Now this is annoying-2nd cracked seatmast

[shovelon]

Both my shiny bikes, 957 and 956LE have cracked below the seat clamp and above the upper water bottle block. To my determination, there is a lot of flexing at the slot because of the flexy carbon seatpost I use on both of them.

The crack is just to the left and right of the slot. I repaired the 957 about 65 months ago and rid the mast of the blocks(genious weight weenie move whist correcting the problem  ). Now 2nd time.

I blame the combination of the blocks in combination of flexy carbon post.

I hope to secure pics of the situation soon.

Terry

[orange]

I used to have a child seat mounted on my seatpost on my xp-8 with a 30lb child on it, yet it never cracked - I'm guessing because I always used an aluminium seatpost? Strange. Least you're in a position to fix these things! If I did it then the bike would have to be binned as I don't know anybody locally who could do it!

[Colin]

I've heard that cracked seatmasts can be caused if you don't have the post all of the way into the mast (i.e. sticking out a bit at the bottom) by causing uneven forces. I assume that this wasn't the case?

Col.

[shovelon]

Seat post is all the way into the mast, but the pinch slot has that block welded over it causing the crack above the block. With the carbon post being so flexy I think it is creating a stress riser there. No matter, as I will chuck the blocks.

[whisperdancer]

So, you don't recommend using a carbon seatpost? I have one to put on the bike and, by some coincidence, I was going to put it on today.
I don't know how flexy it is, since it's brand new and I never tried it...

[shovelon]

So, you don't recommend using a carbon seatpost? I have one to put on the bike and, by some coincidence, I was going to put it on today.
I don't know how flexy it is, since it's brand new and I never tried it...

Do you have the 2 water bottle blocks just under the seat clamp? If so, there may be a problem. I will be posting pics of the situation.

[Willem]

Terry,

I wonder if you're getting cracks for all the reasons you said, plus the radical terrain you're riding...
We just hired a new guy at work from your 'hood and his bikes are THRASHED from barreling down California fire roads and barren hillsides. This guy has a Heckler that looks like it was thrown off a cliff into a gravel pit 3 times a day!
Are you that harsh on your poor XC Proflexies?

Will

[whisperdancer]

Do you have the 2 water bottle blocks just under the seat clamp? If so, there may be a problem. I will be posting pics of the situation.

Yes, I have. My frame is a X57 style frame.
The seatpost I have has no setback, which I believe will help not to crack the frame so easily, as the pressure on the seatpost doens't cause so much torque on the seatmast.
The original proflex aluminum seatpost also has some flex, and it has a quite long setback.
I will give it a try, as it seems to have a thick wall of carbon and was a gift. It's a Look carbon seatpost.

[shovelon]

Do you have the 2 water bottle blocks just under the seat clamp? If so, there may be a problem. I will be posting pics of the situation.

Yes, I have. My frame is a X57 style frame.
The seatpost I have has no setback, which I believe will help not to crack the frame so easily, as the pressure on the seatpost doens't cause so much torque on the seatmast.
The original proflex aluminum seatpost also has some flex, and it has a quite long setback.
I will give it a try, as it seems to have a thick wall of carbon and was a gift. It's a Look carbon seatpost.

I must admit that my easton ct2 posts are probably the problem, as they are pretty flexy, of which I love the feel. They have two flats on the front and reat of the post for some reason, and need a great amount of clalmp pressure. Those blocks are giving the mast a good workout, breaking in time.

I will try to document my repair so you can see the sitution.
Terry

[shovelon]

Terry,

I wonder if you're getting cracks for all the reasons you said, plus the radical terrain you're riding...
We just hired a new guy at work from your 'hood and his bikes are THRASHED from barreling down California fire roads and barren hillsides. This guy has a Heckler that looks like it was thrown off a cliff into a gravel pit 3 times a day!
Are you that harsh on your poor XC Proflexies?

Will

Oh, yeah, that's it!

Nah, it's probably my big butt wreekin' havoc. 
But this weekend is lookin sweet! Sunny and 70 to 80 degrees F. I'm gonna thrash something fer sher.

[whisperdancer]

Well, my seatpost is on and took the first beating today (although not as much as me...).

This is the original proflex seatpost (cut to fit the previous way big frame):



This is the new one:



Old vs New (i needed a bigger seatpost - the new one is already cut to 320mm):



I flexes a bit but not much. I tightened it with a low torque and it didn't move, even going down on top of the seat.

[shovelon]

This is the crack. You can see from flexing over the block that the cracks develope. This time it cracked on both sides before I caught it.

Repair without blocks that propagate cracks.

[kiwi]

do your seatposts  go right through the mast and out the other side???ie right down to the stop.

[shovelon]

do your seatposts  go right through the mast and out the other side???ie right down to the stop.

My seatposts are cut to bottom out on the stop bump on the inside. My carbon seatposts tend to slip a bit after a while, so I use those stops to my advantage.

Terry

[shovelon]

Whisperdancer,
I like that Look post you have. I do like the feel of carbon.

Terry