Seatpost Insertion Question

[DCilliams]

I'm sure this question has been answered before here, but what is considered a safe length of seat post inserted into a classic steel frame?

My seat post doesn't have a minimum insertion mark on it. Right now, I'm at a little over 2 inches. I'm a heavier rider (185lbs) so wanted to stay on the safer side.

[tuscanyswe]

I dont know a rule but personally i would like the seatpost to protrude the toptube junction by a centimer or so but i guess it also depends on how much the seattube extends on top of the toptube. Thats prolly something i picked up from here that made sense .)

[Ozz]

what is the seat post made of? I would assume aluminum?

[wallymann]

for steel, i think they say min insertion should be ~200% of the top-tube OD, although i'm OK with 150% on a lugged frame and weigh about the same as you.

"just the tip" wont do! :-P

prior thread on the topic: https://forums.thepaceline.net/showthread.php?t=169070&highlight=insertion

[DCilliams]

The post is a Campagnolo Record Titanium; hence, my reluctance to try to find a longer post. Right now, it's inserted 60mm, 30mm of which is below the top tube.

[bikinchris]

The post is a Campagnolo Record Titanium; hence, my reluctance to try to find a longer post. Right now, it's inserted 60mm, 30mm of which is below the top tube.

If it is twice as deep as the bottom of the top tube/seat tube junction you should be okay.

[Ozz]

The post is a Campagnolo Record Titanium; hence, my reluctance to try to find a longer post. Right now, it's inserted 60mm, 30mm of which is below the top tube.

It should have a mark:

[Pastashop]

Typically a minimum of three diameters from the bottom should be well supported, but mind where the fastening mechanism is and how things are loaded. I’d say 60 mm is insufficient.

[Ken Robb]

I might push the limit on a Nitto aluminum post but I'd be more cautious with titanium. I wonder if this post has been shortened. That might explain the lack of a minimum insertion mark. Maybe another forumite has a similar post WITH a minimum mark? If so he could measure how far the mark is from the bottom so you would know where you could make a mark on your post.

[Peter P.]

The concern isn't with the seatpost but strain on the frame joint. Extending the post below the seat tube/top tube junction takes the leverage off the joint intersection and distributes it in the seat tube.

If it extends below the top tube at least one seat tube diameter, you're good.

[oldpotatoe]

I'm sure this question has been answered before here, but what is considered a safe length of seat post inserted into a classic steel frame?

My seat post doesn't have a minimum insertion mark on it. Right now, I'm at a little over 2 inches. I'm a heavier rider (185lbs) so wanted to stay on the safer side.

General rule of thumb is bottom of seat post should be twice the width of the top tube below the bottom of top tube. So if top tube is 31mm diameter, the bottom of the seat post should be about 60mm below bottom of top tube. Or about 90mm from top of seat collar. To prevent tube damage.

[Bob Ross]

If it is twice as deep as the bottom of the top tube/seat tube junction you should be okay.
The concern isn't with the seatpost but strain on the frame joint. Extending the post below the seat tube/top tube junction takes the leverage off the joint intersection and distributes it in the seat tube.
If it extends below the top tube at least one seat tube diameter, you're good.
General rule of thumb is bottom of seat post should be twice the width of the top tube below the bottom of top tube. So if top tube is 31mm diameter, the bottom of the seat post should be about 60mm below bottom of top tube. Or about 90mm from top of seat collar. To prevent tube damage.


I'm making this my own personal Sticky!

[d_douglas]

I asked this question about a Moots post on my Desalvo. Mike D said as long as the post is at least 10mm below the top tube junction, you should be fine.

I weigh 210lb and ride my bike off-road with no ill effects.

[EricEstlund]

Just a friendly reminder that minimum insertion marks on the post only tell you the minimum insertion the post needs, not what the frame may require. There are a lot of different seat cluster types (such as taller seat tube collars with dropped top tubes and seat stays) in which the frame design requires an insertion far greater than the posts minimum insertion.

[NHAero]

Reviving this older thread.

I have a Thomson Masterpiece 27.2 post that I cut down for my Strong, and now the Bingham that just arrived has 2 more cm seatpost exposed (by design). So I only have 50mm of insertion length. Thomson asks for 63.5mm of insertion length, and will allow up to 266mm of usable length outside of the seat tube. I would be using about 140mm so less moment arm on the post and frame (and I'm 140 lbs.) The portion of the 31.8 seat tube that is thickened for a 27.2mm post OD is 7cm long - any extra insertion length doesn't help as far as I can see. So I'm 13mm shy of what Thomson asks for and 20mm shy of what's the max possible functional insertion.

What is at risk? The frame or the post or both?

Brad for liability purposes won't say anything but follow the manufacturer's recommendation. Interestingly, the Thomson specs ask for 87mm insertion length for a 30.9 post, which is more than a typical seat tube has in terms of thickened length.

[homagesilkhope]

Wait! Aren't you an MIT bicycle tube engineer? :)

[NHAero]

Wait! Aren't you an MIT bicycle tube engineer? :)

Used to be :)

I figure there's a buncha nerds on here who do these type of calcs daily and can clue me in why that differential matters.

I sent another question to Brad that wouldn't make him say, it's fine...I asked him if I was 200 pounds, would the length of the thicker walled end of the tube (70mm in my new frame) need to be longer for a heavier guy, or would he have used a larger OD seat tube than the 31.8mm on my frame?