Carbon Seat Post Slipping

[Ajax Bay]

In all the Fiber Grip application videos I've seen they are always very liberal with the paste, it is not applied just to the clamping area but to the whole of the post . Fiber Grip creates friction between surfaces - larger surfaces equals more friction. Yes, the post will still need to be clamped but any additional friction created by the paste outside of the clamping area is not zero.

Of course they want you to use the paste up quickly as possible but this liberal application is not unique to the Finish Line video. All I'm saying is the paste is creating friction between the surfaces including surfaces outside of the clamping area.

If you want to ignore the science: feel free.
Any "additional friction" "created by the paste" or otherwise between the section of the seat post more than an inch down the seat tube is minimal (close to zero).
"Larger surfaces equals more friction" only if there's a force operating between the two surfaces: in this case there isn't below the top inch (max).
The seat clamp only compresses the top of the seat tube (metal frame like yours): there is no centripetal force (caused by the compressive force of the clamp) in the rest of the seat tube so even if the paste increases the coefficient of friction between post and tube a tad, any resultant force resisting the seat post sinking will be negligible compared to frictional force in the top (max) inch.

 

[figbat]

If you want to ignore the science: feel free.
Any "additional friction" "created by the paste" or otherwise between the section of the seat post more than an inch down the seat tube is minimal (close to zero).
"Larger surfaces equals more friction" only if there's a force operating between the two surfaces: in this case there isn't below the top inch (max).
The seat clamp only compresses the top of the seat tube (metal frame like yours): there is no centripetal force (caused by the compressive force of the clamp) in the rest of the seat tube so even if the paste increases the coefficient of friction between post and tube a tad, any resultant force resisting the seat post sinking will be negligible compared to frictional force in the top (max) inch.

I understand the principle, but if you undo the seat clamp does the seat drop in the frame? The ‘residual’ friction between post and clamp will vary according to the difference in diameter of the post and tube. Some seatposts are pretty resistant to being moved even with the clamp undone.

I’d also argue that with a bottom on the seat the downward force of the rider’s weight will not be perfectly aligned with the post, so a leverage effect will take place where the seat post pivots about the entry point, pushing the lower portion of the post against the seat tube. Otherwise, why would there be a minimum insertion depth?

 

[Ajax Bay]

Two great 'mechanics based' points.
As you say, some seat posts are tighter than others to push in. It's reasonable to assume carbon paste on the rest of the post will increase the dynamic friction overcome to push in the post. But the force needed to push the post in is surely small compared to the body weight x g force caused by a serious bump in the road which may cause an insecure post to sink.
I hadn't considered the leverage and resultant force at the bottom of the post and the opportunity to 'leverage' swidt that to increase friction half way down the tube. A longer tube would increase that lateral force. The critical scenario is a bump in the road so that would align the body weight force closer to the angle of the seat tube. The ratio of that smaller force and the main force created by the seat clamp moderates my assertion that the length of the seat post doesn't matter. The length of seat post showing above the seat clamp matters as much (edit: because that's the other end of the lever)
As far as 'minimum insertion depth' I suggest that's more to ensure seat post stability in normal use (edit: a lateral opposing force to lateral forces on the seat) rather than any function of generating friction along the axis of the seat tube.

 

[SWMc]

hair spray and builder's sand. (not beach sand as the grains are rounded). Used it on a carbon seat post that was continually slipping and carbo paste did not work.

 

[Chislenko]

Whilst it may not be the answer to your issue I had the same issue (carbon post & carbon frame)

I wanted to swop the post anyway as it was one of those "swept back" affairs that I particularly dislike.

Stuck a longer straight alloy one in (with paste) and it ain't moved since.

 

[Biker BoB]

Whilst it may not be the answer to your issue I had the same issue (carbon post & carbon frame)

I wanted to swop the post anyway as it was one of those "swept back" affairs that I particularly dislike.

Stuck a longer straight alloy one in (with paste) and it ain't moved since.

I get the idea for carbon seat posts and yes they can be comfortable but in experience they generally tend to be trouble.
Alloy is the way....😬

 

[cool_hand]

If you want to ignore the science: feel free.
Any "additional friction" "created by the paste" or otherwise between the section of the seat post more than an inch down the seat tube is minimal (close to zero).
"Larger surfaces equals more friction" only if there's a force operating between the two surfaces: in this case there isn't below the top inch (max).
The seat clamp only compresses the top of the seat tube (metal frame like yours): there is no centripetal force (caused by the compressive force of the clamp) in the rest of the seat tube so even if the paste increases the coefficient of friction between post and tube a tad, any resultant force resisting the seat post sinking will be negligible compared to frictional force in the top (max) inch.

You make valid points, however a simple google search of 'what causes friction between surfaces' explains why friction will be caused between 2 close surfaces with friction forming material applied between them.

How does friction arise between two surfaces?
All surfaces have irregularities, created by their molecular structure. Friction is caused by the bumps on the surface of one object colliding with the bumps of another object's surface.

Yes, it's probably not enough to hold the post in place under body-weight and yes, you will still have to clamp the post but that additional friction will reduce the need to apply excessive torque to the clamp and the risk of damaging the carbon.

And don't forget the fact that a seatpost can become stuck when those 2 close surfaces bind and that's without the application of centripetal force!

Whilst it may not be the answer to your issue I had the same issue (carbon post & carbon frame)

I wanted to swop the post anyway as it was one of those "swept back" affairs that I particularly dislike.

Stuck a longer straight alloy one in (with paste) and it ain't moved since.

I've swapped the carbon post over for the original alloy one I've tightened it to 6nm and it's not moved.

 

[Ajax Bay]

6Nm

 

[cool_hand]

Any recommendations for seatpost brands - either titanium or alloy?

 

[fossyant]

Any recommendations for seatpost brands - either titanium or alloy?

Still not solved ?. Ritchey WCS alloy components are often same weight as the carbon stuff.

 

[cool_hand]

Still not solved ?. Ritchey WCS alloy components are often same weight as the carbon stuff.

Yes, solved. I placed the reflector clamp at the base of the post/above the clamp – this creates enough additional force to stop the carbon post slipping down. This was sort of mentioned in my OP although it's not definitive fix. I'd like something with a better rail clamp - micro-adjustment?