Stormy Archer

I have a Peugeot PGN-10 which has an expansion plug, quill stem style seatpost like this. Is there a good and easy way to modify the frame with a clamp? I've actually seen a few that have a brazed on like a water bottle boss and a tiny screw in the downtube, which seems pretty iffy to me.
I had the idea of cutting a small vertical slit in the downtube, and then using a hinged clamp around it. Good/bad idea? Any other ideas?

dsaul

You're gonna want to cut that slot in the seat tube, not the down tube. That method has been used successfully by some frame builders, who braze a clamp onto the slot. If you can find the proper diameter hinged clamp, it should work. Put the slot on the front of the seat tube to keep dirt and water from being thrown into it by the rear tire.

dsbrantjr

If you decide to slot the seat tube, drill and debur a hole, larger than the slot width, where you want the slot to end. Then cut the slot down to the hole, which will avoid a stress riser at the bottom of the lot.

Stormy Archer

Thanks guys!! right, seat tube Does anyone have any pictures or more guidelines to go on? Thanks.

Stormy Archer

To be clear, my situation is like this. I am looking at making a slot BELOW the seatstay and top tube.

Fissile

This is how it was done on the fillet brazed Schwinns from the factory. Slot in the seatpost tube and clamp-on binder. No brazing or welding required.

WizardOfBoz

Not sure that a closed slot that opens below the frame joint gives you the flex needed to insert a tube AND secure the tube. Might work, though.
But also not sure that you have enough space below the joint to have the "ears" brazed on.

What is the objective? The original seatpost is strong and rigid (one negative: if you tighten the seatpost too much you can bulge the seattube). Because of the seatpost design, the lines of the frame are (IMHO) clean and attractive. What is the motivation for significantly modding a vintage frame?

If you do decide to do this, I'd suggest that you drill and deburr (inside and out) TWO holes, one at each end of the slot. To ensure no stress risers, you'd probably want a machine-cut slot, too. That might be difficult to machine.

fietsbob

What is wrong with the stock post ? looks fine...
, the alternative a plain tubular post with a long stem bold running through it.. pulling same sort of wedge.

WizardOfBoz

If weight is the issue, you could use a tapered plug rather than a wedge. But I think that the wedge has better holding power.

fietsbob

Or saw the post tube at an angle, near the bottom , add a star-fangled nut iside it long bolt, head @ the top,,,

HillRider

Looking at the photo of your frame, I don't think you have enough of the seat tube/top tube lug extending above the top tube to allow for a clamp. I.e., there's nothing for the clamp to go around.

Stormy Archer

Since it makes the seat tube bulge anyway, I think a clamp probably would work, even without the slot especially if i get a really nice fit seatpost, unlike the original.

My problem with the seatpost are as follows:
I do not need the setback
The clamp for the rails is spaced a little bit tighter than normal, so carbon rail saddles are kind of questionable as it pulls the rails inward toward each other
It's a royal PITA to adjust height
The seatpost that came with the frame is a 26.4 but It's loose and I've tested a 26.6 to fit perfectly
Because the seatpost is small, I have to tighten it more causing bulge in my seat tube and exacerbating the problem
Also worth mentioning, the frame was kind of a rescue, I have nothing even close to the original parts besides the seatpost and the paint is a bad respray.

dsaul

Nate Zukas has done this successfully on several of his custom frames.

HillRider

I guess this could be done to the OP's frame but unless he is very attached to this bike, the machining, brazing and repaint costs would probably exceed the value of the bike.

dsaul

Meh... In the OP's case, a dremel to cut the slots and a hinged or 2 piece clamp will work fine with no paint loss.

grizzly59

I would leave that alone. There isn’t enough tube.

FastJake

You don't have enough seat tube sticking out to use an external clamp. What does the seat tube look like from the back? Those fast-back seat stays might make it more difficult to add a slot and seat binder. Though you might be able to drill through them and add a nut and bolt that way, which is how many bikes came from the factory.

On a frame that's already been repainted it's worth doing if you intend to keep it. Quill seatposts are probably the dumbest seatpost idea ever.

Stormy Archer

You schmucks need to read the thread I'm fully aware I don't have enough tube above.

Thank you for this!! It may not have a frame builders blessing but at least now I know it theoretically could haha. Really appreciate it, google couldn't have found this in a million years.

WizardOfBoz

That is a really sweet design. I never would have thought of it cuz, well, seat tube clamp bolts go BEHIND the seat tube. The Zukas design is cool! Good luck.

revcp 

That is brilliant, and it's part of what I love about bicycles, the endless mods and tinkering.

Stormy Archer

Hi guys. I am bumping this thread because I think I'm going to do this. I want to ask opinions about positioning, the length of the slit, and the stress relief. I have long point lugs on my bike, so will have to position it lower - unless It's better to do it off to one side? I'm kind of doubtful that I can do horizontal cuts, so was planning on drilling 4 holes big enough to give some stress relief and kind of connecting them with the slit.

Another question - is this clamp OK? Is it a problem that it's kind of narrow, or will the load theoretically be spread thru the tubing in the whole section of the cut?



Don't ask me why my bike is in the bathroom....

dsaul

For a normal seat tube slot, I make them about 35mm long. You could probably get away with less, say 30mm. I would go as high as you can, probably about 5-10mm below the lug point. Horizontal slots will help the tube to be able to close up on the seat post, it shouldn't be any more difficult than cutting the vertical slot. The clamp you have doesn't have much room to close any further, so it may not close enough to clamp the seat tube. You may want to find something with more of a gap or try to open the gap on that one with a file or the dremel.

Stormy Archer

Ok. I think it will be difficult to cut horizontal slots because the body of the dremel will prevent the cutoff wheel from cutting perpendicular to the seat tube. I think I might wait today and get some more opinions.

Stormy Archer

Also wondering about this picture: Is there a reason why the slot is so wide? should I aim for that? It looks like almost 2mm wide. Maybe wider on the horizontal slots.

dsaul

It just needs to be wide enough that it doesn't close up completely before the seat post is clamped sufficiently. You could start with a narrow slot and open it up more if the sides touch before the post is held securely.