Is it unsafe to have a seatpost "slammed"?

Question

On my hardtail I have my seat post inserted as far as it can go, which is limited by bottle cage bolts. On my full suspension it's a slight bend in the seat tube that limits it.

I'm concerned that I might be putting a lot of pressure on a really small area at the bottom of the post. Sitting with some of my weight on a bolt might lever it around the frame and form a crack. Is this anything to worry about?

My seatpost are droppers with adjustable travel, so I like them fully slammed, but I think the question would stand for a rigid post on a road bike as well if it happened to fit that way. I could reduce travel by 5mm and move the post up, or with a rigid post you could cut off the bottom half inch if this was a problem.

Answer 1

Look at it this way: would you feel comfortable taking your saddle off, grabbing a 1 yard/meter long bit of lumber, and whacking down on the top of the seatpost moderately hard with that chunk of lumber? Not too hard, but in the range where you know nothing would be damaged at all with a seatpost that's not bottomed-out?

Now, do you feel safe doing that with your seatpost bottomed-out, even if it's clamped?

Because I think if your seatpost is truly bottomed-out and stopped by a bottle cage bolt or a bend in the seat tube wall, then yes, you are putting significant stress on that point whether the seatpost is clamped or not.

@MaplePanda's answer assumes if your seatpost clamp is in place, the clamp will take the load. I'm not so sure - if the seatpost is bottomed out when you clamp it, it's still bottomed out. I don't think the clamp in that case is going to take all significant amounts of load off the bottomed-out point and remove the stress being put on that part of the seat tube.

I think @juhist's answer is based on the same premise:

Of course the seatpost needs to be tightened. If the seatpost is put against some hard stop at the frame tube, and you don't tighten it...

If your seatpost is bottomed out, I'd bet every bump you hit is going to transfer significant force to that bottom-out point even if your post is clamped tightly. Under any force the entire length of the seat tube between the clamp and the bottom-out point will compress slightly - but the seatpost won't because there's nothing to compress against - except for that one bottom-out point, where all the force will be concentrated. Every material is a bit of a spring - push down on the top of the seat tube at the clamp and the distance between the clamp and the bottom-out point shortens, if only by a very small amount. But the seatpost itself isn't going to shorten - every bit of force not absorbed by the seat tube compressing will be transferred to that one bottom-out point.

So I'd think every shock your seatpost takes if it's bottomed out will transfer a significant amount of force to that one bottomed-out point, whether the seatpost is clamped or not. The clamp could in theory reduce that a good bit, but maybe not - it all depends on the exact properties of the frame and seatpost material. And even then, I'd think there will still be stresses on the bottom-out point to some degree - I see no way to put a downward force on the saddle that will not be transferred to some degree to the bottom-out point.

Raise that seatpost a few mm off its bottom-out position and then clamp it there.

Answer 2

I have had seat posts that slide down despite being properly tightened. On a post that is not bottomed against anything, you notice at some point, but with a bottomed one, how would you notice? If the clamp somehow became loose and you rode the bike, the seat post would seem to be not moving, bout it would be "banging" whatever it is bottoming against.

For me, that alone is reason enough to raise te s.p. at least 5 mm or 1/4 of an inch. I would also make some discrete marking on it such as it is barely visible when at proper height. If the post slides down a bit, the marking won't be visible, thus an indicator that you should check the clamp.

(I admit the one should do a periodic thorough inspection that should catch such things, but, realistically, how many of us go checking every bolt with a torque wrench before each and every ride?)

My take on Andrew's answer is that using the seat post bottomed down may cause a "micro hammering" effect at the contact point, where the softest metal can get deformed a little. If the seat post develops a bulge product of this hammering, it may become difficult to remove from the frame (I've seen this happen on cheap steel bikes/posts) On the other hand, the constant hitting may make the threaded insert (where the bottle cage mount bolts to) go out of round, making more difficult to insert/remove such bolt. It may also dislodge or move it out of true, depending on bike quality/materials.

Forcefully inserting a pipe inside another that has a bend in it, may wedge it, also making it difficult to remove. Depending on materials, one of the pipes will deform more than de other. (I have played with this while DIY-ing things)

So, in the end, for ease of mind, I would "sacrifice" a few mm of travel and rise the post a bit. After all, the critical position is the upper one. (at least for me) The lower saddle position is often not as critical.

Answer 3

That's not an issue.

The issue of seatposts too high is that you sitting on the saddle forms a long lever and the opposite lever would be very short for a seatpost set too high. If a long lever has certain force, and the same torque is applied at a short lever, the force is multiplied by the lever length ratio. You could easily apply a ton of static force or more into a seatpost that's too high -- and several tons of force on a bumpy road. That's why seatposts have a minimum insertion mark.

There's no such thing as "too low". If you set a seatpost very low, you make the lever at the seatpost end longer, which reduces forces. There's no way there could be too much force, unless an elephant sits on your saddle of course (which would probably ruin your bike instantaneously).

Of course the seatpost needs to be tightened. If the seatpost is put against some hard stop at the frame tube, and you don't tighten it, then gradual movement of the loose seatpost could cause gradual slow wear, which could be dangerous. As long as the seatpost is stationary and cannot move, this is not an issue.

Answer 4

Pull it up a bit, clamp it down. Save yourself a cracked frame later. If the post were supposed to be resting on anything it would have been designed with a stop.

Answer 5

I can only see that being a problem if you regularly ride without a seatpost clamp. Assuming you’re a normal person who indeed has their seatpost secured to the frame, all the force from your butt is being received by the uppermost ~10cm of the seat tube or so. The region you’re concerned about sees no weight.

I suppose you may have an issue in the event that you crash or something and the seatpost slips. This risk could be reduced by firmly tightening the seatpost clamp, but then you run an increased chance of damaging the dropper and/or saddle in a crash. You’ll have to make a risk assessment for your individual situation I guess.