Topic: bracing a CB750

[jaguar]

ive been doing some reading in threads here as well as other places on the interwebs about bracing the stock CB750.

im looking for some pics of bracing on a stock CB750 frame.

ive heard a few things that i have questions about.

is it better to brace using triangles and welding the two sides that touch the tubes and make it solid or is it better to use trapezoids and weld the sides and leave the sharp point open and un welded?

heres what i was thinking
the forward area above the airbox
the triangle below the oil tank(on both sides)
the spot welds on the down tubes.
under the swingarm mount

i understand that if something is over braced it will then over stress some other area and one would run the risk of cracking or other wise not improving the frame.

thanks for any help and more so pictures of proper bracing.

[srook]

Check this thread out: http://forums.sohc4.net/index.php?topic=69141.0

Scott

[jaguar]

thats one of the threads i found.

i like the idea of the addition on a bar above the front motor mounts.

[MCRider]

thats one of the threads i found.

i like the idea of the addition on a bar above the front motor mounts.

To me the concensus priority is, the V above the swingarm pivot, the angle where the airbox goes, the angle at the lower front edge of where the gas tank would be, the steering head gussets on the downtubes should be welded up (they are pop riveted from the factory) Mines been done this way, pictures if you care.

Plate gussets should be laid on the outside of the tubes, and the points should not be welded. The gussets on the downtubes should be welded as upside down "U"s, with the bottoms left open. So I've been told.

[puks711]

i know this is an old thread but is there any chance you could show me some pics of where you have braced your frames please. thanks again

[RSchaefer]

You should get Hondaman's book, he shows pictures of the three specific locations to beef-up the frame.

[MCRider]

You should get Hondaman's book, he shows pictures of the three specific locations to beef-up the frame.

I've flipped thru it twice and couldn't find them. Page references?

[MCRider]

Here's what i did: (Not for performance, as I'll probably never ride it that hard these days. It was just something I'd promised myself I'd do, kinda "bucket list"ish.):
Swingarm gusset (should have been bigger)

Rear of engine compartment (should have been smaller. be careful with this one, see that it clears whatever you are going to use as an aircleaner. Most gusseted bikes use stacks. My pods just clear, stock air box would probably hit)

You can look across the bike to the double sided engine mount supplied by Rob at Voxonda, along with a tighter fitting motormount bolt.
My front of engine compartment are removable bolt ons due to the frame rail kit.

Photobucket has frozen up, I'll finish tomororow. Pic of front bolt on engine mount. Be aware here that on the lft side a gusset will interefere with the stock mount ignition switch which will need a new home. I see that HM puts his behind the cam cover in front of the choke lever using the OEM bracket.

Also will be pics of welding the downtube gussets.

I also had most of the motor mount bolts enlarged, fitted frame to engine and new stainless bolts made.

All for the sake of stiffness.

In a cafe bike, the rearsets can double as swingarm pivot gussets if made right.

[RSchaefer]

Down in Mexico, not w/my Hondaman book  .  I'm sure its there, send Mark a PM!!

[RSchaefer]

Hey MCRider I was looking at stainless all thread for replacement motor mounts for two reasons - I doubled up on the two triangle mount pieces as HM suggested, AND I got some of those small engine guards, cause I'm paranoid.  Oh wait I think I saw them on your pic as well.  Anyway I was concerned about the stainless steel strength issue relative to the motor mounts.  Did you look into this?  No issues using stainless all thread for the motor mounts?

[MCRider]

Hey MCRider I was looking at stainless all thread for replacement motor mounts for two reasons - I doubled up on the two triangle mount pieces as HM suggested, AND I got some of those small engine guards, cause I'm paranoid.  Oh wait I think I saw them on your pic as well.  Anyway I was concerned about the stainless steel strength issue relative to the motor mounts.  Did you look into this?  No issues using stainless all thread for the motor mounts?

Primary reason for stainless is resistance to road crud. In my case i had the work done at a race machine shop and let them pick the material. They took stainless rod turned it down to the new slightly larger diameter, bored the frame and engine,  then threaded the ends with the stock thread.

All thread is fine IMO, but won't achieve any additional stiffness. Either way the engine ain't goin nowhere. 

[RSchaefer]

Yes I see on Grainger were the finer threads have a higher tensile rating then wider threads that makes sense.  Guess I will go ahead with the stainless all thread.  Thanks

[MCRider]

I tried to put the ignition in the gusset, but i missed on the measurements. Put it on the handlebar instead.

Front downtube gusset weldup

[Dream750]

MCRider, nice clean looking frame mods you have!

Found these 750 K2 modified frame pics in my files, but don’t have any info to go with them.

(double click on pics)

[Retro Rocket]

If you are serious about bracing the swingarm pivot area the brace needs to be between the vertical frame rails above the actual swingarm, below the battery box. An "X" type tube brace is probably the best...

[MCRider]

If you are serious about bracing the swingarm pivot area the brace needs to be between the vertical frame rails above the actual swingarm, behind the battery box. An "X" type tube brace is probably the best...

As with all such mods, one eventually runs afoul of streetability. Such a brace negates the use of the inner fender.

I remember seeing such bracing on Tony Foale's lab mule Kaw 750 frame.

Many very successful CB750s, Big Benley for one, ran/run without such bracing. Anything is better than nothing, some a little moreso.

[Retro Rocket]

That is where the 750's flex the most at the rear, i know bikes with and without that brace that have been raced hard, i suppose it comes down to how much flex you like with your ride. I'm using a much stronger swingarm and modern brakes, which will put more stress on that area so i will use that brace, it does't foul the inner fender at all..  Haven't designed the brace yet but the bike will have an inner guard..  I screwed up Ron, i said behind the battery box, its actually under it almost, the brace goes on the vertical tubes the engine mount are fixed to, not the angled pieces that the shocks mount to...

[RSchaefer]

Found it, there are no pictures however, page IIIA-22 & 23