  #21  
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Many years ago I rode a vintage Schwinn 3sp frequently as a townie; I used to go into the vintage schwinn shop about 3 times a year to replace the bent seatpost.
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Old 08-08-20, 08:38 AM
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only $5 !
Not exactly a budget buster ,,
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Old 08-08-20, 08:41 AM
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Just shear bad luck.
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Old 08-08-20, 09:14 AM
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Nice bike.
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