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I have a medium Kona Mahuna 2021 (Seat tube length 420mm and seat post diameter 31.6mm) which I would like to fit a dropper post on. The bike is 'dropper ready' with internal routing.

I've never had a dropper seat before and I was looking at the PNW Loam, would this one with 125mm travel and 385mm seat post length be compatible?

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It’s compatible regarding the diameter and actuator type, but the travel length is something that’s dependent on how much seatpost you have sticking out at pedaling height and how much maximum insertion depth your frame has. Can’t tell you whether it fits or not without that info.

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As long as the bike has internal routing, there are two measurements that you need to check to assess compatibility:

  1. Seat tube length on your bike, in this case 420mm.
  2. Minimum insertion on the post, in this case with the 125mm dropper, the minimum insertion is 90mm.

So that the post will fit.

Once that's confirmed it's important is making sure you're choosing the correct amount of travel. You want the maximum rise on the saddle to be at the comfortable point for pedaling for you, while this is different for everyone, it will be roughly hip height or where you leg is nearly at full extension when the pedal is at it's lowest point. A post that can't fully extend will not allow you to dial in your bike fit and could lead to soreness over time.

The Loam post is 385mm total length, 170mm of this is collar to rails, so 215mm is the maximum insertion length and 90mm is the minimum insertion length.

I used a trigonometry calculator to determine that the top of the seat tube is 405mm from the BB (based on ST angle of 75 and ST length of 420mm). The BB height from the Kona website is 315mm. So the bottom of the seat collar is 720mm from the ground.

In order to make sure this is a good fit for you, you need to confirm that a full extension you will be able to pedal. So, assuming a saddle height of 50mm, in this case the min height at full extension (post fully inserted) will be 720mm + 170mm + 50mm = 940mm and max height at full extension is 720mm + 170mm + 50mm + 125mm = 1065mm.

So, roughly, as long as your hip is somewhere between 940mm and 1065mm from the ground this will work with your bike, and you will be able to dial it in to your size.

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  • Water bottle cage bolts or a bend in the seat tube may not let you insert the dropper fully. Just because the seat tube is 420mm long does not mean you can shove the seatpost all the way to the BB.
    – MaplePanda
    Nov 9, 2021 at 15:48

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