Last week’s Suspension Tech series: How to Pick the Right Dropper Seatpost for Your Bike
The first step in adding or upgrading a dropper seatpost is finding one that fits your bike. Here’s what you need to know:
Getting the Right Fit
Most bike manufacturers don’t specify which droppers fit their frames, but the post makers themselves provide detailed spec dimensions and fitting instructions. With this information, you can take a few simple measurements to determine compatibility.
Dropper Seatpost Sizes
Dropper seatposts come in sizes comparable to regular seatposts, such as 27.2, 30.9, 31.6, and 34.9mm diameters. However, options become fewer as you move towards the extremes of 27.2 and 34.9. The smaller 27.2 is used for less technical disciplines, while 30.9 and 31.6 have become the leading modern standards, offering more stiffness without much weight penalty. The larger 34.9 is ideal for aggressive trail or enduro bikes and heavier e-bikes.
Availability of Droppers in Different Sizes
Not every dropper is available in all four sizes. Most dropper posts are available in 30.9 and 31.6 since these are the most common sizes for mid to long travel mountain bikes. Some companies also offer dropper posts for 34.9 to fit longer travel mountain bikes. However, 27.2mm droppers are less common due to weaker demand and the difficulty of fitting suspension internals into the smaller diameter.
Internal vs. External Routing
If your bike frame does not offer internal routing, you can opt for externally routed dropper posts. Some companies offer external routing options, while others offer wireless electronic remote posts. It is not recommended to drill a hole in the lower seat tube for internal routing, as it can compromise the frame’s integrity. Many bikes without dropper-specific internal routing still have modular internal routing systems that can be repurposed for dropper routing.
Measuring Seatpost Height and Dropper Compatibility
To determine the compatibility of a dropper post, you need to measure three key dimensions: seatpost extension length (SEL), dropper extension length (DEL), and total post length (SPL). By comparing these measurements, you can determine if a dropper post will fit your bike and how much travel it can offer.