I believe I have a 400mm seatpost and I need to lower it my saddle height by about an inch but cannot do so as the bottle cage bolts on the seat tube are stopping it being lowered. Hence, my intention is to buy a shorter seatpost. I realise I can cut the one I have but I want to get a better quality one (it's a _nightmare_ adjusting the seat angle on the stock seatpost that I have).

Now, if I measure just the shaft of the seatpost it's around 370mm long. However, if I measure up to the saddle clamp (where the saddle rails fit) then it's 400mm.

Is this the correct way to determine seatpost length - i.e. measuring from the bottom of the tube to the saddle rail clamp?

Bring the old post to your bike shop to compare length with what they have available.

Can't beat the Nitto. https://www.amazon.com/Nitto-NITTO-S.../dp/B00F1C4A1W


I have 120,000 miles on one and 76000 miles on the other.

We hack saw them off, using the same saw guide as used to cut fork steerers down, so it comes out a straight cut.

as I offer often Brompton Saddle clips are great, they dont slip , and are microadjusrable, then you can use a low cost seat post

a long strong plain seat post I got comes thru Redline/Seattle Sports in 1" (22.2 on the top so saddle clip fits perfect)

which 25.4 is another common ID for seatpost shims that come in all sizes, larger..

I don't think my LBS would have a clue! I've used them a couple of times and I haven't been impressed with their knowledge or service. Also, they don't stock any decent brands at all.

I'm trying to avoid cutting mine down as I don't have a decent hacksaw or anything to hold the seatpost firmly in place. Also, as the bike is new, it's still under warranty so I want to keep the stock seatpost in case I ever need to send the bike back for a warranty repair.

Most measure seatpost length from the bottom of the post to the rail clamps, or to the center line of the rails as they would sit with the saddle installed in the clamps.

LoLing at the other answers here...

Yep, that’s what I figured but I wanted to check. A 350mm one should be perfect as I only need to lower it by an inch or so, and it will also clear the cage bolts. I was a bit surprised as the bike size is Small, yet there’s still a 400mm seatpost on it, the same as they use for their XL sized bikes.

Noting that a number of manufacturers' stock seatposts come angle cut, is there a reason one might want an angle cut vs. straight cut end on a post? I'm thinking maybe it helps reduce wear on the cut end if one removes and reinstalls the seatpost fairly often (making it easier to drive down into a tight seatpost)?

eg. Ritchey


Fizik


FSA

I don't think there is any useful reason for those angle cuts. All of the seatposts I've ever owned (Thomson, Easton, SR, Kalloy, American Classic and a couple I never identified) were cut straight across.

BTW, if you have to "drive down" your seatpost something isn't right. Either the diameter is incorrect or the frame is out-of-round or poorly finished.

The convention for seat post length is from bottom to the top of the section that can slide into the frame. This excludes the head, so you'd typically add 25-35mm to get the height to the rails, though that varies by design.

Cutting the seatpost at an angle makes the bottom more or less level with the ground when installed. Cutting the post straight across is probably easier and cheaper, but that results in a smiley-mouth shaped section at the bottom rear of the post that isn't doing much if any work.

I doubt it makes any difference in real life, but I like to cut my seatposts at an angle when I do it anyway.

I don't think ease of insertion is the reason.


If you're using noticeable force to install a post there's something wrong from the start.


I suspect relentless weight shaving together with a dash of engineering aesthetics.


Or possibly the other way around.


Think about it.
During use, the top of the seat post will see a backwards force.
The post will try to pivot around the clamp.
Meaning the bottom of the post will see a forward force.
Meaning the rear half of the bottom edge of the seat post - the missing wedge - won't be carrying any useful load.
It can be removed w/o any impact on the function of the post.
(also w/o any significant benefit, but that's bicycle development for you...)

Drive down maybe the wrong phrase. Certainly though, if I put the seatpost into the frame, and let go, it doesn't drop down all the way to the saddle unassisted.

When my father was around, I got my traditional 2 bolt Campag seat post extended by joining a same diameter Kalloy
tube seat post ,( that <c>was to be at it's max height line), by fitting the 22.2 top inside machined to fit, the bottom
then welded and machined the bead down.. in his home shop.

those old Campag 2 bolt posts were developed with Brooks saddles (which I use), in mind.




.....

I've bitten the bullet and ordered a 350mm Cinelli Dinamo seatpost. It was hard to find a white 34.9mm seatpost and I got a cracking deal on it - almost 50% off the recommended retail price. I also liked the single-bolt clamp on it, and it has the same 15mm offset as my current stock seatpost.

I've made the assumption that the measurement is from the bottom of the seatpost up to the middle of the saddle clamp.

They were also developed when riders rode significantly larger frames that are common today so they had a lot less seatpost showing. I recall Campy seatposts being 180 mm long which would be much too short for almost any current frame.

horizontal top tube frames are still being made.. not every one wants a sloping top tube.