# Seattube, will this work or will it not?



## mikael_nr1 (Dec 17, 2005)

Hi. I have been lurking this forum for some time seeing quite a bit of discussions regarding seattubes. I am abit confused to say the least, I have also seen some of you experienced builders telling newbies that they have welded theeir seattube the wrong way.

So, my question. Is it the fact that some people weld on the thin part of a butted tube that's the problem when joining Seatstays and top tube to seattube?

And would a completely un-butted tube work good as a seattube? Thinking like 34.9 outer diameter and 31.6 inner?


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## Walt (Jan 23, 2004)

*This seems like a good FAQ topic*

We do get this question all the time. I'll get the ball rolling, feel free (anyone) to jump in, let's try to do a comprehensive answer.
*
So you want to pick a seat tube!*

I'm just going to address *steel* here, but the basic principles are going to be roughly the same for other materials as well.

First, keep in mind that the seat tube of a conventional bicycle frame sees some serious stress - your fat butt in the saddle, hitting bumps at speed, puts a lot of force into the top portion of the seat tube, where the seatpost inserts, and when you start cranking on the pedals, the bottom of the seat tube (where it's attached to the BB shell) also experiences some big-time strain.

**So both the top (seatpost area, and generally where the toptube and seastays join on most frames) and the bottom (BB shell area) of the seat tube need to be really strong.**

This means you want (for steel) at least .9mm thickness or so at the bottom and usually a bit thicker than that (anything from 1mm to 2mm is pretty common) at the top. The top portion needs to be thicker mostly because you need to fit a seatpost into the tube, and to make that happen smoothly, you'll need to do at least a little bit of reaming of the seat tube after you weld/braze everything together, because no matter how good you are, the seat tube won't end up perfectly round when you're done.

As an aside, if you want to be able to ream your seat tube, and don't want to spend hundreds of dollars for a fancy hone, get one of these:
https://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1699920&PMT4NO=0

I have reamed probably 400 frames with mine - they can be easily adjusted to different sizes, are easy to sharpen with a bench grinder, and only cost $40. Don't bother buying a fancy handle for it - just weld on a short piece of old scrap tubing, then stick a slightly smaller diameter tube through to serve as a handle when using the reamer.

*So what are the options?
*

*Option 1 (best for beginners): Buy a single butted 28.6mm tube* (.9mm at the thick end, .6mm the rest of the way) like the RC2ST from True Temper/Henry James. Every manufacturer makes something equivalent, so if you prefer Columbus or Reynolds or Fairing, go for it. This will accept a 27.2mm seatpost in the end, which is most likely what you're going for.

These seat tubes have been around forever - they were originally used for *lugged* frames, where the top .6mm portion of the tube would be surrounded/reinforced by the seat lug. In most cases, mountain bikes no longer use lugs, though, so for these tubes to be usable, we have to simulate that situation.

So get your hands on some 1.25"x.058" 4130 from Wicks Aircraft or elsewhere. This will be a nice slip fit onto your seat tube. You'll want to make a sleeve out of this material that will cover the top 2-5" (depending on your design and preferences) of the seat tube. Here's a picture of what I'm talking about:
https://2.bp.blogspot.com/_5ZOmYbeW-EM/SZtv9fzDw0I/AAAAAAAABLw/zOLJk3mpQZI/s1600-h/IMG_1377.JPG

The sleeve will need to be brazed onto the seat tube - if fillet brazing the bike, do this first (before joining the toptube and seatstays) and if doing TIG, do it last (after welding everything else) so as to avoid contaminating your TIG joints with errant brass/silver/flux.

This method is time intensive but produces a strong, attractive seat cluster that requires (because of the relatively thick walls once the sleeve is installed) relatively little reaming.

*Option 2 *(somewhat lighter weight, less work, but more reaming): Buy an *externally butted seat tube* of an appropriate length. Many companies make these - the easiest to get and use for a beginner are probably the CMST series from Nova Cycles. These tubes are double butted (that is, they're thick at both ends and thin in the middle) and you'll need to select a length that allows you to leave plenty of thick section at both the top and bottom. You should NOT be doing any welding/joining closer than 1" or so from the end of the thick section of tubing!

With an externally butted seat tube, you'll simply cut/miter your tube, then join everything as normal. The top portion of the tube wall is typically 1.1 or 1.2mm thick, so this option will resist distortion less well than a sleeve. You'll need to be ready to do more difficult reaming to make your seatpost fit well, and if your heat control is very poor, it's possible that you'll distort the tube enough that you can't ream it at all (this is rare, though - if you can competently execute the other joints on the frame, you are most likely fine)

*Option 3 (very light, not beginner friendly): Exotic/lightweight seat tubes*

If you're reading this to learn how to build your frame, don't even consider these, but there are some high-end seat tubes that are very thin walled and intended for (mostly road bikes) professional builders who are looking to do lightweight frames. True Temper's HOXPLATST1 (no longer made, but I have a stash) for example is .75/.5/.65 butted. It's VERY hard material and practically un-reamable, so you need to be good at your joinery to make sure that you can actually get a seatpost into the frame in the end. Other manufacturers make similar tubes. Generally speaking, they are not used for mountain bikes, but much more often for road bikes.

Hopefully that's helpful!

-Walt


mikael_nr1 said:


> Hi. I have been lurking this forum for some time seeing quite a bit of discussions regarding seattubes. I am abit confused to say the least, I have also seen some of you experienced builders telling newbies that they have welded theeir seattube the wrong way.
> 
> So, my question. Is it the fact that some people weld on the thin part of a butted tube that's the problem when joining Seatstays and top tube to seattube?
> 
> And would a completely un-butted tube work good as a seattube? Thinking like 34.9 outer diameter and 31.6 inner?


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## pvd (Jan 4, 2006)

If it works, it works. We all have different views on how to get it done but in the end it has to allow functional use of a seatpost and not fail. The rest is up to you.

One important consideration is ending up with a post size that is widely available and produces the proper ride quality. In most cases 27.2mm fits the bill perfectly. You can go larger, but you start sacrificeing ride quality for strength but you also get less choice.


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## Blaster1200 (Feb 20, 2004)

From a beginner's standpoint, that's good advice Walt!
I've completed two frames using option 1 that I've been fairly happy with. I did have to ream the tube, but I also used a fairly inexpensive adjustable reamer (mine was $32 from a local tool shop).

It may be worth noting that when calculating the I.D. of the seat tube there should be a about .15-.2 mm tolerance. For example, a 28.6 mm seat tube with a .6 mm wall has an I.D. of 27.4 but is used with a 27.2 mm post. I've seen other examples where the tolerance is as low as .15 mm.


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## mikael_nr1 (Dec 17, 2005)

Walt, THANK you very much. I did search but couldn't find such informative reply, now you've got that area covered too. Many thumbs up.

I am planning my first ride and I do want to use a large diameter seatpost, 31.6 or 30.0 is what I'm planning to use, I think it'll be 30.0 because I've already got tools for reaming that ID.

This will be my first frame and I do not really care about weight this time, I think I'll just use a standard 34.9mm unbutted tube.

I will try and make some drawings then I'll probably ask more beginner questions along the way, help is greatly appreciated.

/ Mike


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