Hello,

I have a old road bike, that has a welded seatpost clamp. However the previous owner had installed a too small seatpost, causing that the clamp is almost cracking apart. There is maybe 6-7mm space left in the seattube, if I filed the broken clamp off. I was thinking if its possible install a modern seatpost clamp there? Or if someone has gone through this, what other solutions are there?

Thanks!!

henrik: It is certainly possible to do, as clamps are available in a wide variety of sizes. Tou might have to do some machinist work as they need to clamp the frame tube evenly. Bepending on how cleanly you remove the broken clamp you might need the next size up to fit onto the tube. You will also need to get the tube fairly round so that the clamp will bear evenly upon it. You might also need to enlarge and/or lenghten the slot. It should be doable, though.

Your seat tube is most likely 1-1/8" (28.6 mm) outside diameter and there are lots of seatpost clamps that size available. You will have to have enough seattube sticking above the top tube to give the clamp a place to fit. At worst, a thin stainless steel hose clamp may be usable.

There need to be a inch + of seat tube squared off , above the top tube
to use a separate seat post clamp.. 6~mm is insufficient


major frame modification, you might braze another tube in
with a 1" or 7/8" inch ID, fit it with a pinch bolt ear
then you can use a 25,4mm or BMX 7/8: straight tube seat post,
and fit a separate saddle clamp on to of it

Half right. While 6-7mm is probably not enough, you don't need an inch either. 1/2" free height is plenty, and you can get by with less in most cases.

All the action of a seat post collar happens at the back, so all you is enough for the clamp to wrap around the front and stay put. Since clamps are typically 1/2" wide, then all you need is that much cylindrical extension above the weld or braze at the front. You have to be sure that the clamp fits low enough that the tube comes to or very close to the top of the clamp, otherwise it can cam and work upward when tightened.

There is no need to extend the slot, but you do have to have the right ID collar to match the seat tube, and the back area where the ears were welded has to be filed down to the round contour of the tube. If you have a lugged frame it's more complicated because it'll be a non-standard outside diameter, and it'll take more work to establish the cylindrical shape needed. and work upward when tightened.

In a borderline situation, like only 8-10mm workable height, it might be possible to carefully file away from the front of the clamp to create a saddle that fits over the weld, fillet or curve of the top tube. Obviously this will weaken the clamp, but it's at the area of least stress so there's some wiggle room to work with.

If you do file away significantly from the front, you can lessen the chances of the clamp breaking by gluing it to the tube. This will transfer some of the tension to the seat tube easing the stress at the front.

1/2" is 12.7mm 6 is not much. you may have Scrap. a parts source to strip.