# How does a dropper post work?



## HollyBoni (Dec 27, 2016)

I'm interested in how a dropper post works exactly. I'm interested in the type that uses an air/gas cartridge. From what I understand, there are two types, one that uses a sealed cartridge, and one where you can pump up the cartridge(?).
How does the cartridge look inside? I guess the air pressure holds up the post(?), but what happens exactly inside the cartridge when you push the lever on the bars? Does the air transfer into a different chamber or something, and then it goes back to the previous chamber when you press the lever again, and that makes the post go up (just a guess, I could be completely wrong)? 🍻


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## Timothy G. Parrish (Apr 13, 2014)

If you go to thr FOX website, under the help section, you can view all the design drawings for their Transfer dropper post. Each brand is different, with FOX being unique compared to most others, but looking at the drawings will show you how it functions.

Sent from my moto g(7) power using Tapatalk


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## RustyIron (Apr 14, 2008)

Keep in mind that the sealed cartridge and the pump-up-able posts both use a compressible gas (either air or nitrogen) to push the seatpost up. But compressible gasses are... compressible. If the gasses were all there was, the seatpost would sag every time you sat your giant rump on it. It would bounce up and down like your fork. In seatposts, a non-compressible fluid (oil) is used to control the movement of the seatpost. When you actuate the lever on your bars, the OIL is allowed to move from one side to the other. The air is merely compressed. The oil must be kept separate from the air, or squishiness will result. It's the same way with the dampeners forks and shocks.

Take a look at this animation from BikeYoke. It might not be exactly the same as the seatpost you have in mind, but the idea is the same.


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## HollyBoni (Dec 27, 2016)

RustyIron said:


> Keep in mind that the sealed cartridge and the pump-up-able posts both use a compressible gas (either air or nitrogen) to push the seatpost up. But compressible gasses are... compressible. If the gasses were all there was, the seatpost would sag every time you sat your giant rump on it. It would bounce up and down like your fork. In seatposts, a non-compressible fluid (oil) is used to control the movement of the seatpost. When you actuate the lever on your bars, the OIL is allowed to move from one side to the other. The air is merely compressed. The oil must be kept separate from the air, or squishiness will result. It's the same way with the dampeners forks and shocks.
> 
> Take a look at this animation from BikeYoke. It might not be exactly the same as the seatpost you have in mind, but the idea is the same.


Thank you for the explanation! So if i'm understanding right, all sealed cartridge and pump-up-able posts droppers have oil in the cartridge. The air is the spring which makes the dropper go up, and the oil is what keeps it up when you sit on it.
For some reason I thought there is no oil, just so much air pressure that the post can't compress under your weight, like if you over inflated your fork/shock. But yeah, thinking about it now that's not exactly a good design. 😄


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## Aglo (Dec 16, 2014)

Just to add, that some droppers use some kind of mechanical brake to lock the dropper in place.


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## HollyBoni (Dec 27, 2016)

Aglo said:


> Just to add, that some droppers use some kind of mechanical brake to lock the dropper in place.


Interesting!

BTW, is it okay to use droppers at not full extension, if let's say I lend the bike to a friend and it's slightly too large for them? (I know in that case they would have to dial in the seat height every time they use the dropper, which is a pain in the butt)


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## mhoopes (May 8, 2007)

As far as I know, they all work like a shock with a lockout feature, using a compressible medium like air or pure nitrogen as a spring.

The “infinitely adjustable” ones use a hydraulic lockout, and the “indexed” ones use a mechanical lockout.

Advantages:
Hydraulic: more fine adjustment, lower lever force, damped spring.
Mechanical: much easier maintenance. That said, I never had to service my hydraulic KS LEV 100 during 6 yrs of use.


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## Aglo (Dec 16, 2014)

You can use the dropper at any height you want, is not uncommon for people to use the dropper with 1 inch drop on some trail sections and then drop it fully just to go fully extended next and then drop it to half travel. That's the advantage of a dropper post, you can pick how much you want to drop to best suit the situation.
Most of the current droppers are infinite adjustable, but are still some that have indexed positions.


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## Taroroot (Nov 6, 2013)

Aglo said:


> You can use the dropper at any height you want, is not uncommon for people to use the dropper with 1 inch drop on some trail sections and then drop it fully just to go fully extended next and then drop it to half travel. That's the advantage of a dropper post, you can pick how much you want to drop to best suit the situation.
> Most of the current droppers are infinite adjustable, but are still some that have indexed positions.


Its just not recommended you lift/pull up on saddle if the post is not fully extended. There is a risk that air will get sucked in throught the seals into the oil.


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## kapusta (Jan 17, 2004)

There are droppers that use a coil spring. My Gravity Dropper and PNW Cascades are all coil spring.

EDIT: Never mind, I see you asked specifically about air.


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## 006_007 (Jan 12, 2004)

kapusta said:


> There are droppers that use a coil spring. My Gravity Dropper and PNW Cascades are all coil spring.
> 
> EDIT: Never mind, I see you asked specifically about air.


From pnw website:

"
*WEATHERPROOF SEALED AIR CARTRIDGE*
Like all of our droppers, the Cascade rocks a sealed air cartridge that is known for its performance in below freezing temperatures. This design also makes the post easy to self service when it comes time for maintenance."


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## kapusta (Jan 17, 2004)

006_007 said:


> From pnw website:
> 
> "
> *WEATHERPROOF SEALED AIR CARTRIDGE*
> Like all of our droppers, the Cascade rocks a sealed air cartridge that is known for its performance in below freezing temperatures. This design also makes the post easy to self service when it comes time for maintenance."


I think that must be new then.

I remember when looking around, before I bought mine, the Cascade was not an air cartridge, which was part of the reason I bought it. And why the post is so heavy.

Of course if it's sealed and there's no way to access it from the outside, it could well be air in there without a coil
and I wouldn't know any better.

But I'm nearly certain that some of their droppers used to be coil.


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