PNW Dropper Posts: Rainer Gen 3 Dropper 34 x 125mm

It's not as trivial as "check your seatpost diameter" like some people have said, though that is the first step: make sure you get the right diameter (you can either measure your current seatpost or check the bike specs).

The next thing to measure is the fully extended length of your post. Using your current seatpost, install it at the highest riding position (for the person riding the bike) and then measure the length between the seatpost collar and the saddle rails. Then using this measurement, you need to check the specs on the dropper post to ensure that it will extend that far while still meeting the "minimum insertion depth" that is given.

Then, assuming it goes high enough, you then need to check that the post will actually install deep enough into the bike frame (using the max height position you measured in the previous step). The bike specs may list the maximum insertion depth, otherwise you need to measure this -- using another long seatpost is easiest, as sometimes there are things that would block the seatpost from going all the way in that aren't obvious. A hardtail marlin will probably accommodate a long seatpost, but you never know -- in one of my bikes, the seat tube wasn't actually bored to size as deep down as it would seem.

And of course, if you're going internally routed, you need to make sure your bike can run the cable internally and that there's still enough space for the actuator at the bottom of the post.

A dropper is certainly one of the best things you can add to a mountain bike, but finding the right one does sometimes take some careful measurement and fitting. I've got a 210mm dropper on my modern enduro bike, but my ~6 year old size large trail bike is limited to just 125mm (while my wife's size small is limited to just 100mm).

Yep, I have a "bike dropper post" deal alert I've been waiting years to trigger... and it didn't trigger on this deal because nothing says bike or bicycle.

For those who just picked up the Axum deal, post size is 30.9