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  1. #1
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    Added disk tab - feedback?

    Hi guys,

    I decided to upgrade my wife's 20-year old Rocky Mountain Hammer Race with a disk brake tab, and tried TIG welding for the first time. While I have experience with lugged frame building and brazing, I only have access to a TIG welder now...

    Here are the results:
    Added disk tab - feedback?-2016-04-08-18.01.28.jpgAdded disk tab - feedback?-2016-04-08-18.01.48-large-.jpg

    Not the prettiest results but I didn't want to get too carried away - I figured I would get a couple strong welds in there then fill in the rest with solder for cosmetic purposes.

    I realized I should have removed more paint, I underestimated how far the heat would travel. Notice how the brass around the original dropout is exposed now?

    So do you think this is strong enough? Or would you recommend going back with the TIG welder and added a few couple more tacks?

    Thanks guys!

    cheers,
    dave

  2. #2
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    I think you should braze in a brace between the seatstay and chainstay. And then cross your fingers, because you cooked the crap out of that seatstay. Good luck!

    -Walt

  3. #3
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    whoops, I was afraid of that...thanks for the advice!

    I had the TIG welder set at ~60Amps - too hot? Or was I just not fast enough? The disk tab is 6mm, the tube is whatever Rocky put on their Race series bikes...

    Ok, I'll look around for a brazing setup to avoid doing any more damage. I'd better do the first test ride too, before painting!

    cheers,
    dave

  4. #4
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    A brazing set up isn't a panacea for cooking the tubes -- you can do just as much damage with a flame and brass as the TIG if your heat control is... out of control.

    Definitely concur on the brace.

  5. #5
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    Well, things either weren't shielded well or got WAY too hot - that's why the bead is grey instead of shiny/colored. Second, you've got enormous blobs and low spots, which will mean stress riser city. There's also not much weld bead overall, meaning the tab is doing to put a lot of force on those spots that are welded.

    It will probably be fine and I'd say just ride it and see what happens, but you really should have practiced on some scrap a bit before tackling the disc tab project. There is a decent chance you killed the seatstay here, but there's also a chance it'll work for years.

    -Walt

  6. #6
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    Thanks again for the advice! Looking around now for a short length of tube to use as a brace. Figure 10mm dia with 1.0mm wall...

    Was I wrong in trying to limit how much to weld in the first place? All of the braking force is into the seatstay, right? Theoretically it could just sit on the seatstay, like some early aftermarket options that are held on by the axle... (?)

  7. #7
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    There is a considerable amount of twisting force as well, because the caliper hangs over to the side of the stay where it grabs the rotor. So no, you can't just put the tab on and let it sit there with nothing holding it on and have it work.

    If you don't weld most of the tab, you are concentrating all the force in the places you *did* attach it.

    Again, it will probably be fine. But you used way too much heat/not enough shielding gas and probably did some damage to the stay, and you also didn't weld much of the tab. Those are both bad things overall and the lesson here is that you should practice practice practice next time to get your technique down before trying things on an actual bike.

    -Walt

  8. #8
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    Wow, it's been a few years since I started this project!

    In the meantime I had the bike powder coated and built it up with 1x11 drivetrain and mechanical disk brakes. My wife is super happy with it and it's riding like a champ.
    Added disk tab - feedback?-2019-06-10-17.55.01.jpg

  9. #9
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    I didnt realize this was an old post when i started reading... Then I got to see the end result 2 years later! that was fun! And good to hear its holding up!

  10. #10
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    That's cool. It looks like you welded a bit more on it, but not much. Did you end up brazing much? Is the support between the stays brazed on?

  11. #11
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    The brace was brazed on (by a buddy who knows how), but nothing added to the disk mount itself.

  12. #12
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    Quote Originally Posted by cdn-dave View Post
    The brace was brazed on (by a buddy who knows how), but nothing added to the disk mount itself.
    If you have not done loads of welding these kinds of joints are tricky, its always when you are welding thin wall to something comparably thick that can catch you out, too much heat into the tube rather than the mount and its easy to make a mess.

    Powder coated up it looks good. I reckon you will get a good amount of use out of that

  13. #13
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    Effect on Brass/Brazed Dropout

    Looking for input and this seemed like a good feed to ask on. I have a rear disc tab all fit up and getting ready to tig weld on to a road bike I'm converting to disc brakes. I was planning on welding all the way around the tab, but was thinking about the effect the heat would have on the brass/brazed dropouts and thinking I might want to leave a small section no welded by the brass. Thoughts?

    Tom
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  14. #14
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    QUOTE=thomasauction;14155583]Looking for input and this seemed like a good feed to ask on. I have a rear disc tab all fit up and getting ready to tig weld on to a road bike I'm converting to disc brakes. I was planning on welding all the way around the tab, but was thinking about the effect the heat would have on the brass/brazed dropouts and thinking I might want to leave a small section no welded by the brass. Thoughts?

    Tom[/QUOTE]

    Hi, I am by no means the expert on this, but have you tried tig brazing? I am using it a lot now as acetylene is hard to get for home use in the UK. Basically the same process as Tig but use a silicon bronze rod like sif 968 and low amps.

    This doesn't melt the base metal and in my tests I break the tube not the weld.

  15. #15
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    Hi, I am by no means the expert on this, but have you tried tig brazing? I am using it a lot now as acetylene is hard to get for home use in the UK. Basically the same process as Tig but use a silicon bronze rod like sif 968 and low amps.

    This doesn't melt the base metal and in my tests I break the tube not the weld.

    I know of one builder who used TIG brazing to attach disc tabs and ended up having problems with cracking and fork failure.

    The issue is that if you end up melting the base metal into the braze material it causes embrittlement and subsequent cracking.

    TIG brazing lends itself well to sculpture/metal art and heavier duty applications like gate or fence making. It's not so well suited to more optimized structures that are subject to cyclic stresses although I am sure there are exceptions to this.

    Alistair Spence,
    Seattle, WA.

  16. #16
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    Quote Originally Posted by Noah240t View Post
    I didnt realize this was an old post when i started reading... Then I got to see the end result 2 years later! that was fun! And good to hear its holding up!
    I asked Dave to share his results (like most of us, he had forgotten to put a follow-up). It's actually much more useful to know if the tab and welds held up after a fer years of use, than just a few days after. That and I wanted to see that little Mapple Leaf on the tab again. Rates pretty high on coolness factor...

    Quote Originally Posted by thomasauction View Post
    Looking for input and this seemed like a good feed to ask on. I have a rear disc tab all fit up and getting ready to tig weld on to a road bike I'm converting to disc brakes. I was planning on welding all the way around the tab, but was thinking about the effect the heat would have on the brass/brazed dropouts and thinking I might want to leave a small section no welded by the brass. Thoughts?

    Tom
    I just did mine, plain welding with a MIG. Both the tabs and the brace.

    From what I've investigated, the bike's frame is made of medium carbon steel (1040) and I'm pretty sure it has has some sort of heat treatment (at least the main triangle, not sure about the stays). It didn't burn or fall apart. But if your bike is made of something more exotic (like 4130), I'd consider brazing. Alloy steel anneals with heat and since the whole point of making it out of 4130 is to reduce the wall thickness of the tubes, it may end with less support than needed and snap.

    About the looks... don't blame me. I hired a local shop and for the price they charged me, I can't really complain. But hey, it works!




    This is my bike. Yes, it's a Schwinn. But I must say, a super-Schwinn by now (I upgraded the hubs, switched from freewheel to cassette, etc.).



    I swapped the fork to do the front side. I could have welded tabs on the old one, but I figured out it would be easier and safer to just do the swap. Besides, the old one came in threaded and it was the opportunity to upgrade to threadless.

    The new fork is taller (29er, while the bike is 700C). But hey, it's a thick chromoly one. Should last more than the rest of the bike, I guess. I'm still thinking what I'm going to do to alleviate the dorky look. Open to suggestions...
    Attached Thumbnails Attached Thumbnails Added disk tab - feedback?-20190619_200715b.jpg  

    Added disk tab - feedback?-20190619_200731b.jpg  

    Added disk tab - feedback?-20190619_200740b.jpg  


  17. #17
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    Quote Originally Posted by A. Spence View Post
    I know of one builder who used TIG brazing to attach disc tabs and ended up having problems with cracking and fork failure.

    The issue is that if you end up melting the base metal into the braze material it causes embrittlement and subsequent cracking.

    TIG brazing lends itself well to sculpture/metal art and heavier duty applications like gate or fence making. It's not so well suited to more optimized structures that are subject to cyclic stresses although I am sure there are exceptions to this.

    Alistair Spence,
    Seattle, WA.
    Interesting take on it, I have only so far used this for braze ons and adding brake mounts, no issues so far. I plan to make a full frame this way just to see how I get on as an experiment.

    Starting is the tricky bit, you need to melt the filler before the tube, once you have this it seems to flow very well. I've chopped a lot up and haven't seen any base metal melting yet, but this is using pulse also.

    When I braze with oxy/acet I am basically using the tip of the neutral cone to create the fillet, if you get this wrong you have a similar issue where by the base metal ends up mixed? Just my thoughts.

    I Always appreciate another view on something, it makes you stop and think.

    Thanks

  18. #18
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    Quote Originally Posted by mikeey_boy View Post
    Interesting take on it, I have only so far used this for braze ons and adding brake mounts, no issues so far. I plan to make a full frame this way just to see how I get on as an experiment.

    Starting is the tricky bit, you need to melt the filler before the tube, once you have this it seems to flow very well. I've chopped a lot up and haven't seen any base metal melting yet, but this is using pulse also.

    When I braze with oxy/acet I am basically using the tip of the neutral cone to create the fillet, if you get this wrong you have a similar issue where by the base metal ends up mixed? Just my thoughts.

    I Always appreciate another view on something, it makes you stop and think.

    Thanks
    It's not my take on it. This is the received wisdom I've come across over the the 20 years or so that I've been learning about frame building, from people who know a lot more about metal fabrication than I do.

    Ask yourself, how many people are constructing bicycle frames using TIG brazing? I don't know of any, but I'm not as much in the loop these days so maybe there are a few? Even so, that the technique isn't prevalent should be a clue as to how appropriate it is.

    That being said, I can see how it could be fine for braze on's in lower stress areas of the bike.

    As for fillet brazing. Bronze filler melts at around 1750 F. Steel melts at 2800 F. You'd have to be doing something seriously wrong to accidentally start gas welding, when what you meant to do was fillet brazing.

    Alistair.

  19. #19
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    DO NOT build a frame with silicon bronze/tig brazing.

    Full stop.

    You (or your customer) will get hurt.

    -Walt

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