Kai-ming's kitchen

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justkeepedaling
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by justkeepedaling

If you're doing a cleansheet design, wall thickness is far better to manipulate to get light weight and the flex that you are looking for. This is the only modification possible to an existing thick/heavy seatpost, but it's far from ideal when starting from scratch. Seatposts used to be fluted, drilled, etc etc, by many different manufacturers.

http://bikethomson.com/seatposts/masterpiece/

Nowadays, they can machine for oval internal shaping. Strength requirements are a must when making mass production with quantitative numbers figures for fatigue and strength.

I think a concern that manufacturers had (besides possible stress risers) was that water can get into the frame. Just a guess

daj
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by daj

A construction with a shim may not be wise to install on frames that are designed around the extra support that a seatpost gives a frame at the top of the seat tube junction (e.g. Cervelo R5). Since the seatpost adds support, the frame can be built with less material in that area.

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kulivontot
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by kulivontot

Weight before/after? I would think if you're suggesting it's worth the effort or risk you would want to demonstrate that the weight reductions are significant. This is weight weenies after all. Scale pic or it doesn't count!

crohnsy
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by crohnsy

Read the thread kulivontot, he's not concerned with weight (207g with holes) or measuring the increase in deflection. He is too busy riding for that, you'll have to take is word that the comfort increase is significant.

:beerchug: :noidea:

kulivontot
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by kulivontot

Nobody reading weight weenies is unconcerned with weight savings.

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bobbyOCR
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by bobbyOCR

daj wrote:A construction with a shim may not be wise to install on frames that are designed around the extra support that a seatpost gives a frame at the top of the seat tube junction (e.g. Cervelo R5). Since the seatpost adds support, the frame can be built with less material in that area.


The 'shim' should need to extend at least to the bottom of the top tube/seat tube junction but apart from that, the concept is sound. If this could be made as a one piece tube with adjustable collars and wall thickness/tube shape as flex control, this could be a very, very good idea.

Although kai, from the few posts I've seen, can grate people pretty bad, he's forward thinking. It's always refreshing to see people look past the parts on the shelves and see how far the envelope can be pushed.
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Phill P
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by Phill P

I'm with Bobby (good to see you back mate) its refreshing to see somebody take some risks and come up with ideas. Kai-ming is certainly more of a risk taker than I am!

Also quoting he is an engineer, but ppl getting upset because he hasn't done any math on his CONCEPTS is irrelevant. He is applying engineering judgment and concepts to his experiments.
OK I like his idea on the seat post more than his idea of removing metal from a set of 550s, but just because we have a different point of view doesn't mean I'm going to abuse the guy. There is a difference between having an opinion and offering input, and flamming somebody.

Most concepts are prototyped in some way to prove a concept, then the CAE and testing is done to refine the design.

My thoughs:
-Drilling holes in the seat post will let water into the frame. Make sure you have a drain in you frame's BB shell.
-CF is weak in compression, you may want to consider not drilling out the rear of the seat post as much
-You've cut a lot of CF stands. Not only are the holes stress risers you've interrupted the fibers. A 90deg weave might not be the best for this, 45deg or 30/30/30 triangular weave might work better for this application and help you tune the flex.
-with the two sleeves you don't need a round seat post to fit into the round frame tube. You can make your inserts D shaped to match a D shaped seat post.
-I'd use a two bolt seat clamp to reduce the risk of cracking the seat post. But then the sleeve is already helping distribute loads. But with the drilling in the clamp anbd bending moment area you want to reduce compressive loads.
-frames and seat posts are designed with a minimum insertion length. I wonder if only having the bending moment resisted at the frame at two points rather than along a length would make a difference
-Might be a better design in Ti or Stainless steel as they don't have the compression loading issue of CF, but do have a few more weight issues.

Mate - ideas get straight out stolen in the bike industry with no credit (or cash) given in return. Very little chance of somebody who commercializes something like your seat post naming it after you as well!
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BobSantini
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by BobSantini

I'd give you naming rights for that idea. It's cool.
What about the strength?
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jsinclair
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by jsinclair

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Last edited by jsinclair on Thu Sep 02, 2021 11:08 pm, edited 2 times in total.

socratease
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by socratease

^^
Good post

elSid
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by elSid

Let us know if you have any more concept design ideas. You can borrow my hammer.

aerozy
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by aerozy

Sorry mate but I wouldnt ride that seat post. Im a qualified structural engineer as well and those cut outs dont seem to be very clean. You might have singular hotspot stresses that might develop into cracks over time (fatigue). Good luck and safe riding!
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Leviathan
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by Leviathan

Im another lapsed Ex Engineering student (UK, MIMechE). Can you explain to me why drilling the holes helps with the vibration profile given that the post is effectively held ridgid at both ends?
Any simple KMC/ Diff analyses model would help. Thanks.

aerozy
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by aerozy

For a SDoF (fixed at one end and a mass at the other) the natural frequency is given by the following equation:

w^2 = k/m

K being the stiffness of the beam and m the mass of the weight. Guess with all the holes the lateral stiffness of beam has been largely reduced so the natural frequency will be much larger, thus providing a smoother ride. I'd be concerned though of its overall strength and specially concerned of its fatigue life.
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Leviathan
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by Leviathan

of course, but the point is the post is effectively held at both ends (or at least its a planar joint at the bottom) so the only thing drilling will change for vibration, I think, is the overall mass of the system? But given I didnt "major" in vibration analyses (did we even HAVE majors at UK engineering courses??) I dont know.

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