wedge style seatpost

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SimonCelsa
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wedge style seatpost

Postby SimonCelsa » 25 Jul 2018, 10:56am

A pal has just taken receipt of a refurbished Peugeot steel road frameset. I went around for a look this morning to discuss sourcing various parts etc.

The first thing that became apparent was how to clamp the seatpost?

seat lug.jpg


I see there are a few BMX wedge type seatposts available; https://www.ebay.co.uk/itm/Animal-Wedge ... SwjjpbBB5Y

However, I would be grateful should anyone have knowledge of similar, longer, more traditional looking style posts which may fit the bill. I measured the seat tube at 26.6mm internal diameter.

The frame has been nicely finished so it would be a shame to butcher it with an external clamp or try some other modification. Anyway, thanks in advance for any advice,

Cheers, Simon

brynpoeth
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Re: wedge style seatpost

Postby brynpoeth » 25 Jul 2018, 11:06am

Did it have a clamp that was cut off?
One could use a jubilee clip as discussed elsewhere
A wedge style seat post, is that with an expander bolt like a stem?
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Brucey
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Re: wedge style seatpost

Postby Brucey » 25 Jul 2018, 11:20am

BITD a few frames used expander or wedge type seat pins. However even if you find such a thing it is unlikely to be very long, because that wasn't the fashion then. About 200mm would be the longest I think, and bear in mind that for it to be secure, you may need to have more of it in the frame than usual.

Image

Thus you might struggle to find one at all, and even if you do it might not be long enough to suit you. If this is the case then you may as well make your own. This can be as simple as robbing some parts from a quill stem and with a few other odds and ends, plus correct use of drill, hacksaw and file etc, you can come up with a workable solution.

cheers
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SimonCelsa
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Re: wedge style seatpost

Postby SimonCelsa » 25 Jul 2018, 11:23am

I think the jubilee clip wouldn't look so elegant!! I wasn't aware of the expander type seat post either until I was searching this morning - it seems quite popular with the BMX crowd, less to knock your nutz on when a trick goes wrong I'd imagine.

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SimonCelsa
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Re: wedge style seatpost

Postby SimonCelsa » 25 Jul 2018, 11:28am

Yes, home made bodge version is the way ahead I think. I must say, notwithstanding the added hassles of adjusting seat post height, the frame certainly looks sleeker without the external clamp.

fastpedaller
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Re: wedge style seatpost

Postby fastpedaller » 25 Jul 2018, 12:34pm

I guess these need (very?) careful tightening so that a thin seat tube isn't 'expanded'?

Brucey
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Re: wedge style seatpost

Postby Brucey » 25 Jul 2018, 2:30pm

yes; the wall thickness of the seat tube is ~1/2 or even ~1/3 that of a steerer.

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SimonCelsa
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Re: wedge style seatpost

Postby SimonCelsa » 25 Jul 2018, 4:05pm

Good points, the tentative plan is to purchase a straight old style seat post and cut the bottom part off to form the wedge. A length of M8 stud, some flat washers and suitable nuts should see a reasonable prototype. Similar to this here on Retro bike; http://www.retrobike.co.uk/forum/viewto ... &mobile=on. Unfortunately the rather elegant lightweight frame is getting heavier!

Would fitting a seatpost shim lend some extra strength in the wedge area and allow more leeway for hamfisted tightening? He hasn't purchased any parts yet so a slightly smaller seatpost and shim may 'reinforce' the thin tube wall.

Cheers for the comments so far.

GodfreyPurvin
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Re: wedge style seatpost

Postby GodfreyPurvin » 25 Jul 2018, 5:56pm

You'll notice that the Campag post pictured by Brucey (originally for a "binderless" fillet-brazed Moser frame) has a very long wedge - much deeper than those found in stems. This is so that the clamping effect isn't concentrated on a small area of the frame's relatively fragile seat tube. If making a "bodge", the wedge part needs to be the same diameter as the post, so help from a friend with a lathe (you do have a friend with a lathe...?), for whom making a suitable wedge should be a very simple job, should be sought. I would strongly recommend using Finish Line Fibre-Grip (or similar) friction-enhancing mounting paste (from any good bike shop) when fitting the modified post - and not grease. This will significantly reduce the amount of expansion force needed to securely bind the post into the frame. Grip paste also protects against the electrolytic reaction that often results in unwanted, permanent binding of posts into frames. This can be a particular issue with "binderless" frames, as the top of the frame is not pinched together and water gets down the gap between post and frame very easily. For this reason, the post must not be fitted dry. Proceed with caution. Getting this wrong could lead to either a post that slips down into the frame when riding, or permanent damage to the frame.

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531colin
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Re: wedge style seatpost

Postby 531colin » 25 Jul 2018, 6:45pm

There isn't a grub screw somewhere, is there?

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SimonCelsa
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Re: wedge style seatpost

Postby SimonCelsa » 25 Jul 2018, 7:08pm

The grubscrew idea did cross my mind but I have visions of Frankenstein's bolt going through both sides!

I have access to lathe, pillar drill, milling machine and other workshop equipment but unfortunately they are aboard a ship which is currently off Taiwan

The current plan is to use a wedge from a 1-1/8" quill stem, buy a 25.4mm seat pillar and shim for 26.6mm seattube. An old fashioned saddle clamp and suitable length wedge bolt should complete the kit. A bit of permatex gasket sealant or similar to assist with friction would help. Suck it and see, not much else I can think of without easy access to workshop equipment. It'll be good!!

Brucey
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Re: wedge style seatpost

Postby Brucey » 25 Jul 2018, 8:20pm

a variation that might be a bit kinder to the seat tube, but requires a longer insertion depth, is to make a clamp that exploits the high bulk modulus of rubber.

In this design you take along seat pin and make about six vertical slots through the wall that extend from 3/4" to about 3" above the bottom of the seat pin. Inside the seat pin there is a slug of solid rubber about 3" long, that you can compress lengthwise using a long bolt, in combination with suitable spacers inside the seat pin.

The bulk modulus of (solid) rubber is about the same as that of steel; thus most of the 'give' in the system is created by the rubber extruding into the gaps and so forth. With a following wind you can create a seat pin that is quite resistant to loosening and imposes fairly benign loads onto the seat tube.


A length of studding, a tube spacer, locknuts and washers, and a suitable set of rubber bungs (doorstops?) are all that is required. You can cut the slots with a Dremel tool, or, if you only have a hacksaw, you can cut the slots quasi-helically instead. If it doesn't work to your satisfaction, you can probably just chop the bottom off the thing and convert it to a wedge instead.

cheers
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SimonCelsa
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Re: wedge style seatpost

Postby SimonCelsa » 26 Jul 2018, 8:41am

Yes I like that idea but for simplicity, instead of cutting slots I am thinking more of simply cutting the correct sized seat pillar. Insert 2 or 3 suitable diameter rubber bungs / door stops between the cut parts and tensioning with a long bolt, nut and a couple of washers. See what's available on eBay.

Brucey
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Re: wedge style seatpost

Postby Brucey » 26 Jul 2018, 11:00am

that will probably secure the post lengthwise but may not prevent it from rotating well enough. Easy enough to try though. Suggestion; why not cut the end of the seat post so that (once the rubber is bearing against it) it can't so easily turn, and have something just within the seat post that will bear against the rubber parts?

BTW if the seat pin is a good fit, cuts that break out of the end of the seat pin might be OK, i.e. so that the seat pin flares slightly at the end when the bolt is done up.

cheers
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brynpoeth
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Re: wedge style seatpost

Postby brynpoeth » 26 Jul 2018, 2:22pm

Make sure it can still be adjusted and removed later if need be
If you do some complicated shedgineering (+1!) it might be worth putting a sticker on the frame in case someone else dismantles it later
Entertainer, kidult, curmudgeon
Cycling-of course, but it is far better on a Gillott
We love safety cameras, we love life "597"