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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 9:25 am 
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Joined: Sat Aug 27, 2011 3:26 am
Posts: 390
Location: Sydney, Australia
kai-ming wrote:
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:


Im not going to argue with you anymore, you only respond with snide remarks where others are trying to have a meaningful discussion about the technical aspects of your design. Nothing ive seen so far suggests that you have any engineering experience.

Whatever, im over giving a crap. drill holes where you like. my bike is fine without your spaghetti seatpost.


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 Post subject: Re: Kai-ming's seatpost
Posted: Tue Aug 14, 2012 9:25 am 


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 9:53 am 
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Joined: Sat Jun 02, 2007 1:10 am
Posts: 1845
Location: Melbourne, Australia
My feeling is the upper clamp point is effectively a torsional spring loaded pin joint. The lower shim is a less torsional loaded spring joint.

The stiffness of the seat clamp area on the frame will determine how stiff this point will be, and how much flex is then permitted inside the seat tube. If you look at the straw flex image on the last page you can see a lot of rotate at what would be the seat clamp area. If this were stiffen and not allowed to rotate you would get very little bow in the red area.
Because the torque on the seat tub is concentrated more on the seat tube area this may be an issue for the material and welds.

The area between the shims will flex, but no where near as much as the area above the seat clamp. It would be interesting to strain gage the inside of the seat post to see where the flex is occurring. It could be that 90% of the flex occurs above the seat clamp (just an thought not a quotable prediction). If that was the case then designing the flex into the post for when it is exposed would be more practical than trying to design for flex inside the frame.

However this could still be a clever way to reduce the amount of reduction shim used when putting a smaller OD seat post into a stiff large ID frame seat tube.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 12:32 pm 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
jsinclair wrote:
kai-ming wrote:
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:


Im not going to argue with you anymore, you only respond with snide remarks where others are trying to have a meaningful discussion about the technical aspects of your design. Nothing ive seen so far suggests that you have any engineering experience.

Whatever, im over giving a crap. drill holes where you like. my bike is fine without your spaghetti seatpost.

I thought if you are ok with this ''you need to re-read ... your first year statics textbook.'', you should be ok with my reply. All I am trying to do is to make people fully understand the works. All bikes will wear and fatigue when ridden.
Let us not argue but accept that we see things differently.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 1:14 pm 
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Joined: Sat Jun 02, 2012 10:43 am
Posts: 66
Phil, your view of the mechanics of this set up is correct. The top is indeed a very stiff torsional spring. Ki-ming picture of it as a freely rotating pinned joint is not correct. Also, the majority of bending is going to occur on the cantilevered part of the post - i.e. above the clamp.

As for credentials, I have a Masters in Mechanical Engineering and work on structures nearly every day.

Kai-ming, it appears we are "neighbors" as I am in Guangxi right now. If you have never ridden near Guilin, you should given the chance...the scenery is spectacular. Give me a shout if you make it up this way and we can go for a spin.

For the record, I wouldn't ride that set up. Doing a quick mental FMEA on that post looking at the likelyhood of a failure of a drilled composite structure and the effect of snapped post plus I value my tender vittles waaaaaay to much to take that risk.
:beerchug:


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 2:01 pm 
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Joined: Tue Apr 18, 2006 3:03 am
Posts: 1882
Location: Islip, NY
I'm surprised the Trek IsoSpeed concept hasn't been brought up yet.
http://www.youtube.com/watch?v=yel3yhN_vy4" onclick="window.open(this.href);return false;

Here are my suggestions for improvement that would use a similar concept on any bike.

The seatpost should attach to the bike at the clamp with a ball/socket interface instead. The bottom of the seatpost beam can have a bushing to match the seattube inner diameter. This would make an inexpensive bushing a wear item and not the frame. The distance between the bushing and the ball/socket interface would be a set length. The beam can be made rather thin below the ball/socket.

Then you could use a seatpost topper similar to Trek where you have a mast sticking up (the flexible beam) that is cut to size and a topper similar to those used by ISP frames go on top of that. This way you have adjustability without interfering with the design that attaches to the bike.

I'm no engineer, but this makes sense to me.

Image

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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 3:02 pm 
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Joined: Sat Mar 17, 2007 10:49 am
Posts: 794
Location: Mallorca, Spain
Phill P wrote:
My feeling is the upper clamp point is effectively a torsional spring loaded pin joint. The lower shim is a less torsional loaded spring joint.

The stiffness of the seat clamp area on the frame will determine how stiff this point will be, and how much flex is then permitted inside the seat tube. If you look at the straw flex image on the last page you can see a lot of rotate at what would be the seat clamp area. If this were stiffen and not allowed to rotate you would get very little bow in the red area.
Because the torque on the seat tub is concentrated more on the seat tube area this may be an issue for the material and welds.

The area between the shims will flex, but no where near as much as the area above the seat clamp. It would be interesting to strain gage the inside of the seat post to see where the flex is occurring. It could be that 90% of the flex occurs above the seat clamp (just an thought not a quotable prediction). If that was the case then designing the flex into the post for when it is exposed would be more practical than trying to design for flex inside the frame.

However this could still be a clever way to reduce the amount of reduction shim used when putting a smaller OD seat post into a stiff large ID frame seat tube.

I think this is right, and the issue is, if Ive understood it right, in order for this type to design to work the conventional sp collar needs to be less clamped to allow the post some flex. Imagine the "beer glass" held the white joint absolutely solid at this point and youd get no flex out of the red part of the "seatpost"...Id imagine this very requirement would create stress concentration on the frame at this point, as well as (Im guessing here) seat post slipping issues.

***edit**think Dozer said this 2 hrs ago better than me but I missed it! ****


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 3:28 pm 
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Joined: Sat Jun 02, 2012 10:43 am
Posts: 66
Leviathan wrote:
***edit**think Dozer said this 2 hrs ago better than me but I missed it! ****


I was just 7 time zones and 3 beers ahead of you, so I got lucky at the line. :thumbup: :beerchug:


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 14, 2012 3:31 pm 
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Joined: Sat Jun 02, 2012 10:43 am
Posts: 66
ergot, your idea is sound. But, just make the whole seat post flex, not just the seat post. It just needs to have a pinned joint (pinned means it can rotate freely, like an axle) where the seat tube, top tube, and seat stays meet.

Isn't Trek or Specialized doing this exact thing on a bike they just released?


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Mon Aug 27, 2012 7:01 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
Dozer wrote:
.....Specialized doing this exact thing on a bike they just released?

http://www.youtube.com/watch?v=ipjuWSTMDkA&feature=relmfu 4:08 - 4:14


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 28, 2012 12:46 am 
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Joined: Sat Jun 02, 2012 10:43 am
Posts: 66
No, not that. That Cervelo video shows a standard frame during a simple strength test. The seat tube and top tube are fully constrained with each other.

It's the Trek Domane IsoSpeed decoupler I was thinking about.

http://www.youtube.com/watch?v=yel3yhN_vy4


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 28, 2012 1:32 am 
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Joined: Thu Aug 16, 2007 3:16 pm
Posts: 617
just buy a thud buster


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Tue Aug 28, 2012 11:45 pm 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
Dozer wrote:
...

Kai-ming, it appears we are "neighbors" as I am in Guangxi right now. If you have never ridden near Guilin, you should given the chance...the scenery is spectacular.....

Done a ride there in 2004, here is my report:- http://forums.roadbikereview.com/genera ... -1571.html
Still go cycling trip in China from time to time. Guangxi is one of my favourite provinces.


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Wed Aug 29, 2012 1:16 am 
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Joined: Tue Apr 18, 2006 3:03 am
Posts: 1882
Location: Islip, NY
I drew the titanium collar and the sleeve of the seatpost to fit as a pivoting joint. The collar would be two piece and it would wrap around the sleeve once installed.

_________________
www.ergottwheels.com


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Wed Aug 29, 2012 1:47 am 
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Joined: Mon Dec 03, 2007 12:53 pm
Posts: 94
ergott wrote:
I'm surprised the Trek IsoSpeed concept hasn't been brought up yet.
http://www.youtube.com/watch?v=yel3yhN_vy4" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Here are my suggestions for improvement that would use a similar concept on any bike.

The seatpost should attach to the bike at the clamp with a ball/socket interface instead. The bottom of the seatpost beam can have a bushing to match the seattube inner diameter. This would make an inexpensive bushing a wear item and not the frame. The distance between the bushing and the ball/socket interface would be a set length. The beam can be made rather thin below the ball/socket.

Then you could use a seatpost topper similar to Trek where you have a mast sticking up (the flexible beam) that is cut to size and a topper similar to those used by ISP frames go on top of that. This way you have adjustability without interfering with the design that attaches to the bike.

I'm no engineer, but this makes sense to me.

Image
ergott wrote:
I drew the titanium collar and the sleeve of the seatpost to fit as a pivoting joint. The collar would be two piece and it would wrap around the sleeve once installed.

I think your idea is an improvement to my design. I would probably buy one if you put that into manufacture provided the asking price not too expensive :beerchug: .


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 Post subject: Re: Kai-ming's seatpost
PostPosted: Wed Aug 29, 2012 10:20 am 
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Joined: Sun Jul 10, 2011 9:03 am
Posts: 292
The key to the concept lies in constraining the seatpost at two points allowing it to bend along its entire length. If you go back to Kai-ming's original and redesign the upper collar so that it has an elastomer lining that would provide ample play. Rather than designing the lower part of the post to do the major part of the bending just design the whole post to be a bit more flexible than usual. You could tune the flex somewhat by using two lower collars both independently and adjustably positioned on the post.

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 Post subject: Re: Kai-ming's seatpost
Posted: Wed Aug 29, 2012 10:20 am 


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