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Thread: Budget dropper posts
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10-18-2019, 11:19 AM #251Registered User
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Sure. What situations will i be in where that seatpost has a good amount of shear force or torque applied to it when in the up position? Maybe im thinking about it wrong, but sitting and spinning, with occasional out of saddle efforts all while going less than 5mph doesnt wouldnt seem to impart much non-normal force on the post.
Physics would say that the ratio you talk about doesnt really matter as the pivot point of the moment arm is at the seatpost collar- so 6" above with 6" inserted is the same moment arm as 6" above and 2" below.
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10-18-2019, 11:32 AM #252
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10-18-2019, 11:32 AM #253
Thread has devolved into a physics discussion.
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10-18-2019, 11:34 AM #254
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10-18-2019, 11:43 AM #255
You can apply only normal force when you ride a sagged / suspended bike’s seat tube??? When climbing uphill?
What magical frame is this that sags to a 105 degree effective seat tube angle???
The fun part about physics and engineering is that we can actually calculate estimated shear force and bending moment caused by a 200 pound rider carrying a 10 pound pack, distributing anywhere from 1/2 to 2/3 body weight force onto an 8” post with 2” of insertion, for a 71-degree sagged full suspension bike climbing up over a root at a 20-25 degree pitch angle. And that’s just a static load, just wait until we analyze the weight transfer from bottom bracket to seatpost and calculate the dynamic effects! But if you’re talking Columbus SL steel then there’s no point, because that shit is magic.
Since all my graduate texts are downstairs in storage and I don’t have Matlab installed anymore ... I’ll just armchair e-chuckle at the ensuing discussion instead._______________________________________________
"Strapping myself to a sitski built with 30lb of metal and fibreglass then trying to water ski in it sounds like a stupid idea to me.
I'll be there." ... Andy Campbell
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10-18-2019, 11:49 AM #256
From your example, the force from that 6" lever is distributed over either 6" or 2" of plastic.
Put another way, you have 6 inches of leverage on the frame while it has 6 inches of leverage on you, or you have a 4 inch advantage over the frame.
But I agree that it *might* not matter.
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10-18-2019, 12:00 PM #257Registered User
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I see where you are coming from. My thought is that the post below the seatpost collar is 'loose' within the frame. The seatpost collar acts as a fixed connection, thus not allowing any moment below. If the whole seattube, all the way down, clamped down on the post, then i would agree with you, but my understanding is that only a small amount of the seattube, right around the collar would actually be holding the post in place. The seatpost collar is not a hinged connection, it is a fixed connection, that assuredly does not extend 4-6" past the collar.
In anycase, I just don't see much force (relative to the rest of MTB) being transferred to my seatpost when sitting and spinning on fairly smooth trail.
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10-18-2019, 12:02 PM #258Registered User
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10-18-2019, 12:17 PM #259
Fine, how about seated 15 degrees over small root/rock step-up - I do that all the time. The rest of my pontification is still largely valid, especially the dynamic force transfer part of it.
But mainly we are talking about bike park seated climbing, like when doing the low angle traverse to get to your trail - you're not exactly wrenching on it, and you're not looking for your optimal highest seated position on a heavy bike with heavy wheels built to stand up in the bike park. So in that use I'm sure it's fine.
For roots and all that other shit, I wouldn't fuck with 2" of seatpost insertion (well maybe I personally would I weigh 135 lbs). I'm also still on the fence about whether 2" of insertion would be okay in the gravel grinder situations (assuming no roots and all potholes being avoidable), but again if that shit is good quality steel then I could be convinced after seeing the numbers showing dynamic load transfers wouldn't get too close to factor of safety on the seat tube's yield strength. For the seatpost, a softer modulus and higher yield strength tube is also probably better for dampening dynamic load transfers to the frame's seat tube._______________________________________________
"Strapping myself to a sitski built with 30lb of metal and fibreglass then trying to water ski in it sounds like a stupid idea to me.
I'll be there." ... Andy Campbell
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10-18-2019, 12:22 PM #260Registered User
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Maybe ive been biking wrong all these years, but im not plopped on my saddle when riding over roots, steps, or anything remotely tech. My dead weight is only in the saddle when its pretty darn smooth. Otherwise im always moving around the bike with only a portion of weight on the saddle.
Another random thought... how much of your actual weight is on the saddle when pedaling up a steep climb. A lot of it has to be on the pedals and a good amount on the bars.
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10-18-2019, 12:35 PM #261Rod9301
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10-18-2019, 12:37 PM #262
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10-18-2019, 01:18 PM #263
Oh for fucks sake, if dude doesn’t want want to run a dropper - let him. He’s clearly dug in - we’re not gonna convince him.
It seems that he also can’t be convinced that standard 3” insertion depth is not an Illuminati plan to sell longer seatposts, and that’s fine too
Sent from my iPhone using TGR Forums
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10-18-2019, 01:46 PM #264
Aww, what a buzz killer. This stay at home dad needs more entertainment when the baby is sleeping. Back to Netflix and bike building I guess ...
_______________________________________________
"Strapping myself to a sitski built with 30lb of metal and fibreglass then trying to water ski in it sounds like a stupid idea to me.
I'll be there." ... Andy Campbell
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10-18-2019, 01:49 PM #265Registered User
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Do you also strictly believe that more than 3 mounts on a ski is unsafe for EVERYONE because that is industry standard? The seatpost insertion thing to me smells of unnecessary overdesign for most peoples situation. Lots of industry standards out there that can be broken (within reason) with no ill effects if you arent pushing the equipment to its absolute limit. Personally, im not pushing a carbon 6" superbike to its limit while seated spinning up a smooth climbing trail. YMMV.
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10-18-2019, 02:04 PM #266
I’d run way more post inserted than you’re suggesting. Countless broken steel, aluminum and carbon frames came through. Even on light weight kids.
Sent from my iPhone using TGR ForumsI rip the groomed on tele gear
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10-18-2019, 02:06 PM #267Registered User
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10-18-2019, 02:11 PM #268
Yeah.
Cracked frames at the top tube seat tube junction.
I would say post needs to be an inch below the bottom of the top tube minimum. Personally I go way more.
Sent from my iPhone using TGR ForumsI rip the groomed on tele gear
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10-18-2019, 02:21 PM #269
FTR, I agree that for JRA you'd likely be fine. I've done way crazier stuff with motorcycles and I'm still alive. But you'd have to be conscious of it rather than not even thinking about it, which is the state I'd rather be in - especially if it's a long way back to the trailhead.
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10-18-2019, 02:24 PM #270
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10-18-2019, 02:25 PM #271Registered User
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Interesting. If i am understanding correctly, the argument is that the seat tube relies on the addition of the seatpost to gain its structural integrity. Meanwhile everywhere else on the frame (including all the normal high stress spots) are fine with the normal hollow frame? I am just trying to envision how it would fail, and how a seat tube inserted all the way down actually would help.
From what i understand, in order to gain full design strength, you would just need to insert the post far enough that it extends the full length of clamping surface. Since i know that the whole length of seatube isnt clamping, i would assume it wouldnt be much more than an couple inches from the actual collar. Additionally, each frame (and material) would have a different length of clamping surface, so why do all posts come with a standard insertion length?
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10-18-2019, 02:28 PM #272
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10-18-2019, 04:32 PM #273
Probably it’s more like a seat tube that extends above the top tube and doesn’t have a gusset relies on minimum insertion depth to transfer shear load to the top tube.
_______________________________________________
"Strapping myself to a sitski built with 30lb of metal and fibreglass then trying to water ski in it sounds like a stupid idea to me.
I'll be there." ... Andy Campbell
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10-18-2019, 04:43 PM #274
That's definitely it. I was just being glib.
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10-18-2019, 04:59 PM #275
I should take Joe's advice and be done here, but since you seem genuinely curious I'll bite. You are fundamentally misunderstanding how the seat post acts as a lever inside the tube, as well as the scale of the forces created by a 200 lb guy just sitting on it. I don't feel like explaining why further when someone with a graduate degree in engineering and someone who has seen these failures repeatedly in a shop both disagree with you.
It is entirely possible that you may not break your frame running 2" of insertion. But, you are absolutely running a very real risk of doing so, and for what gain (I am not arguing that you should run a dropper)? If you have a warranty, that is something that Rocky probably would not cover.
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