Bicycle Mechanics - Slipping seatpost fix.

I have a problem with my seatpost slipping down. The bike is carbon with a carbon post. The post looks like there may be an aluminum insert, but I don't want to tighten any harder then I have already for fear of crushing the post. My thoughts are to replace the seatpost with a Thompson Elite, and then upgrading the collar to a Salsa Lip-Lock. I was hoping that might fix the problem, but was hoping for some input here.

http://www.competitivecyclist.com/product-/2009-tacx-dynamic-carbon-assembly-paste-3321.html

It feels gritty when you smear it between your fingers.
It has fixed slipping seatposts, *and* a slipping headlight mount for me.

you need to buy a torque wrench. since you own a carbon frame you should already have one. this way you can tighten everything correctly/securely without the chance of doing damage.

did the post come with the bike?

+1 on the torque wrench, they are available inexpensively from Harbor Freight, Northern Tool and others. A must on carbon components.
+1 on the carbon assembly paste.

I have a torque wrench,so I have that covered. The post is the one that came with the bike. I was placing an order today, so I will add assembly paste to the rest of what I am getting. Thanks everyone!

Carbon assembly paste, or if you have a friend who does auto you can beg a smear of coarse lapping compound.

Either will improve traction preventing slippage at lower clamping force. Lapping compound is basically grit in a grease base. The grit bites into both the post and frame providing a mechanical interlock. It's the same principal as why you use sand to improve traction on an icy road.

Carbon paste varies from brand to brand. Some are very similar to lapping compound, others use softer, non-cutting media with high friction properties.

If you use a grit base product, be sure not to move the post unless you've fully loosened the clamp, otherwise you'll leave scratches in the post. I also suggest you avoid inserting the post beyond the desired depth to avoid marring the visible area.

coarse lapping compound??? good to know... thanks!

coarse lapping compound??? good to know... thanks!

I suspect that some of the assembly pastes are simply that repackaged, though they may be slightly different.

I did not know about this product, found some in the internet and i thought the same thing, the purpose in mechanical industry is different but pretty much is the same product with a different label, as you say maybe just plain repackaged.


I suspect that some of the assembly pastes are simply that repackaged, though they may be slightly different.

How good is the fit of the seatpost? Does it wobble in the frame before you tighten the binder screw?

I have solved this issue by buying a seatpost 0.2mm larger than the original. For example, if the original seatpost is 27.2mm, get a 27.4mm seatpost.

Thomson makes a 27.4mm seatpost.

How good is the fit of the seatpost? Does it wobble in the frame before you tighten the binder screw?

I have solved this issue by buying a seatpost 0.2mm larger than the original. For example, if the original seatpost is 27.2mm, get a 27.4mm seatpost.

Thomson makes a 27.4mm seatpost.

This would only make sense if the frame was oversize. The ID of seat tubes below the slotted area is fixed and should match the published spec with a tolerance of only +.25mm. For example, a frame taking a 27.2 post should have an ID of 27.25mm and no more. The tune below the slot has no capacity to expand and therefore it would be nearly impossible to jam the next size post in there.

This would only make sense if the frame was oversize. The ID of seat tubes below the slotted area is fixed and should match the published spec with a tolerance of only +.25mm. For example, a frame taking a 27.2 post should have an ID of 27.25mm and no more. The tune below the slot has no capacity to expand and therefore it would be nearly impossible to jam the next size post in there.

Manufacturing defects can and do happen.

I have never seen a bike with a 27.4 seatpost. I think that Thomson, and others, make a 27.4 seatpost for exactly this reason.

Manufacturing defects can and do happen.

I have never seen a bike with a 27.4 seatpost. I think that Thomson, and others, make a 27.4 seatpost for exactly this reason.

27.4 was a fairly common size many years ago. It's for light gauge steel frames, such as some made by Reynolds. Of course it can also be used in cases where seat tubes are reamed oversized by people who don't know better. OS seat tube defects are very rare because the ID is rarely touched by the builder. The dimension is as provided when the raw tubing comes from the mill, and normally not touched later except to deburr the slot.

27.4 was a fairly common size many years ago. It's for light gauge steel frames, such as some made by Reynolds. Of course it can also be used in cases where seat tubes are reamed oversized by people who don't know better. OS seat tube defects are very rare because the ID is rarely touched by the builder. The dimension is as provided when the raw tubing comes from the mill, and normally not touched later except to deburr the slot.

Yet, I have had instances where the nominal size of the seat tube and seatpost are the same and the seatpost wobbles.
Fixed by going 0.2mm bigger on the seatpost.

0.2mm = 0.007 874 015 748 inch

metal posts in metal seattubes weren't perfect either. it's remarkable that an expoxy coated post (some with plastic decals on them no less) made of carbon in a carbon seattube can withstand the torque necessary to keep it in place at all!

I would never use Valve Lapping compound on my Carbon Fiber parts.
Carbon assembly paste comprises tiny plastic balls suspended in a grease. The compound is designed to increase friction.
Valve lapping compound is an abrasive grid made for cutting metal and finer grit will polish by the same process.

I used carbon paste and it still slipped a little, read somewhere to sand inside of seat tube lightly, that with carbon paste worked well.