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  #1  
Old 02-15-2016, 11:08 PM
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Mucci Mucci is offline
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Stronger to weld on bungs or pass through frame?

I'm looking to add 2 identical mounting points on the left and right side of a motorcycle frame backbone (to mount a bolt on seat subframe). Would it be stronger to:

A. Weld 2 independent threaded bungs to the exterior of the backbone, or
B. Drill a hole on either side, pass one long threaded bung all the way through so it sticks out either end and weld it in place?
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  #2  
Old 02-16-2016, 12:21 AM
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JaysinSpaceman JaysinSpaceman is offline
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Strength wise I think it would be about the same. I suppose if it is very thin wall tube then one continuous bung might have more strength then two surface mounted bungs. If the two bugs need to be well aligned then running one bung all the way through might be easier. There is much to consider, maybe post a picture.

Jaysin
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Old 02-24-2016, 11:51 AM
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Quote:
Originally Posted by JaysinSpaceman View Post
Strength wise I think it would be about the same. I suppose if it is very thin wall tube then one continuous bung might have more strength then two surface mounted bungs. If the two bugs need to be well aligned then running one bung all the way through might be easier. There is much to consider, maybe post a picture.

Jaysin
Here's the idea. The 1" subframe (black) bolts to the 2.18" frame backbone (silver). This is regarding the upper left most mounting point.



Here's what I'm thinking as a cross-section view:



The M12 threaded bung is welded to the frame. This can either be 2 independent bungs or one female threaded pipe that pierces the frame on both sides.

The 1/2" counterbored bung is welded to the subframe. (1/2" because I can't find anyone who makes metric versions. 1/2" is 12.7mm so it'll fit the M12.)

I guess I shouldn't just ask which would be strongest but also which would be easiest to align.
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Old 02-24-2016, 12:36 PM
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Graham08 Graham08 is offline
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My thought on this is if your subframe is wide enough that your welded bungs can be long enough to have adequate thread and not require a hole in the main frame tube, I would go the route you've drawn, with the threaded piece welded to the outside of the frame as two separate pieces.

If you have to drill a hole in the frame tube anyway, I would make one long threaded bung that goes all the way through. That way it's sealed up without having to make blind tapped bungs, which are a pain compared to one with a through hole.

Another thought is to make it a smooth bung all the way through (no threads) so you can use a bolt and a lock nut. While the bungs for socket head screws look really clean, it makes it tough to mechanically safety the fastener so it doesn't vibrate out. The way it's drawn, you would be relying on torque and possibly Loctite to hold the bolt in. If the bolt goes all the way through, you could use a Nylock nut, which is less prone to vibrate loose.

My two cents, anyway.
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Old 02-24-2016, 01:53 PM
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Quote:
Originally Posted by Graham08 View Post
My thought on this is if your subframe is wide enough that your welded bungs can be long enough to have adequate thread and not require a hole in the main frame tube, I would go the route you've drawn, with the threaded piece welded to the outside of the frame as two separate pieces.

If you have to drill a hole in the frame tube anyway, I would make one long threaded bung that goes all the way through. That way it's sealed up without having to make blind tapped bungs, which are a pain compared to one with a through hole.

Another thought is to make it a smooth bung all the way through (no threads) so you can use a bolt and a lock nut. While the bungs for socket head screws look really clean, it makes it tough to mechanically safety the fastener so it doesn't vibrate out. The way it's drawn, you would be relying on torque and possibly Loctite to hold the bolt in. If the bolt goes all the way through, you could use a Nylock nut, which is less prone to vibrate loose.

My two cents, anyway.
Thanks for the great feedback Graham. This is a custom build / show bike so fit and finish are going to be important. It'll still be ridden though. I'm hoping loctite and a lock-washer will be enough to keep those bolts in place.

...could always get fancy with some safety wire too. I do love safety wire.
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Old 02-24-2016, 03:22 PM
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cbrogers cbrogers is offline
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I would think maybe a saddle for the tube that you would weld the bungs to. It would allow you to align the threaded section and modify width as needed.

Being that the large tube is the main backbone for the bike I would worry about putting a pierce through it causing a crack point or with a lot of welding adding a larger than necessary HAZ. If that bike sees a lot of flexing then it would be very easy to start a crack there. If possible I would put a saddle flange there and then have the welded on threaded area above the tube if possible. Kind of like the 2 lower mounts on that picture.

If you have to pierce that main tube try to make it go through the center-line of the tube. That will help to keep flexing even top and bottom.

Carl

Last edited by cbrogers; 02-24-2016 at 03:24 PM.
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  #7  
Old 02-24-2016, 11:15 PM
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JaysinSpaceman JaysinSpaceman is offline
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As to the thought of the nyloc nut making it a safer attachment... There are nyloc bolts too. I don't have a supplier off the top of my head but I do know the aircraft industry uses them. Maybe check with Aircraft Spruce. Then you have the best of both worlds, a clean install and a secure fastener.

Jaysin
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Old 02-25-2016, 11:17 AM
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Mucci Mucci is offline
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Quote:
Originally Posted by JaysinSpaceman View Post
As to the thought of the nyloc nut making it a safer attachment... There are nyloc bolts too. I don't have a supplier off the top of my head but I do know the aircraft industry uses them. Maybe check with Aircraft Spruce. Then you have the best of both worlds, a clean install and a secure fastener.

Jaysin
No kidding...

Quick search found this on the Nylok website:



"Nylok® Torq-Strip® self-locking elements are permanently embedded in the threads of a fastener. When mating threads are engaged, the tough, resilient Blue nylon element is compressed and a counter-force is created to establish a much stronger contact and positive resistance to vibration and loosening."

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  #9  
Old 02-25-2016, 11:35 AM
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Graham08 Graham08 is offline
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I forgot about these. I think you can get them, or something similar, from McMaster-Carr.
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  #10  
Old 02-27-2016, 09:37 AM
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kkroger kkroger is offline
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You can bore a shallow hole in the side of a bolt and put a nylon rod in to yourself too, no need for fancy bolts. Or Put a patch of plastic on the side of the threads. They are used that way in industry all the time.
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