When seatpost manufacturers specify a minimum insertion line on their seatposts, is it at point A or B on the frame?

I would say point “A” is OK so long as the bottom of the post extends to below the bottom of the top tube junction.

I'd have my minimum insertion mark inserted at least as far as the weld below 'B'. Cantilever force can snap off the protruding seattube if there's too much seat post above that weld and not enough below it. I've seen frames ruined because that section between what you have marked 'A' and 'B' broke off because there wasn't enough seatpost below it.

Cheers

If I wasn't at least half way through the top tube I'd be looking at a longer post.
More if I were big & strong

answer is C , bottom of the seat post at least in as far as the bottom of where the top tube joins the seat tube..

On my touring bike build I had a classic Campagnolo seatpost I wanted to use, since it was close to max ( because of sloping top tube)
I had a welder add a length of a common Kalloy seat post, that diameter, then get the weld bead machined down smooth..

I really want to see that. That seems like extreme measures but awesome and well worth it.



If you are a larger rider than get the seatpost in farther and get something of length.

The Machinist was my father (1920-2000) and his friend had a welding shop.

Ok, mine is 3 millimeters below point B. Which is safe totally. Thanks everyone. Thread closed.

Good luck with your Assumption ..

That doesn't square with post #5 above.

Glenn

When buying used bikes first thing to look at after evidence of front end damage is condition of seat lug. Or on a frame like one in top post, top of seat tube. They are very very frequently damaged from inadequate insertion. Back when all frames were sorta similar the rule was 6 to 7 cm inside the frame. Make that 2-1/2” to 3” if you can’t speak metric. Modern frames with sloping top tubes will have huge amounts of seatpost showing. Means you need more post inside frame. Frames break and bend at this point all the time. There is not ever any reason to skimp on seatpost insertion.

On old steel frames it was possible to do lots of bending and repair that damaged seat lug. Sometimes it would be worth it to get out the torch and some brass. Modern frames mostly they get scrapped. Anyone thinking about how little insertion they can get away with will soon be scrapping a frame. Plus medical bills.

To be clear; the minimum insertion line on the seat post is to be at the B height ,
so the rest of the seatpost below that line is below that point?

How long is the length of that post below that minimum line ?

and as I offered above, it should be the spanning the miter width of the top tube .....

unless the op means that the minimum insertion mark is 3 mm below point b

This is in chapter 10 of my Park Tool School instructor manual.
Tim

In this case, it's point A. But mine is 3mm below point B. So, I think I'm good. My seatpost is 400m length.

Correct, 3mm below point B. Seatpost is 400mm length.

I measured two Campagnolo posts. Both, a Chorus Aero and a NR, measured 57mm (2.25:") from the top of the line to the end of the post. I believe that may be enough to get the end to the bottom of the seat lug. Your risk.

Agreed!
Tim

Any seatpost that was inserted only to Point B would have a lot of leverage/torque on that part of the seattube that's above the toptube of the bike. The more seatpost above the toptube and the heavier the rider the more leverage/torque there is. Its a failure waiting to happen. I tell people to make sure that at least TWO INCHES of their seatpost is BELOW the top of their toptube. I'm glad you did that. Having a seattube break off on a long ride is unpleasane at bast dangerous to the rider, and often the ruination of the frame.

Cheers