Integrated seatposts in steel - wall thickness?

[cfrisia]

Hi Folks,

I've been lurking around for quite a while, I built a frame to my fit a year ago (massive tubes, 160mm HT, 520mm TT) and while I'm happy how it turned out, I want to built another one. Lighter, more custom options, not so much OS.
This forum and the late framebuilding-forum helped me a lot lot lot, I'll try to give it back in the coming time by illustrating my low-budget process that I think still produced nice tolerances etc.

So here's the thing, since the frame geo will be very customized anyways (540mm TT, ~560mm ST) I'd like to build an integrated seatpost, since nothing says personal more that the inability to adjustment ;) but honestly, I might change the seat's angle from time to time, but height will only change when I begin to shrink in 20-30 years.

It'll be a steel frame, so the post will be made from 28.6 tubing. Now the seatmast will extend over the toptube, which wall thickness should I go for?
I'm tempted to take two Columbus Life seattubes, cut them at the 0.38 part and silver braze them together with an internal steel sleeve.
Would you recommend to join them at the butted section instead? Or is a 0.38 seatmast too thin even to withstand the bending forces of my 78kg astral body?

Kind regards

Chris

[steve garro]

.


It'll be a steel frame, so the post will be made from 28.6 tubing. Now the seatmast will extend over the toptube, which wall thickness should I go for?
I'm tempted to take two Columbus Life seattubes, cut them at the 0.38 part and silver braze them together with an internal steel sleeve.
Would you recommend to join them at the butted section instead? Or is a 0.38 seatmast too thin even to withstand the bending forces of my 78kg astral body?

Kind regards

Chris

I cannot imagine this to be a good idea.
i wouldn't trust less then .049"/1.2mm for anything like that.
.3mm is the thinnest you can draw steel - that's a high stress area.
- YRMV - Garro.

[jon_norstog]

I cannot imagine this to be a good idea.
i wouldn't trust less then .049"/1.2mm for anything like that.
.3mm is the thinnest you can draw steel - that's a high stress area.
- YRMV - Garro.

My own thought is integrated seatposts belong on NAHBS show bikes. I can think of a lot of things that could happen with this sort of arrangement, maybe half of them leading to the bike's having to be surgically removed from the rider.

jn

"Thursday"

[Jonathan]

I'm hesitant to post because I've never made an ISP, but I've looked at the idea. I think a small or large diameter relatively heavy tube is designing away from the sweet spot of steel. I know some guys have had tubes drawn to meet their needs but barring that their might not be a butted alternative that works well in the size you want to build. Tom Kellogg is a smart guy, somewhere on his Flickr page is his version and how he handled the problems I mentioned.

[cfrisia]

My own thought is integrated seatposts belong on NAHBS show bikes.

All the crabon bikes do it. And it looks very very good on fillet brazed bikes.

I'm thinking:

Tensile strength of an integrated steel seatpost is much higher than on an aluminum inserted seatpost.
Anyways, tensile strength doesn't really matter here, since I won't bend the post, I'm much more likely to buckle it.

So Euler said

4be263c8f28b018d391d88fafff8a4e8.png

I don't need to know the force or length, since both are the same, wether I take an aluminum post or a steel post. That leaves me to compare E*I of steel and aluminum, elastic modulus and area moment of inertia (...bear with me).

For area moment of inertia I'll assume:

Aluminum: 2 mm wall thickness on a 27.2 seatpost (23.2 ID)
Steel: 0.5mm wall thickness on a 28.6 seatpost (27.6 ID)

I_steel = Pi * (OD^4-ID^4)/64 = 4358 mm^4
I_alu = Pi * (OD^4-ID^4)/64 = 12648 mm^4

The elastic modulus of steel is about 215 GPa, for aluminum it's about 72 GPa.
215 GPa / 72 GPa = 2,986

To compare, I multiply

I_steel * 2,986 = 13013 mm^4

=> Larger than I_alu, therefore 0.5mm wall thickness on a 28.6 seatpost should be fine. I might go for 0.6 so I could take a spirit seattube and not ream/cut the top butt.

[echelon_john]

I'm just a caveman; your mathematics and bright lights confuse and frighten me.

But I've seen more buckled seatmasts in the last 5 years than I have bent seatposts. I come at this from the perspective of not wanting a failure winning out over visual concerns or 50-100 grams.

.6 is very thin for this. Don't forget that the ST is going to have a HAZ from likely 2 heat cycles, from both the top tube attachment and the seatstay attachment if you're fillet brazing. That's where the failure will occur; right in the HAZ on the rear of the tube above the seatstays.

All the crabon bikes do it. And it looks very very good on fillet brazed bikes.

I'm thinking:

Tensile strength of an integrated steel seatpost is much higher than on an aluminum inserted seatpost.
Anyways, tensile strength doesn't really matter here, since I won't bend the post, I'm much more likely to buckle it.

So Euler said

4be263c8f28b018d391d88fafff8a4e8.png

I don't need to know the force or length, since both are the same, wether I take an aluminum post or a steel post. That leaves me to compare E*I of steel and aluminum, elastic modulus and area moment of inertia (...bear with me).

For area moment of inertia I'll assume:

Aluminum: 2 mm wall thickness on a 27.2 seatpost (23.2 ID)
Steel: 0.5mm wall thickness on a 28.6 seatpost (27.6 ID)

I_steel = Pi * (OD^4-ID^4)/64 = 4358 mm^4
I_alu = Pi * (OD^4-ID^4)/64 = 12648 mm^4

The elastic modulus of steel is about 215 GPa, for aluminum it's about 72 GPa.
215 GPa / 72 GPa = 2,986

To compare, I multiply

I_steel * 2,986 = 13013 mm^4

=> Larger than I_alu, therefore 0.5mm wall thickness on a 28.6 seatpost should be fine. I might go for 0.6 so I could take a spirit seattube and not ream/cut the top butt.

[David Tollefson]

So Euler said

4be263c8f28b018d391d88fafff8a4e8.png

Isn't that for an axially loaded column?

[Jonathan]

I have a sg .6mm 28.6 DT in a old Columbus PL "event" tubeset. I can say with certainty it would not last as an ISP very long. You can figure that out by holding it in your hand.

[bellman]

.6 is very thin for this. Don't forget that the ST is going to have a HAZ from likely 2 heat cycles, from both the top tube attachment and the seatstay attachment if you're fillet brazing. That's where the failure will occur; right in the HAZ on the rear of the tube above the seatstays.

Good points here and it was exactly what was in my mind when i added a long point steve garro style seattube sleeve on my own isp bike.

To the op in regards to an isp; a lot depends on how much seatmast you need above the top tube.
a fairly level tt bike is probabally ok, My own ride uses a full length piece of 28.6 x .035 for this and seems to be ok after 3 seasons of cross with my feathery race weight of 84kilos. A similar bike I made for a gorilla of a man who is the same size but much stronger and faster than I used a seatpost inside full length untill the bottom was into the correct insertion point anyway.

I've built them for lighter riders using long butted seat posts but always reinforcing the junction with a sleeve.


hope this help,
jake

[cfrisia]

echelon_john - I didn't want to frighten anyone or pose as a wise ass .. I'm seeking discussion because calculations do not replace real life experiences. your input on sleeving is much appreciated :)

David: depending on the degree of freedom, this formula gets some coefficients, but nonetheless, it all stays the same except E and I

jake: very good input too, I think I might do some more CAE investigations.

It's really hard to dimension this by gut feeling. Therefore I tried to push this conversation a bit with my calculations... Anyone with more first-hand-buckling experiences?

[bulgier]

I built a race bike with an ISM in 1978 (though we didn't call it that back then). Pic here.

As you can see, level top tube, so not too much stickout. I used Reynolds 531, 9/8", 19/22 gauge SB so it's nominal 0.7 mm wall at the seat lug. Probably brass-brazed. (Reynolds used to make an extra-long SB seat tube, 75 cm I think?) I weighed about 175 lb then and I rode it for maybe a year and raced it a little, then sold it to another guy who was bigger and stronger than me, ex-racer who was a Cat 2 road and track. He didn't buckle it either, but I don't know what miles he put on it. Last I heard he gave it to his son who was riding it. Other than fatigue endurance, the number of miles doesn't matter much, if the question is just whether hitting that first pothole with your butt on the saddle will buckle it, right? Though I suppose fatigue endurance could become an issue too.

The seat rail grabber parts are from a Weyless post (original '70s version). The fulcrum that it pivots on to set the angle was replaceable, to give some adjustability. The fulcrum is kinda tall in the photo because I made the frame a bit small, to be able to accommodate a taller saddle like a Brooks leather, and/or longer cranks. It looked cooler when I had 10 mm longer cranks on there, and the fulcrum was 10 mm smaller. (I liked 180s but the cranks on for the photo were 170s)

I later came to the conclusion that the ISM was a dumb idea (which I believe to this day) and never made another one. I was 20 yo so I chalk it up to youthful indiscretion.

Mark

[cfrisia]

Hey Mark, thanks for your input!
Yes, fatigue endurance is an issue. I'm not sure how to simulate this... from what I know, repeated microscopic buckling (even within elastic elongation) could lead to hairline cracks.

Why do you think it was a dumb idea to build an ISM? Because it won't fit to other people?

regards

Chris

[Mark Kelly]

Regarding your application of Euler's formulae:

This isn't a simply loaded column, the likely failure mode is shell buckling not column buckling as you have supposed.

The critical stress for shell buckling from Koiter's circle is given by t / R * E / sqrt(3(1-ν^2))
Taking ν as equal for steel and aluminium the stress is proportional to t /R * E. As you note E steel / E alu is about 2.9 but in the example you give t / R for alu is 2/27.2 = 0.074 and steel is .6 /28.6 = 0.021, the ratio is thus 3.5 so the steel post will have the lower critical stress. In the case of the tube you originally talked about using this is even worse as its t / R ratio is .38 / 28.6 = 0.013 so it's about half as strong as the alu.

[cfrisia]

Thanks for pointing it out Mark! I was thinking already that my simplified model leaves out shell buckling. In my calculation, a large diameter post with, say, a 0.05mm wall thickness could still be superior to the aluminum post, as long as I keep increasing the diameter ;)

btw I think the R is for radius, of course it doesn't change the ratios though.

let me get back to you when I measured the actual thickness of a mediocre seatpost I have here.. it's a close call

[devlin]

I think you are confusing you decimal points when talking mm. 0.05 is very thin and I doubt tubes could ever be drawn that thin. A normalish seat tube is 0.8mm at the bottom and 0.6 at the top (I'm talking very general pedants, ok). I would suspect you'd need to either have localised butting or reinforcing where the seat tube leaves the cluster of tubes. More for a little bit of over building insurance than anything.

[Mark Kelly]

btw I think the R is for radius, of course it doesn't change the ratios though.

Yes. My bad.

[cfrisia]

Devlin, i was just explaining how my false assumption that buckling on a seapost would only be dependent on elastic modulus and area moment of inertia makes no sense since such unpractical numbers would still produce a positive prediction.

solution to this: I'll take either a 0.7 or a 0.8 tube. The weight penalty for a 150mm post is about 20-40 g compared to 0.6 mm walls.

A cheap Ritchey seatpost I have here has 2.1-2.2 mm walls by the way.

[devlin]

Ahh, gotchya. I wasn't reading it deep enough, just skimming.

[steve garro]

"I'm an engineer, won't break here/won't break here!"
NOMEANSNO, I think?
How about the HAZ and how long it's held at heat, which is dependent on your joining skill......
I like to back think things..........what if all bikes had an ISP and someone made a light Al one, that *WOW!* would adjust, and be removable if it bent - what a revelation!
- Garro.

[ScottB]

Garro raises a good point about time and temperature of the HAZ. My metallurgy experience is limited to a few classes in college, but from looking at CTT charts for 4130, holding the steel at elevated temperatures gives it more time to form Ferrite and Cementite. While this microstructure has a lower yield strength than others, the failure mode is more likely to be ductile rather than brittle. I think this would be useful, especially on a part of the bike where having the frame “surgically removed from the rider” is a concern…

If there are others with more background in metallurgy or materials science who can weigh in, I’d be interested in hearing it.

FWIW, I don’t care either way about ISPs. Approaching framebuilding with some science, in addition to years of experience, is important to me.