Carbon/Carbon Seat Post Creak Question

Secteur

I removed and re-greased my carbon-on-carbon seatpost last night using Finish Line carbon assembly paste, and tightened the fixing bolt correctly using a torque wrench, and today it creaked for the latter part of my ride. It is correctly installed in terms of length (the minimum length markers are well inside the seat tube).

In the garage I could easily replicate the creak just by straining the post.

The saddle has carbon rails, and I have checked that these are torqued up ok. The creak was definitely coming from the seat post bolt area.

I regreased (with carbon assembly paste) it and now the creak has gone when I manually stress it (will be taking it back out on the road tomorrow).

My questions are;

1) Clearly the creak was something that I had done, but all I can think of is my greasing - does creaking indicate over or under greased? (I put a modest amount of carbon paste on but most of that squirted out when the post slid into the seat tube. Today I re-installed it with much more paste, again most of which squirted out!)

2) any other thoughts when fitting a carbon-on-carbon post?

I dont think there's a problem, and I have inspected everything for cracks - there's none (that I can see) and the post is now silent when I strain it, but I just wanted to learn more about this specific area of bike building that I am new to with never having had a carbon bike before. I like to really understand everything I work on.

Many thanks.

sungod

mine creaks if there's not enough grease and if it's not clamped quite tight enough, it only takes a minute amount of movement between surfaces to cause a creak that can resonate through the whole frame (cf post in xcr frame)

as i have to remove the post whenever taking the bike overseas i had plenty of practice on how to ensure it's creak free on refitting, what works for me...

ensure the both the post and seat tube and clean, after wet rides fine dirt can get in

squeeze about an inch of tacx assembly paste (no gritty feel) onto the tube, smear it over the entire length that will be inserted, then a smaller amount of finish line paste (feels gritty) and repeat the smearing (why both pastes? if i use just one it slips, the combination doesn't, i've tried a few times, it's consistent, no idea why, don't care as long as it works!)

insert post, almost all the grease will squish out

tighten clamp to exactly the point i've learned to tighten it to for no problems

Secteur

Well it creaked again from about an hour into yesterdays ride, so something is amiss since I fiddled with it!

Perhaps I need to tighten it slightly more than the 5NM suggested?

sungod

is the 5Nm suggested for the post or for the clamp?

mine needs 6Nm to avoid slipping, this is the maximum for the post but the frame has an unusual clamping method and has no torque specified

given the number of clamp designs it's hard to see how a rating on a post can be valid for all of them, but if your post+frame are a set then i guess you need to be wary of exceeding the recommended figure (if the wrench isn't calibrated it could be quite a bit off anyway)

maybe try a different assembly paste, back when i was experimenting i found the tacx held better than the finishline

Secteur

yes it's 5nm specified by the bolt on the frame that secures the clamp.

it's a funny sort of integrated wedge that gets pushed down & tightened against the seat post as you turn the bolt.

I will give it another good and more careful strip and inspection tonight - I am probably missing something simple!

ic.

Have you checked the post for damage?

I had this problem with my carbon synapse. In the end I found the only thing that would shut it up was smearing grease over the whole length as sungod said above. Even though you only clamp the top, the post can creak at any point it's in the frame if it's a tight fit (which it is if the grease is forced out when you insert it) so grease the whole thing.

Secteur

Tightened the post to the recommended 5nm, and then a bit more (6nm) and creak gone!

So maybe my Park torque wrench is out (though its well looked after and always stored after being reset to zero)

sungod

hurrah!

even zeroed a torque wrench will drift, they're pretty clunky mechanical systems, if it's not regularly calibrated the accuracy could be way off

fastener condition has an impact too, the conditions for recommended torque are rarely specified (i always assume clean and lubed) but the difference between dry or dirty bolts and lubricated ones can be a few Nm