Mechanically limiting dropper post return travel

Question

My daughter has a new MTB with a dropper post. It has 150mm travel, and is fully inserted up to the post collar into the frame. When it fully returns to its raised position it's about 32mm too high, telling me she should instead have a 100mm travel post. I don't really want to drop more money on this. So, my idea: Put on a cable of some sort that limits the return travel of the post and then just push the button. So, one end of the cable would be secured to the underside of the seat or (probably better) to the top of the dropper. The other end would be attached to the frame or the collar of the dropper. Then when the release is triggered the post would return to the height the cable allows, not the full height of the dropper.

If it matters this is a TransX dropper. I don't know the internals of these devices, so maybe there's a reason not to use it at anything but fully dropped or fully raised positions. Thank you.

Answer 1

A few comments:

• Most people looking to change droppers are looking for longer ones, not shorter. Kids growing and needing a longer dropper to suit their longer legs, people buying new frames with shorter seat tubes than their old frame, people giving up on the concept of saving a bit of weight by running a short dropper, etc. You should be able to find someone willing to trade with you, possibly even with cash on top (paid to you) or them paying for shipping or another sweet benefit like that.
• TransX makes a bunch of different models of droppers, but you can usually open any of them up by unscrewing the seal collar (that bulbous section where the static part meets the moving part) and add more of these brass "keys" (pictured below). This will limit the extension length while also giving you better durability. In fact, a lot of droppers are marketed as being length-adjustable precisely by doing this. Note that your dropper may use wider pill-shaped keys instead of these skinny cylinders, but the concept is the same. These keys are readily available and quite affordable (<$10), and the installation will take like 10 minutes.
• There's nothing mechanically harmful about limiting the travel for this type of dropper. The hydraulic lockout mechanism works the same regardless of position. Maybe reduce the air pressure if you do decide to reduce the travel--the effective air pressure will be higher if the resting position is now shorter. That will also help lighter riders (such as children) compress the thing under their body weight.

Overall, I'd only recommend trying the cable option if you're unable or unwilling to consider these alternatives. Having the dropper slam into a hard stop gives some aural feedback that it's fully extended. A cable may not provide that, and plus it's another thing that could scratch the dropper stanchion, get sucked into the rear wheel, etc. Also take into consideration how reliable your cable joining method will be.

Answer 2

Generally dropper saddles should be fine at any height. The mechanism used to lock the position of the saddle height isn't too concerned about the saddle position in any droppers I'm aware of.

There could even be an argument made for not fully extending a saddle to maximum extent, reducing mechanical advantage of sideways lever forces... but I wont be the one to make it.

Would suggest setting up cable that limits the saddle height to be symmetrical, and taking care to stop it getting caught/tangled.

A strong rope like climbing rope could be more suitable than cable, it'll have a tiny amount of give, and avoid scratching the bike.

Could be worth considering a secondary elastic/spiral cable to keep the primary limiting cable from swinging around.