Carbon Seat Tube Considerations

I’m working on a new version of my Ti/carbon gravel bike for a local friend. She’s interested in a traditional seat tube as opposed to a seat mast. I’m curious if the CF-knowledgeable folks here have input on if this sort of arrangement is sensible. It would use a traditional seatpost clamp and a Ø31.6mm seatpost.

The tube is this one: Bicycle Top Seat Tube

Questions I have:

  • Is making this sort of slot in CF easy-ish? My gut says to just do it like I would a steel one with a slotting cutter but at high speed and with masking tape on both sides.
  • Is there enough space between the slot and the lug beneath it?
  • Is the tube I’m using strong enough?


I can’t comment on the slot but most of the builders I sold carbon tubing to at ENVE purchased a fiberglass insert to glue in so they could easily ream the tube, use thinner wall thicknesses and offer 27.2 post compatibility. Firefly actually does that for all their ti bikes too. I have no idea what the status of Origen is but it would be worth reaching out to see if they still offer it.


That’s a great tip! I reached out to Scott at Origen, I’ll report back with what he says. It seems like that’s a sensible way to prevent galvanic corrosion and add some extra strength.

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@JMY made a clever tool to cut ST slots in carbon using a dremel:


My eyeball FEA says that it is on the brink of too close to the lug for safe operation, the rapid change in stiffness at the lug combined with the flex of the slotted tube makes me cringe a bit… Can’t you extend the upper part of the lug and then incorporate a binder bolt as well? That would make the whole stiffness transition a bit smoother I guess…


That is my gut reaction too. I originally had the binder integrated into the lug but redesigned that to make for easier assembly. Maybe I should revisit that thought - it’s certainly the most elegant way to do it.

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Also, here are some of Origen’s CF tube offerings:

Generic Frame Tubes.pdf (87.6 KB)

This really makes me want to build a carbon hardtail. @liberationfab the last frame was great. Can’t wait to see this one.

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Kinda like this? I’m debating whether to go with an M6 bolt as pictured or do two M5 a la the Engin clamp


Yeah that was what I had in mind, looks a lot smoother stress wise!

I personally find these Campagnolo binders very elegant:


I like this much better from a design and aesthetics point. looks right to me.

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Just wanted to pop in and thank the thread for the discussion. I am working on a side project of a lugged Ti frame and found the info here useful.

We are going to start with thicker material, then machine a lip to prevent the carbon tube from being inserted into the distorted section of the tube:

We are also going to try to use the 31.6mm seat tube ( but sleeved with titanium (34.9 OD)

Finally, I reached out to Ben of July for some thoughts on wall thicknesses, tolerances, and adhesives.

The tubes you linked should work out great. 1.5-1.75mm wall thickness on the main triangle is what I shoot for, seat stays thinner. I’m sure you saw it, but the layup schedule is listed the bottom, 0/45 for the top tube and 0/30 for the seat stays.
I use either DP420 or Henkel Loctite E-20HP for carbon to carbon. There is ‘aerospace’ stuff out there, but those two are plenty strong. I ordered some ti dropouts from Nick Crumpton and he recommended DP420. In terms of fit, I shoot for ~0.005”-0.010” when bonding carbon to carbon and then reinforcing with an overwrap. I’ve found the most important thing for strength is surface prep. I lightly sand and scuff the surface to be bonded, then clean with solvent, several times. On your last question… don’t know if it matters straight or mitered, but feel like the end of the tube should be butted up / bonded to a surface so mechanically it has nowhere to go. Jonathan at frameworks mills a straight shoulder into the lug, I think others miter the tubes, but idk?

Thanks everyone :+1:


The timing of this thread is incredible. I’ve long wanted to incorporate a carbon ST and possibly a HT into a steel build but not sure of steel sleeve sizing, carbon tube sizing etc. I also prefer to use a seat post as well.

Rockwest has a ST that looks like it should accept a 27.2 post with a 31.8mm OD I know that 0.058 wall steel tubing will sleeve into each incrementally larger size diameter tube, but is that also the case when using a carbon tube?

And what to do for a HT sleeve? Would you just sleeve the carbon tube but make it about 15mm shorter on the top and bottom for the headset cups to press in?

I’m so excited to give this a go, but want to try to minimize my wasted costs if possible.

Thank you all in advance.


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I was thinking about this as well.

It does not look like there is a fractional-inch tube with an ID of 1.25in. It seems like a builder would need to make a custom “topper” and a lower socket for the seat tube.

A 34mm OD 31.8mm ID tube would give a wall thickness of 1.1mm

A 1-5/16 OD tube gives a wall thickness of ~.77mm

I personally would feel safer with the 34mm tube. But that might negate the weight savings of the carbon tube… Both of those sizes have available hole saws.

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Do you not think that tube from Rockwest would work?

Wicks has this tube that might work.
R1-3/8X035-41 1-3/8 0.035 1.3050