I’m about to trim my seat post down. I just want to make sure there’s
enough left after i’m done. How much post do i need to have in the
tube ? I’ve seen various numbers… But i want to be sure. Or maybe
it just doesn’t matter… buaha.
My default rule is to only cut off what you need to for the post to fit. That is, unless you’re pretty sure you may want it lower in the future. This is true for when you’ll have shorter people riding it, and also if you plan to ride on rough terrain, for instance, where you’ll want a lower seat. But in general, it’s always easier to cut some more off, than to add some on.
If the goal is to shave weight, I’d say about 2", though the real answer probably depends on your frame design, post material, clamp size, etc. You want plenty of tube going down past the bottom of the clamp. If you really beat on your unicycle, keep more post in there.
Lastly, remember if you want to raise it in the future, leave room!
My understanding is that you want enough seatpost to go at least the diameter of the post lower than the bottom of the slot on the seat tube. This might not be true if the seat tube has butting or reinforcement below that point. If it is reinforced you might need to have it go the diameter of hte seat tube past the reinforcement, otherwise it will set up a weak spot. Personally I wouldn’t cut it any shorter than I need for the reasons stated in the prior reply.
Looking at the KH assembly guide this morning it suggests a minimum of 50mm of seat post in the frame.
I leave as much length as I can on the seat post i.e. cut it to the right height for the longest crank I use, then raise the seat post from there for shorter cranks. For me that means the seatpost only raises ~25-35mm, leaving ~70mm-80mm in the frame, depending on the uni.
If it worked out to be any less than that in the frame, I’d probably keep a spare seat post cut to an alternative length and swap it when swapping cranks.
I would go with about double the diameter of the seat post or more. I don’t have experience that would back this up, but my reasoning comes from commonly used conventions with screws and fasteners and engagement lengths that I’ve touched on through my schooling (engineering).
Otherwise I’d also go with leaving as much on as possible. If you need for it to be shorter so you have this much possible variation then cut it just enough to provide you with what you need, no more.
If that 2 inches of excess seat tube weight is hurting your performance, maybe it’s time to shed a t-shirt, or take a poop… Really, have you weighed a few inches of tube latley? We’re talking a few ounces here…
And yes, I speak from experience. My other hobby is building, tweaking, and maintaining my best friends dirt late model race car… We changed battery brands for a 2 pound shave = FAIL, did nothing!
i fully understand what your saying,ive look and found i could save about
80gr by cutting my post down.ive got heaps in my frame and will never need to raise my post.the length of my seat post in the frame is about 6in
im currently at 5kg with my uni and i got new parts coming that will drop the weight down to 4.5kg and thats without any titanium,thats next on the cards
I’m trying to get how more post would make a frame more likely to break? Maybe it depends on the type of frame/how it’s built. For example, on my Wilder frame, the seat tube goes all the way down, where the post can drop down against the tire. The strongest arrangement would seem to be having the post go all the way down.
But if your seat tube doesn’t go all the way down, or isn’t designed for the post to go through, use caution. Sometimes there’s stuff down there that your post can get wedged in, and make it really hard to get it back out. Or if you have a Coker frame (new style), they put a little bar in there, for unknown reasons, that keeps your post from coming out the bottom. Gives you less leeway for your post, which can be a problem for bigger wheels.
Several people have told you the minimum amount. Use less and you increase the risk of frame damage. Frame damage is a lot more expensive than post damage. An inch of aluminum seatpost weighs how much? It’s your call…
a seat post inserted into a frame creates a hard point, a place where the strength of an assembly changes. I flew model planes and gliders (including 10lb racing gliders made from composites) and when a plane breaks from flight stress or a crash the break almost always occurs at the end of doubler, or at the end of spar. So the point where the seat post ends is a change in the strength/flex characteristics of the seat tube.
If the end of that seat post is near a point where the frame is under stress anyway that seat tube inhibits a gradual stress of the metal and the stress becomes concentrated at the end of the seat post. Of course it is the seat post that is trying to bust out under certain kinds of torquing stress, and you are right that having a seat post inserted further is going to be stronger since it inhibits the torquing on the seat post. (also the amount of seat post exposed is going to be a factor)
As an example of avoiding a hard point look at the design of bike frame lug like in the picture below.
On some of the glider wings I made in my laminations schedule I would have a layer of glass in the shape of a long triangle pointing down the wing on the top of the wing (like in the lugs below). This may seem counter intuitive since a break in a spar assembly is usually visually seen in the stretched side but the stress of a spar is actually in compression. With no failure of compression there is no stress in, I can’t think of the right term, so I will call it a failure in elasticity (the stretched side). Break a wood match or popsicle stick and you can see what a mean. Or even in a book binding. No crease in the binding and no loose pages.
But with all that said, has anyone here seen a failure at the end of the seat post? In a Uni or in a bike frame? I never have.