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Old 05-09-07, 07:09 AM   #1
juan162
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Long Seatpost and Flex

Hi All,

I'm trying to change the position of my seat for and aft, so I bought a Brompton saddle adapter pin. If you don't know what that is, look here:
http://www.foldabikes.com/CloseUp/bike/sapMain.html
I excitedly attached it to my seatpost, but when I got on, I noticed flex in the seatpost. I understand the physics behind this...I'm effectively making a cantilever, which puts much more shearing force on the seatpost. I sort of already know the answer to my question, but should I be worried about the flex? The follow up question is, how can I get rid of the flex?

Juan

Last edited by juan162; 05-09-07 at 07:22 AM.
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Old 05-09-07, 08:15 AM   #2
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Interesting. The website to which you linked says it "... works well even for very heavy riders of up to 300 pounds if adjustments are made by using the stronger seat post options and by reinforcing a portion of the seat pillar with a close fitting insert." Helpful, eh? I guess it depends on how strong your seatpost is, how far it's extended, how much you weigh, &c.
In my limited experience, a seatpost is most likely to bend just above the point where it enters the frame, making it very difficult to get the bent and distorted post out of the frame.
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Old 05-09-07, 09:29 AM   #3
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If the saddle pin helps with positioning, I'd think you're better off reinforcing the seatpost, which will make your bike heavier of course. But heavier is better than messing around with the consequences!
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Old 05-09-07, 09:56 AM   #4
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Juan:

I flexed the seatpost on my DT a little, and it made it difficult to extend the seatpost up and down. I reinforced it by inserting a wooden dowel (fashioned from an old broom handle) into the hollow core of the seatpost. Didn't add too much weight and I feel much more confident on the saddle now.
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Old 05-09-07, 09:13 PM   #5
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SesameCrunch,
I think I might try a similar solution. For anyone that is wondering, I bought one of those 16" Kent folding bikes and am trying to get the saddle more aft. If I can get that dialed in, I think it is worth it for me to do some other, inexpensive upgrades. I am not too worried about adding a couple of pounds to the bike, even though I might be carrying sometimes.
Thanks All, Juan
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Old 05-09-07, 10:11 PM   #6
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Juan

Can't a Setback Thomson seatpost solve your seatpost problems?
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Old 05-11-07, 09:12 AM   #7
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Juan

Can't a Setback Thomson seatpost solve your seatpost problems?
Unfortunately, all the layback seatposts are too short.

Juan
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Old 05-11-07, 11:41 AM   #8
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Quote:
Originally Posted by juan162

Quote:
Originally Posted by caotropheus
Juan

Can't a Setback Thomson seatpost solve your seatpost problems?
Unfortunately, all the layback seatposts are too short.

Juan
Even if you could get a setback seatpost, I don't see how that solution would actually be different from that offered by the Brompton saddle pin adapter. Either way you're going to place your weight in a different place in relationship to the frame, meaning stresses on the frame that it wasn't designed for; and the effect will be more pronounced if (like me) you have your seatpost at or above "minimum insertion." Maybe your seatpost can handle it; maybe not. Maybe your frame can handle it; maybe not. How to tell?

It would be nice if we could look all this up in a database of stresses known to cause failure in different frames... I wonder if ANY manufacturer has that kind of thing.
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Old 05-11-07, 04:23 PM   #9
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Rhm,

I agree with you. I think my best bet is using the Saddle Adapter Pin with the original seatpost and somehow reinforcing the seatpost. I'm going to start with something like what SesameCrunch has done with wood. If it seems to work well, I will either leave it that way or have my bro-in-law machine a steel or aluminum reinforcement to serve the same purpose. I don't want to bother him until I've tried it in wood and have more confidence that it will work out.

As far as taking a risk and experimenting, with myself as the guinea pig, well, I'll give it a go. It's not like I'll be using the bike for downhill racing and I do wear a helmet. If it works with the wood, I probably will eventually have my bro make the steel or aluminum version.

Juan

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Old 05-12-07, 07:26 PM   #10
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The adaptor pin is designed to reduce the folded size by positioning the saddle further forward. Putting the saddle further back will increase the bending moment applied to the seatpost.

The telescoping seatpost uses a thicker wall and I believe the extended seatpost does also, both of which will minimise flex. There is supposed to be a factory option (rarely mentioned) for stronger seatposts for those who bend Brompton posts.
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Old 05-12-07, 10:08 PM   #11
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Quote:
Originally Posted by LWaB
The adaptor pin is designed to reduce the folded size by positioning the saddle further forward. Putting the saddle further back will increase the bending moment applied to the seatpost.

The telescoping seatpost uses a thicker wall and I believe the extended seatpost does also, both of which will minimise flex. There is supposed to be a factory option (rarely mentioned) for stronger seatposts for those who bend Brompton posts.
I know this already, but I'm 'experimentin!'

Juan
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Old 05-15-07, 10:57 AM   #12
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Quote:
Originally Posted by juan162
I know this already, but I'm 'experimentin!'

Juan
Hey, wait a sec', if you're up for another spearmint, why not get an old banana seat, complete with the stuts that support it? That way you can distribute the weight anywhere you want it. And it would look sooo coooooool, you may want to avoid certain neighborhoods while riding it. Oh, okay, never mind.
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Old 05-15-07, 05:55 PM   #13
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Quote:
Originally Posted by rhm
Hey, wait a sec', if you're up for another spearmint, why not get an old banana seat, complete with the stuts that support it? That way you can distribute the weight anywhere you want it. And it would look sooo coooooool, you may want to avoid certain neighborhoods while riding it. Oh, okay, never mind.
RHM,
I know I get looks when I ride my folding bikes as it is. I think the banana seat might be the one step too far. It would solve my setback problems, though. I wonder what that banana seat would feel like in an aero riding position...ouch!
Juan
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