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Old 08-19-12, 11:40 PM   #1
kamtsa
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How to spec a custom long aluminum seatpost?

Hello mechanics,

My Brompton bike (monotube folder, 16" wheels) has this stock seat post

http://www.sjscycles.co.uk/brompton-...sp0p-prod3014/

http://www.nycewheels.com/brompton-colors.html

The stock seatpost is made of chrome plated steele and tends to slip down during the ride. I am thinking of replacing it with a custom machined aluminum seatpost with slightly rougher surface.

How should I go about spec'ing the custom seatpost? For example, what material? Any special treatment? What inner diameter Anything else?

Safety is important, weight is less important (though it will be nice to keep the weight reasonable). Also, the seatpost interface with Brompton Pentaclip sadle adapter so I need to preserve the shape of the top end (this excludes most off the shelf after market seatposts).

Thanks,

K.
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Old 08-19-12, 11:46 PM   #2
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I may have an easier solution for you. Most of the time when you cant get enough clamping its because the slot for the clamp has "closed up. Squeeze it together with a clamp and resaw the slot with a hacksaw. This will allow more tension from the clamp to hold your post tight.

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Old 08-19-12, 11:56 PM   #3
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What is the diameter of the seat post you need and the appoxemiate length you need this will help a lot in finding one that is right? If your looking for just strait strength and holding you may want to look into a chro-mo BMX seat post maybe even vintage.
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Old 08-20-12, 12:09 AM   #4
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Quote:
Originally Posted by speedy25 View Post
I may have an easier solution for you. Most of the time when you cant get enough clamping its because the slot for the clamp has "closed up. Squeeze it together with a clamp and resaw the slot with a hacksaw. This will allow more tension from the clamp to hold your post tight.
Thanks Speedy. The stock slot is very wide, at least 3 mm when it is clamped.

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What is the diameter of the seat post you need and the appoxemiate length you need this will help a lot in finding one that is right? If your looking for just strait strength and holding you may want to look into a chro-mo BMX seat post maybe even vintage.
31.8mm, ~500mm total length (400mm above the top of the clamp). I have a friend with a machine shop that can do the job but I need to know how to spec the job for him.
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Old 08-20-12, 12:42 AM   #5
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I reckon I would go with 6061 tubing. 1-1/4 is your OD. I'd have to measure some seatposts to see what ID to get. Then probably get some aluminum rod to press fit and pin it into the tube, then turn down to a decent diameter for the seat clamp. Not sure what that would be off the top of my head.
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Old 08-20-12, 03:50 AM   #6
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Quote:
Originally Posted by kamtsa View Post
Thanks Speedy. The stock slot is very wide, at least 3 mm when it is clamped.



31.8mm, ~500mm total length (400mm above the top of the clamp). I have a friend with a machine shop that can do the job but I need to know how to spec the job for him.
Damn thats one big seatpost you may be able to do 400-450 looking around but 500 is likely a custum job.
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Old 08-20-12, 08:23 AM   #7
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Before spending serious dough making a post which may not solve the problem anyway, try to find out why your post is slipping, and solve the problem at the source.

Thousands of Bromptons have been with the same post and obviously they don't all slip. So what's different with yours? Possibly you can solve the problem with something as simple as cleaning both the post ind inside the frame's clamping area (top 2-3 inches). Otherwise the problem may be a cracked clamp, or ear on the frame, or a bad cam on the QR.

If all is 100% and you still have slippage, possibly because you're heavier than average, here's a cheap fix using the same concept they use to make non-slip stair treads. Buy some carborundum in a medium grit, say 120, and some clear nail polish. You're going to paint a traction band in the 2" clamping area. The beauty of this is that it's a cheap, reversable fix which can save you serious dough.

Start by marking the post above the frame tube when it's at your riding height. Clean the area completely with acetone, or another solvent that evaporates 100% dry. Paint the area with a mix of the paint and grit. There's no one best way to do this, you can dip the wet brush and pick up grit, or you can paint the post and apply the grit into the tacky paint. Once the grit paint is dry and cured (24hrs) you should find you have excellent hold, at a cost of fifteen minutes and a few dollars.
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Old 08-20-12, 11:25 AM   #8
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Originally Posted by FBinNY View Post
Before spending serious dough making a post which may not solve the problem anyway, try to find out why your post is slipping, and solve the problem at the source.

Thousands of Bromptons have been with the same post and obviously they don't all slip. So what's different with yours? Possibly you can solve the problem with something as simple as cleaning both the post ind inside the frame's clamping area (top 2-3 inches). Otherwise the problem may be a cracked clamp, or ear on the frame, or a bad cam on the QR.

If all is 100% and you still have slippage, possibly because you're heavier than average, here's a cheap fix using the same concept they use to make non-slip stair treads. Buy some carborundum in a medium grit, say 120, and some clear nail polish. You're going to paint a traction band in the 2" clamping area. The beauty of this is that it's a cheap, reversable fix which can save you serious dough.

Start by marking the post above the frame tube when it's at your riding height. Clean the area completely with acetone, or another solvent that evaporates 100% dry. Paint the area with a mix of the paint and grit. There's no one best way to do this, you can dip the wet brush and pick up grit, or you can paint the post and apply the grit into the tacky paint. Once the grit paint is dry and cured (24hrs) you should find you have excellent hold, at a cost of fifteen minutes and a few dollars.
Thanks FB. The frame and the quick release release are in avery good shape. A google search for 'brompton seatpost slipping' suggests that this is a common problem, possibly due to the smooth chrome plating. I had good success with Finish Line Carbon Grip but got tired applying it periodically since I move the seatpost up/down on each ride.

At the time the dealer, with extensive Brompton experience, suggested to drill a through hole and insert a support pin. He said he done it himself but I don't want to risk a mechanical failure.

As for your suggestion, will the pain stick to the chrome plating? Any special pain to use? I tried for example to draw a hight marker with permanent marker but it did not stick. Will grit wrapped with thin tape work better?

The custom seatpost will be done by a friend so the cost will be reasonable. Any suggestion how to spec it?
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Old 08-20-12, 12:08 PM   #9
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Quote:
Originally Posted by kamtsa View Post
\....

As for your suggestion, will the pain stick to the chrome plating? Any special pain to use? I tried for example to draw a hight marker with permanent marker but it did not stick. Will grit wrapped with thin tape work better?

The custom seatpost will be done by a friend so the cost will be reasonable. Any suggestion how to spec it?
I've found nail polish and a number of rattle can spray paints to stick to chrome reasonably well. The key is to have a 100% dry, oil free surface. Use acetone (not nail polish remover), or naphtha, to totally strip any oil or wax film then you can mask at just below yout target height so it wont show. Toy might get by with some grit sprinkled on 2" cello packing tape then wrapped around. The grit will cut through and engage both the tube and the post and frame and do it's job.

If you want a better job you can prep the post a bit by gripping medium sand paper around it and twisting a bit while squeezing hard. Unlike most sanding work, the object isn't to get a nice finish, but to leave scratches which will be anchor points for the paint.

Normally I use coarse lapping compound on slipping posts because it's easier, but Knowing your application calls for raising and lowering often, II suggested a painted band that would stay put and not make a mess, by smearing all over the post.

As for making a post, I'd suggest 6061 T^ as about ideal, but you have to know the extended weight to calculate the bending moment. One way to approach it is to take a reliable post like a thomson, measuring the usable length and wall thickness, then increasing the thickness in proportion to the added length (proportional to the bending moment, assuming Thomson got it right), then adding 15% for the jell of it as an added safety factor. Be aware however that the friction properties of anodized aluminum aren't that much better than steel (or chrome) so unless your situation close to marginal, you still may not solve the problem.
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Old 08-20-12, 02:36 PM   #10
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I resolved the slipping chrome seatpost with a second QR, band around the seat post itself..
they are made with a lip around the edge,that usually
sits against the top of the seat tube end, of MTB frames.
I filed off the lip so the band slid over the seatpost,
upside down, that is the flat side..

It sits on top of the frame, both levers open to fold bike,
slide the seat post down, almost as quickly..

pulling up the seat post to height, then sliding the second one
down on top of the frame , and closing the 2nd QR lever
is another brief move.


FWIW,
Brompton makes alternative seat-posts used with their x model
lighter bike offerings..
Aluminum and Titanium.. in the telescoping and standard and extended length versions

Last edited by fietsbob; 08-20-12 at 02:56 PM.
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Old 08-20-12, 02:57 PM   #11
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I like fietsbob's solution. They're usually called seatpost clamps or seatpost collars. , but yeah, get one and file down the lip. I'd probably get a non-QR version personally but that's up to you.

Hit up BMX shops for the widest array of available colors.
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Old 08-21-12, 12:04 AM   #12
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I've found nail polish and a number of rattle can spray paints to stick to chrome reasonably well. The key is to have a 100% dry, oil free surface. Use acetone (not nail polish remover), or naphtha, to totally strip any oil or wax film then you can mask at just below yout target height so it wont show. Toy might get by with some grit sprinkled on 2" cello packing tape then wrapped around. The grit will cut through and engage both the tube and the post and frame and do it's job.
I think I will give my stock seatpost once more chance. Any idea where I can get medium grit powder? I searched on the internet and found only 5lb. Will find sand do the job? Can I scrap it from a sand paper?

Quote:
Originally Posted by fietsbob View Post
I resolved the slipping chrome seatpost with a second QR, band around the seat post itself.
This will actually work for me. Typically I don't push the seatpost down but pull it out, before lifting the bike to the car. In this case, the clamp can stay on the tube.

Quote:
Originally Posted by fietsbob View Post
FWIW,
Brompton makes alternative seat-posts used with their x model
lighter bike offerings..
Aluminum and Titanium.. in the telescoping and standard and extended length versions
Are they still available? Could not find any. Nyce lists them as available but I tried to order twice in the last year and it was not in stock.
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Old 08-21-12, 01:23 AM   #13
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Nyce lists them as available but I tried to order twice in the last year and it was not in stock.
Order, call on the phone, rather than click on 'add the basket'
and that will get them to fill that order from their sources, even though not maintained in stock.
in anticipation of every possible thing ..
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Old 08-21-12, 08:52 AM   #14
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Order, call on the phone, rather than click on 'add the basket'
and that will get them to fill that order from their sources, even though not maintained in stock.
in anticipation of every possible thing ..
I think that Brompton discontinued the aluminum seat posts. Will give them a call though, just in case.
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Old 08-21-12, 09:09 AM   #15
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FWIW,
Quote:
Any suggestion how to spec it?
I'd leave the tube wall pretty thick.. , bore out ID just to fit another seat post
into, it, for the adjustable head.

or say some 7/8" OD tube.. to continue using the Pentaclip.
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Old 08-24-12, 12:18 AM   #16
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A quick followup, I abandon the idea of having a custom seat post and installed a seat clamp per fietsbob's suggestion. My seat post has no flare at the bottom and I remove it when folding the bike so the clamp simplified the height adjustment. I also moved one of the Brompton o-rings to below the clamp to protect the frame. Overall it works great. Will take it for a test ride this weekend.

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Old 08-24-12, 12:44 PM   #17
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Blast the seat tube with glass bead and put it back in,no more slipping.It will look like aluminum.If it needs to be shiny,use a second clamp on the post.
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