Snapped the saddle rail bolt on my seatpost

[__PG__]

Apparently this is quite common. The clamp was old (i.e. original equipment). It all fell off in a rush while riding home yesterday and I couldn't find all the pieces. I found the top of the rail clamp, which still had half of the bolt sitting in it...the bolt had just sheared off.

My LBS says finding replacement parts for single bolt rail clamps is impossible and its time for a new seatpost.

I'm googling around to see if I can find replacement parts, but outside of a 2nd-hand shop (e.g. bicycle recycle) I think he's correct.

[rkelsen]

Must have been a painful ride home...

Did you find the bottom half of the clamp too? If so, I'd just drill it out and put a stainless steel bolt in.

Otherwise, you can get brand new seatposts fairly cheaply on ebay, unless it's a special one.

[__PG__]

Nope, only found the top half of the clamp. Riding home without a saddle is achievable but someone offered me a ride home (there were only a few kms left) so I took it. It is funny how more unstable the bike is when you aren't using your bum as a contact point.

I'm looking at the 3T Dorico Pro as a cheap alloy setback post, although some people suggest that the clamping mechanism on it is a bit finicky. Any other suggestions? Ritchey Pro? BBB?

EDIT : Now looking at 3T Palladium Pro or Thomson Elite setback. Might start a thread in the buying forums.

[rustychisel]

it happens. Sometimes a sign that the bolt was too darn tight, but [shrug] it happens.

Replace like with like? Most LBS stock a Kalloy alloy seatpost in a range of diam. for about $25

[ekib]

__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.

[Mulger bill]

What sort of post was it? Someone may have spares gathering dust.

[Stepr]

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.

[__PG__]

__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.

[ekib]

_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

Ekib.

[pacra]

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.

[toolonglegs]

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 .

[FXST01]

Just had this happen to me on my commute home.

[rkelsen]

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.

[find_bruce]

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

[rkelsen]

Why stainless rather than a steel bolt ?

Primarily strength.

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

[find_bruce]

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.

[rkelsen]

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.

[Nobody]

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. 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.

[find_bruce]

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


The pictures I have seen of seatpost bolt failures all show fatigue due to repeated bending, eg like

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

[rifraf]

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 .

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.

[Nobody]

...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&" onclick="window.open(this.href);return false;

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

[rifraf]

...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&" onclick="window.open(this.href);return false;

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.

[Mulger bill]

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

[singlespeedscott]

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.