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Help - Welding / drilling seat post to frame

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Help - Welding / drilling seat post to frame

Old 11-12-23, 05:00 AM
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Help - Welding / drilling seat post to frame

Hi all,

struggling to fit a seat post to this frame.

It’s part of a personal refurb job… the frame has been cut too short so cannot use a clamp.

FYI It’s only for light use…

Other than drilling the frame and seat post to put bolts through at different positions, or welding the seat post to the frame… does anyone have any suggestions ??

Thanks for any help.
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Old 11-12-23, 06:52 AM
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Pictures please…
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Old 11-12-23, 07:09 AM
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Well, he can't post pics in a thread, but if he posts them in a gallery on his profile, we can move them here.
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Old 11-12-23, 09:10 AM
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There were a few vintage bikes built which used a seatpost with an internal expander bolt, sort of like used for old adjustable quill stems (before aheadsets). Either search and find a suitable size/length expander seatpost or modify an existing seatpost to work with an expander.

Welding together is a bad idea, especially if dissimilar frame/seatpost metals. Epoxy might be a better one time semi-permanent technique but loosing ability to ever adjust height is just a bad idea. If frame were steel, it should be possible to slot the top of seattube and add a binder bolt however short the seattube were cut.
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Old 11-12-23, 11:01 AM
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Originally Posted by GrayJay
There were a few vintage bikes built which used a seatpost with an internal expander bolt, sort of like used for old adjustable quill stems (before aheadsets). Either search and find a suitable size/length expander seatpost or modify an existing seatpost to work with an expander.

Welding together is a bad idea, especially if dissimilar frame/seatpost metals. Epoxy might be a better one time semi-permanent technique but loosing ability to ever adjust height is just a bad idea. If frame were steel, it should be possible to slot the top of seattube and add a binder bolt however short the seattube were cut.

This!
I've seen (long ago, forgot where) a repair with a machined insert that is bonded in the seat tube. This insert extends above the top tube enough to have it's own binder and slot. The replacement seat post is sized to fit the new insert (or other way around, you get the idea). Not hard to do if you have a lathe. Expensive if you don't Andy
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Old 11-13-23, 04:31 AM
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I posted this seat tube from Nate Zukas in the last thread about this issue. If you have the skills to do this, it is the best solution to the problem.

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Old 11-13-23, 04:52 AM
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Originally Posted by unterhausen
Well, he can't post pics in a thread, but if he posts them in a gallery on his profile, we can move them here.
Hey,

I have uploaded the frame and issue to my gallery if you can copy across please?

it’s an old We The People BMX

Thanks!
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Old 11-13-23, 04:55 AM
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Originally Posted by dsaul
I posted this seat tube from Nate Zukas in the last thread about this issue. If you have the skills to do this, it is the best solution to the problem.
What about running bolts directly through the outer frame and inner seat post ? Would this work just as well?

I don’t have the tools to do the above.

(it doesn’t need to be pretty)

thanks
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Old 11-13-23, 06:53 AM
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Originally Posted by Benjwg
What about running bolts directly through the outer frame and inner seat post ? Would this work just as well?
Would but also weaken seatpost a bit. Maybe not enough to worry about, but then...

Originally Posted by Benjwg
I don’t have the tools to do the above.
Neither do I but it sure is a neat solution to a fairly frequent problem! My '70's era MB GR's seatpost clamp shows its age and results of my efforts to get it secured. Clamp ears are rather close together now after the surrounding metal they're attached to has stretched. At this point there's still enuf clearance between 'em to keep post secured. If I keep riding it I may have to resort to taking a die grinder to the ears' inner edges to bring back a little more space.

Here's your pic BTW:



Looks like that frame's seen some bodging for repairs? Evidence of brazing showing, besides the rough texture after attempts at smoothing out fillets?

Last edited by spclark; 11-13-23 at 06:58 AM.
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Old 11-13-23, 08:46 AM
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Definitely taken some bodging to get the old seat post out.

aluminium post in a steel frame corroded so a combination of heat, cold, drilling and cutting to set free. Work to do to neaten this up.

What about some kind of clamp used lower down the frame to tighten around the frame and post - any ideas if this is possible ?

Thanks!
thanks!
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Old 11-13-23, 09:24 AM
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Originally Posted by spclark
Would but also weaken seatpost a bit. Maybe not enough to worry about, but then...



Neither do I but it sure is a neat solution to a fairly frequent problem! My '70's era MB GR's seatpost clamp shows its age and results of my efforts to get it secured. Clamp ears are rather close together now after the surrounding metal they're attached to has stretched. At this point there's still enuf clearance between 'em to keep post secured. If I keep riding it I may have to resort to taking a die grinder to the ears' inner edges to bring back a little more space.

Here's your pic BTW:



Looks like that frame's seen some bodging for repairs? Evidence of brazing showing, besides the rough texture after attempts at smoothing out fillets?
Find someone with TIG or braze rig, it would be all of a 2minute job that you can trade for a 6pack to make a good repair. Does not need to be an experienced frame builder, most anyone with welding/brazing skill should do fine. Once the binder boss is attached, use a hacksaw with blades doubled-up or a cutoff grinder wheel to cut through the binder boss and form the slot. The zukas mid-tube slot could work but if you still have a rear slot below the cut then that would probably be easier.
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Old 11-13-23, 01:30 PM
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I would slot it below as shown above and use a front derailleur clamp to tighten it.
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Old 11-13-23, 02:09 PM
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Originally Posted by Canaboo
I would slot it below as shown above and use a front derailleur clamp to tighten it.
Or a couple of pipe clamps.
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Old 11-14-23, 03:08 AM
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You could probably braze or weld a pinch bolt on the back where the slot is? I make these on a minilathe: get some round bar, drill and tap it to M6, weld it on, then cut it in half in place, then drill out one half of the threads so the bolt just goes in and threads on the other half.
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Old 11-14-23, 07:18 AM
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Originally Posted by guy153
You could probably braze or weld a pinch bolt on the back where the slot is?
That'd work as long as whatever's left of the slot – that we can't see in that pic – is long enough below where you'd fix that pinch bolt bit.

There's a lot of metal in that intersection that'd resist pulling a gap in back much closer together.

Looking at the bodgework done on that tube joint I'd:

1) Find a piece of steel pipe close to the ID needed;
2) Cut a piece maybe 1-1/2" long;
3) Braze (or weld) that onto the cleaned-up stub above that joint.

Then do what you suggest: add a pinch-bolt above the new weld. Lastly cut a new slot into extension tube and pinch bolt with a thin cut-off wheel.

Or maybe just square off that stub end, weld one half of one of these on top, go ride:


Last edited by spclark; 11-14-23 at 07:27 AM.
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Old 11-15-23, 09:16 AM
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Originally Posted by spclark
That'd work as long as whatever's left of the slot – that we can't see in that pic – is long enough below where you'd fix that pinch bolt bit.

There's a lot of metal in that intersection that'd resist pulling a gap in back much closer together.

Looking at the bodgework done on that tube joint I'd:

1) Find a piece of steel pipe close to the ID needed;
2) Cut a piece maybe 1-1/2" long;
3) Braze (or weld) that onto the cleaned-up stub above that joint.

Then do what you suggest: add a pinch-bolt above the new weld. Lastly cut a new slot into extension tube and pinch bolt with a thin cut-off wheel.

Or maybe just square off that stub end, weld one half of one of these on top, go ride:

One of those would be perfect for a slot below the cluster. They even come in double. I had not seen these before. Not sure if the right size would be available.
They look like a nice clamp for normal use too.
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Old 11-15-23, 10:21 AM
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I use shaft clamps on a few of my building and alignment tools. Do know they are pre made in a limited range of inside diameters which need to be very close to what they will be slid onto. There are a wide variety of other aspects, number of splits/bolts, width, materials. I get mine from McMaster-Carr shaft clamps | McMaster-Carr. Andy
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Old 11-15-23, 06:11 PM
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Originally Posted by Canaboo
One of those would be perfect for a slot below the cluster. They even come in double. I had not seen these before. Not sure if the right size would be available.
They look like a nice clamp for normal use too.
They are, and if need be order a size slightly smaller than the OD of the tubing you'll me marrying 'em to. The ID can always be machined larger (for a payment for the work to be done of course) but making them smaller just adds to the weldment / braizng fillet needed to make 'em fit.

Advantage over a pinchbolt being you're using a full-circumference clamping surface rather than pinching a thin-section tube to sustain the friction needed to keep a seatpost in place. There's a lot more metal behind most split shaft collars than in the typical downtube.
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Old 11-16-23, 11:39 AM
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Shaft clamps are a good idea. McMaster has them in 1 1/8", but I suppose the paint would add some thickness.
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