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Thread: HT angle changing after brazing

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    Default HT angle changing after brazing

    Master Builders!

    I've been building some practice frames and after getting the front triangle all brazed up I noticed that the HT angle changed. The first one changed by 0.7 degrees from 72* to 72.7*. The second changed from 72* to 72.5*. Both frames had extremely tight miters and the alignment was spot on after tacking as well. I've not encountered this before in any of my previous frames to date and I'm racking my brain trying to figure out how this happened. My current thought is that either I'm not using enough heat, and because of this I'm spending too much time brazing the HT/DT joint, or, I'm using too much heat.

    I start at the DT/HT joint and braze 90* around the joint, then move to the opposite 90 diagonally across, then the third 90, and finally the last.

    I've timed myself and I'm sitting around 5 minutes per joint, but due to the extra bronze on the underside of the DT/HT, I'm at about 8 minutes at this joint. That needs to come down no doubt.

    What exactly could be happening to cause that much movement?

    Any and all thoughts and advice is welcomed.

    Thanks.

    Brandon
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    Bueller......
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    These triangles are being brazed while in a fixture, or held freely in space?

    If it's a lugless assembly, or even if it isn't, and you *want* to buy back that missing .7 of a degree, simply add a millimeter or too to your fork span.

    Or, next time - assuming this is a trend - don't trim the bottom of the head tube and leave it long a bit. That'll get you the same result as a longer fork.

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    Default Re: HT angle changing after brazing

    This isn’t a snarky question. How do you know the angle changed?

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by e-RICHIE View Post
    These triangles are being brazed while in a fixture, or held freely in space?

    If it's a lugless assembly, or even if it isn't, and you *want* to buy back that missing .7 of a degree, simply add a millimeter or too to your fork span.

    Or, next time - assuming this is a trend - don't trim the bottom of the head tube and leave it long a bit. That'll get you the same result as a longer fork.

    Good questions. Let me answer. The frame was only tacked in the fixture and brazed in free space. I used a Paragon HT that I had and really can't leave it longer to "raise" the front end up to slacken out the angle.
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by Curt Goodrich View Post
    This isn’t a snarky question. How do you know the angle changed?

    No worries. I know it changed because I put it back in the fixture to get some CS's on it and the HT wouldn't fit back in it. I had to adjust the cups to fit correctly and then measured the new angle.

    I haven't seen this happen before which has me perplexed. I "think" I know what happened but not 100% positive.
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
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    Default Re: HT angle changing after brazing

    so the head tube angle increased but nothing on the rest of the frame went with it?

    i'm going to suggest you lean the frame back in the refixture process, essentially create the angle you want, have the top tube raise a red hair or three, and move the saddle forward .6466464 millimeter to adjust for the revised setback that results.

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    Default Re: HT angle changing after brazing

    In both cases it sounds like the lower junction of the head tube/ down tube miter sucked in some. You can revisit your tacking and brazing sequence, and confirm those miters are tight and tubes don't float in the fixture.

    That's not a ton of movement, and if it's habitual and in that half degree range you can compensate against it earlier in the process.

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by e-RICHIE View Post
    so the head tube angle increased but nothing on the rest of the frame went with it?

    i'm going to suggest you lean the frame back in the refixture process, essentially create the angle you want, have the top tube raise a red hair or three, and move the saddle forward .6466464 millimeter to adjust for the revised setback that results.
    Thanks. I completely understand what you are suggesting. What if this frame had a carbon fork paired with it? Since I wouldn't be able to change the fork length, how would I (or anyone else) get the HT angle back to the desired angle? I apologize if these are simple questions, but I'm just trying to fit it all together in my (little) brain.
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by BigPoser View Post
    Thanks. I completely understand what you are suggesting. What if this frame had a carbon fork paired with it? Since I wouldn't be able to change the fork length, how would I (or anyone else) get the HT angle back to the desired angle? I apologize if these are simple questions, but I'm just trying to fit it all together in my (little) brain.
    You simply won’t know or notice the difference.

    I can’t know why the angle changed if you don’t know. Or since I don’t know your tools. Or sequences. Or experience.

    If it matters here, I never measure or build to angles, nor would I ever fully trust the engraved hash marks on a fixture’s backplate.

    Heat, metal, your temperament, the quality of the pile of stuff on your bench, and fate all play a role. You can only hope to play a part in the whole. You’ll never fully tame the beast. So maybe find a positive in that current assembly and tee off on it. A part of an angle is less than one bar of air in either of your tires.

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by e-RICHIE View Post
    You simply won’t know or notice the difference.

    I can’t know why the angle changed if you don’t know. Or since I don’t know your tools. Or sequences. Or experience.

    If it matters here, I never measure or build to angles, nor would I ever fully trust the engraved hash marks on a fixture’s backplate.

    Heat, metal, your temperament, the quality of the pile of stuff on your bench, and fate all play a role. You can only hope to play a part in the whole. You’ll never fully tame the beast. So maybe find a positive in that current assembly and tee off on it. A part of an angle is less than one bar of air in either of your tires.
    Many thanks Richard. I appreciate your words of wisdom. I do have the same thoughts as you mention, but I'm also a perfectionist and have a bit of OCD. A nasty combination. I "think" that I may have spent too much time at the HT/DT junction which is what caused this. But that is the thought of a fledgling builder of only a few years. I just want it to be right. If in the future I build a bike for a customer and this happens again, I won't have them come back to me and say the HT is 72.5 not 72.
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by Eric Estlund View Post
    In both cases it sounds like the lower junction of the head tube/ down tube miter sucked in some. You can revisit your tacking and brazing sequence, and confirm those miters are tight and tubes don't float in the fixture.

    That's not a ton of movement, and if it's habitual and in that half degree range you can compensate against it earlier in the process.
    Thanks Eric. I appreciate your thoughts. I tack from front to back down the centerline and then will go to each side of each junction. After tacking, I will throw the frame on the alignment table and check to ensure all is well. Then I start brazing. I will start at the HT/DT and do 90* around one side, then go to the opposite side diagonally on the same junction and braze 90*. Then finish up the junction.

    I'll then move to the TT/HT and move around in the same manner since all of the heat is still in the HT. In these frames I then moved to the BB junction and start at NDS side of the ST, braze all the way around the DT to the DS of the ST, and up and over the ST/DT. I leave the back of the ST for when I put the CS's on. Lastly I finished up with the ST/TT in the same manner as the HT joints, doing 90* at a time.

    I pride myself on my miters and they were/are damn good and tight. After tacking, there was no movement of the tubes.
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    Brandon, these are fillet brazed frames right? Your are putting a lot of heat on one side of the head tube while not putting any on the other. Put a good straight edge against the head tube and see if it is bowed. I'm betting it is and the reason for the angle change.

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by BigPoser View Post
    Many thanks Richard. I appreciate your words of wisdom. I do have the same thoughts as you mention, but I'm also a perfectionist and have a bit of OCD. A nasty combination. I "think" that I may have spent too much time at the HT/DT junction which is what caused this. But that is the thought of a fledgling builder of only a few years. I just want it to be right. If in the future I build a bike for a customer and this happens again, I won't have them come back to me and say the HT is 72.5 not 72.
    It’ll never be right.
    Accept it.
    If it’s ever right, stop.
    Because you’ll never repeat it.

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    Default Re: HT angle changing after brazing

    Quote Originally Posted by BigPoser View Post

    I pride myself on my miters and they were/are damn good and tight. After tacking, there was no movement of the tubes.

    Perhaps your miters are too tight and the misalignment is a result of interference fits that have no room to move when the expanding/contracting moments arrive.

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    Default Re: HT angle changing after brazing

    You are doing the most accute joint that needs the most heat and the longest workingtime first. I'm not supprised it's sucking in (closing the angle between the bottom of the HT and bottom of the downtube).

    1. Don't lose sleep over fractions of a degree
    2. Do some mental gymnastics on tacking sequence and brazing sequence and see if you can anticipate and correct movement.

    I have found good success with going three tacks on the ht to tt and ht to dt juntions. Triangles man, gotta love em.
    Last edited by Eric Estlund; 08-07-2021 at 04:11 PM.

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    Default Re: HT angle changing after brazing

    Thank you everyone for your replies. I'll take your advice and put it to use on the next one to see if that makes a difference.
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    All,

    I just wanted to take the time to thank you for your replies, advice, knowledge. I put it to use and it made everything better. Thanks again!
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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    Default Re: HT angle changing after brazing

    At the risk of being repetitive:

    The instant the flame hits the tubes everything changes and will never go back exactly to pre-braze perfection.

    Solidifying filler pulls acute angled pairs together more than obtuse angles; each tube has one of each and the main "triangle" (it obviously isn't one) has four sets; adjust tacking/brazing procedures accordingly.

    Sweat joint structural competency, not slight dimensional changes.

    If you build lotsa frames you might figure out a recipe that reduces dimensional changes; reduces, as in not eliminates.

    Or don’t worry about it (recommended!), pretzel avoidance assumed.

    Being a person who aspired to perfection in certain endeavors (including this one), I now fully subscribe to what Richie said...but safe money says I’ll never need to cross that bridge: If you ever build a perfect frame, stop. You’ll never repeat it.

    When I built our house I figured I could frame it to 1/8". The framers who helped me said, more than once, "it ain't a piano". They were right. And a piano ain't perfect either.
    John Clay
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    Default Re: HT angle changing after brazing

    Quote Originally Posted by jclay View Post
    At the risk of being repetitive:

    The instant the flame hits the tubes everything changes and will never go back exactly to pre-braze perfection.

    Solidifying filler pulls acute angled pairs together more than obtuse angles; each tube has one of each and the main "triangle" (it obviously isn't one) has four sets; adjust tacking/brazing procedures accordingly.

    Sweat joint structural competency, not slight dimensional changes.

    If you build lotsa frames you might figure out a recipe that reduces dimensional changes; reduces, as in not eliminates.

    Or don’t worry about it (recommended!), pretzel avoidance assumed.

    Being a person who aspired to perfection in certain endeavors (including this one), I now fully subscribe to what Richie said...but safe money says I’ll never need to cross that bridge: If you ever build a perfect frame, stop. You’ll never repeat it.

    When I built our house I figured I could frame it to 1/8". The framers who helped me said, more than once, "it ain't a piano". They were right. And a piano ain't perfect either.
    I totally understand this, but I also want to build a frame that spec's out to what I've outlined with what a customer may request. If we build something that has a HT angle of 72*, it should be 72* and not 72.5*. Chances are they may not notice the difference, but I feel that it needs to be right since that's what said customer is paying for. That's my perfectionist side speaking.
    Brandon Poser
    BAHL Cycle Works
    brandon@bahlcycle.com
    www.bahlcycle.com
    Instagram: @bahlcycle

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