Crankycrank

I say go for it. You sound like the kind of person who can do this IMO. Hell, if there were no tinkerers with some ingenuity we would probably have about 2 bike mfrs. in the world.

Andrew R Stewart 

Sorry to miss understand your use of the "pin", not at all what I was taking from my reading. I've seen many posts with a top clamp inserted into the post portion and secured with a pin. I build frames and sometimes work on machinery so my take on a pin is different.

Have fun on this project. While I understand the want to make your own stuff some here have said "don't make what you cab buy". Andy

Russ Roth

Sounds like a fun idea to screw around with. Not certain the full validity of this statement but I'm under the impression that you have to be careful bonding straight carbon to aluminum due to the aluminum reacting to the carbon for some reason. From what I understand this was the reason for early aluminum lugged carbon frames failing at the joints. The solution, from what I remember, was to put a layer of fiberglass in between the two materials that served to stop the process. I believe Calfee had something about this at one time on their page since they were doing a lot of frame repairs on this style.

Jonneh

Cheers Andrew. My main task at the moment seems to be working out how well 2.5--3 mm multidirectional carbon can withstand compression from the cross piece in the axis of the layering. If the answer is "well", I may not need to bond the insert in at all.

Jonneh

From what I understand, you do have to be careful. The epoxy used to bond the two surfaces can serve as the barrier, but one then needs a method of maintaining this separation along the length of the bonded section. I don't know about the fibreglass system, but another method for achieving this is by adding fine glass beads to the epoxy mixture. These suspend the outer layer while bonding and ensure that separation, if only on the order of 0.1 to 0.2 mm, is maintained. This may well have been a more recent evolution.

Love those webbed lugs on the Calfee frames btw! The classic look will always be classic, but I'm looking forward to the more organic shapes emerging from generative design---the Calfee lugs have something of that to them.

Koyote

This experiment may end up as a great story for some lucky proctologist.

Kimmo 

/eyeroll @ all the naysayers

Sy Reene

Sounds a bit iffy. After all, we're still wondering what happened to the poor guy in this thread: https://www.bikeforums.net/bicycle-m...them-down.html

veganbikes

I own a bunch of Thomson posts, would highly recommend them, have had zero problems with the 5 I own and wouldn't hesitate to buy more as needed. Quality sometimes costs a little extra but it is worth it for peace of mind.

rosefarts

The seat clamp I think you are describing, needs to be fairly beefy or it might break.

I had one that style fail on its first ride. There is a lot of pressure on some fairly small areas. I suppose it’s possible to build a reinforced version but I don’t like the design.

Just fwiw, I’ve broken 2 seat posts in my time surprisingly (140lbs). I was never hurt or even wrecked but holy cow, it’s a looong way home without a seat.

CliffordK

I think my Graftek may also benefit from a 25mm seatpost if I remember right.

A couple of thoughts.

You can purchase 25mm OD, 20mm ID tubes on E-Bay.
https://www.ebay.com/itm/142615908405

That gives you 2.5mm walls or so. If that isn't enough, get a 20mm OD, 16mm ID, and epoxy it inside the 25mm.

I'm not sure the minimum epoxy gap. You can sand the tubes down slightly if needed (a sanded surface would bond better anyway).

For your "mast", calculate the length of seat post above the seat tube. I'd cut the mast part as long as practical (even all the way down to sit on the seat tube clamp). If you epoxy the mast onto your post, that will maximize the contact area.

=======================================================

@Crankycrank found a couple of 25.4mm posts. You could also calculate the inner diameter of the post. Insert an internal strengthening sleeve. And, sand the post down to 25.0mm to fit your frame.

=======================================================

You mentioned a 30.25 mm ID "mast" as being too much of a gap to shim. You can purchase 30mm OD, 25mm ID tubes that could be used for a shim. Or, 30mm OD, 26mm ID.

https://www.ebay.com/itm/143536660123

Unfortunately I fear such a configuration would be a bit unwieldy.

Nothing is cheap, and I fear that ultimately you'll be spending as much on your hack as just purchasing the more expensive finished seatpost.

Jonneh

That's good to know. I'm curious about how exactly it failed---did the pin start compressing the edges of the holes in the CF tube? If so, do you remember the approximate wall thickness of the post?

Jonneh

Thanks for the various musings Clifford. My thoughts at the moment are leaning towards idea (1) rather than the mast idea (4) for reasons of elegance: as you say, the mast could end up quite unwieldy with all the telescoping tubes and shims. That said, depending on the failure mode of rosefart's post, the tube for (1) may have to be reinforced somewhat (perhaps with a bonded insert) or else have a decent wall thickness at the outset, which will be prudent anyway because I'm on the tall, high-leverage side.

Jonneh

A final thing to keep in mind is the retention mechanism on the Look seatpost clamp: it uses a binding bolt similar to this one but split symmetrically. I don't think this is the best design for carbon posts, given that the compression forces are concentrated on the bolt and not spread around the circumference of the seat tube as they would be in common clamp designs, but the design was used as late as the KG 481, which was well into the era of carbon seatposts, so Look apparently gave the go-ahead. Still, another motive for a generous wall thickness!

rosefarts

https://www.ebay.com/itm/OMNI-Racer-...-127632-2357-0

It really takes some looking around to find that style of clamp. A few years ago, all the cheapie posts had them. Makes me think there have been a lot of failures.

You see up top where there is the threaded aluminum bit that would clamp the top half of the seat rails? Well, right at the threads the entire piece split in half. This rendered it unfixable, either mid ride or at home.

If the entire upper section was one piece, that then had steel to thread into - exactly how Thomson does it - the design should work. It’s not the hole through the post that is the weak spot, it’s the under built clamping section.

Jonneh

That's interesting, and frankly, reassuring. I've been using a KCNC post with a similar (but clearly sufficiently different) design for years now without problem. Each yoke has a pretty substantial vertical height near the threads (about 15mm), which may be more than yours had.

I've got some 2.5mm-walled carbon tube on the way, with which I'll fashion a saucer to fit the KCNC cup (no reinforcements for the time being), drill holes for the cross piece, and stress test.

A remaining variable to decide on is the angle at which the holes are drilled. The two common variants are perpendicular to the seat post and parallel to the saddle rails, shown here, respectively:





I would think that the latter places more equal stress on each hole and less bending strain on the bolts, but the former is more common, for whatever reason. EDIT: the latter does seem to be more common on newer posts.

rosefarts

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