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.
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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