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It is a standard column bicycle seat and I realised maybe I should have it raised to top (I can still stand the bicycle up using my toes) but the saddle rocks as there is no column to stop it moving and it doesn't have any other supports.
I don't want it fixed forever but still want the saddle to be stable, with minimal movement. How can I do that?
The movement is 1 mm. I will correct this later.
Your seat post is not all the way up — it's too far up.
If you need the saddle at that height, then you need a longer seat post. As mentioned in comments, there is usually a "minimum insertion" point marked on the post.
Don't ride the bike with the seat post too far out of the seat tube. It's dangerous as the seat post may collapse under you, and you risk damaging the top of the seat tube even if that does not happen.
Seat posts are usually marked with a line indicating a minimum insertion point. Extending the seat post past that point poses a risk of the seat post bending, or cracking the frame. While 250-300mm seems to be the normal original size there are extra long seat posts available. Lengths of 400mm are fairly common. I did see a 700m telescoping (two sections) post for a folding bike. You must also match the seatpost diameter to the bike's frame.
If you are lucky you can buy a longer post and replace your present one. As mentioned in other answer, each post has a clear mark indicating the maximum usable length, and exceeding it is not a smart idea.
Visit a bike shop and search for a longer post fitting your frame. If the diameter is not weird, you should fine one.
I mentioned the weird diameter out of personal experience: on one of my previous bike the post had a larger diameter in the sub millimeter range than the new one I bought based on the millimeter size, and could not fit.
Have had the same issue..seatpost hight enough is too high..on the boundary from medium to large frame and have to opt for large and short stem instead to get a sturdy saddle height..
As above mentioned not observing the minimum insertion is incredibly dangerous and costly(will possible write the frame off)..if nothing else works go up a frame size..and shorten the stem a bit..works a treat for me(100mm instead of 120mm stem for example)..
More info
The post diameter is often stamped into the post. There are a number of diameters, some very close together, using a post slightly too small in diameter can result in a clamp that won't tighten enough allowing your post to gradually sink into the seat tube or turn easily. Not a problem if you like to ride side saddle or with your knees in your face :) A post that's just slightly too large in dia probably won't fit the seat tube. If you've ever extracted a stuck post, it's obvious they're meant to fit tightly.
ebay has lots of long steel and aluminum posts; I bought a 400 mm long post from ebay a few years ago.
BTW, the longer the post, the farther back you sit and the easier it is to lift the front wheel. May not be noticeable, the seat can be moved forward on its rails some to compensate if it is.
Enjoy!
As other answers said, you should not ride w/o at least a few inches of seatpost inserted into the seat tube of the bicycle.
If you cannot locate a seatpost that is long enough (which you should be able to do), or otherwise want to use the seatpost you have, seatpost extenders exist that might be able to foot the bill. Essentially, the are two parts -- a long tube that inserts into your seat tube (as a normal seat post would do) and an upper part that acts like another seat tube and into which you can slide your existing seat post.
I can only copy all the other answers. Do not pull the seatpost too far up.
There is a minimum insert mark on the seastpost. The reason is the post is not vertical so while sitting on it you are bending the post. When the post is mounted too shallow, it can bend or break the frame.
I cannot recommend using longer seatpost either, because longer post means higher bending momentum. You can bend or break the post itself, damage the frame or both.
Wery nasty injuries come from seat and seatpost failures!
If you need to pull the saddle up that much, the frame is too small for you. I would strongly recommend giving the bike to someone smaller tha you are and getting yourself bike that fits you.
