Minimum seatpost insertion

[salem]

I bought a new-to-me light seatpost for my road bike, a 287mm length to replace an excessive 350mm length. The new post has the max height line at 100mm while the old one was at 80mm. Checking an even older 310mm post, the line was at 65mm.

I understand this; there has been a trend towards extending seat tubes further and further above the top tube, so the post has to go in further to engage the structure of the frame (top tube and seatstays) that can handle the load. For example, that last old post is in a frame with 65mm of seat tube above the top tube, so I would never run that post anywhere near the max height line on that frame.

Back to the new post and it's corresponding frame. The post will be riding about 1.5cm above the max height line (or effectively 5mm more insertion than the max height line of the old post), and I'm completely confident that is sufficient give the frame's short seat tube extension and my light 140 pounds. I'm curious if there is a standard rule of thumb for how much post should extend below the top tube junction. Of course, this should probably be a percentage of the extension (lever) above.

As a related aside, some twenty years ago, I had a winter loaner bike to ride until team bikes were ready. It was a size too small, and after riding with the seat low for a week or so, my knees were feeling it. I called the company about options, but this was in the days before super long posts were readily available. One of the engineers told me to just run the post high (about an inch above the old school short max height line); he was curious to see if I could break the frame. It made it a couple months without a hitch.

[AngryScientist]

there can really be no rule of thumb for posts in general. too many variables, including the material of the post, post construction, frame material, construction etc.

[old_fat_and_slow]

The minimum insertion line for a seatpost is dependent on the individual design of the post, and must be determined by the manufacturer based on their proprietary design. There cannot be an accurate "rule of thumb" due to the wide disparity in seatpost construction and design. The minimum insertion level is assigned based on analysis done by the individual manufacturer's designers/engineers to ensure that the stress levels on the inserted portion of seat post are below the strength capability the post was designed and (hopefully tested to, plus some margin of safety).

[AngryScientist]

i'll add to what we said the following visual, note the cross section of a thomson post, and how the fore and aft portions are thicker than the midsection. this is to allow the post to handle it's highest levels of tension and compression better while still allowing relative light weight, excellent design.

take another lightweight alu or carbon post and you likely wont see the same construction, but rather uniform thickness around the circumference, but still lightweight. likely adequate construction method, cheaper but the margins will not be the same with a foot and a half of the post in the air.

this is all before you take into account the weight of the rider and frame construction.

http://4.bp.blogspot.com/_urSQl6wUA5g/ScmrXPgcQpI/AAAAAAAAGUw/lXE5_xABc1k/s400/DSC03295.JPG

[rice rocket]

Very neat, I never knew that about Thompson posts. Just hope you don't lay it down with a body on top.

[oldpotatoe]

there can really be no rule of thumb for posts in general. too many variables, including the material of the post, post construction, frame material, construction etc.

There actually is a rule of thumb, no caps..the bottom of the seatpost should be twice the width of the top tube, below the bottom of the top tube.

So if top tube is 30mm in width the seatpost bottom should be no higher than
60mm below the bottom of the top tube. The seatpost probably won't break but there is a good chance the frame would if the SP isn't far enough in. SP, particularly some aluminum, like Thompson are far stronger than the carbon frame they are in, seat clamp/cluster wise.

[Peter P.]

My rule is along the lines of oldpotatoe's method.

The seatpost should insert into the seat tube such that it extends a distance below the top tube equal to the diameter of the seat tube.

For instance, regardless of how far the seat tube extends above the top tube, if the seat tube is say, 30mm in O.D., then the seat post is in the frame past the seat tube extension, past the top tube, then another 30mm. Here's why:

Maximum leverage of the seat post in the frame is at its bottom end. If it doesn't reach beyond bottom of the top tube, your bodyweight is leveraging the top tube/seat tube/seatstay joint, and bad things can result.

A tube has its maximum stiffness when its length equals its diameter; any more length insertion gains no extra strength. Hence the 30mm in the example above.

In the old days of lugged steel frames, seatposts usually had 50 of minimum insertion marked on the shaft. That's roughly the diameter of the old, 1" top tube and the old, 28.6mm seat tube; same result as above.

[salem]

Thank you to Peter P. and oldpotatoe; these guys understand the nature of the question and more importantly the engineering reasons for max height lines. For those that answered stating it is seatpost dependent, thank you for your efforts, but you aren't understanding how the forces are being applied.

Peter P.'s explanation is a good one, but if that still doesn't convince you, think about where a seatpost bends if it does: right at the point of insertion. This point (where the seat TUBE ends) is where the post will see it's highest load, and the amount of insertion is irrelevant to this point the post itself. Also, for a given strength post and exposed extension, a 400mm post would bend just as easily as a 200mm one. Insertion is just to protect the frame from the leveraged loads exerted by the seatpost.

Again, thank's for expanding my knowledge base.

[salem]

A tube has its maximum stiffness when its length equals its diameter; any more length insertion gains no extra strength. Hence the 30mm in the example above.


Rereading this, I realized it is the same rule of thumb we used for choosing bolt length when I was designing mechanical equipment: thread engagement of at least the diameter of the bolt. Thanks for answering two questions, including the one I didn't think to ask.

[93legendti]

Can you insert a post too far into the seat tube? I have a 54cm Ti Concours and the seat tube extends a bit above the top tube.

I am using a Easton EA70 aluminum post. Unlike Easton's EC-70 carbon post, this has no max insertion line. The shaft of the post protrudes 65mm above the seat tube. This style of Easton post has a long aluminum clamp bonded to an aluminum shaft.

Thanks.

[old_fat_and_slow]

For those that answered stating it is seatpost dependent, thank you for your efforts, but you aren't understanding how the forces are being applied.



Pete erased the attack items for they are a NO NO...yes I am back to forum for meds are lessening... :crap: PETE


attacks will lead to bans,,,,WE ALL CAN BE NICE TO EACH OTHER AND MANY ANSWERS from folks address the same quesion. THANKS PETE

[AngryScientist]

For those that answered stating it is seatpost dependent, thank you for your efforts, but you aren't understanding how the forces are being applied.



i believe i understand very well how the forces are being applied. this borders on what i used to do for a living once upon a time, but i will concede that post construction is likely the least of the factors involved. :)

http://wwwstaff.murdoch.edu.au/~dfarrow/images/BrokenFixie2.jpg

[rugbysecondrow]

Thank you to Peter P. and oldpotatoe; these guys understand the nature of the question and more importantly the engineering reasons for max height lines. For those that answered stating it is seatpost dependent, thank you for your efforts, but you aren't understanding how the forces are being applied.

Peter P.'s explanation is a good one, but if that still doesn't convince you, think about where a seatpost bends if it does: right at the point of insertion. This point (where the seat TUBE ends) is where the post will see it's highest load, and the amount of insertion is irrelevant to this point the post itself. Also, for a given strength post and exposed extension, a 400mm post would bend just as easily as a 200mm one. Insertion is just to protect the frame from the leveraged loads

I am glad these Old Pot and Peter were able to sufficiently answer your question especially since the other guys were to Eff'n stupid to do so. :rolleyes: Classy.

So, what you have neglected in this is 1), what is the rule of thumb? It seems Old Pot has one thumb while Peter is using another. 2) RE: insertion just being important to protect the frame, is this true for carbon? I might be a dullard like the other guys, but I don't think a carbon seat post will bend are damage a frame like a Thomson will.


Not exactly the most humble "Thanks" I have seen.

[AngryScientist]

i'll also add that these so called "rules of thumb" may have been fine with standard gauge seat tubes and beefier posts.

today's lightweight technology uses all sorts of tricks to reduce weight, as i was alluding to in my first post. some of these tricks include "butting" of tubes or increasing the gauge in certain areas, reducing it in others.

if you have a trick carbon frame and a super trick lightweight post, unless you specifically know it's construction, you're not really safe guessing where to clamp it, or how far to insert it.

i know OP and Peter are smart guys, no doubt about that, and that rule of thumb are good places to start, but before you write off a manufacturers warnings, you should have all the facts.

[AngryScientist]

but if that still doesn't convince you, think about where a seatpost bends if it does: right at the point of insertion.

yup, that's always where they fail. every. single. time.

http://2.bp.blogspot.com/_V54WWNeyyp4/SQdCD1ClgiI/AAAAAAAAA0A/bdQG6Zxln-0/s400/2969039343_b70ce17c1d_b.jpg

http://2.bp.blogspot.com/_urSQl6wUA5g/S1RBEqMhLFI/AAAAAAAAHs8/mkaZ1DXHeQk/s400/look+ergo+seatpost+failure+1.jpg

http://2.bp.blogspot.com/_V54WWNeyyp4/SQ87xxq5sEI/AAAAAAAAA1w/EjzykTARB64/s400/2969607348_91178de2ca_o.jpg


http://klausenrussell.com/GTVgallery/bikes/index.php?resize=1&image=3TTT%20seatpost_0434.jpg&screenwidth=1280

[AngryScientist]

one more, because it's a good one:)

http://farm4.static.flickr.com/3164/3089816119_c01bcc9ff3.jpg?v=0

[oliver1850]

.

[salem]

First,
I will genuinely apologize to those I offended. I don't buy into the modern internet ideal of not being concerned with real people on the other end.

Not to incite further, but one response to the photos: I did specifically say "bend" not "break," and that was not by chance. Just this winter, I had a post break about an inch or two about the seat collar, but this was where a scratch caused a stress riser. My guess would be a similar situation (prior damage) caused the seatpost failures pictured.

Now I better get my little butt out of here on that extended post before I make a bigger ass of myself.

[Pete Serotta]

dependent on the material and specs of the material. A very good rule of thumb is never above the minimum insertion. Crap happens and this is one you do not want to have a chance. Years ago when the "lightness" started there cracks and broken seat bolts...... Just my view and I am not a lightweight. :crap:


The minimum insertion line for a seatpost is dependent on the individual design of the post, and must be determined by the manufacturer based on their proprietary design. There cannot be an accurate "rule of thumb" due to the wide disparity in seatpost construction and design. The minimum insertion level is assigned based on analysis done by the individual manufacturer's designers/engineers to ensure that the stress levels on the inserted portion of seat post are below the strength capability the post was designed and (hopefully tested to, plus some margin of safety).

[Pete Serotta]

After riding 30 plus years, I have seen "fads" on lightness and also som exposures from too light. I agree with with Old fat and slow (but getting faster) :beer: Pete


The minimum insertion line for a seatpost is dependent on the individual design of the post, and must be determined by the manufacturer based on their proprietary design. There cannot be an accurate "rule of thumb" due to the wide disparity in seatpost construction and design. The minimum insertion level is assigned based on analysis done by the individual manufacturer's designers/engineers to ensure that the stress levels on the inserted portion of seat post are below the strength capability the post was designed and (hopefully tested to, plus some margin of safety).