Bike geometry, dropper posts, and seat clamps-
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    Bike geometry, dropper posts, and seat clamps

    This could easily go in the Accessories forum, but I wanted to share with you guys what my experiences and mistakes have been with selecting dropper posts, and how you can learn from that, even if you selected a dropper you found out later was too short.

    Let me give you a few examples of starter bikes under $1000 and what dropper post lengths are appropriate for someone (the rider's height can also be a factor so you many need to try some at a bike store first):

    This is a low-end 26" bike. Not the best for hard riding. Notice how the top tube is almost parallel to the ground. This means the seat tube is higher than normal, in this case it's 18 inches even though the rest of the bike is pretty small. For droppers, 100mm is plenty of travel for putting the seat way up on steeper climbs, probably a bit too much.

    This next bike looks smaller (lol) but it's a 27.5 bike. Notice how the top tube starts to angle down from the front to back. The two sets of rear frame parts (chainstay and seat stay) are still short like the 26" bike above, but the top tube is relatively more modern geometry (or maybe I should say relatively less ancient geometry). A 100mm dropper post will work just right with this seat tube height.

    This third bike, another 27.5, has the top tube angled down even more than the Diamondback Overdrive above. Look how the seat is up more than the 2nd bike. A 100mm dropper post will not cut it on this bike. It needs to be at least 125mm, if not 150mm.

    But what if you already have one (or more) 100mm dropper posts? Do you just throw those in the trash when you get a better bike that has a wider and lower seat tube? The 27.2mm wide dropper posts will work on the first two bikes but not on the third bike, because that one has a seat tube that's 31.6mm wide. But you buy a 27.2 to 31.6 shim adapter, and it turns out that works just fine. But you still have the 100mm travel problem. If the post is fit all the way down to the seat clamp, steeper climbs are going to be a lot harder because you can't raise the seat up to where it should be.

    The solution? Most bikes now come with seat post clamps like this:

    You can raise the 100mm post 1-2 inches for climbing, and lower it at the top of the hill for descending (to get a lower center of gravity), but having to take out an Allen key from your pack every time is a pain. So if you messed up and bought a 100mm dropper post, or carried it over to a newer, more modern bike, there is a better option:

    A lot of these are already on cheaper bikes, but if you don't have one, they are great whether you have a dropper post or not. They really should be used for all seat post clamps except if someone has the correct length of dropper post travel.

    But doesn't adjusting the post height defeat the whole purpose of the dropper post itself? Not at all. It only takes 10 seconds or so to lower the post down to the QR seat post clamp at the top of the hill, and then the rest of the time you put it up 1-2 inches above the clamp. You will still use the dropper post a lot downhill, and even sometimes when climbing. You just use the extreme ends of travel by 'artificially' extending the travel past 100mm by adjusting the fixed part of the post with the QR seat post clamp.
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