Author Topic: Carbon seat post with internal bit - can I cut it...?  (Read 556 times)

Carbon seat post with internal bit - can I cut it...?
« on: July 09, 2020, 04:46:13 pm »
I am needing a lot of setback at the moment, mainly due to using an ISM saddle. 

While idly browsing eBay I saw one with 40mm, which is pretty rare.  It wasn't expensive so I bought one.  It looks good:



Complication is that it has an internal cross-piece...



...which is where I want my Di2 battery to go. 

This bit is pretty strong and, while I don't know if it is structural, it would take some cutting out.

I would be having the seat post a long way in, such that the bottom 15 cm or so would not be needed, so I could just cut it down and stick my battery underneath it.  Or I could try to take bottom 10cm of the the bit out cleanly, with hacksaw and dremel, etc, to make a space for battery. 

Any thoughts before I give it a shot?

Re: Carbon seat post with internal bit - can I cut it...?
« Reply #1 on: July 09, 2020, 04:51:01 pm »
My guess is, that the web is in there for a reason.
You don't generally put time and material into something for the fun of it.
Sorry, Frank. That's probably not what you wanted to hear.
Also, I think that removing the web 'cleanly' would be problematic - but that's by the by.

LittleWheelsandBig

  • Whimsy Rider
Re: Carbon seat post with internal bit - can I cut it...?
« Reply #2 on: July 09, 2020, 05:08:24 pm »
The web is only to increase resistance to bending moment. Maximum bending moment is at the seatpost clamp. At depth within the seat tube, bending moment is zero.

The web could be removed from the deeply buried section without problem but if I wanted to fit a seatpost battery, I would keep the web upwards from 50mm below the clamp.

Cutting carbon has its own issues.
Wheel meet again, don't know where, don't know when...

Re: Carbon seat post with internal bit - can I cut it...?
« Reply #3 on: July 09, 2020, 05:15:40 pm »
I'm struggling to argue with LWaB's more in-depth (no pun intended) view than mine.

Re: Carbon seat post with internal bit - can I cut it...?
« Reply #4 on: July 09, 2020, 05:29:20 pm »
I'm struggling to argue with LWaB's more in-depth (no pun intended) view than mine.

That’s cos he’s right lol.
We are making a New World (Paul Nash, 1918)

Re: Carbon seat post with internal bit - can I cut it...?
« Reply #5 on: July 09, 2020, 07:53:38 pm »
I agree with LWaB's approach but I note that the length over which the bending moment is transferred between seat pin and seat tube will vary depending on how the stiffnesses of the two compare with one another. If the seat pin is stiff relative to the seat tube the load will be transferred over a long length but if the seat pin is flexible vs the seat tube the load will be transferred over a much shorter length.   I would normally expect the seat pin to be seeing a significant bending moment at least 3" into a frame.

FWIW I don't see why the battery can't stick out the bottom of the seat pin, if necessary using an extension to the seat pin which is bonded into the last couple of inches? This would leave the reinforcing rib largely intact....?

cheers

LittleWheelsandBig

  • Whimsy Rider
Re: Carbon seat post with internal bit - can I cut it...?
« Reply #6 on: July 09, 2020, 08:05:58 pm »
Because a seatpost is somewhat undersized relative to the seat tube, there is only good moment transfer between the two at the seat clamp.
Wheel meet again, don't know where, don't know when...

Re: Carbon seat post with internal bit - can I cut it...?
« Reply #7 on: July 09, 2020, 09:11:24 pm »
Because a seatpost is somewhat undersized relative to the seat tube, there is only good moment transfer between the two at the seat clamp.

this only happens if the frame is stiff relative to the seat pin, and the seat pin is a tolerable fit.

This isn't always the case.

Furthermore if the seat pin is undersize vs the frame the usual consequence is that the binder may clamp locally but the seat pin is able to see high bending loads deep within the frame and may therefore move around. This 'three point bend' (with the binder trying to do something impossible in the middle)  is the usual reason why some seat pins don't stay put and gradually work their way downwards in the frame.

So in summary re the length over which the load transfer occurs;  'it depends'. 

Two main things upon which it depends are

a) quality of fit and
b) relative stiffness of the parts (even if the fit is good)

If you put a seat pin 2" into a frame and load it (in bending), often you can see local (hopefully elastic) deformation in a (flexible) frame when the seat pin is loaded up.  Since this doesn't occur with a longer seat pin, it is pretty clear that the load is being transferred differently, i.e. over a longer length.

cheers

LittleWheelsandBig

  • Whimsy Rider
Re: Carbon seat post with internal bit - can I cut it...?
« Reply #8 on: July 09, 2020, 09:30:31 pm »
Bending moment in a seatpost is highest at the seat clamp and decreases with distance away and we agree on that. Cutting off an overlong seatpost, retaining only the equivalent minimum insertion length, does not worsen the stresses within the seatpost compared to having the seatpost at maximum extension. I think we can agree on that.

Removing the internal stiffener reduces the post’s stiffness locally. Moments are concentrated within and preferentially transmitted by stiffer sections.

Given that minimum insertion lengths on seat posts for sensibly designed frames are commonly 75-100mm, the moment in the seatpost at half that length will approximate half the maximum moment.

That seatpost is likely to have a constant cross-section over its length. That particular stiffener is unlikely to more than double the moment capacity of the seatpost unless something very clever is occurring. A cheap seatpost will not be designed that cleverly.

Individuals can choose to do something that is more or indeed less conservative than I would be happy to do. I don’t have a problem with that.
Wheel meet again, don't know where, don't know when...

Re: Carbon seat post with internal bit - can I cut it...?
« Reply #9 on: July 09, 2020, 10:00:58 pm »
Thanks all, interesting.

I was working on the assumption that, as long as I had at least the same length of intact post in the seat tube as the minimum insertion level, I could compromise the excess with impunity, and it sounds like I can, so it answers my question.

The battery can't go lower than the seatpost or it would snag on the bottle cage rivnuts.

The relative stiffness point is interesting: with a stiff post in a floppy frame, could leaving too much seatpost inside the frame actually damage it by making part of it more rigid than it is meant to be?  So if my frame was floppy (which I don't think it is) would it actually be prudent to cut down a stiff seatpost?

LittleWheelsandBig

  • Whimsy Rider
Re: Carbon seat post with internal bit - can I cut it...?
« Reply #10 on: July 09, 2020, 10:08:04 pm »
If you are maintaining at least the minimum insertion length without modification, I don’t think whatever you do to the post at greater depth is likely to damage the frame.

Sometimes a long seatpost can be straighter than the seat tube. That situation tends to jam the end of a too-long seatpost into the frame at depth, causing high point loads which might locally damage the frame but probably also squeaks over bumps.

Moulton avoided that problem on APBs and TSRs by either swaging down the seat tube to clamp the post over a comparatively short length or by fitting a seatpost shim and a significantly narrower post. Both methods meant there was clearance between frame and seatpost at depth, so no creaking.
Wheel meet again, don't know where, don't know when...

Re: Carbon seat post with internal bit - can I cut it...?
« Reply #11 on: July 10, 2020, 12:59:58 am »
re the rivnuts;  IIRC the battery itself is a good deal slimmer than the seat tube (eg SM-BTR2 is about 16mm dia), and is normally mounted centrally within the seat tube.  It occurs to me that if you are using a non-standard battery mounting (eg below the seat pin), it can be offset within the seat tube so might better clear the rivnuts. A 16mm battery inside a 27.2mm seat tube ought to allow  rivnuts protruding up to ~9mm into the seat tube to be used.

It also occurs to me that it isn't difficult to remove the extant rivnuts and replace them with shallower ones, if needs be.

A final comment is that you can minimise the seat pin weight vs strength by slash-cutting the base, such that the seat pin is longer at the front than at the back. This also allows bending stresses from a fairly stiff seat pin to be transferred into a thin-walled seat tube in a more benign fashion. In this case there won't be much weight to be saved but  it might work out  better if you intend to use an offset battery mounting.

cheers

frankly frankie

  • I kid you not
    • Fuchsiaphile
Re: Carbon seat post with internal bit - can I cut it...?
« Reply #12 on: July 16, 2020, 08:59:31 am »
Cutting carbon has its own issues.

I've cut down several seatposts and they cut like butter.  I'd do it with a simple hand hacksaw in a windless environment, in a way that collects the little pile of residue safely.
It's not dark yet but it's getting there.