Creaking seatpost
05-26-24, 09:09 PM
#76
Respectfully, I have many weird *questions* about lubricants, when I don't have the answer, and am always open to those that know more about a subject (I think I recognized your knowledge in a very recent other thread). I think my comments above were asking just such a question. Because the book says greased static friction is very low, but greased posts seem to hold. However I have not yet done the calculation on clamping force for what it's supposed to be, so it's still up in the air for me.
All of the weirdness with seatpost lube came about because of carbon posts, and their problem is really that they change shape when clamped the way metal posts do not. So if you tighten the clamp enough to hold them, you are on the verge of breaking the carbon. Enter carbon paste to grab the post with friction instead of with clamping force alone.
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05-27-24, 01:52 AM
#77
Clamping forces really don't care about whether there is a lubricant - especially when the lubricant doesn't have any particulate to prevent the clamping forces from squeezing the lube out of the affected area. Friction is not really a factor.
All of the weirdness with seatpost lube came about because of carbon posts, and their problem is really that they change shape when clamped the way metal posts do not. So if you tighten the clamp enough to hold them, you are on the verge of breaking the carbon. Enter carbon paste to grab the post with friction instead of with clamping force alone.
All of the weirdness with seatpost lube came about because of carbon posts, and their problem is really that they change shape when clamped the way metal posts do not. So if you tighten the clamp enough to hold them, you are on the verge of breaking the carbon. Enter carbon paste to grab the post with friction instead of with clamping force alone.
"Seguir virtute e canoscenza." (Seek virtue and knowledge.) Cano-what? Cano-worms?
Last edited by Duragrouch; 05-27-24 at 01:56 AM.
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05-27-24, 02:53 AM
#78
Senior Member
Thread Starter
This is a very strange logic as it would be highly improbable statistically that you had several faulty bikes. A more careful consideration of the issue would focus on the common element to all these allegedly faulty bicycles, to a more statistically plausible cause. That common element is you.
You're really helpful man.
05-27-24, 05:27 AM
#79
Senior Member
05-27-24, 11:25 AM
#80
OK, good info... I'm a little confused, but not in a bad way... I said previously that I wondered whether the grease was being squeezed out to the point of metal to metal, and it sounded like you were saying I had things all wrong, but it looks like I sorta guessed right (even a blind sow finds an acorn every once in a while), and that explains a LOT. So it sounds like, for an engineering table that lists static friction like it does, there should be a qualifier that the greased values no longer apply beyond a certain unit area pressure, and a path for the grease to be squeezed out. And then, knowing a relationship between that and the seatpost clamp force and a way to calculate it with respect to bolt torque or tension, would give minimum clamp force needed based on weight, though an estimate of multiple for shock loads would be good. Or, just assume dry clamp and that if it holds, the grease has squeezed out. My seatpost clamp does not list a torque because it's a cam lever, but I'm sure I can convert between bolt torque and lever force with a few calcs. But my guess is, bolted metal post clamps, use a value that is sufficient to overcome the grease and have good metal-to-metal contact, without massive overkill on the torque. The Park Tool guide says 4-6.8 N-m or 36-60 lb-in, thread size unknown, just says Campy for part used, and note, "Seatposts require only minimal tightening to not slip downward. Do not overtighten." Hmm, OK. I'll have to determine common bolt thread for that, and calculate the bolt tension versus my cam lever collar, because mine is levered pretty tight (even with lubed cam surfaces), as the post was slipping down with lesser clamp force, and mine is clean and dry, no lube. (Which is why I mark the position with black sharpie to check.) It'll be awhile before I do that calc, I need to be in the right frame of mind these days.
"Seguir virtute e canoscenza." (Seek virtue and knowledge.) Cano-what? Cano-worms?
"Seguir virtute e canoscenza." (Seek virtue and knowledge.) Cano-what? Cano-worms?
Greased metal seatposts of proper dimension don't slip, and if they do it isn't the fault of the grease.
And you can't calculate clamping forces from hardware torque directly. For example, the distance from center of the post to the bolt varies and varies clamping tension with it.
So you aren't going to derive a rule from the half information you are trying to work from.
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05-27-24, 01:41 PM
#81
05-27-24, 08:30 PM
#82
I think you're looking at this wrong. You keep starting with external engineering information and then try to derive bicycle practices from them. But all of this stuff is decades old, and doesn't require an engineering explanation - especially if you process keeps causing you to suggest the opposite of common/best practices.
Greased metal seatposts of proper dimension don't slip, and if they do it isn't the fault of the grease.
And you can't calculate clamping forces from hardware torque directly. For example, the distance from center of the post to the bolt varies and varies clamping tension with it.
So you aren't going to derive a rule from the half information you are trying to work from.
Greased metal seatposts of proper dimension don't slip, and if they do it isn't the fault of the grease.
And you can't calculate clamping forces from hardware torque directly. For example, the distance from center of the post to the bolt varies and varies clamping tension with it.
So you aren't going to derive a rule from the half information you are trying to work from.
