As a newbie to this sort of thing, how do I decide length? My main concern is that I have enough post in the frame whilst still maintaining the minimum insert.
I need 285mm between top of seat tube collar and the middle of the seat rails plus what ever is needed for minimum insert. The frame is small for my legs so I could live with the post not dropping too far or there ebing some outer post sticking out.
I have just a touch more than that distance inside the frame . Any pointers? The various calculators don’t address minimu insert. (I think)
Cheers

If you’ve got 285mm stack height between the collar and rails, and just over that in the frame (this is the important measurement) then you’d be able to fit a 210mm one up.

With that much room you don’t need to be worrying about minimum insertion depth, go for 200mm as a minimum.

One up typically give the best ratio of drop to post length and their post specs on the website give all the info you need to calculate what will fit.

On a small I suspect the maximum insertion depth will be your limiting factor so you’ll need to work that out then you should be able to match everything up on the size guides.

So, lets say a 200m post is viable. Is that how they are sold?
My worry is that they are long enough. Never seen one out of a bike . Only sat on one a couple of times.

Got about 280/290mm of insert usable. On One Scandal so just a curve lowish down.

A 210mm dropper has an insert depth of 297mm and a stack height of 243mm, if you have 285 stack height then you should be fine. And the drop can be reduced by 20mm if needed.

You need to do a load of measuring and then look at the stats that the companies put out for minimum insertion, etc. Some are better at that than others. If you’ve got a plain, oldschool seatpost of the relevant size that’s really useful as it’s full thickness all the way down, so will pick up on slight narrowing of the tube, small weld intrusions etc.

You also have to take into account the actuator length, not just the seapost length.

It’s also pretty useful to choose a height adjustable post! I thought I had all the maths right with my last Oneup, somehow I didn’t, but it was OK as I could still use the included spacers to shorten it a little. Not an option with all posts.

Ah thats another one then. Don’t dropper posts stop at the point you let go of the lever? The one I used could be stopped anywhere I wanted it from topped out to just above minimum.
It is the minimum height I can’t find in most cases. Always a line but no details of distance from rails to minimum insert line.
Of course I might not understand the terminology.

Height adjustable in this circumstance means the maximum drop can be adjusted  (reduced) internally so a 240mm drop post can be adjusted to 230.

I did the maths on this with one up droppers. I had a 180 drop post in my 420mm seat tube. There was 60mm under the collar so in theory I can swap it for a 240mm drop post. But… the longer post is longer under the collar as well as above and I would need to run it slammed. The minimum insertion of the 240 post is 337mm. There wasn’t quite 337mm of space in my frame between the top of the seat tube and the cable entry point. So I would need to have about 5-10mm of post out of the frame. I could still use the post by adding a spacer inside to reduce its fully extended height by 10mm. So I’d still be able to pedal with it all the way up.

in the end I got a brandx 200mm post. It has a thicker collar and so about 25mm out of the frame. I can’t imagine needing more drop!
it was also half the cost of a one up.

Measure the distance from cable port to top of seat tube and add this to your 285 to give max total post length. I suspect anything apart from a one up 240 will be fine. Your seat tube may be a bit short for the longest post.
Dimensions are all here- brandx also publish theirs.

https://www.oneupcomponents.com/products/dropper-post-v2

Just remeasured with my glasses on, in the light without guessing and using the seat post. 225 space inside the frame so a One up of 180mm would go in accodring to their tables. I assume this is with the top of the outer part against the frame.

Cheers all.I guess I need to look at one out of a bike. I would have assumed that length includes any cabling brackets etc. Getting more confused the more I go on.

It will fit but not with the collar touching the frame. The link says 267 insertion length needed for it to be fully slammed. For your pedalling height with rails 285 above the seat tube you’ll have 70mm below the collar outside the frame. Total length of that post is 480. You have 510 to play with (285 outside and 225 inside the frame). Next size up in a one up would be too long. I think a brandx 200 would be too long but a 170 ok for abut less £££

https://www.chainreactioncycles.com/brand-x-ascend-xl-dropper-seatpost-170-200mm-/rp-prod159172

mattsccm
Free Member

Ah thats another one then. Don’t dropper posts stop at the point you let go of the lever?

Yes but it’s rubbish if you can’t have max length be your correct length- if you’re depending on stopping it early it’ll never end up twice in the same place and never be quite right. And sometimes you’ll accidentally overextend it and it’ll be bloody awkward.

Cheers. I think I meant dropped height.
Getting there folks.

So, lets say I went for that 170mm Brand X.
Need 285 to the seat rails so I would have 115 sticking out of the seat tube full time . Am I correct? If I went for the 200 I would need 85mm which wouldn’t matter as it is 59mm too long anyway for the seat tube.
Sorry, my understanding is appaling.

One up have a calculator to help you. It shows the longest post for theirs and other makes. https://can.oneupcomponents.com/pages/oneup-dropper-post-selector-v2

For the dimensions you have given (285mm/225mm). It says a 180mm one up post will fit.

So, lets say I went for that 170mm Brand X.
Need 285 to the seat rails so I would have 115 sticking out of the seat tube full time

kind of… but stack height is not the same as drop. There is the height of the collar (27mm for brand x) and the distance from top of shaft to rails. Total 🟰 probably 55mm? Think of a fully dropped dropper. The seat rails are not touching the seat tube.
so I think you’d have about 60mm  of seatpost visible under the collar with either a 180 one up or 170 bx. Nixie’s link illustrates this nicely