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 Post subject: Re: Kai-ming's seatpost
PostPosted: Mon Aug 13, 2012 10:45 am 
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Joined: Sat Aug 27, 2011 3:26 am
Posts: 382
Location: Sydney, Australia
yeah, if the points at which the post is held were free to rotate then i gather the idea of the holes is to reduce its stiffness to facilitate bending. the design fails for the aformentioned reasons.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Mon Aug 13, 2012 1:30 pm 
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Joined: Thu Sep 04, 2008 9:48 am
Posts: 741
Location: Brighton, UK
The holes are reducing bending stiffness in all planes, in and out (its a symetrical cross section so its stiffness is the same in both planes). If I were to model this problem I would consider it fixed at one end (bike) and free at the other end. Your butt is not fixed, it moves with the saddle, which I would consider as a mass.

The seatpost will have far less axial capacity as well with the varrying cross sectional areas. It could also buckle under compression load. Generally speaking I think its a bad idea if you havent backed it up with calculations. And to drill like that in carbon? Ouch! Carbon is a brittle material. When a crack appears it wont give you much time before it fails completely. I'd insure the boys downstairs before I'd ride that. Have you told your wife about this seatpost? :D

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 Post subject: Re: Kai-ming's seatpost
Posted: Mon Aug 13, 2012 1:30 pm 


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Mon Aug 13, 2012 1:37 pm 
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Joined: Sat Aug 27, 2011 3:26 am
Posts: 382
Location: Sydney, Australia
sorry yes, fixed at the lower end and in the middle where the collar is, with the upper end free to deflect with the mass. since it is clamped a the mid point, im not sure the holed section would be under any measurable compression.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Mon Aug 13, 2012 4:41 pm 
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Joined: Sat Jun 13, 2009 4:02 am
Posts: 1951
Location: NoVA/DC
I think you guys arent realising how much movement a seatpost gets in a frame. If one were to imagine the seat tube having a much larger bore than the post's shims, and the post clamp pinching down on the upper shim, it's easier to see that the seat tube in fact "gives" more locally and less uniformly. Reality is similar to this, but on a smaller scale.
Many, many frames get "creaking" from excess seatpost hanging out inside the frame moving around slightly, even frames with a constistent interior shape all the way down.

One thing i would have changed, however, is that i would not have drilled any holes in the front edge of the post, as those fibers are completely in tension. I would have focused on the rear 2/3rds of the post , with a gradual increase of hole size as i got to the back edge.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Mon Aug 13, 2012 4:55 pm 
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Joined: Wed Oct 31, 2007 10:32 pm
Posts: 7446
Location: Los Angeles
It certainly needs refinement, but the concept by itself is some very innovative thinking. :beerchug:
I'm glad to see the collective efforts here on WW lead to an improvement of this concept.

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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 3:51 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
jsinclair wrote:
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.

As I have said, I am not playing cards here whether engineering/psychology/racism or not. All I wish to do is to show my works, explain/make it clear and open to receive responses/comments, good or bad. I just wish to eliminate possible pollutions to make the discussions straight and simple. Racism exist all over the world, is one that I do not wish to see polluting the board. Similarly, quoting that I am an engineer is to avoid the scorn in the first place. As in the earlier thread about shaving rims, I did not mention my qualification until someone said he/she is an engineer.
It appears that you have not read my posts thoroughly. This may be what you have missed – the upper and the lower shims are fixed to the seatpost permanently as stated in my first post. There is no need to clamp the seat tube collar clamp too tight, conventional clamp has to be strong to stop seatpost from slipping, just enough to stop the seatpost/saddle from turning should do. A weak and super-light collar clamp could be used for further weight reduction. There has to be movements at both shims though it may be tiny. The clearance/ free movements (axial and lateral) at the lower shim inside seat tube had been checked while the seatpost was half inserted into the seat tube. Grease had been applied at the lower shim/seat tube interface. As I have said, I have ridden it for a year, it make my ride smooth, proved to myself that it is durable and solid for everyday ride. I have purposely ridden over small ramp/bad road surfaces with all my weight rest on the saddle. I suppose carbon has good fatigue life.
Image
The black section - lower shim at the bottom end of seatpost.
The red section - flexible section of seatpost inside seat tube.
The white section rested on the handle of my coffee cup - upper shim sit on top of the seat tube of bike frame.
The pink stuff - the load of a rider.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 3:51 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
Sorry, double posted.

justkeepedaling wrote:
......
water can get into the frame. Just a guess

Phill P wrote:
...
-Drilling holes in the seat post will let water into the frame. Make sure you have a drain in you frame's BB shell.
...

If you look at the first picture, you may notice that I have taken a little care on not letting water in the frame. I have turned the upper shim so that the cut slot is facing to the front, epoxy to fill the upper part of the cut slot. The upper shim is longer than the cut slots of the seat tube, cut slots of the seat tube are on left/right sides, holes of the seat tube camp are away from cut slots. I hate water get into my frame, I have checked from time to time, no water found.


Last edited by kai-ming on Tue Aug 14, 2012 4:25 am, edited 2 times in total.

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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 4:41 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
thisisatest wrote:
.... with a gradual increase of hole size as ...

with a gradual increase of hole size/holes towards the lower end of the post - you may notice that in the first picture.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 4:47 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
prendrefeu, it certainly needs refinement - no doubt about it, could not agree with you more. :beerchug:

There are ideas flash in my mind. :idea:
Image


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 5:27 am 
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Joined: Sat Aug 27, 2011 3:26 am
Posts: 382
Location: Sydney, Australia
kai-ming wrote:
It appears that you have not read my posts thoroughly. This may be what you have missed – the upper and the lower shims are fixed to the seatpost permanently as stated in my first post. There is no need to clamp the seat tube collar clamp too tight, conventional clamp has to be strong to stop seatpost from slipping, just enough to stop the seatpost/saddle from turning should do. A weak and super-light collar clamp could be used for further weight reduction. There has to be movements at both shims though it may be tiny. The clearance/ free movements (axial and lateral) at the lower shim inside seat tube had been checked while the seatpost was half inserted into the seat tube. Grease had been applied at the lower shim/seat tube interface. As I have said, I have ridden it for a year, it make my ride smooth, proved to myself that it is durable and solid for everyday ride. I have purposely ridden over small ramp/bad road surfaces with all my weight rest on the saddle. I suppose carbon has good fatigue life.


It appears you need to re-read my post, and your first year statics textbook.

You are modelling the shims as pin joints that are free to rotate, when in reality they act as rigid joints resisting rotation. This is very basic first year mechanical statics. I dont doubt that they rotate a little bit, but like i said earlier, this will cause friction wear and fatigue in your seat tube. Your little bent stick photo does not reflect the reality of what is happening inside your frame. The white and black sections ARE NOT free to rotate like you are showing them.

Also from your photo you can see that the majority of the bending is taking place in the red region which is inside the frame with the exposed part showing relatively little. It should be obvious that you would want the bending to occur on the exposed part where it will not damage your frame.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 5:48 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
jsinclair,
''in reality they act as rigid joints resisting rotation'' against ''dont doubt that they rotate a little bit'' - I give up. :noidea:

kai-ming wrote:
...The clearance/ free movements (axial and lateral) at the lower shim inside seat tube had been checked while the seatpost was half inserted into the seat tube. Grease had been applied at the lower shim/seat tube interface....
Worry about wear and fatigue, don't ride your bike then. :mrgreen:


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 5:59 am 
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Joined: Mon May 24, 2010 4:18 am
Posts: 329
Location: Australia
I keep wondering why people would want to change their position on the bike when they take so long and spend so much time perfecting it?

Provided your position on the saddle remains constant, having setback "pivot" effectively varies the distance from a defined point on the saddle to the centre of the pedal axle (remember: bdc is not full extension on the bike - 5 o'clock is) albeit only by a few mm which I wouldn't expect would be a good thing from a power development standpoint (especially where every half % counts).

Extreme example:
Q. From a power development standpoint, is a [vertical] suspension seatpost a good idea.

A. Nope. Varying saddle height is detrimental to power development and power is most efficiently developed through certain ROMs of the muscle (and varying the saddle height can take the rider out of the optimum envelope).

Just my thoughts.

Cheers,
I


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 6:27 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
Correct me if I am wrong. I think the saddle position is a compromise, in particular the fore/aft position. I keep changing the position of my bottom on the saddle depends on how I wish to ride. I also don't put all of my weight on to the seatpost/saddle most of the time.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 6:37 am 
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Joined: Sat Jun 13, 2009 4:02 am
Posts: 1951
Location: NoVA/DC
kai-ming wrote:
thisisatest wrote:
.... with a gradual increase of hole size as ...

with a gradual increase of hole size/holes towards the lower end of the post - you may notice that in the first picture.

if you quote my entire sentence, you may notice where your modification and my suggestion disagree.


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 Post subject: Re: Kai-ming's seatpost
Posted: Tue Aug 14, 2012 6:37 am 


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 7:03 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
thisisatest wrote:
kai-ming wrote:
thisisatest wrote:
.... with a gradual increase of hole size as ...

with a gradual increase of hole size/holes towards the lower end of the post - you may notice that in the first picture.

if you quote my entire sentence, you may notice where your modification and my suggestion disagree.

Is idea A similar to your suggestion?
I have done the drawing before I saw your post.


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