(newbie q) rear disc tabs - how strong does the joint need to be?

[hgrd]

Hi! I am hoping to add (braze) a disc mount to a frame where the dropout is constructed in such a way that it seems hard to attach the lower end to the seatstay. Instead I was thinking about whether it would be a stupid idea to attach that end without a fillet it to the actual dropout, in which case there would be room for a capillary joint of 6X20mm on that end. I find it hard to estimate how much lateral force these things are subjected to with a pair of BB7 and a 160 mm rotor. Trial and error is an option, but thoughts would be greatly appreciated:)

[jon_norstog]

Hgrd,

W/O a picture it is hard to visualize your problem. Do you have access to Don Ferris' "FengShui" disk tab fixture? If so, use it.

Once you have the disk tab blank bolted up in Feng Shui, you can rotate the whole schmutz to find an alignment that gives you bronze contact along the length of the tab. You will have to mark, cut and file. Depnding on the outcome you may wish to triangulate the chainstay and seatstay to keep things from breaking.

Good luck

jn

"Thursday"

[hgrd]

Hi, and thanks for the answer! Here is an embarrassing photo, and I know what I probably should do is get a proper disc dropout:) In my defence I wasn't planning on using disc brakes when I ordered the dropouts, and given that I now have them I thought I'd see if I could save some money by trying to use them. From the look of things the braking force would go straight through the top joint, and there would definitely be a tube from seatstay to chainstay at this location. With the 15 mm or so offset of the disc the lower joint would however probably take quite a bit of stress, which worries me a bit given the design. I also don't know if it is a problem to focus so much stress on the top joint, even though many tabs seem to be constructed this way. It guess the socket-part of the dropout could be used for attaching a tab, contrary to my previous post. Either through tweaking this one or makig another, but I think it might look ugly so then I would rather bite the bullet and order some expensive paragon dropouts:) (ps. this bike is obviously not meant to ever be used by anyone but myself) P1000418.jpg

[Dornbox]

Hgrd,

W/O a picture it is hard to visualize your problem. Do you have access to Don Ferris' "FengShui" disk tab fixture? If so, use it.

Once you have the disk tab blank bolted up in Feng Shui, you can rotate the whole schmutz to find an alignment that gives you bronze contact along the length of the tab. You will have to mark, cut and file. Depnding on the outcome you may wish to triangulate the chainstay and seatstay to keep things from breaking.

Good luck

jn

"Thursday"

+1

With a CX tire I'd assume that the socket style dropout will provide plenty of support for the SS and a brace under the top of the disc tab between the SS to the CS should be fine assuming you haven't had to hog out the sockets on the dropout too much to make angles or slightly larger stays work.... Even then I think the triangulated brace would still add a ton of support. I' would check the location of you disc tab though as it seems way too close to the axel.

Good luck!
Chris

[hgrd]

Thanks! Might be my absent photo skills that twist things:) I took the measurements from my 2002 Gunnar rockhound, and found them to be 40mm/78mm with no offset from the dropout.
I have a rusty homemade device that allows me to rotate the tab with the triangle 40/78/51 intact. From your answers I take it there is not that much to worry about when it comes to strength of the lower joint, and what to mainly consider is the strenght and support at the top(?). Thanks again for helping me out!

[devlin]

I used this www.paragonmachineworks.com/cgi-bin/commerce.cgi?preadd=action&key=BK2009 to retrofit to the rear to an older Diamondback Axis Pro frame. I didn't want to put the triangulation in purely for aesthetic reasons and saw this. The reason is to add a bit of meat to the area and keep the lines a little. I had to do a little filing and tweek the extension sideways to track the top of the eat stay but it has work out reasonably well. I'll get a photo when I get a chance.

[Eric Estlund]

I'm won't comment on these particular parts for your application, but I will say as a rule of thumb don't make the deign work for the parts in hand. Design the bike, then source or fab the parts needed to make it without compromise.

Best of luck!

[hgrd]

@devlin: That is an interesting suggestion! It sure looks like it would do the job well in the rear aswell!
@Eric: That is sound advise, which I WANT to follow. I guess what I'm trying to assess is if this design is a compromise which is not strong enough, or one which is only less strong than it could be. The reason (apart from being cheap:)) I even considered using the parts for this is that I thought that my eyes would like the outcome, provided it worked, so in that sense it was not a compromise;)
Thank you both!

[hgrd]

This is what it turned out like. So far so good, but still a bit nervous.
P1000557.JPG

[steve garro]

Disc tabs put on with silver alloys will bust off. I learned this the hard way.
- Garro.

[dbohemian]

Disc tabs put on with silver alloys will bust off. I learned this the hard way.
- Garro.

Were the parts in the picture stainless? That would work best with a silver alloy or possibly nickel/silver. Maybe a combo of both (nickel rod on stay and higher alloy silver on drop)

[Andrew R Stewart]

Disc tabs put on with silver alloys will bust off. I learned this the hard way.
- Garro.

Due to poor fit up and surface area? I've had the same with a stand off fender braze on during shipping. Andy.

[steve garro]

Due to poor fit up and surface area? I've had the same with a stand off fender braze on during shipping. Andy.

Nope. Tight as a tick, just like everything I make.
- Garro.

[jon_norstog]

FWIW, the rear drops and the disc tab is a place I like to use Welco 17. It's a little stronger than bronze, a little more aggressive, and it works on a lot of stainless steels.

jn

[hgrd]

Welco 17, will have to look into that. In the picture I used 45% silver throughout. Suppose the day will come then when I will need to replace the whole thing (especially since the drive side also has been through hell in the form of course-alien objects bending the hanger all over the place) Is silver not good for these applications because it is inherently brittle, or what is the cause of what Garro stated?

[dbohemian]

Welco 17, will have to look into that. In the picture I used 45% silver throughout. Suppose the day will come then when I will need to replace the whole thing (especially since the drive side also has been through hell in the form of course-alien objects bending the hanger all over the place) Is silver not good for these applications because it is inherently brittle, or what is the cause of what Garro stated?

Welco 17 is just a form of nickel silver or german silver which is a nickel/copper bearing rod and has no actual silver in it. Because of the nickel content which has an affinity for the nickel in many stainless steels it will wet out and stick well to them when most bronzes will not. High content silver fillers like many of us use are not brittle but do not have any greater shear strength than bronze and they cost a lot. Everything being equal, that is clean stock with proper preparation silver has a similar bonding strength to bronze. The tensile/yield of bronze and lower bearing silver alloys (40-50%) are similar.

So why the bad rap for silver? IMHO...here is it. Generally people do not build similar sized fillets in silver as they do bronze and you need a minimum fillet size of 3-6 times the wall thickness of the part you are bonding too. If two fillets of exact same size are created they will have reasonably equal strength levels. Silver though costs a ton and people do not build it up enough where it needs to be built up. It may be subconscious because you have 150 bones sitting in your rod hand vs. 30c. Do I recommend silver for non stainless joints? No, not really, seems a waste of money to pile a bunch of silver up where bronze will work every bit as well. Only times I recommend it is either stainless parts or where you have really thin section parts where you want to minimize distortion etc. Nickel fillers work well too but to have a higher melting temp than bronzes and can cause cracking and distortion. They work well with a little practice though and the right flux, you can't get away IMHO with any ol' flux, you need one optimized for nickel/silvers.

So, will your silver bonding probably work. Yep. If you were clean about things and had a good fitup (which should be the same either way) and the same sized fillet you would have built in bronze it will be all good.:)

[steve garro]

I do many, many stainless dropouts.
the only thing I have had bust is an "add on" disc mount like the one above - they thing is, fit on those always involves filling a big gap - they hang over the round stay, ect. The one I had bust off, to be fair, was bent and it was only in trying to re set it it made a weak crunch and came right off. A bronze fillet, I believe, would have torn the stay off before it sheared.
So, what I'm saying is that if you are building a stainless bike, use a dropout that has an integral disc tab and save work, filler and lost sleep.
I'm only trying to save others woes i have had.
FWIW I use 45% filler on all my stainless dropouts with my own blend of fluxes.
- Garro.

[dbohemian]

I do many, many stainless dropouts.
the only thing I have had bust is an "add on" disc mount like the one above - they thing is, fit on those always involves filling a big gap - they hang over the round stay, ect.

So, what I'm saying is that if you are building a stainless bike, use a dropout that has an integral disc tab and save work, filler and lost sleep.

FWIW I use 45% filler on all my stainless dropouts with my own blend of fluxes.
- Garro.

I agree Steve. With the gaps that normally occur on some of these tabs it would take a fair bit of silver (same amount as bronze at least) to adequately fill the area at a great cost. No need to do that unless it is a special circumstance. Whole-heatedly agree with integrated solutions such as those from Paragon ( I have my own version). You have your own secret sauce for flux...? If you have not used our stainless light I will send you a tub for free. You know it is made by actual scientists with PHD's and stuff

I also have something we call stainless extreme and stainless light in pure powder form now. A very nice addition to the arsenal. Apply some via a dry powder to your rod and blow through even the worst shite. (often have to help do that with a student)

[Andrew R Stewart]

Steve, David- The fit up is exactly what i was referencing with my comments about a braze on fender eyelet breaking off. The eyelet was on a stand off made of steel strip about 1/8"x3/8" and about 1/2" tall. it was brazed onto the rear drop out just behing the stock eyelet. During shipping the bike box was pierced and the eyelet bent. The Silver brazing broke when the assembly mechanic (a supported tour, we were arriving too late for me to do the assembly myself, mistake #2) tried to straighten the stand off.

That I used silver to attach it, the small surface area in contact and the less then best fit up were all the reasons that after tow major stresses it broke. Andy.