Help With Frame Repair

[Gbrebes]

Hey Everyone,

I wanted to ask for some help from you guys with ideas on repairing my frame. I home-built this frame using 1 1/8" inch tubing for the top tube, cross bar and bottom tube. All the joints were interior lugged. I developed this crack in the bottom tube in between my rear motor mount and the bottom bearing bracket. That's the first picture.

The second picture is of two drawings I did for repair ideas for this crack. In the top drawing, I would use a sleeve that is 5" long or so to repair the broken tube. In the bottom drawing, I would use a much longer tube as the repair sleeve. It would join near the bottom bracket and then again on the bottom tube nearer the head tube.

I am making all the joint welds using an acetylene torch and brazing. I am not a frame or welding expert by any means, so I am completely open to suggestions regarding more gusset work or any other engineering addition to make this area of the frame stronger.

Thanks in advance for your opinions,

Gilbert

 

[cobrafreak]

Do you have a wire feed welder? I know that wire feed is a lower temperature weld, which is good for not weakening the tensile strength of the frame.

 

[Gbrebes]

Thanks for the reply cobrafreak. I do not have a mig welder. But I always thought that brazing was lower heat than a mig welder? I could be wrong. The location of the crack does look like a tensile strength failure, but I am pretty sure that the downtube in between the crack and the bottom bracket is interior lugged up to, or near the point of the crack.

After having pondered my quandary for a while, I came up with a new drawing. In this drawing, I have the rear motor mount extending all the way to the bottom bracket. When I look at the broken tube, I see that the strongest structure in this area is the bottom bracket, so I figure if this motor mount plate connects to both the bottom tube and the bottom bracket, it will be a strong enough structure to prevent failure in the future.

I welcome any other opinions,

Gilbert

 

[Gbrebes]

Oops, for some reason, my attachment did not work on the last post. Retry.

 

[culvercityclassic]

Is the motor supported at more than one area, like is the head supported to the top of the frame? Post a good picture so I can see what's going on.

 

[Gbrebes]

I already pulled the engine, exhaust, fuel tank, and jackshaft. Here are some older photos of my build. The motor is only mounted from the four bolts on the bottom of the engine to the two plates that are brazed to my down tube. it is not attached to the cylinder in any way.

Thanks in advance for the help CCC,

Gilbert

 

[Gbrebes]

There are also a bunch of pictures of this build in this thread.

 

[trug]

Thanks for the reply cobrafreak. I do not have a mig welder. But I always thought that brazing was lower heat than a mig welder? I could be wrong.

no, fillet brazing is done at a much lower temp. http://en.wikipedia.org/wiki/Brazing

To say that mig is a low temp weld is not really the whole story, it all depends on your set up, and skill. I weld slow and over heat everything whether it is gas, arc, mig, or tig. The key is setting up your rig for what you are doing and moving your hands. If you can do that the process won't matter as much.

 

[trug]

is the motors mass balanced from one side to the other? how does the crankshaft's center line line up with the center of the bike?

If i were to guess as to what caused the crack i would say the up and down motion of the piston is rocking your mount and tube. That fatigued the weak point where the motor mount ends.

I think your drawings are on the right track maybe consider boxing in the tube back to the bottom bracket and raising the bottom tube as high as it can go like in the bottom of the first picture. Running two smaller tubes like a motorcycle might handle the stress better as well (second pic).

But If I am right about the torsion on the motor mount it might just shift the stress and tweak your bottom bracket or brake the tube out in front of the motor mount.

 

[cannonball2]

You are more than likely focusing the engines vibes to a very localized area just ahead the bottom bracket which is pretty rigid, Also the engine plates being in 2 pieces will allow more flex(however small) that a 1 piece. The engine is likely rocking of sorts with the loading and unloading of torque flexing the frame ahead the rigid bottom bracket. Trying the mount to the BB is a good idea, but it needs to be subatantial enough to transfer the load, angles however ugly work well. Approved aircraft tube splicing has the overlay tubing cut at angles to increase the weld area. Brazing is good as the brass if properly done flow by caplilary action inside the joint. I would overlay the mounts with a single plate, this will tie the front of the frame to the BB negating any opossing flex from the wheel/steering loads.

 

[Ibedayank]

out of balance flywheel is not helping

 

[cobrafreak]

out of balance flywheel is not helping

Oh heck yeah, I just noticed this too. Dude, you need to get a new flywheel or you are going to keep breaking frames and possibly damage your engine. To grind off fins you need to do it while the engine is idling for perfect balance. You have an external weight on the flywheel so you can't do this. New flywheel time.

 

[Gbrebes]

Even though it doesn't look like it, the flywheel is balanced. I used a lawn mower blade balancer as recommended by msrfan. There is extra meat on the fins opposite the flywheel magnet in order to balance it. Thanks a lot for all the help guys.

Gilbert

 

[Mr.B.]

Perhaps you could just sandwich the tubes with some simple welded gussets?

It would tie everything together and add a lot of strength.

-Kirk

 

[culvercityclassic]

Perhaps you could just sandwich the tubes with some simple welded gussets?

It would tie everything together and add a lot of strength.

-Kirk

I agree with Mr. B I would also add a bracket to the top of the motor to keep it from twisting. I had to do this with my last build and it sure helped.

 

[vintagebiker]

You could try this style of motor mount.
It seems to work well with the flat bottom lawn mower type engines.
The thicker metal parts were mig welded but the thin tubing areas were brazed.

 

[Lurker]

I think Mr. B is on to something. I would do exactly what he has sketched out, but I would add a upper brace and also connect the two mounting plats, there is no reason not to connect them. Having them separate only encourages twisting, thus you have unneeded stress to the frame.

 

[Gbrebes]

I think Mr. B is on to something. I would do exactly what he has sketched out, but I would add a upper brace and also connect the two mounting plats, there is no reason not to connect them. Having them separate only encourages twisting, thus you have unneeded stress to the frame.

Yes, I think Mr. B's sketch is a wonderful design, and I will probably be using it. But I do not understand what you mean by "adding an upper brace". I also think it is a prudent idea to make the motor mount one piece, instead of two. I was just trying to save weight, for what reason, I do not know, as it sits this bike is darn heavy. I want to thank everyone for all the ideas and help with this. I really do not have anyone to go to for this kind of assistance, so I appreciate it.

Gilbert

 

[Lurker]

Sorry, by upper brace I was talking about making a brace that would connect the top of the engine to the frame directly above it. lots of race bike and street bike of the day did this to further eliminate frame twisting. Essential by doing this your are creating a structural triangle that allows the engine to be a structural part of the bike. I know Pat has done this with bikes and I will soon doing this on my v-twin build as well.

 

[culvercityclassic]

This is a simple bracket that I installed. It sure helped as before I could see the motor twist as I accelerated.
http://motorbicycling.com/showthread.php?t=32314&page=15