Triple Backbone Question!

[amat91]

So i finally have been able to find time to tear apart my cb750. I bought a king sportster gas tank that i want to make fit, however i'd have to modify the triple backbone to a single. Ive looked around for information, and some sources say i should replace the main center tube with a thicker walled tubing (better safe than sorry). But others say that id be fine with just cutting the outside struts completely and leaving the stock center tube there.

So essentially, my question is, would i be fine if i left the stock center backbone in? Or should i go ahead and put in the work and install a thicker walled backbone completely to be safe? I havent started cutting yet so i have no idea the thickness of the stock tubing. Maybe someone could give me that info as well?

[LukeM]

Those on this forum with welders need to step up. Should be an interesting thread.

Luke M

[TOOLS1]

I know of several bikes with the two tubes removed, and have never heard of any problems. Cycle one's hardtail kits require the removal of them, and just weld to the existing backbone with a short slug. So in my opinion, cut them off, and don't worry about it.
TOOLS
P.S. Check out my "Mortal Vindication" build.  

[shinyribs]

Tools,havent you removed those tubes on more than just your Mortal Vindication build?

Also,if i recall the auto's dont have them to begin with.

Sounds pretty much like a cut 'em off and toss 'em ordeal

[amat91]

Thanks guys! thats what i needed to know! Tomorrow morning those damn things will be coming off, just as soon as i extend the back bone about 3 inches. I greatly appreciate the information!!

[TOOLS1]

The matics do have them. However they have a bend in them that allows the valve cover, and head to be removed without having to remove the engine from the frame.
amat91 If, you are going to extend the backbone, be sure to slug it. Also are, you going to extend the front down tubes also?
TOOLS

[amat91]

No i will not be touching the neck/downtubes. Ill be extending the backbone towards the rear. Essentially, what i will be doing is in white in the picture

my goal is to be able to fit this tank on, and have it sit properly on the back bone

[TOOLS1]

Just clean up the tubes, you cut off. Then bend them to go from the cuts to the backbone. That would look clean, and the tank should fit nice. What, you are doing is very similar to "Konging" a frame.
TOOLS

[LukeM]

Thanks, TOOLS and sparklyribs.  

Amat, I like what you're doing: planning it out, asking the right questions, and getting it done. Keep us in the loop.

Luke M

[shinyribs]

Oh,autos have BENT tubes.Thanks Tools

[amat91]

And.......... Here it is! I cut 'em bastards off and as extednded the center backbone quite a bit towards the rear. Hell of a lot harder than i thought it would be to get a good weld; ended up looking like shit. Thank God for grinders!

[Dredd]

Thanks for this thread! Im doing the same thing with an old peanut tank! Are you keeping the swingarm too? I am I want a better ride than the hardtail kits. My old back cant take the beating anymore!

[Dredd]

Can you post a couple more views/angles?

[callmephil]

i have a 75cb750  no one wants to mess with it i have a (.120) wall tube to.replace the center one because i was told the center tube will not be strong enough alone to support all the weight my main question is what type of weld do i need to put this new tube in and is there anyway of doing this without disabling my whole bike down to the frame please help im a confused 20 year old and this is my first bike thanks

[shinyribs]

I ditched the two upper cradle tubes:the ones that run over the top of the engine,on either side of the back bone. I did make some removable cuffs where I could bolt the tubes back on just to be safe. After several thousand miles of riding I can personally say that the two tubes have not flexed the least bit since the paint has not been disturbed at all. If it had been flexing it would have scarred the paint to some degree. And I will admit that I am not some knee-down kind of rider,but I do push my bike pretty hard in the curves. Simply,if you ditch those tubes altogether you'll never know it. Unless you're gonna take up road course racing I wouldn't worry about it. And even then you'd probably be ok

[TOOLS1]

As stated previously in this thread "you do NOT need to replace the backbone tube when removing the two outer tubs" The back bone is made out of some pretty hard steel, and is plenty strong. Also it would take a lot of experience working with metal, and fabricating to cut out the backbone, and replace it; keeping everything straight, and strong. As for what type of welding process to use. I used to build frames (also high pressure boiler systems) with oxy/accy welding. Then there is "stick welding" which is more then strong enough, but requires some skill. Mig welding has pretty much become the standard today. Just about anybody can Mig weld. However the internet Gurus will always scream, and holler that it must be Tig welded. Although Tig is probably the best process for welding, it requires a lot of skill. So I would recommend you use what you got.
TOOLS  

[callmephil]

thank you guys for the great advice my dad is just worried about me lol

[callmephil]