Please teach me about seat tube collars

Hi all! Probably a dumb question that I’m overthinking, but here goes -

I keep seeing references to and advice about adding collars to seat tubes. I don’t know if I’ve seen a good explanation for the why though.

I understand that the goal is that your finished seat tube has a proper fit with whatever flavor of seat post you choose. I guess I’m wondering when and why a framebuilder would use a collar in the first place.

For my first frame, I purchased this guy - the ID was all set for reaming and it was an easy experience so collars never crossed my mind.

  • Are collars literally just adaptors for when other tubing styles/diameters are too large/small for the final desired ID?

  • Are they meant to provide thicker material for seat stay welds?

  • Are they typically made to match the OD of the seat tube? Or is it on a case-by-case basis?

Thanks for your help!


For me (and for most builders from what I’ve seen), the biggest use case is for mountain bikes where we’re looking for a few things:

  • Dropper post compatibility
  • Ability to easily bend the seat tube without wrinkling
  • Strong!

Dropper posts have mostly solidified around the 30.9mm & 31.6mm diameters which means we need a tube larger than that. The next size up in the imperial system is 34.9mm (1-3/8"). Of course a tube with a 1.65-2mm wall thickness would be overkill so we use 0.9mm wall tube for 90% of the tube and then add a little topper at the end to reduce the diameter to whatever is needed. A straight 0.9mm tube has the added advantage of being easy to bend to whatever angle is required whereas custom butted tubes could wrinkle.

The thicker material for top tube & seat stay welds is a nice bonus! It means there will be less distortion in the seat tube and that joint will be overall stronger.

Almost all weld-on collars are sold to match a specific OD - usually 27.2, 30.9, or 31.6mm.


I think the root of this dilemma is that seat post diameters were the result of the tubes and construction methods at that time. That is why we have such weird sizes: 27.2, 30.9, 31.6, etc… I am not oldschool, so I can’t speak to the history of lugs, brazing, and TIG. I am sure someone else can.

Weld-on collars were a hack to get the OD of the seatpost to match the ID of the seat tube. This was a bigger problem with dropper posts that needed really weird 31.6mm ID’s.

However, tube suppliers have caught up. They make seat-tube specific tubes that have thicker butts built-in:

I think there are a few reasons people still use weld on toppers:

  • The custom-butted seat tubes don’t exist in titanium (yet)
  • The butting profile might not work with really tall or short seat tubes
  • People might not know butted tubes exist

Maybe someone who has been around longer can fill in the blanks of the history. @mark_pmw ?


One addition to Daniels reasons: If you are having pivot points for rockers or shocks on your seattube the usual butted seattubes probably have the thinest wall right where the shock put most force and a heavier non butted seattube with a weld on collar works best. Although I have also milled aluminium or stainless sleeves so I didn’t have to weld on a collar.


So looks like your looking for seat tubes for 31.6 post , which in general is for the dropper post. As mentioned above previously there was no butted options available so the solution was weld in a seat collar into 35mm x .9 tubing.

At BFS we stock multiple options , two are out of stock at the moment but restock will happen soon in June/July

35mmOD /31.6 post light seat tube July restock

Straight 35mmx 1.65/.9 x 600 - for 31.6 post we should have these restocked by end of the month

35mm /31.6 post Bent Seat tubes - lots of options , for butted which requires specific bent location notes builder provided , and stock offered options which require a weldin seat collar , but doesn’t require a pre determined bend location .

Also we do now have a Titanium butted 35mm seat tube for 31.6 post see here we can bend this tube as well , again requires pre-determined bend location


One reason the collars are nice (although it may just be incidental) is you end up with only a couple inches of area that actually ‘bites’ the seatpost, and everything beneath it has extra clearance. That makes it much easier to ream to the right size, and have enough material to grab the post effectively. It also makes it less likely to get stuck / seized up over time.
It’d be fun to get a slammed dropper in and out if the whole length of the seat tube was reamed to a tight fit, as you can imagine.


@Daniel_Y @ben.land101 nailed it with this comment! It’s not a hack, but it was a well thought out way to get relatively small diameter seatposts that were available in the dark ages to fit a HUGE seat tube, like 1-1/8" or 1-1/4" diameter.
There have been a number of old school builders (they will remain nameless to protect the lazy and cheap) who didn’t bother with a butted tube or collar. The frames that didn’t break ended up with the seatpost hopelessly stuck in the seat tube. A collar on a hand built frame has always been the sign of a quality frame that will enjoy a long life.

Fortunately, that’s changing with the availability of decent butted seat tubes.


Thanks Mark! I meant “hack” in the ingenious and creative meaning of the word!

@mark_pmw Do you know when Paragon started to produce seat collars? I think it’s fun to study the fossil record of frame building.

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I guess I’m too old to use “hack” in the current vernacular!

We made seat collars for a few shop-built bikes way back, but as an in-stock catalog item, it’s been about 15 years or so. We never planned to make them, because it’s such a simple part, it was assumed that builders would make their own as needed. Turns out not everyone has a lathe or the time to make them.