Topic: CB550 Frame bracing help needed!

[MCRider]

Honda didn't "finish" the welds on the head tube for good reason... All frames flex.... The area that are not welded are there to allow flex.... If they are welded 100% then you've given the frame no where to release the energy. Usually resulting in cracks at the weakest area(tubes)

All very true. But to complete your statement "...area not welded are there to allow flex", its also there as to weld it would've cost more with no benefit at the production level, the extra cost could not be recouped in additional price.

Too much flex was the result of Jap bikes in general scrimping on things like welding up the headstock, opting to pin them with rivets instead. The magazines of the day lamented the "flexi-flyer" frames and anecdotally many riders were killed because of too much flex being left in the frames.

Honda stopped short of 100% or 80% or 60% for production cost reasons, not just design reasons. So reasonable amounts of additional welding will strengthen the frame without taxing it. As the many racers do, without cracking it.

[754]

 I am pretty sure Honda used MIG on at least the later 750 frames, I have seen the tell-tale wire stub..
 On the sandcast the footpeg mounts (that bolts on) appears hand welded not so on later ones.
 Honda was one of the first to use Cell manufacturing or building comples machinery to accomplish single task.. ie the cases went thru such a machine, came out finished..
 If anyone could absorb the early teething cost/price of relatively new forms of welding equipment. (and robot-welders).....Honda Motors could amortize it far quicker than many others..jmo..

[Mtmooradian]

Honda didn't "finish" the welds on the head tube for good reason... All frames flex.... The area that are not welded are there to allow flex.... If they are welded 100% then you've given the frame no where to release the energy. Usually resulting in cracks at the weakest area(tubes)

All very true. But to complete your statement "...area not welded are there to allow flex", its also there as to weld it would've cost more with no benefit at the production level, the extra cost could not be recouped in additional price.

Too much flex was the result of Jap bikes in general scrimping on things like welding up the headstock, opting to pin them with rivets instead. The magazines of the day lamented the "flexi-flyer" frames and anecdotally many riders were killed because of too much flex being left in the frames.

Honda stopped short of 100% or 80% or 60% for production cost reasons, not just design reasons. So reasonable amounts of additional welding will strengthen the frame without taxing it. As the many racers do, without cracking it.

Your right, but can only make something stiff if the materials can support it. I'm not any sort of expert in cbs, but I see after building my bike that the tubing isnt the greatest(not that many bikes had great tubing) its probally the equilvent of sch-5 water pipe..... I'm just saying don't think about making your frame as stiff as an alum Gsxr frame and not have cracking issues.

[MCRider]

Absolutely, point taken. There is only so much that can be done. But there is room for easy improvement on the CB era frames.

I'm taking my cues from Tony Foales and the like. In particular, scroll down to the "Frame Stiffening" article: kawa 750.
http://www.tonyfoale.com/Articles/index.htm

[Mtmooradian]

I'm going to start doing my research.... My frame is almost ready for paint and I love to improve on it.

[MCRider]

I'm going to start doing my research.... My frame is almost ready for paint and I love to improve on it.

I just checked out your thread, nice stuff.

On the 750, the most common gussets are: The "V" above and behind the swingarm pivots; the interior angle behind the carbs, both sides, the interior angle behind the downtubes (your frame rail kit may preclude this) and weld (no gusset needed) the headstock upside down U's. I've been told not to weld these across the bottom, just the vertical runs. Don't know why.

That will take the silly wigggles out of it. More than that gets into the problems you mention.

[SF]

I'm no welder by trade and no master of vintage motorcycle frames but the first thing i did was clean up and weld(MiG) any seams and add gussets on my 75's. it just makes sens to. my 72 r5 will definetly need some improvment in her frame!