Rebuilding rear section questions... what size tubing to use, etc, etc

[therichisgood]

So I want to finally tear into my bone stock CB400. I would like to start with re-doing the rear seat/shock mount section to accommodate for the cafe style seat while giving everything the look I want.
What would be a suitable tubing size (diameter & wall thickness) to re-do the seat section? And what about for the hoop that goes around the battery cover area? I want to cut that out as well and have the tubing be more at the angle of the front down-tube, as seen in the second picture.

That being said, I'm good in with the mechanical and electrical side of things but know nothing about welding so I have a friend who is proficient in all types, to deal with this part of the build.

Now pics for clicks..

My bike:

What I want to achieve with the rear:

 

[VonYinzer]

That's a tall order for your frame, but if your buddy is really a competent fabricator he would know what the correct material would be.

 

[oldskool]

That's a tall order for your frame, but if your buddy is really a competent fabricator he would know what the correct material would be.

lol

 

[VonYinzer]

Wasn't trying to be a dick, but the guy building it should really be deciding the materials used based on the job at hand. We can tell you what we might use, or what might work for hypothetical fabrication jobs, but again... The guy who's building it should have the final say.

 

[oldskool]

Wasn't trying to be a dick, but the guy building it should really be deciding the materials used based on the job at hand. We can tell you what we might use, or what might work for hypothetical fabrication jobs, but again... The guy who's building it should have the final say.

i know,just fucking wit ya brother,lol

 

[VonYinzer]

Hahaha... I like the jokes.

 

[oldskool]

That's a tall order for your frame,

shoulda just quoted that part,lol. it's the same as the cm,lol.

 

[therichisgood]

Oh for sure we'll be taking a closer look at what we're dealing with soon, I just thought I'd ask for some advice here since you have the experiences. He's a great welder, I was just looking for tips/tricks that we may not have thought of being new to frame modifying.

VonYinzer, what did you mean by 'for your frame'? Is this actually do-able or am I dreaming? I've noticed the frame on these bikes are a little strange but I'd love to be able to go ahead with this.

 

[jungalist]

just make sure to use slugs when welding 2 pieces of frame tubing together.

and also make sure you level the frame and secure it well before you start welding to make sure your frame is welded straight.

you should be just fine

 

[therichisgood]

Thanks jungalist. Sweet build by the way, I'm feeling inspired!

 

[SONICJK]

How much of the rear end are you going to redo?
I use .120 wall 1" tubing but thats overkill and I like overkill. People use .06 and a lot use .095 as well.

If you're going to be welding to existing tube and not cutting the whole subframe off your best bet will be to cut where you want to cut, then measure the frame diameter (probably 1") and thickness and get similar tubing to weld to it.

 

[Ppleasants]

The frames on these CB400Ts doesn't have a lot of potential. The only way to really fix them is to cut and weld a new subframe so I think you're on the right track. I did the same thing with my bike.

like someone already said, you'll have to use plugs to join the new tubing to the old. I ordered my tubing before I started cutting into the frame so I ended up with a thicker wall than the stock which means that I had to make a plug with a step down from the original ID to the new ID. I dont' remember the wall thickness off hand so I can't help you much there but you can always cut off a small section you know you won't be keeping.

Once you've made plugs and with them in place, drill through the tubing and into the plug just enough to make a dimple. Your buddy will need to weld through this hole to secure the plug to the old tubing. Once it's welded in place, do the same with the new tubing positioned over the plug. It won't hurt to do this 2-4 all the way around the plug, on the old tubing and new.

Remember to bend your tubing before you weld it on and test fit/measure numerous times with reliable positions on the frame to make sure your new subframe is symmetrical. Welding can also warp the metal so try to tack weld on opposite sides of your tubing so that the weld will pull evenly. If you can, weld temporary cross-braces to resist the warping.

This was my first time tearing into a frame modification and one of my first few times MIG welding so it's not over your head but if I could do it again I would have practiced a lot more to make my welds cleaner and stronger. Having your experienced buddy do the welding is a good call. Good luck and looking forward to seeing it when it's done... by the way, that holiday cycles 450 was one of my major inspirations as well

 

[therichisgood]

Ppleasants awesome info man, thanks. That's exactly the section I want to re-do as well. Do you have pictures of the final product?

 

[Ppleasants]

Glad I could help some. The bike is still under construction over a year later but this is the latest shot several months ago. It's been put on hold until I get some more free time to devote to it.

 

[oldskool]

nice looking subframe!! I did mine like this