Reader Story: Designing & Prototyping an Internal Mechanical Dropper Post
A while back, I designed and prototyped my own custom dropper post.
I came up with an idea for a mechanical infinite-travel post that locked in place using plates that bind along a rail, similar to a caulking gun or bar clamp... At the time, I hadn't heard of the Crank Brothers Kronolog dropper post from 2012, which uses that same bar-clamp concept, but it was externally clamping on the top post, whereas the mechanism I designed is fully internal.
A housing part holds steel plates with springs underneath them, pushing them at an angle into the center rail. This binds the plates into the rail at rest (that is connected to the top post) and holds everything in place. A brake cable runs from a lever on the handlebars into this mechanism, and when pushed on, it pulls down the center link, forcing the two plates to spread apart, releasing them both from the rail, allowing the post to move up or down.
The housing assembly sits stationary on top of a cylinder in the base post that holds the air chamber and air spring. The moving rail of the top post has a piston and O-ring assembly on the end, so when the top post is pushed down, it compresses air inside the air chamber at the same time. The air spring then can push the post back up when you release the brake.
I CNC machined a lot of the parts myself, and also reused the bottom post of an old RockShox Reverb (modified by machining an internal thread into the bottom of it).
Also, the bushings and wiper seal parts were used from the reverb as well inside the top portion. Then I used some off-the-shelf O-rings and aluminum tubing from McMaster-Carr for the air spring assembly, and designed the piston to work with those and seal properly. There's also a Schrader valve in the bottom to pump air into the air chamber with a shock pump to get the pressure high enough to function.
I only got as far as a working proof of concept, but it was a cool project I learned a lot from. I can ride around with it; the brake holds my weight, and the air spring works, but it definitely needs some updates to improve the reliability and function. The plates slip sometimes under load, so the design needs some serious improvements to get it good enough to actually ride on a trail..... So hoping to work more on it one day and see if I can get it trail-ready.
Check out the full video on YouTube if you're interested in seeing more on the design, machining, testing, failures, and other stuff. And let me know what you think you would change to improve on it.
Author Info:
Westford, MA, United States
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In later designs, we used a wedge system and was infinitely adjustable, used a mechanical spring but still had 3" of travel, which at that time seem plenty. It also used a pull knob, but we were working on a handlebar remote at the time I sold my company back in 2002.
My company at the time Hurricane Components, was arguably the first one to make a modern dropper post, we called it the "Elevator Shaft"
I have a long history of dropping my saddle starting in the late 80's, using a Hite-Rite and the Elevator Shaft was derived from that idea. Actually there is a long story of how this all became to be.
Utter game changer though.
shop.vecnum.com/gb/6-dropper-post
Idk why they're not more widespread, quality and service has been topnotch for everyone afaik
Great job, well done !