LarrySellerz

Today on the way home from work I felt something give in my saddle. I stopped to investigate, thinking it was coming loose, and to my horror I found that the seatpost was cracked and barely holding on! I still had 4 miles to go, and when I got home it was hanging on by a thread. It barely took any force to break it off (I got a video of myself kicking it off and it goes flying.)

it must have been a bit too high. it was previously installed way too high on another bike and could have been damaged then, or it could have been damaged in a crash I had last Friday. Hard to say. I don’t have the metric Allen to pull it out and see how much is left in the frame.

a bike store told me previously that putting a long lever like I had could crack the aluminum frame. Would you be worried about the frame, or is it probably okay if it looks undamaged?

A bike shop told me a horror story of someone getting seriously injured by a broken seatpost. Do any of you have any horror stories?

mstateglfr 

Installed above the min insertion line, eh?

Seriously though, I cant imagine how much abuse was needed to make that happen. I suggest you stop crashing so frequently.

tomato coupe

I guess the manufacturer knew what they were doing when they put that "minimum insertion" line on the post.

LarrySellerz

Can't imagine the crash had anything to do with this, and a mechanic at the shop installed this (really doubt it was negligently) It was well over the min line on another bike and has been on this one for about 9 months. I'm guessing I compromised the integrity then and nobody noticed until it failed. Salty that I crashed because I didn't even want to ride that day because it was raining and I was slightly sick, had a bad feeling about riding and ended up crashing.

The plan as of now is to ride to work tomorrow without a seatpost. 15 mile round trip... last time I tried riding without a saddle in an attempt to get a full body workout it was harder than I thought it would be.

tomato coupe

Why do I suddenly have an urge to re-read "On the Origin of Species"?

Eric F 

Look to me like a high probability that the seatpost clamp was cranked down too tight. A crash causing excess irregular forces on the seatpost could also be a significant contributor. Being above the min. insertion line, as well. I would also guess that this has been in the process of cracking for a while, it just finally reached a critical level.

jaxgtr

I looked at that picture and said the same thing about the min insertion line. SMH.

jaxgtr

If a mechanic installed that, they should be fired. Anyone worth their salt, would have told you, you need a longer seat post if you were at min insertion.

wolfchild

I witnessed two carbon component failures over the years. Both happened on a mountain bike group ride in very rough rocky terrain. One was a carbon handle bar the other was carbon seat post. Luckily no injuries and those failure didn't occur as a result of a crash, they just snapped for no reason at all.

LarrySellerz

Yeah never riding with a carbon seatpost again if I can help it... overtightening that bolt can cause the seatpost to just break like that? That's a horrific failure mode. Especially when tightening that bolt is the first thing you do when the seatpost is slipping down.

jaxgtr

They have a new tool to get the rest of that post out..

Its the new Park Tools cracked seat post installed above minimum insertion point removal tool

CSPIAMIP 1.0

Maelochs

I had a couple Bontrager CF seat posts break on me many, many years ago .... too much exposed post for the load. (I found a source with NOS posts, lightweight and at a good price .... didn't really pay off.)

I can get away with maybe nine inches, but on this bike, because of frame size and shape, about a foot of post was exposed .... and not enough inserted also. For anything really long I go with aluminum nowadays.... the length is enough to damp out most of the harshness.

Rider weight and riding style is a big deal here ... I am an ultraclyde, so I favor 400-mmm posts with a lot inserted--- 275 and just at the min-insertion mark is pretty sketchy. Also, do you ride light over bumps and such?

But yes .... not enough inserted, too much exposed, or both .... I had both posts break in the first hundred yards of my ride, while pedaling slowly, which is a gift because if i had been seated at speed ... i cannot imagine how many ways I could have been hurt.

Is this the bike with the kick-start derailleur?

Maelochs

Also see Erik F's post .... CF seat posts like all CF parts, do Not appreciate high compression clamping. That post was barely long enough (IMO---I would Not have used it) and if on top of that you tightened it too much ... on top of having it even more exposed on a different bike for a while .....

Anyway, you didn't get hurt and you did get your bike to a bike shop, so win-win here.

LarrySellerz

Thats not very far above the minimum line...and its not his fault, hes a great mechanic. I was pushing the envelope though

LarrySellerz

Yes, you can see how badly cross chained it is in this picture. Maybe I should take it back to the shop that installed the seatpost and ask them to fix the derailleur lol. I didn't get it to a bike shop, I kicked the seat off it snapped and went like 15 ft.

Maelochs

NO.

Fir4st thing you do is Google "Carbon paste" (https://www.amazon.com/carbon-paste/s?k=carbon+paste) and then you get a torque wrench and make sure you are not overtightening.

Seat posts don't usually slip ... they are sort of manufactured to fit snugly. Even when lubed (which I recommend for metal posts in metal frames) they shouldn't slip a lot .... and of course with a metal frame and post, you can crank away. But with CF the grit in carbon paste helps keep things from sliding.

Maelochs

Ummm .... have you changed the front derailleur cable?

jaxgtr

They have a min insertion point for a reason.... it does not matter how many micrometers it is above the line.... this could have been a devastating accident that could have injured or killed and should have never left the shop that way period. Now if you want to change post shop, that is on you.

This is why they make you sign when you leave a hospital against medical advise.

What size post is that, I might have a 400 AL post in my bin of spare parts.

Eric F 

There's a reason why torque specs are printed on parts. There's also proper use of stuff like carbon paste when installing a carbon seatpost to keep things where they're supposed to be at proper torque values. If you aren't willing to follow these kinds of simple assembly protocols, then the fault of the failure rests squarely on your shoulders. Because of this, I would also highly recommend you stick to parts that have a higher tolerance level for carefree cranking of squeezy bolts.

If the seatpost was slipping down, it could very well be from getting crushed a little when over-tightened previously. Tightening the clamp even more to stop the slipping of a damaged post only increases the rate of damage. What you might see as sudden catastrophic failure might have actually started a long time ago.

jaxgtr

Sorry, the only post I have a very old Specialized carbon post that is only 250mm.

LarrySellerz

The slipping seatpost was on another bike and I ended up washing the lube off and sanding the surface. I don't want to deal with torque wrenches and finnicky stuff when doing something potentially on the fly like a saddle height adjustment. What if I want to let my friend ride the bike quickly or something. Most of the bikes ive had were quick release lol.

This could have been REALLY bad and im now super skeptical of carbon seatposts. They seem like a bad idea. Thanks Jax im sure I have something i can take off another bike

Maelochs

You don't want to have to make careful adjustments on the fly .... but you are okay with breaking your seat post mid-ride?

Also .... your seat post was slipping ... so you Sanded it?

Dude .... everything is dangerous if you use it wrong.

My miniature torque wrench is about four inches long and fits neatly in a seat bag .... but usually I find the right seat height after a couple adjustments and don't need to carry it on every trip.

If you want to loan the bike to a friend ... he is at your house. He isn't standing on the side of the road waiting for you to ride by. So if you had a torque wrench at home, it would be right there.

And what kind of "friend" would loan someone one of your death-trap bicycles?

Anyway ... please try a little harder not to tear up nice stuff, and even more important, please be careful so you don't tear up yourself.

Eric F 

For people unwilling to be reasonably accurate with how much they tighten bolts (experienced mechanical folks can usually get fairly close to a target torque without a torque wrench), something like a carbon seatpost or handlebar might not be a good idea. I have carbon seaposts on 6 of my 7 bikes, and started using carbon seatposts over 20 years ago without a failure. Same with handlebars.

jaxgtr

So you are a lab tech, but decided sanding carbon was the best way to remove carbon paste or whatever lube used , versus maybe just wiping it down, and as mentioned, using carbon paste. I little tube will last you a lifetime. I'd say ask your mechanic for about a 20 cents worth dollop, but they installed a seat post over min insertion point, so that is out.

LarrySellerz

I never said I sanded a carbon seatpost, and sanding a metal seatpost to roughen to surface if it is continuing to slip is not weird. Come on