Cyclist0108

I have a steel frame and a carbon FSA post. They are of the right size (and it came with the bike). I get slow sipping. I have used carbon paste, changed to a Salsa clamp, but still have not eliminated it. I have torqued it to 8 Nxm, and would tighten it more but I am afraid of cracking the seat post (but at this point I'm almost ready to ditch it for an aluminum one).

Any suggestions to get this to stay put? Will over-torquing it (say to 10 or 12 Nxm) crack the post? I'm guessing the clamp and frame spread the pressure over a wide area, so it isn't as likely, but I don't want to find out I am wrong the hard way.

cpach

I never have but I've also never significantly overtorqued a post. If it were my bike the next thing I would try would be a very thin shim (probably a cut aluminum can) and more carbon paste.

exmechanic89

I would try shimming it with aluminum can as well.

Troul 

Slide a section of [cut to appropriate length] CF square tube that meets the outer edges of the seat's tube to which goes down the seat post tube, that will bottom out.

Prior to inserting the square tube, drill a thru-hole going from one side of the square CF tube to the other approx. mid-way of CF square tube, feed one end of a strong thin piece of poly rope down the center & pull the poly rope from the center coming out one of the holes, then back in using the other hole. Grab the poly rope that you just slid in thru the second hole & pull it up the center of the square CF tube.

Tie a secure knot of the two loose poly rope ends & stuff it lightly within the square CF tube.


By doing the above, you will maintain the height for you & the lite weight, same material type of square CF tube will be removable should you need to ever take it out.

HillRider

Despite the seat post having come with the bike it seems to be undersize, possibly due to manufacturing tolerances in either the seat post or seat tube adding up the wrong way. Other than fabricating a bottoming stop as Troul described, you are probably going to have continuing problems unless you replace the seat post.

Cyclist0108

The fit is so tight that I think the problem might be I am scraping the carbon paste off before I can get the post into the frame. There is definitely no room for a shim.

The post also holds my Di2 battery, so I am afraid Troul's suggestion can't be implemented.

Cyclist0108

I've pretty much resigned myself to this.

masi61

Cyclist0108

Salsa's spec is 8 N x m. (Sorry everyone I left out all these details last night. I do really appreciate everyone's responses.) The other thing is that the (rather beefy) Salsa bolt has blue lock-tight on it, and I wonder if that could be interfering with an accurate torque measurement? (In other words, is the torque applied to the bolt really reflective of how tight the clamp is squeezing the seat tube?)

Last night I cleaned all the surfaces and re-applied carbon paste, and torqued it to 10 N x m.

fietsbob

No.. have assembled bikes with them in shop, bikes shipped in by owners
preparing to tour the coast.


There are separate values (numbers)
on typical automotive /industrial torque charts
for dry and greased bolts being torqued.

As I recall..


IDK about old cured ,dry thread lock vs freshly applied, wet..

equivalent, ? maybe .. FAA certified mechanics might know ..




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Cyclist0108

I'm going to gradually crank it up a Nxm or 2 every time it slips until it either stays put or cracks. I just hope it doesn't shear off while I am riding and I impale myself on the splintered, broken seat-post.

fietsbob

Not carbon, but I have used a 2nd clamp directly gripping the seat post, itself,
sitting on top of the seat post clamp that squeezes the frame, around the seat post.

and that cured a slipping post..

here is a threaded collet clamp, around a seat post, (27.2) gipping it
and a band clamp around a frame with a 1.25" diameter (aka 31,8..) OD.

https://www.sjscycles.co.uk/seat-pos...272-to-318-mm/






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superdex

I have. I've found torque recommendations are conservative, I have a feeling you can get away with 10-12Nm.

To wit, my next post is Al, for my budget, there are no weight differences....

masi61

rm -rf

I think the biggest problem with overtightening a clamp is that the expansion slot at the top of the seat tube might dig into the carbon post. Make sure it's edge is smooth.

When I bought my previous bike, the LBS installed one of those secondary clamps around the post. Most seatpost clamps have a milled collar to keep it in place on the top of the outside of the seat tube. This one was designed to fit on the seatpost itself. I expect a bike store could order one. It was similar to this small clamp at Jensen, but from other photos, that one might have a collar edge, but that seems odd, since the narrow clamp would be right at the top edge of the seatpost.

On my new Ti bike, the top section of the seat tube was milled inside. It was too smooth, and the aluminum seatpost creeped down. I used some wet-dry 240 sandpaper wrapped around my finger and roughened it up with a circular motion. Then the carbon assembly pasted worked a lot better. No problems now.

Cyclist0108

Salsa makes one. They call it a post-lock.

fietsbob

a rack mount seat clamp band ? Salsa is a brand not a manufacturer,
(was a guy in Nor Cal , making bikes , but QBP paid him for his company and trademarks)


another, with famous logo, though probably not made by them,


note how it wont line up with the slot in the frame seat tube, intentionally.

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Cyclist0108

The Cinelli one looks ideal. Thx.

Troul 

didnt know you had a Di2 battery.

rm -rf

It won't work on the seatpost.
It's designed to fit over the frame, there's a small lip around the top of the opening to keep it from sliding down. So it can't clamp directly onto the seatpost.

Cyclist0108

Sorry, I should have been more detailed. I really appreciate the thought, not to mention the creativity!

Troul 

can you slip a larger dia tube (CF or comparable composite) over the seat's post prior to inserting the seat's post down in the frame side?

Cyclist0108

I think the set-back would make it tricky, but the clamp-collar suggestions are fairly similar and possibly more doable.

pdoege

I had this issue on a Specialized carbon frame when I changed seat posts .

You can use a double clamp such as: https://www.ebay.com/i/263385206452?...r=562396655151
You can shim with very thin double sided tape. The adhesive helps where the carbon paste is rubbed off.
You can shim with normal cello tape, but the carbon paste tends to need multiple passes and you have to apply it to the inside of the tube.
You can adjust your tape thickness to really min/max the clearance if you have sufficient OCD.

Lastly you can move the Di2 battery into the seat tube itself with a cord/zipties to fish it back out. A bit tricky because the water bottle bolts have to be sized to not damage the battery. I did this in conjunction with the tape method above to mount a CF3 in the above mentioned frame.

I would definitely not over torque the clamp.

fietsbob

Not on carbon, But, by filing off the lip,
(which is there for sitting against, your frame's
square cut top of the seat tube to rest against that.)

I managed to get my folding bike's seat post (chromed steel)
to stop slipping down..
It is a QR version, not that exact one.






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