what wall thickness is desired for a brass fillet braze? is columbus spirit too thin?

The shaped Spirit with 5/3/5 walls? It's higher level stuff as far as fabrication, you have to really have your skill set refined to pull it off. If you're going to braze it, Fillet Pro (with it's lower working temp) is the way to go.
Btw, like all the light weight materials, there is a rider weight limit for Spirit.

I remember Mike Appel built a fillet-brazed frame from Ishiwata Alpha tubing (also 5/3/5) and was able to ride it for several years before it failed.

You're really pushing the limits with tubing that thin, even if you know what you're doing (which Mike certainly did). Is there a reason you don't want to use lugs?

if you are asking the question
you are not ready

I ask, what value is there is using tubes .5/.38/.5 in your frame?
100 to 150 grams !!!!!!!!!
use .75 or .8 walls
and have some crash resistance
long life
and also it will cope with your skills
Harsh maybe be
but the best path

and I have never used .5/.38/.5 tubes in the main triangle as it serves no useful purpose

don't forget that there is Spirit for lugs, which isn't quite so thin as the shaped stuff.

how and where did it fail? I've thought about building myself a bike with Spirit or TT S3, just because. I would use fillet pro, I assume Mike used brass.

On frameforum I remember reading to stick to 0.7 mm and above for brass brazing.

i like to use .035" straight gauge cro-mo. dirt cheap and no worrying about butt length!

The down tube separated from the head tube.

Yes, it was brass. Fillet Pro wasn't available back then.

"i like to use .035" straight gauge cro-mo."

So do I, but that is kinda the other end of the spectrum. Which is what he should be thinking about...

I got kicked off the other forum for suggesting it, but there is also .028 tubing for those who like what they see in the mirror.

Schwinn built tens of thousands of fillet-brazed frames between 1938 and 1978, and used straight gauge 4130 chromoly tubing with pressed-in reinforcing sleeves at the joints to act as butting. The finished frames were strong and durable, and relatively light (my 61cm Super Sport frame weighs 7 pounds without headset, BB, or kickstand). The wall thickness of the tubing measures a hair over 1mm using a caliper.

I agree with everyhing you said except for:

It does have it's place and it can build into a super light, sweet riding frame...it will just have a finite lifespan if it's ridden by a strong and/or heavier (180#) rider.

I don't know if you have checked recently but 180 is the average weight of women these days, let alone men. If something has a finite lifespan that means it is working past yield (I'm just guessing here), that sounds kinda squishy to me. That doesn't mean it shouldn't be used, but I do wonder about it. Also sounds lkue a pretty poor fortress liability wise.

And the number of bikes built with Spirit tubes you've ridden is....?

I agree with you on the lifespan issue though. I build bikes with longevity/durability in mind and never use anything thinner than 7/4/7 and that's been for women and little guys.
I have, however, ridden/helped fab 5 different Spirit tubed bikes and they were great! Plenty stiff too.

BTW- 180 average for women? Dude, you need to move to a different town!

As a new builder I get most all my knowledge either through trial and error or asking guys like yourself who have more experience. I have built 3-4 frames out of TT S3 luckily without issue yet, I only do fillet brazing, so if we have these thin walled tubes produced and sold that are clearly not for lugs how else can you join them other than fillet-braze with brass? I am not a tig-welder but I have been told that trying to tig this thin stuff is almost impossible and I would think it would damage the tube way more than brass. Also, I have heard of Fillet Pro although I haven't used it but it hasn't been around for as long as the ultra-thin tubing has it? I guess my question is how did the producers of these super-thin tubes intend for us to join them if they aren't made for lugs? Are the S3 frames that I've built just living with a limited lifespan because they are 6/4/6, are they gonna fail just for that reason? I can understand they may fail if they were joined improperly, overheating, etc. but are they gonna fail just because they are thin wall?

Oh yeah, and those are some big girls you're hanging out with Peter Pan.

A fatigue failure does not mean that there are stresses that exceed the yield strength of the tubing. That's the misunderstanding that caused the Brits to lose all those Comet passenger jets back in the '50s, the airframe was built to accept a load far higher than it would ever see in service.

Thanks on the engineering!

On the spirit tubing, I try not to use anything i can't get at home depot. I wish.

Oh come now, TIG guys have had their way with killer thin tubes for years. Not my cup of tea either but that's not to say that you can't build a decent frame from the stuff. Old school ELOS and similar had .4mm center butts, and the metal used wasn't as hard as modern alloys so those frames dented really easily yet lots of people loved them.

OK, fair enough. So if you exclude Fillet Pro, tig welders can tig it and fillet-brazers can use their brass, was there an intended method for joining these thin- walled tubes by the producers of them and just because they are thin-walled does this mean there lifespan is limited no matter how they are joined. If they are joined correctly and the rider weight limit is in effect why wouldn't the frame last? The S3 stuff rides pretty good. My road bike is all S3 and a buddy of mine is riding the first S3 cross frame that I built with no issues yet and he abuses it, granted hes only 140lbs and has only a little over a year on it. I guess I just don't want to believe that the frames I built out of thin-walled tubes are destined to fail.

Depends on the intended application and the strength of the rider too. Strong riders seem to break more stuff then heavy ones do. But you're right, S3/Spirit can make a long lasting frame when used correctly.
FWIW, I would never build someone my size (6'2" 190) a frame with tubes that thin.

thank you for the awesome answers. I havent brazed it yet just was curious. you guys are super awesome.

Yeah, thats what I thought. I believe the rider weight limit on S3 is 190 but I would never use on someone over 180 just to play it safe. I am 6'2 and 165 and it feels pretty good to me. The first time I used it was for a racing buddy of mine and he just wanted to try it on a cross bike just to see what would happen, I cringed at the idea but it has held up thus far and I am hoping that it last forever, we'll see.

Hey Live Wire I was checking out your blog and it looked like you had a little accident. How are you feeling? I would have told them to insert some bionics in my legs then we'll see how this thin stuff really holds up. Twist that stuff up like a Christmas ribbon.

Ouch, was the guy on his phone. I'd say the Doc putting in the screws did a pretty good job, my leg looks worse 10 years after being hit at 160 mph by a 20" tamarack tree.

Speaking of light bikes. Been looking at this bike with seatstays made out of lightweight 3/8" 4130. Don't seem to give the builder any trouble, but that is some light tubes. I wonder what the

Thanks for the thoughts guys. It's getting better- I finally got clearance to put some weight on it this week, so I've been more mobile (w/a cane-my son says I look like Dr. House!)and being able to stand at the bench makes framework a lot easier. Still 6 weeks from being able to ride.
No complants though. It could have easily been much, much worse, plus, now I have an excuse to build me a new mtb!

Have you talked to a lawyer? (no answer required).

Mitred for 54cm frame

Spirit 31.7 TT .5/.38.5 = 168 g
Spirit 38mm DT .5/.38/.5 = 220 g
Spirit 28.6 ST .6/.38/.5 = 166 g
Total main tubes = 554 g
(the original .5mm CS tubes were removed from sale, did not meet with success in the real world, failures etc)

Mitred for 54cm
Life 31.7 TT .65/.4/.65 = 206 g
Life 38 DT .65/.45/.65 = 264 g
Life 28.6 ST .75/.4/.6 = 189 g
Total main tubes = 668 g

You save 114 g.
Is that really making the frame "super light"?
Selection on three tubes alone does not make a frame light.
Select the other parts of the frame with a reliable reduction in grams is Ok to a point, but the three main tubes gain buggar all for a lot in the loss column.

An empty 750mm plastic water bottle = 85 g

I see no value on making a frame with the three main tubes thinner than .65/.4/.65, no matter what diameter.
considering the consequences of some times simple crashes, not just TT dents fro handle bars but the main triangle crumple! (I have seen them)

I see no value in the reduction of 114 grams even if you believe or perceive it has some performance benefit, and this is only a perception as it can not be measured because it cannot be measured in the real world. (I worked for many years with the Australian Institute of sport .....................)

The value of light is in marketing brochure text.