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My then 8year old at 50.5" started with a 120mm OneUp v2 on a 24" Spawn Rokk. It was perfect for him, with the dropper fully inserted into the seat post. But a year later @52.5" tall we swapped it out for a 150mm since he wanted the seat up higher, but still be able to go as low as before.

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It will totally depend on the frame (seat tube and crank lengths) and kid (inseam), a bit on the post (bottom of collar to rails height fully compressed) and a tiny bit on the seat (rail to saddle top height). My 49-50" son could probably get 10mm or so on the Vitus nucleus, but he could have 35mm on the giant stp and he could have had around 150 mm (just an estimate: I didn't measure) on the 20" Cannondale quick that he rode until a month ago.

ETA: those are all maximum drop. Obviously you can use a shorter dropper.
 

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As was stated, this is very child and bike dependent (inseam, shoes, seat tube length, saddle thickness, pedal thickness, etc.). Having said that, we modified my son's bike to accommodate the Brand-X Kids internal dropper as it was the only one that could get his saddle height within a few mm of his previous saddle height with the collar slammed all the way down. We have since raised the post about 5mm as my son continues to grow.

Based on our own experience, I would say that 70mm is sufficient drop for kids on 24" bikes. Would we take more if we could get it? Of course. But on bikes of this size, you don't have nearly as many options. Short legs and short seat tubes will likely limit your options. Unless you are willing to have your kids saddle too high in the up position.
 

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Here's our experience in doing this at 20" and 24" and my own bikes:

1- Definitely figure out how to do the math and include the actuator length as well as a little buffer space at the bottom for housing curves. You can dial in nearly the exact length you can fit if you use an existing seat and measure like crazy. Lots of youtube videos to help with that process.
2- If your seat is a pivotal seat, that won't work...you'll need to have a seat with rails to test out where the static seat is at...then get your measurements off of that for what works for his leg length
3- Different seats have different stacks if you are swapping seats too.
4- DON'T get droppers that use the external housing to actuate rather than the internal cable. OneUp v1 and Revive Devine use this. It can work fine I guess if you have room in the frame but even then it still sucks because its hard to move the lever. I've tried all this and they are less than ideal. I had a OneUp v1 and v2 with both different actuators. Note that most droppers won't work with a kids light weight. The KS Lev droppers are by far the best in my testing of like 5 different droppers. I can run literally 82PSI and they still work great with ultra buttery lever action. One a OneUp, I had to run at least 240PSI for the post to extend right. Night and day. The Brand X kids one does work tho when we messed with it in the store. Only downside is we couldn't use it since the diameter was quite small (27.7 or something) and our bikes were 31.9. Shimming might have worked but that'd be a pretty sizable jump.

5- Drop amount: this is difficult. When we've messed around with it, especially for my younger son, the amount of drop was somewhat important. This is because when you factor in the stack of the dropper collar and the head and the seat...it already eats up quite a bit of space. So if you drop it like 50mm (2") the damn seat is still definitely in the way. I worked hard to find something with at least 100mm. Our 24" bike had 120mm drop and my youngest boy has 100mm drop on his Spawn. Fitting a 100mm dropper on a spawn for a 49" kid was challenging. I had to remove the BB to route the cabling through it (very tight) and more importantly it was about 10mm to high (I knew this going in). Thankfully I just hacksawed off 10mm of the exposed seattube, got a very low stack seat post clamp and Bob's Your Uncle it works like a dream and the seat gets out of the way plenty. You have more room for less or more drop with a kid on a 24" bike because their legs are longer etc tho. Good luck!
 

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Hey CE, how did that OneUp psot works as far as him being able to actuate it? I'm looking at that same 120mm post for my 4-11 kid moving onto a 27.5 bike. I think that amount of travel will work with the bigger bike. My concern is if he can actuate it with his hand standing next to it (at the 275PSI). He's 80lbs.


My then 8year old at 50.5" started with a 120mm OneUp v2 on a 24" Spawn Rokk. It was perfect for him, with the dropper fully inserted into the seat post. But a year later @52.5" tall we swapped it out for a 150mm since he wanted the seat up higher, but still be able to go as low as before.

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Hey CE, how did that OneUp psot works as far as him being able to actuate it? I'm looking at that same 120mm post for my 4-11 kid moving onto a 27.5 bike. I think that amount of travel will work with the bigger bike. My concern is if he can actuate it with his hand standing next to it (at the 275PSI). He's 80lbs.
It's worked great for last couple years since day 1. Not sure what else to say aside from never having issues with it. Highly recommended. I regularly slick honey all our seatposts to ensure they run well but aside from that I don't really maintain them.

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So no issues for him actuating with just his hand (standing on the side)?

I assume you are running 270ish psi?

It's worked great for last couple years since day 1. Not sure what else to say aside from never having issues with it. Highly recommended. I regularly slick honey all our seatposts to ensure they run well but aside from that I don't really maintain them.

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So no issues for him actuating with just his hand (standing on the side)?

I assume you are running 270ish psi?
Hmm. Can't remember for either question. I set the pressure as low as possible at the time, ensuring it was still a fast pop up. He knows how to get on the bike even if the seat's all the way up, so I can't recall if he ever actuates it by hand from beside the bike. I do know whatever he does hasn't caused any complaints, whining, or hindrance else I'd know it was a problem I never have to push it down for him.

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