Is it there to protect the seatpost or the frame?

I am replacing an alloy seatpost with carbon (round, 27.2) on a carbon frame bike. Manufacturer recommended max is 6nM for the OEm aluminum post. Does this hold for the carbon replacement?

Thanks

I like to use the least torque that holds the components without slipping. That's often 20-30% lower than the recommended max.

I have an aluminum post in a carbon frame. That range of 6 nM works fine on my bike, no slippage. (I'm probably at 4-5 nM). I put a narrow tape strip on the post, just above the frame, to monitor for any slippage.

I use carbon assembly paste, which works great. My tube of Tacx assembly compound is 12+ years old, and I still have half the tube. It's a gel with tiny plastic grit particles. ( Tacx seems to be unavailable? The Park Tool Supergrip looks like a similar gel. )

I use the carbon paste on my aluminum stem to aluminum handlebar too. Note--carbon paste will dull the paint that's within the clamped area, the tiny particles are digging into the paint. That's just cosmetic, of course.

The tape marker is one I use as well.

As a clyde, I've had problems with seatpost slippage if I didn't have the binder bolt tight enough. I've also broken a bolt by over-tightening (took a trip to a hardware store to keep riding on that vacation!). So try not to overdo it.

This is no doubt good policy, but I am trying to determine if the recommended max changes when a carbon post is substituted for the original aluminum post.

I assume the recommeded 6nM is to prevent damage to the frame, rather than the alloy post. What I am curious about is whether substituting a carbon post reduces the recommended max out of concern for the post itself.

In my brief time as a design engineer we would set max torque for the assembly, which would include the frame and post materials and what max torque the set screw could take in the clamp. A change in the post material certainly could reduce the max and the suggestions above are reasonable. I suspect there may be carbon posts that could take 6Nm and many that might fail at far lower than 6 Nm.

For me, using campagnolo-style binder bolts in vintage frames with a variety of posts/post materials, it's always been the bolt's breaking point that determined the torque. Something about that design led to a lot of broken bolts over the years.

I agree with the post that recommends lowering the torque as you are changing the material from what it was initially spec'd for using. 20 to 30% less & the use of carbon paste would be what I would do.

I've shortened an inexpensive carbon seatpost maybe 12 years ago. And cut off carbon steerer tubes too. Those short cutoff tube sections are really strong. The 6 nM won't hurt the seatpost with the compression from the frame clamp.

At random, I checked Ritchey's website for carbon seatposts.

Their $58 Comp 2-bolt says "Seat collar torque: not specified - follow frame/seat binder specification"
Their $99 WCS Link says "Seat collar torque: not specified - follow frame/seat binder specification"

Their $270 light Superlogic post says: "Seat collar torque: 6Nm (max) / 7Nm (max) for Ritchey steel frames" And it's description says: Made from a lightweight carbon fiber layup developed for Pro Tour teams at the highest level, the SuperLogic Link post offers more vertical compliance with no loss of lateral or torsional stiffness.

I don't think the carbon version of the Ritchey WCS Link seat post can be had for only $100; that must be the aluminum model.

I agree with above recommendations to tighten to the least torque without slippage. I also agree 6 Nm should be fine for most CF seat posts. The torque limit is probably there to protect the CF frame more than the CF post; if you do not believe me, keep torquing it and see which cracks first.

The lower the tolerances & lower grade of component the higher the allowable (& necessary) torque to do the job.

I've have never, ever torqued a seat post clamp or binder bolt above 5nm. Even greased carbon to carbon interfaces. Only once did I have a problem with seat post slippage. It turns out the mechanic at the bike shop mechanic erroneously swapped the proper frame-matched collars. Swapping the right collar back to the frame fixed the issue.

When switching to a carbon seat post, it may pay dividends to rotate the clamp around 180 degrees so that the bolt is in front of the seat tube. Doing so prevents the sharp corners at the cut-out from being driven into a finite point into the post by the bolt standing off some distance. The bolt being located opposite the slit encourages a circumferential squeeze of the post.

Some premium posts can be very susceptible to improper torque & have well defined insertion & weight limits. Darimo springs immediately to mind. If you have to ask, you probably don't have an engineered (delicate) enough post for this to be a concern.

Friction paste, the binding bolt away from the slit & 5nm ought to be good enough for whatever it is you have.

Thanks all.

This is my version of an off road, gravel maybe, Litespeed Ti road bike that I felt guilty for not adding that I use a "belt and braces" approach, note the seat post collar above the seat post clamp, torqued with a 4 Nm torque driver and the clamp itself 5 Nm.