# stress distribution in seattube - best location to add internally-routed dropper



## high_desert_mud (Jul 22, 2017)

The basic task I'm looking at, adding an internal-only dropper to a frame with no routing, has been asked a few times:

http://forums.mtbr.com/frame-building/drilling-hole-frame-dropper-post-cable-bad-idea-805222.html 
http://forums.mtbr.com/banshee-bikes/drill-hole-internal-seatpost-cable-1030388.html 
http://syntace.my1.cc/liteville/pdf/RockShox_Reverb_Stealth_an_Liteville_engl.pdf 

but reading all posts on mtbr and elsewhere, I haven't found much serious information about frame design in that area.

I'm familiar with stress risers, and how to drill thin sheet metal. What I have no practical experience with is relative stress in the seat tube area, how that is distributed over its length (near the BB, near the top-tube, in the middle), etc.

Anyone have any experience with FEA of frame design (or practical experience) that can comment where on the post would have the least stress, and what are the most common seat-tube failure modes (buckling versus cracking (where), etc).

Which is a better trade-off: slot of minimal width (more cable wear, slightly less water intrusion protection) or slightly larger slot with purpose-made grommet?

Thank you!

(Frame is a Surly Ice Cream Truck. There are no bottle bosses / rivet-nuts, or fixed deraillleur mounts on the seat tube. Rigid steel frame, with a wide (~130mm) press-fit BB. 4130 steel, likely butted tubes)


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## high_desert_mud (Jul 22, 2017)

I did find this:
https://sheldonbrown.com/rinard/fea.htm

very interesting, with some relevant summaries at the bottom, especially relating stored energy in each frame member during high pedaling stress (starting on a hill).

I was a bit surprised to see the seat tube 2nd on the list of total stored energy (energy stored in elastic deformation of member). The greatest is downtube, storing 40~50% of total stored energy, seat tube has 20~25%. "stored energy" is broken down by axial / in-plane-of-bike bending, out-of-plane bending, and torsion.

For my question, I see that the seat-tube's load is 80+% out-of-plane-bending, and the remainder torsion (negligible other deformations).


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## Walt (Jan 23, 2004)

You can drill holes in the ICT seat tube all you want, it's fine.

-Walt "FEA, schmef-EA" Wehner


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## pvd (Jan 4, 2006)

Surely bikes are built using Schedule 80 pipe. No need to worry.


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## high_desert_mud (Jul 22, 2017)

Walt said:


> You can drill holes in the ICT seat tube all you want, it's fine.
> 
> -Walt "FEA, schmef-EA" Wehner





pvd said:


> Surely bikes are built using Schedule 80 pipe. No need to worry.


I have heard similar references to surly durability in the past. It's one of the things that initially attracted me to the brand (that and the existence of the BFD).

Can you give me any comparisons on the range of (steel) tube wall thicknesses found on other brands.

For me measuring my bikes wall thickness - I haven't heard of any steel bikes using fancy drawn tubes with varying thickness, have you? The seat tube isn't double butted either (from pics, I'll receive the bike in about a week)


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## pvd (Jan 4, 2006)

We sorted all this out long ago.

https://www.peterverdone.com/drilling-holes-in-perfectly-good-bike-frames/

http://www.peterverdone.com/drilling-holes-in-perfectly-good-carbon-fiber-frames/


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## pvd (Jan 4, 2006)

Surly is an extremely low end brand. They do very little to make their bikes well.


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## high_desert_mud (Jul 22, 2017)

pvd said:


> Surly is an extremely low end brand. They do very little to make their bikes well.


based on the fact this seems blatantly wrong, must be sarcasm? Surly seems to have a rather large contingent of followers (who aren't there for cheap prices) ... and price-wise Surly doesn't sell any super-high-end bikes but is by no low-cost either (ICT is ~$2800 complete). Ah, the internets.



pvd said:


> We sorted all this out long ago.
> 
> https://www.peterverdone.com/drilling-holes-in-perfectly-good-bike-frames/
> 
> http://www.peterverdone.com/drilling-holes-in-perfectly-good-carbon-fiber-frames/


More importantly though -

The threads you link and others do not address the question. At least per what i read in the threads, no thought has gone into *WHERE* (and *why* there) to locate the hole from a *structural *context. They consider only what is most convenient for the cabling.

Structural consideration is the question from my OP, should be obvious from the text.


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## high_desert_mud (Jul 22, 2017)

high_desert_mud said:


> The seat tube isn't double butted either (from pics, I'll receive the bike in about a week)


Re-reading Surly's product page, this appears wrong. It says the triangle is butted, actually.

Unfortunately this is bad (for my question) as it probably makes it more important where I locate the slot (Since I haven't found any info yet saying it really isn't a practical concern, i'm still operating under the assumption location matters).


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## alexbn921 (Mar 31, 2009)

I contacted calfee about drilling a hole in my carbon frame and reinforcing it on the seat tube in the same location of the new bikes. They said no problem. I didn't end up doing it, but it can be done. There price was reasonable too.


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## Walt (Jan 23, 2004)

I think you got your answer. If you want to run an FEA on the whole structure, go for it. I can tell you that people drill these all the time down at the bottom of the seat tube, and that it will be just fine unless you do something really dumb. Fatigue and a failure propagating from a badly drilled hole would be a bigger concern. If you are really worried about it, send it to me and for $50, I will do it and install a little reinforcing patch and guarantee the work.

Otherwise just drill the damn thing, it's a very cheap and very strong frame.

-Walt


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## Eric Malcolm (Dec 18, 2011)

OP, it is well known that Surlys are bullet proof. they are constructed from heavier wall tubes than what Walt and Peter refer to. Hence, they are cheaper, this is not said to offend, it is a statement of fact in the tube specification sense. 
What is also missing in the understanding here is that if butted, where does the butt end on the seat tube? This is such a variable dimension that we cannot say, even from tubes we might use ourselves to make a custom frame. There are many options of butt lengths available and in differing diameters that the mix becomes a very muddy area to answer. And we would have to discect your Surly to find out, not an option I think.
So we go to the more practical way of answering your question, just drill the hole, we don't see Surlys falling apart because they have a hole in them.

Eric


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## scottzg (Sep 27, 2006)

Eric Malcolm said:


> OP, it is well known that Surlys are bullet proof. they are constructed from heavier wall tubes than what Walt and Peter refer to. Hence, they are cheaper, this is not said to offend, it is a statement of fact in the tube specification sense.
> What is also missing in the understanding here is that if butted, where does the butt end on the seat tube? This is such a variable dimension that we cannot say, even from tubes we might use ourselves to make a custom frame. There are many options of butt lengths available and in differing diameters that the mix becomes a very muddy area to answer. And we would have to discect your Surly to find out, not an option I think.
> So we go to the more practical way of answering your question, just drill the hole, we don't see Surlys falling apart because they have a hole in them.
> 
> Eric


That said, there's a 50% chance it's an externally butted seat tube. If that's the case it's easy to find the butt; it will show up when you shine a light along it or run your hand down the tube. Don't put the hole in the butt. Teehee.

I'd just run an external route post. Infernal is dumb.


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## pvd (Jan 4, 2006)

I forgot to weld about 1" of a seat tube to BB joint. Found out after paint. I rode it for a few years with no issue.


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## high_desert_mud (Jul 22, 2017)

it is done. acceptable results. I'll either make a silicone gasket, or try one of the ones online.








I'll try to remember to post back if it breaks  lets hope NOT.


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## Meriwether (Jul 26, 2007)

My friend ghost rode his Surly off a 4 story parking garage and it was fine. They expect people to do stuff like that so a lil slot won't hurt it. But I would get a Cinelli grommet:
https://m.ebay.com/itm/CINELLI-Fram...oaABIN7zrCneBJZO&_trksid=p2489528.m4335.l8656

Sent from my iPhone using Tapatalk


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## Eric Malcolm (Dec 18, 2011)

high_desert_mud said:


> it is done. acceptable results. I'll either make a silicone gasket, or try one of the ones online.
> View attachment 1162657
> 
> 
> I'll try to remember to post back if it breaks  lets hope NOT.


Nicely done. Tidy job.
Go and ride the bike, clean it occasionally to inspect, otherwise get the grommet to keep the BB internals dry.

All good.

Eric


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## fone (Mar 14, 2007)

high_desert_mud said:


> it is done. acceptable results. I'll either make a silicone gasket, or try one of the ones online.
> View attachment 1162657
> 
> 
> I'll try to remember to post back if it breaks  lets hope NOT.


Are you still alive?


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## high_desert_mud (Jul 22, 2017)

probably?


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