Joshua asks “I ride a size small mountain bike and want to install a 170mm dropper post. How do I know which dropper posts will work with my frame?”
We can’t say it enough: having a dropper post is the ultimate luxury for all mountain bikers. As dropper posts approach ubiquity, long travel options are available now that once seemed unimaginable. This means that not all dropper posts will work with all frame designs and sizes.
To ensure you purchase the correct size post for your bike, you’ll need to know the diameter, overall length, stack height, and insertion length of the post. All of these measurements are listed in the Features section at Art’sCyclery.com.
Dropper post diameters are currently 27.2, 30.9, or 31.6 millimeters, though it appears that 34.9 millimeter seatposts may be on the horizon. Make sure you match the seatpost diameter to your frame’s seat tube diameter. You must use the correct diameter seatpost, otherwise you’ll experience slipping and likely damage your frame upon installation. You can find your seatpost’s diameter by pulling the one currently in your bike and looking for the engraved or printed numbers on the post, or by looking up your bike’s specs online. If you can’t find the correct diameter post, seatpost shims are an option, though this is not an ideal solution and options are limited.
At Art’s Cyclery, we measure overall length in a straight line from the bottom edge of the post to the middle of the seatpost clamp at full extension. Most stated lengths from the manufacturer measure to the top of the clamp cradle, so they end up being a bit short.
Stack height refers to the distance from the bottom of the post’s collar to the middle of the saddle clamp. If you insert the post into your frame as far as it will go, stack height is what shows above your seatpost clamp.
Insertion length is how much room is required inside your bike’s seat tube to accommodate the post. This is mostly a concern for long travel posts on small frames. Insertion length is the distance from the bottom of the seatpost collar to the bottom edge of the seatpost. If you are using an internally routed, or “stealth,” post, you’ll have to add a bit to account for coupling mechanisms on the bottom of the post. This varies across brands, but figure about 50 millimeters for Rock Shox Reverb posts and 30 millimeters for Kind Shock posts. To determine your insertion length requirements, measure from the routing port or frame bend in your seat tube to the top of the seat tube.
The last step is to measure your current saddle height in order to make sure that your new dropper post will allow you to match that, taking into consideration the measurements that we’ve just discussed. As a refresher, saddle height is measured in a straight line from the center of the bottom bracket to the center of the saddle rail.
Lastly, here are some best-practice tips. As cool as it might seem to run your post’s collar flush with the top of your seat tube, this leaves almost no opportunity for change or room for error and we’d strongly advise against cutting it close. Even if it means having 25mm less travel in your dropper post, err on the side of caution. Finally, for anybody wanting to run a longer post on a smaller frame, always beware of the minimum insertion line and never install a post beyond it’s minimum insertion.
Before buying your next dropper post, grab a tape measure and a pencil, measure your bike, and find the dropper post you need at ArtsCyclery.com.