Snapped the saddle rail bolt on my seatpost

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Re: Snapped the saddle rail bolt on my seatpost

Postby ekib » Thu Dec 13, 2012 1:11 pm

__PG__

I would propose another possibility. That is that the bolt may have actually been loose.
A loose bolt can fail very quickly from fatigue.

Apparently about %85 of bolt failures are due to fatigue.

Have a look here if you're interested;
Saddle bolt failure



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by BNA » Thu Dec 13, 2012 10:26 pm

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Re: Snapped the saddle rail bolt on my seatpost

Postby Mulger bill » Thu Dec 13, 2012 10:26 pm

What sort of post was it? Someone may have spares gathering dust.
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Re: Snapped the saddle rail bolt on my seatpost

Postby Stepr » Sat Dec 15, 2012 1:13 pm

had this problem a while back. you could either find some spares at the local tip shop. or go to a nut and bolt supplier and get an 8mm socket head screw.
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Re: Snapped the saddle rail bolt on my seatpost

Postby __PG__ » Sat Dec 15, 2012 9:47 pm

ekib wrote:__PG__

I would propose another possibility. That is that the bolt may have actually been loose.
A loose bolt can fail very quickly from fatigue.

Apparently about %85 of bolt failures are due to fatigue.

Have a look here if you're interested;
Saddle bolt failure
ekib.

Surely if the bolt was loose I'd notice the saddle creaking and moving around?
No idea what the post originally was, all the markings/logos on it rubbed off years ago.
Have scored an old 2nd-hand post from a mate as a temporary fix. Am planning to go into the LBS for a quick saddle fit and then will look at how much setback I need.
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Re: Snapped the saddle rail bolt on my seatpost

Postby ekib » Tue Dec 18, 2012 1:15 pm

_PG_,

yes you're right, you would expect to notice a loose seat post bolt.
Or perhaps you wouldn't,
From the article,
"Because the lower part of the clamping system is serrated, it is not free to move if the bolt is only slightly loose."



I might try loosening my seatpost bolt and see, or maybe I'll loosen someone else's seatpost bolt :wink:

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Re: Snapped the saddle rail bolt on my seatpost

Postby pacra » Tue Dec 18, 2012 9:39 pm

I have had a saddle bolt failure (ala catastrophic, unfortunately for me at the time).
I got the bolt checked at Monash Engineering and it showed

1 Work hardening very similar to the failed bolt in the article leading to a critical crack then failure.
2 but also when the bolt was measured under magnification it had necked over time (years) from continual tightening.

As you cannot detect necking with the naked eye the moral of the story seems to be to lightly lubricate the bolt at regular periods and possibly consider replacing the bolt every couple of years.

The bolt that failed was an alloy bolt on a Chorus seat post. Steel bolts would be more durable.
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Re: Snapped the saddle rail bolt on my seatpost

Postby toolonglegs » Tue Dec 18, 2012 9:53 pm

I have had 5 or so go over the years... since switching to Thomson posts I have never had a failure.
All of my bikes have Thomson posts now, but I find they do have an annoying problem over time of changing shape or wearing ever so slightly around the clamping area... real PITA especially on mtb and CX bikes when they slip down a touch... hence I now have dual clamp seat post clamps... finally I am happy :wink: .
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Re: Snapped the saddle rail bolt on my seatpost

Postby FXST01 » Thu Jun 06, 2013 5:05 pm

Just had this happen to me on my commute home.
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Re: Snapped the saddle rail bolt on my seatpost

Postby rkelsen » Fri Jun 07, 2013 9:57 am

pacra wrote:The bolt that failed was an alloy bolt on a Chorus seat post. Steel bolts would be more durable.

Alloy bolts have their uses. This ain't one. Can't beat stainless steel for this application, IMO.
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Re: Snapped the saddle rail bolt on my seatpost

Postby find_bruce » Fri Jun 07, 2013 11:30 am

rkelsen wrote:
pacra wrote:The bolt that failed was an alloy bolt on a Chorus seat post. Steel bolts would be more durable.

Alloy bolts have their uses. This ain't one. Can't beat stainless steel for this application, IMO.

Why stainless rather than a steel bolt ?

Either way you are assembling dissimilar metals & I would be using an anti-galling compound
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Re: Snapped the saddle rail bolt on my seatpost

Postby rkelsen » Fri Jun 07, 2013 11:41 am

find_bruce wrote:Why stainless rather than a steel bolt ?

Primarily strength.

I bent/broke several steel bolts before I tried a stainless bolt. Problem solved.
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Re: Snapped the saddle rail bolt on my seatpost

Postby find_bruce » Fri Jun 07, 2013 1:48 pm

rkelsen wrote:
find_bruce wrote:Why stainless rather than a steel bolt ?

Primarily strength.

I bent/broke several steel bolts before I tried a stainless bolt. Problem solved.

Um, last time I checked, stainless has a lower tensile strength than steel, as well as a greater tendency to gall. Of course the fact that you are clamping aluminium alloy means that the ultimate tensile strength doesn't matter much - if you are breaking a steel bolt, any steel, in an aluminium alloy part, you are using junk bolts. By the way I wouldn't use high tensile bolts for this application either - you do not need the tensile strength & they are more brittle & hence prone to fatigue.

edit should have said I was referring to grade 8 steel bolts.
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Re: Snapped the saddle rail bolt on my seatpost

Postby rkelsen » Fri Jun 07, 2013 2:44 pm

You're probably right. I'm not a metallurgist. But it is obviously a lot stronger than the steel bolts that I had tried in there (including the original one)... It's been there for over 3 years. Admittedly, the steel bolts I used were regular ones from Bunnings. I don't think they were Grade 8, or anywhere near it for that matter.
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Re: Snapped the saddle rail bolt on my seatpost

Postby Nobody » Fri Jun 07, 2013 3:22 pm

rkelsen wrote:But it is obviously a lot stronger than the steel bolts that I had tried in there (including the original one)... It's been there for over 3 years. Admittedly, the steel bolts I used were regular ones from Bunnings. I don't think they were Grade 8, or anywhere near it for that matter.
No surprises there. If the steel bolts weren't marked on the head, then they were probably grade 1 or 2. My understanding is stainless is more brittle than steel in general. That is why I was impressed they could use stainless in spokes. But then my DT DB black spokes started to rust (I ride close to salt water) and then I found they could attract a magnet. :roll: Obviously not a proper grade of stainless like 304, just "stainless". Impressed no longer.

I'd be surprised if I saw any grade 8 bolts on a bike. High grade bolts are usually found in car suspension parts where the torque settings can be quite high, like 160Nm. Try torquing one of them up when on your back under a car.

__PG__ might be better off with a two bolt clamp design next time. Those bolts should be flexed less.
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Re: Snapped the saddle rail bolt on my seatpost

Postby find_bruce » Fri Jun 07, 2013 8:18 pm

Don't sweat it rkelson - one of my buttons is misconceptions about bolts & which ones to use where - comes from working on my race car, my bedtime reading includes Caroll Smith's nuts, bolts, fasteners & plumbing handbook. It also helps that SWMBO is a metallurgist.

A failure due to tensile overload has a very typical necking, like this
Image

The pictures I have seen of seatpost bolt failures all show fatigue due to repeated bending, eg like
Image
Breaking at the start of the threads is also typical of this sort of fatigue.

About the only place I can think of that might use grade 8 bolts is for square taper cranks & even then it is probably not necessary. You certainly aren't going to use one in a seat post with an aluminium alloy nut - the nut would strip long before you got anywhere near the required torque for the bolt.

As your experience shows, a stainless bolt is adequate for the task, just that IMO you get a slightly better fatigue margin by using a good quality grade 5 steel bolt
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Re: Snapped the saddle rail bolt on my seatpost

Postby rifraf » Thu Jun 13, 2013 1:50 am

toolonglegs wrote:I have had 5 or so go over the years... since switching to Thomson posts I have never had a failure.
All of my bikes have Thomson posts now, but I find they do have an annoying problem over time of changing shape or wearing ever so slightly around the clamping area... real PITA especially on mtb and CX bikes when they slip down a touch... hence I now have dual clamp seat post clamps... finally I am happy :wink: .

Hi Toolonglegs,
can you possibly link to a pic of a dual clamp seat post clamp?
I've added a Thomson to my Surly Ogre having picked up an as-new Masterpiece in the for sale forum.
As I'm doing a Wiggle purchase, I was glad to spot a Thomson seat-post spare bolt kit, consisting of bolt, washer and one of those Thomson barrel nuts.
I added one to my order which included a bbb torquewrench to assist me not to over tighten said bolts. :lol: :lol:
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Re: Snapped the saddle rail bolt on my seatpost

Postby Nobody » Thu Jun 13, 2013 10:25 am

rifraf wrote:...can you possibly link to a pic of a dual clamp seat post clamp?

http://www.carboncycles.cc/?s=0&t=2&c=73&p=851&

I've got one of these by MB/Shaun's recommendation. It works well. TLL has similar.
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Re: Snapped the saddle rail bolt on my seatpost

Postby rifraf » Thu Jun 13, 2013 2:48 pm

Nobody wrote:
rifraf wrote:...can you possibly link to a pic of a dual clamp seat post clamp?

http://www.carboncycles.cc/?s=0&t=2&c=73&p=851&

I've got one of these by MB/Shaun's recommendation. It works well. TLL has similar.

Thanks heaps Nobody.
I'd not seen these before.
On my wish list now. :D
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Re: Snapped the saddle rail bolt on my seatpost

Postby Mulger bill » Thu Jun 13, 2013 8:59 pm

You won't regret it Aidan. I tried everything short of a welder to get that particular post to hold height, nothing worked.
Don't forget to check your sizing, there's a few different ones.

Shaun
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Re: Snapped the saddle rail bolt on my seatpost

Postby singlespeedscott » Thu Jun 13, 2013 9:13 pm

I know of no Campagnolo seatpost that uses an alloy bolt. Everyone I own (which is few, 1970's thru to 2000's) all use steel bolts.
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