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Broken seat post

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Old 10-12-15, 12:13 AM
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jyl
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Broken seat post

A friend brought me his bike tonighrt. The seat post broke off, snapped fully through, right below the seat clamp. He took a hard spill, may have broken his wrist. Scary stuff. I'm going to replace his seat post, do some other work on his bike.

Anyone have a similar thing happen?

My seat clamp broke a few months ago, but in such a way that the seat was still retained, but just tilted nose up, and I was going 3 mph at the time. Lucky.

Mine was an old JPR seat post. His was a no-name aluminum post, the kind where the seat clamp itself clamps around the round top of the post. I'm going to find him a used post, and hopefully it will be solid.
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Old 10-12-15, 12:23 AM
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If it was a plain pin type post of this type, and made of aluminum, I'm not all that surprised that it snapped at the saddle clamp. This is a classic well proven design that has been around for decades in steel, but aluminum posts of this type haven't been around as long.

The issue is that a poorly made saddle clamp can cut into the post, or crush is slightly, establishing a stress riser. Then it's only a question of time. I suspect that we don't see that many failures of this kind only because the bikes these posts are usually on, simply aren't ridden as much as better bikes, and so the posts don't see that long a service life.

Don't read this as saying these posts are doomed to ultimate failure. It takes a combination of the post, and a poorly made or over-tightened clamp, or lots of twisting of the saddle, to score the post and start the process. Even then it depends on the weight of the rider and an unfortunate distribution of forces, ie the saddle forward or back, to bring about a failure.
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Old 10-12-15, 06:27 AM
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Originally Posted by FBinNY View Post
If it was a plain pin type post of this type, and made of aluminum, I'm not all that surprised that it snapped at the saddle clamp. This is a classic well proven design that has been around for decades in steel, but aluminum posts of this type haven't been around as long. ...
Well, not to nitpick, but aluminum posts of this kind have been around a pretty long while. I had a 1950 Norman Rapide bike that came with a Stratalite 27.2 post, certainly original to the bike. Hilary Stone always has some Reynolds aluminum alloy posts from the 40's on his site:


This is the first such failure I've heard of. That said, the fact of OP's post failure cannot be denied, and your explanation makes a lot of sense.
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Old 10-12-15, 08:14 AM
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I have had two seat posts break on me.

The first was something stupid I did. I scratched a height line on the post so I knew what height I needed if I took the post out. Well that scratch line was more than a scratch and eventually the post broke off right at the frame. Couldn't get the piece out and had to send the frame to a frame builder to get it out. Lesson learned.

The second one was a Syncros post that just failed while was just riding along.

Bottom line, components break sometimes.
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Old 10-12-15, 09:13 AM
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[Touch (knock) wood] none in 4+ decades of riding.. But I'm not one to count grams of weight reduced from that component Part.



I can see the bite into the upper section of aluminum seat posts, by a steel saddle clamp.. making a mark ,
and potentially a stress riser , which the scratch becoming a fracture line clearly demonstrates..

though over the years , better saddle clips have been made, current favorite is Brompton's

It is aluminum itself mostly..

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Old 10-12-15, 09:49 AM
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i had a set of flat MTB handlebars break right at the stem. they were light weight (too thin to use for 10+ years) and the stress at the clamp eventually created a crack. the brand was SPECTRA or SPECTRE or something. came stock on the early '90s Jamis Dakota MTB. it was low speed and i didn't fall. so... good times.

this summer I bought a Gunnar Rock Hound with a stuck post. I dissolved the post out of the frame with 100% LYE (sodium hydroxide). this works fine if:
1. you're going to paint the frame (there's a bit of heat generated, and it's caustic soda, so i think it trashes the paint. i had pre-trashed the paint with a torch, so not sure how much damage the lye does)
2. the frame is steel (the lye dissolves aluminum, not steel or plastics or rubber gloves)
3. you have a couple of hours to stand around repeatedly pouring lye and water into a frame plugged with something (like balled up latex gloves)

good luck.
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Old 10-12-15, 10:04 AM
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There were some gouges on the pin post from the saddle clamp. I couldn't tell if any intersected the fracture, but I suspect you guys correctly explained the cause. I feel bad that I never inspected the old post on the previous occasions that I serviced his bike, but I didn't know about this failure mode.

He says his wrist will heal, it is in a brace and hopefully it is just a bad sprain. Going to keep him off the bike for awhile, I think.

I replaced it with a used post from my parts box. Not a pin post. A rather heavy "Kalin" post, think I got it off a curbpicked bike.

Going to replace his chainrings today, he's been having chain skipping problems and I've already replaced freewheel and chain. I told him to ride in the big ring and see if that reduced the skipping, it very much did, so I think that confirms worn little ring and might as well replace his big ring too. I will look in my parts box, I may have some lightly used chainrings in there.

Edit: and yes, I had some used chainrings that look okay, we'll see how they work out. Trying to save him some $$$.

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Old 10-12-15, 11:42 AM
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You can see that it broke at the transition from the shoulder to the top section.

That type of machining (sharp edges, no radiusing) creates a stress riser just as surely as a deep scratch...
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Old 10-12-15, 12:01 PM
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I had an Avocet seatpost break through the clamp and the seat fall off. 3 miles before, I was riding no-hands down a steep concrete slab downhill (celebrating the near finish of a very hard ride) when I felt a click through the whole bike. Everything seemed fine (except my heartrate) so I kept on riding.

If you have any Avocet machined aluminum parts, seriously consider replacing them. Every piece of their's I have owned has broken except the chainrings (which just wore out as chainrings do) and the left crank (retired after the right broke). And all have broken JRA.

Ben
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Old 10-12-15, 01:08 PM
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Originally Posted by mrv View Post
this summer I bought a Gunnar Rock Hound with a stuck post. I dissolved the post out of the frame with 100% LYE (sodium hydroxide). this works fine if:
1. you're going to paint the frame (there's a bit of heat generated, and it's caustic soda, so i think it trashes the paint. i had pre-trashed the paint with a torch, so not sure how much damage the lye does)
2. the frame is steel (the lye dissolves aluminum, not steel or plastics or rubber gloves)
3. you have a couple of hours to stand around repeatedly pouring lye and water into a frame plugged with something (like balled up latex gloves)

good luck.
Did it last year with my surly after trying every thing else to get the broken and stuck seatpost out of it. Didn't hurt my paint but the paint wasn't pretty to begin with.
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Old 10-13-15, 09:42 PM
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Originally Posted by jyl View Post
Originally Posted by rmfnla View Post
You can see that it broke at the transition from the shoulder to the top section.

That type of machining (sharp edges, no radiusing) creates a stress riser just as surely as a deep scratch...
Is that a solid tube that is machined so it is thin at the top and thick at the bottom? Plus smaller in diameter. So the thin part would be the weakest point.

I avoid any of those posts with clamp on seatpost clamps. Always integrated post/clamps if I can find them, although there are a few different styles.

Most of my miles are with the old Campy post which I've been riding for more than 3 decades, and perhaps over 4 decades old now.

Let me get this straight. The post failure caused the crash, and not the other way around, the crash causing the failure. Road bike or MTB? Bumps?
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Old 10-13-15, 10:09 PM
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Correct, my friend was just riding along, and the post broke, sending him to the ground. Old Univega Sport.

Funny story. He fell outside the office of Rogue Brewing. Some guys there rushed over, helped him up, checked his injuries. One of them put my friend and his bike in a SUV and drove him home. They exchanged business cards. When my friend looked at the card later, he realized the good Samaritan was the CEO of Rogue Brewing.

Met my friend for a beer today. His wrist is okay. His bike is running well. All is well.

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Old 10-14-15, 11:32 AM
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Originally Posted by jyl View Post
Correct, my friend was just riding along, and the post broke, sending him to the ground. Old Univega Sport.

Funny story. He fell outside the office of Rogue Brewing. Some guys there rushed over, helped him up, checked his injuries. One of them put my friend and his bike in a SUV and drove him home. They exchanged business cards. When my friend looked at the card later, he realized the good Samaritan was the CEO of Rogue Brewing.

Met my friend for a beer today. His wrist is okay. His bike is running well. All is well.
Hope the brew was Rogue.
I'll look for it.
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Old 10-14-15, 02:33 PM
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It wasn't, but I'm going to start trying Rogue beers now too.
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Old 10-14-15, 04:42 PM
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Originally Posted by CliffordK View Post
Is that a solid tube that is machined so it is thin at the top and thick at the bottom? Plus smaller in diameter. So the thin part would be the weakest point.

I avoid any of those posts with clamp on seatpost clamps. Always integrated post/clamps if I can find them, although there are a few different styles.

Most of my miles are with the old Campy post which I've been riding for more than 3 decades, and perhaps over 4 decades old now.

Let me get this straight. The post failure caused the crash, and not the other way around, the crash causing the failure. Road bike or MTB? Bumps?
Exactly...
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