Without a doubt, dropper posts have changed how we ride trail bikes, and most bikes out there can have a dropper fitted to them. However, it's not as simple as buying one and sticking it in the bike.
There are three major measurements you need to know from your bike, so you can get the right dropper for you.
- Seat tube diameter
- Extended seat post length
- Frame insertion depth
So, let's take a look at these measurements and what you need to know about them in order to get the right dropper.
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Seat Tube Diameter
This one's super easy! The diameter of your current seat post will be the only size your frame can fit. There are 3 common sizes, 27.2mm, 30.9mm, and 31.6mm, with the latter two being the most common by far. 34.9mm is also becoming increasingly popular.
To find this measurement, you can:
- Look at your current seat post. It is likely that the diameter is printed on the post.
- Look at your bike's specs online.
- If none of this is available, then you can take this measurement yourself with a ruler, tape measure, or for the most accurate results, a set of digital vernier calipers.
31.6mm for this PNW Rainier IR
Extended Seat Post Length
This measurement determines how much drop you can fit, and more is always better here. Just because you have 150mm available however, doesn't mean you can fit a 150mm post. Droppers have a collar and a head that add length to the post. You'll need to compare your available height with the size chart of the dropper you're interested in.
To find this measurement:
- Set your seat height at the highest, comfortable pedalling position
- Grab a tape measure
- Measure from the top of your seat tube to the saddle rails
- Compare this to the size chart for the post you're considering to confirm availability
Saddle rails to seat clamp
Frame Insertion Depth
This is the last part, and one of the trickiest bits on some bikes. You need to know how far into your frame your post can go. It's all well and good if you can fit a big long 185mm drop going off the extended length, but if you can't fit the rest of your post in the frame, then you're going to need a shorter post unfortunately.
There are a few different ways to figure this out:
- Lazy Method: have a look at your bike from the side, and see if you can spot any bends or obstructions in the seat tube. Suspension pivot bolts, water bottle cage bosses and other things in the way won't allow the seat post to go past. Measure from the top of your seat tube, down to this point. This is only lazy, because it means you don't have to remove your current seat post just yet, and it isn't as exact as the other methods.
- Seat Post Method: This one, you undo your seat clamp, and stick your current post as deep into your frame as it can go. Once it stops, mark the position of the top of your tube on the post, and pull the seat post out. Measure from your mark to the bottom of the post, and that's your insertion depth.
- Most Accurate Method: Now, if you can stick your current seat post all the way in your bike without it bottoming on anything, then you're going to have to remove the post, and try this method. With the post out of the bike, grab your ruler or tape measure, and stick it down into the frame until it hits an obstruction. Then that is your insertion depth.
Seat clamp to frame obstruction; in this case, a pivot bolt
Now, let's take a look at some examples!
As you can see, this bike has a completely uninterrupted seat tube, a full 300mm of available frame for a dropper to sit into. This means that any length post will fit the bike. However, the bikes rider is only short, and to have a comfortable pedalling height, the saddle rails are 195mm from the seat clamp. This means, to fit a PNW Rainier IR, they can only go for the 125mm travel post.
Let's move back to the Transition.
The Transition's seat tube is interrupted by a pivot bolt at 210mm. This is reasonably short, and slightly limiting. A 150mm Rainier IR has an insertion depth of 254mm, so it won't fit all the way in this frame. However, the rider of this bike has an extended seat post height of 245mm, which means the post can be run extended far enough out of the bike for it to fit.
How to choose the correct size on MTB Direct!
Dropper posts are the best thing to come to mountain bikes since suspension; they completely change the way you ride the bike and greatly improve your skills and your confidence. Grab a tape measure, bring up a size chart, and get yourself measured up for the best bike innovation of modern times!