Kai-ming's kitchen

Discuss light weight issues concerning road bikes & parts.
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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.


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

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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.

by Weenie

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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!

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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:

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by kulivontot

Nobody reading weight weenies is unconcerned with weight savings.

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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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by BobSantini

I'd give you naming rights for that idea. It's cool.
What about the strength?
r o y g b i v

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by jsinclair

Ill preface this by saying that i am not an engineer, I did half an aeronautical engineering degree at uni before moving into architecture. I don't feel i have anywhere near the expertise that a real engineer would have, but significantly more than your average commerce or arts student might possess. So yes, i do have more than enough basic engineering knowledge to question your design.

Conceptually, this could be thought of as a cantilever beam with one end fixed in a pin joint, one pin joint in the middle, and a free end that is allowed to deflect under load towards the rear of the bike. The section between the middle pin joint and the fixed end has been drilled to facilitate more deflection and the diameter of the OD of the seat post is smaller than the ID of the seat tube, allowing this deflection to take place inside the seat tube. This model relies on one thing, that the joints fixing the seat post are pin joints that don't resist torque in the desired range of motion. This however is not the case. The post is fixed with two sleeves that look to be about 1 inch long each. The reality is that when the clamp is tightened, these connections become fixed joints that resist torque, hindering the comfort dampening that is supposed to be taking place. It is still a tube in a tube, as opposed to a ball in a socket which would be ideal. It is likely that these sleeves are moving a little as the post deflects, but whether that movement is relative to the seat post or the seat tube, i cannot tell. Either way, it is probably doing damage to one of them as the edge of the shim repeatedly puts pressure on a small area. In a tolerance critical area such as a seat tube, this wear and tear will eventually lead to some kind of failure. If we assume that the shims are not moving, then the only deflection taking place in the seat post is above the clamp because the shims are resisting the deflection below this point. Obviously this means that the seat post is behaving in a manner no different to the original design, and all the holes drilled in it are not really achieving anything. So at least to me it seems as though you've got a seat post that is either doing damage, or not doing anything at all different to a normal seat post, except:

A seat post stays in place due to the friction between the post and the inside of the seat tube (the use of carbon paste is a good example of this). The clamp increases the static friction up to a point where it is unlikely to be moved in normal use. The problem with this design is that the contact surface area between the seat post and the seat tube has been reduced so much that it would probably take a significant amount of clamping force to hold it in place. Experience tells us what happens when a seat clamp is over tightened on a carbon post/frame. The way that you have haphazardly drilled your seat post clamp right in the middle of the load path suggests to me that this might not be the best collar to try this with.

Another thing i take issue with is the drilling without much thought for the carbon layup of the post. Carbon fibre is not an area i am completely familiar with but i don't believe you can just drill though it willy nilly and expect it to behave like an alloy and flex more since there is less material. As someone else has said earlier, you would have severed countless strands of carbon, greatly reducing the posts strength. with a bit of bending, the post would almost certainly fail at one or several of these holes. The irony is that this deflection probably isn't taking place because the shims are holing the bottom half of the post straight, and if the post was working as intended it would probably break.

Dont get me wrong, i think what you are trying to do is a good idea. But to me the best place to build comfort into a seat post is above the clamp, not below it.

I would like to hear you, as a self proclaimed engineer, respond to these points without blowing it off and suggest i am trying to flame you. Im not. I just think that a real engineer would not have overlooked these issues. just saying...

Also, please don't paint us all out to be racists because some of us don't agree with your ideas, its a pathetic line of argument. This is an international forum with members from all over the world, and seldom do we see anything approaching racism on here. suggesting that euro members might not like your username is just a weak way of defending your ideas.
Last edited by jsinclair on Mon Aug 13, 2012 3:58 am, edited 1 time in total.

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by socratease

Good post

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by elSid

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

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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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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.

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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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by Weenie

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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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