Saddle slipping back on carbon seatpost

[robjh]

I recently acquired 2nd-hand a bike with a Selcof Monocoque MC04 carbon seatpost

Looks great, but every now and then when going over a bump there will be a loud crack, and the saddle has shifted backwards on the rounded platform at the top of the post, and now has its nose in the air.
No amount of tightening the 2 bolts seems enough to hold it for long, and in fact has got worse recently as I've been taking it apart and refitting it to look for the problem.
Should I be looking for some kind of carbon gripping paste, or are there simpler mechanical tricks that I'm missing?

Some more pictures of the thing :

1) saddle clamp in position, after slipping backwards (hence funny saddle angle)

2) the rounded platform that the saddle clamp sits on, with 2 holes for bolts. The position on the platform determines the angle of the saddle. The surface is roughened but no ridging or anything.

3) underside of the part of the saddle clamp that fixes against the seatpost surface(above). Again, slightly roughened but no more than that.

4) the 2 fixing bolts, from underneath. The 2 square washers are not entirely flat, but shaped to fit against the non-flat cavity surfaces.

 

[Smokin Joe]

You could try carbon paste and see if that works. Unusual for the top of the post not to have a ridged surface to provide some grip.

If the paste doesn't work I'd maybe look at a new seatpost.

 

[Sea of vapours]

That's a design inherently likely to slip in exactly the way you're experiencing. I'd try carbon paste all over the curved surface too (though I'd not have bought a seatpost with that design in the first place as it's always going to be liable to move if you apply force to the front or back of the saddle, which is pretty much inevitable).

 

[Milkfloat]

Are the grippy washers grippy on the other side too? If not, I would be tempted to turn them over.

 

[robjh]

Are the grippy washers grippy on the other side too? If not, I would be tempted to turn them over.

thought of that, but they are not flat, being slightly curved around the edges as if to fit into the two long cavities in the head of the post, this way up only. Unless of course they have moulded themselves to the shape under pressure, and originally should have been the other way up.

 

[Pale Rider]

The washers might be flexing a little, making it difficult to nip the assembly up tight.

Might be worth doubling them up with steel ones.

Carbon assembly paste, as said.

I can't see any harm in scuffing the bearing surfaces with emery cloth as well - might help a bit.

A tip with the paste: put a bit in the end of the seat post - it will be there when you need it next time.

 

[Yellow Saddle]

The washers might be flexing a little, making it difficult to nip the assembly up tight.

Might be worth doubling them up with steel ones.

Carbon assembly paste, as said.

I can't see any harm in scuffing the bearing surfaces with emery cloth as well - might help a bit.

A tip with the paste: put a bit in the end of the seat post - it will be there when you need it next time.

Nice tip, I like it. Now, if I can only find the little sachet of paste so that I can do it.

 

[Yellow Saddle]

There are some places where carbon should not go in the first place. But, I'm sure you are trying to save the situation. Turn the washers over as Milkfloat said and put on some carbon grip paste or, if you can find some, course griding paste of the type used to lap valves in a car workshop. That's got more grip than plastic baubles in carbon paste.

How much are you torquing the bolts?

 

[cyberknight]

I have the exact same issue with a carbon stem, the bars rotate if you catch a decent bump even with paste.

 

[robjh]

How much are you torquing the bolts?

As much as I possibly can at the moment - but just by feel, I don't have a torque meter

 

[Pale Rider]

@Yellow Saddle's idea of valve grinding paste sounds good to me, it must surely provide more grip than the assembly stuff which is designed to lubricate as well.

Here's a traditional 'coarse one end/fine the other' tin:

https://www.machinemart.co.uk/p/051110553/?da=1&TC=GS-051110553&gclid=CIPbtp6XjdACFQ4o0wodriEPuA

 

[Smokin Joe]

@Yellow Saddle's idea of valve grinding paste sounds good to me, it must surely provide more grip than the assembly stuff which is designed to lubricate as well.

Carbon paste is no lubricant. It contains fine particles to prevent CF components from sliding against each other, the opposite of a lubricant.

 

[Pale Rider]

Carbon paste is no lubricant. It contains fine particles to prevent CF components from sliding against each other, the opposite of a lubricant.

Well, the little plastic balls roll about a bit it and it certainly lubes my finger tips when I use it.

Whatever, grinding paste is likely to do a better job in this situation.

 

[Yellow Saddle]

As much as I possibly can at the moment - but just by feel, I don't have a torque meter

OK, but I can honestly say that you should not waste too much time with that post.
Get a proper one with two bolts - one fore, one aft - (not adjacent) and a strong aluminium head.

 

[robjh]

Are the grippy washers grippy on the other side too? If not, I would be tempted to turn them over.

thought of that, but they are not flat, being slightly curved around the edges as if to fit into the two long cavities in the head of the post, this way up only. Unless of course they have moulded themselves to the shape under pressure, and originally should have been the other way up.

no, tried that and no improvement.