Crafting an inline seatpost

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Stolichnaya
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by Stolichnaya

So, I had to change my position recently due to, well... I am getting old.
I am now on an inline post on most of my bikes.

My older Kuota came with a setback carbon post that is actually acceptably light but has a crappy head and was never really used. The thought was to lop off the top and craft an inline post by sanding it down with a dremel to fit a cradle properly.

I plan to order the KCNC cradle / yolk kit for carbon saddles (the post is a 30.0 so I'll order the larger cradle size.)
Also in the order will be the appropriate KCNC center pin.

The question for the engineers and those who have done this before is - bolt length.

I could order the KCNC M6 bolts but those seem short at 37 mm (front) and 47 mm (rear).
I cannot support this with any data, but it would seem that the post is less compromised (under my weight) when there is a longer distance / more carbon between the center pin and the cradle.
So, would M6 bolts from POP in the longer lengths of 50 to 60 make more sense for this project?

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GMF
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by GMF

You are definitely thinking in the right direction. Not knowing your post, you'd have to get an educated guess at the shear strength of the carbon, and the measure the wall thickness, etc. and do some fairly basic calculations to get your enginerd on. The problem with doing this to carbon is that it doesn't like cut fibers, so typically there is a metal ring (usually aluminum for weight) that reinforces the whole area and distributes that shear stress (pin trying to pull out of the top of your post) to a much larger area through the glue joint.

However, if you have an extra 60mm of seatpost length, sure... give it a go. And if it cracks/breaks, cut off the top and go the slightly harder reinforced ring way.

Good on ya for experimenting!

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Stolichnaya
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by Stolichnaya

GMF, thanks for the comments. I spoke to the long lost FelixOR, and he is also of the same thinking. The post is quite long and the carbon rather thick. I believe it will hold with longer bolts as a safety measure. If something goes awry, I calculate I have two tries before I run out of post to use. I'll order the longer POP bolts this weekend and the KCNC cradle and pin are hopefully on the way already.

sungod
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by sungod

i'm assuming you are going to cut two recesses to take the cradle and match its radius, these need to be concentric, which i'd think would be tricky with a dremel

how about...

cutting the recesses a bit bigger, then putting a cf 'cap' over the top - which you can form to match the cradle by pressing it in and clamping until the resin sets (having coated the cradle in release compound first!)

this way, for the price of a few grams extra weight, you get an exact match of radius, concentricity, increased load bearing surface, and a bit of reinforcement at the top of the tube

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prendrefeu
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by prendrefeu

If the post is circular, you can use the portion of the cut off post as the cradle itself.
So... take the post, cut down a piece long enough that the length is longer than the length of span across the saddle rails and add a few mm to each side for good measure. Then drill through that piece-of-post two holes, all the way through, in parallel, with a diameter matching your saddle rails. Then cut that piece-of-post in half and you've got a cradle.

The pin which holds the bolts is also at an angle through the post, so perhaps use a jig & drill press to get it just right?
I forget the angle off hand - it may be 6° but I am not certain, may need to double check.

Also, using a circle-saw drill bit in a drill press may be the best way to ensure a perfect half-circle to hold the cradle and easiest way to make sure you're matching diameter size.
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Stolichnaya
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by Stolichnaya

sungod, prendrefeu, these are really appreciated comments. Both excellent approaches. I am awaiting the pin, bolts and yolks. Once they arrive, I'll size up what I can actually do without butchering the post and eventually putting any body parts in harms way. :wink:

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Stolichnaya
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by Stolichnaya

So, after an inordinate amount of time trying to launch this project, I have dusted off the box of bits and am awaiting a new drill press/stand.

In terms of drilling the post clean through, any suggestions on the drill bit to use? I have not seen any carbon specific bits (like the carbon specific saws that are out there) so my plan was to use standard drill bits for wood and start with a smaller diameter and work up to the size needed to match the pin's size.

In terms of the pin angle, I will measure what it is on the KCNC inline post I have and use that.

I will skip the step of installing a carbon cap between the cradle and post as I will not get to it ever, I have to be honest.

Any thoughts on the drill bit choice would be welcome.

Rudi
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by Rudi

I used a Ti-Ni coated set for drilling a frame (not seatpost). They didsn't say if they were for wood or a different material on the blurb on Amazon. They key thing I found when researching this was to use as sharp (i.e. new) as possible.

I did do the small dimeter and work up thing as you described.
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Marin
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by Marin

Any normal drill bits work for me. Use high speed and small pressure and drill a smaller hole 1st.

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Stolichnaya
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by Stolichnaya

Thanks Rudi and Marin!

alcatraz
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by alcatraz

Have you decided on a method to reinforce the seat tube around the pin? I'm curious how to do this as simple as possible.

Nice project

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Stolichnaya
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by Stolichnaya

Honestly, with the first attempt, I did not plan to reinforce at all other than perhaps applying a thin layer of epoxy to the drilled surfaces. The post I am working on is a relatively thick tube and I have enough length for two tries.

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