Topic: Frame Bracing

[spiritof67]

Does anyone have a diagram or pics of the standard must-do bracing for a stronger CB750 frame? Is a brace behind the steering head or in the front down tubes recommended? Please let me know...

[spiritof67]

Anyone? Anybody got diagrams, pictures, recommendations?

Spirit

[70CB750]

Hondaman mentioned bracing in his book:

http://www.lulu.com/us/en/shop/mark-paris/my-cb750-book-hardcover-edition/hardcover/product-16107855.html

I don't remember where and what since I was never planning on doing it.

[MCRider]

Anyone? Anybody got diagrams, pictures, recommendations?

Spirit

In my experience, the main bracing goes as follows: The angle where the original ignition switch would go, and the rt side; follow that lower horizontal tube back to the angle behind the air box, and the rt side; travel down the tube to the "V" above the swingarm pivot, both sides. Finally the downtubes from the steering head to the engine, are connected with a pressed fit and riveted gusset. This should be welded shut on the vertical, not the horizontal. You can go nuts but that would make it plenty stiff for competition. re: Tony Foale.  Pictures to follow.

[MCRider]

Top right was a little big, above swingarm a little small. Points of triangles should not be tacked down.

Front downtube gusset


My front frame gussets are bolted on, to accommodate my removable frame tubes.

[MCRider]

More, better pics here:
http://forums.sohc4.net/index.php?topic=108300.0;nowap

[MRieck]

That is some nice welding on the kickstand pad Ron.

[MCRider]

That is some nice welding on the kickstand pad Ron.

Thanks: Coourtesy "Bob the Welder" at Saldana Racing PRoducts. Unfortunately, I measured wrong and it was too long to use. I went thru several, CB450/350 with different cut and welds. Never could hit it.

So I bought an adjustable one from Soupy's in Michigan.
http://www.soupysperformance.com/catalog/item/4408900/7420224.htm

He actually didn't make one for the CB750, only modern sport bikes. But a little begging and a few photos later I had it. It has some nice welding too, attaching the footpad. Its mostly carved from billet.

Here's the final with the optional large footpad, and Brown's chrome.



It'll go 1/2" shorter and about 1.5" longer, so no matter which shock or tire profile I use it fits.

(sorry for the hijack)

[Don R]

I put a gusset on the bottom of the swingarm tube to the frame. You can't really see it unless you get under.

[spiritof67]

I note that in all cases no one did any bracing around the oil tank area or triangulated the tops of the frame tubes at the back of the regular fuel tank area. I can see a lot of reasons to do that, just as obviously you can't make the area around the steering head too strong. Any thoughts on this ? Perhaps something tying together the upper frame tubes and the crosstube for the rear swing arm?

[MCRider]

I note that in all cases no one did any bracing around the oil tank area or triangulated the tops of the frame tubes at the back of the regular fuel tank area. I can see a lot of reasons to do that, just as obviously you can't make the area around the steering head too strong. Any thoughts on this ? Perhaps something tying together the upper frame tubes and the crosstube for the rear swing arm?

Huh? My example has a gusset at the bottom V under the oil tank. and at the back of the fuel tank area. Welding the headstock gusset to the downtubes shores up the steering head.

Tony Foales discusses these and a "pyramid cross" behind the carbs tying the swingarm pivot tube to the side downtubes. There comes a point where enough is enough. But its all in fun anyway, mostly.

[JohnN]

I was hoping to see more suggestions so I'll revive this thread. I have found lots of ideas and suggestions on this subject but not much theory on what is best for the CB750 or based on trial and error. I installed an x-brace on mine per Hondaman's description in his book. I'm not sure if I made the x big enough so perhaps Mark will comment on the picture attached. I've had some stability problems on the track so I may add a brace to the front downtubes next. I'm also adding a second bracket to the upper rear engine mount. I already doubled the front mount as Mark has described.
John

[MCRider]

Here's more.
http://forums.sohc4.net/index.php?topic=84383.0
Also search "Big Benley" for tried and true mods. Theory or no.

IT's all a matter of degree and riding style. There's no end to how stiff you can make it. And for some, more is better, for others, not so much.

[bwaller]

The purpose of the "x" brace is usually to add support to the swingarm pivot area and should be as close as possible to the pivot. Saddle type bracing in those triangular areas is the strongest method and can be of lightweight material. A 1" piece of tubing between the rails near the upper shock mounts adds strength there.
Here's a comparison of my latest 550 stock frame upgrade. The lightweight chromoly tube replacement of the pressed steel backbone offers greater rigidity & much increased steering head support.

 

[calj737]

Bwaller - thats such a beautiful frame you've got there. With all the modifications and added bracing, I wonder why not complete the "upgrade" with a completely new seat hoop and cage? Was it driven purely by cost or was it stated by the frame builder that these mods were sufficient and wouldn't overwhelm the original areas left of the stock frame?

One thing is for sure; today's modern tubing is both stronger and lighter than the factory metal from 35-40 years ago. I think if I were upgrading the HP substantially and putting my butt in a race environment, I'd pursue this same level of strengthening/upgrades.

[bwaller]

I would have preferred a completely new lightweight frame, but to comply with class rules the frame must be stock based. With a transverse tube between the shock mounts the rear frame isn't a problem.

Honda chose to support the steering head with formed steel on many of these frames, the stock 500 model is quite good, but if there is a weak point it's this backbone. Without major surgery it is difficult to improve this. Turboguzzi built a top engine mount to help stiffen this area, and I have seen a few CB750's with a similar "head steady". I bent a frame this season and the weakness showed, that's why I chose this path.
The other area that probably helps to stiffen for a race bike is that area around the swingarm pivot.

Like most people, my first attempt at frame bracing was mostly guess work and maybe only added weight. I'm no expert, but I have a friend who is a builder and he has helped educate me a little.  I do know the frame I used originally was not straight in several areas, it is surprising how this is possible. I'm certain it is now and wheel alignment is perfect, something I struggled to understand previously. 

[calj737]

Clearly see your points. I've often wondered if the rear engine mounting flanges themselves weren't underperformers already. I've seen many who upgrade the hangar bolts, double side the bracket on the right side, but the flange on the left always struck as me as very weak.

Would removing and replacing that with higher grade steel violate class rules in your situation? Not suggesting anything, only inquiring.

[MRieck]

That looks really nice Brent. You are going to give the competition fits.

[bwaller]

Clearly see your points. I've often wondered if the rear engine mounting flanges themselves weren't underperformers already. I've seen many who upgrade the hangar bolts, double side the bracket on the right side, but the flange on the left always struck as me as very weak.

Would removing and replacing that with higher grade steel violate class rules in your situation? Not suggesting anything, only inquiring.

No, I have higher grade bolts. I had envisioned another possible solution after reading what others have tried. The engine would be more solid support for the S/A pivot if the hanger bolts were a "reamed fit". I thought about reaming the engine lugs/frame mounts slightly larger, inserting thinwalled tubing then better bolts but in the end I doubt it's any better than a frame support. Removing the bolts would like become a nuissance.

I wanted to upgrade to a hollow 20mm pivot axle and install the frame bosses slightly lower to help straighten the swingarm angle, but it's a big job. I'm quite certain I'm not fast enough to notice the difference anyway!

Mike, the only thing I know for sure is that finally everything is in alignment. I suppose the rest is up to me. 

[MCRider]

My frame enhancements are purely for show. My theme was to do all the things that I never had the resources to do bak when it would have made a difference to me.

One of those things besides the bracing, was to tighten up the engine mounts bolts and holes, per your comments bwaller. Somewhere I'd heard years ago that was done by those who were serious about it. Besides using Rob Voxonda's double palte rear mount, he provided a new turned down hanger bolt which is a slight friction fit thru the frame and engine, I welded the case guard on the right side to the motor mount, making a double out of it. Then I increased the bolt size from 8 to 10mm. For the 2 main lower bolts, I had the frame and engine holes enlarged and matched, then new stainless bolts were turned down to a friction fit. This required enlarging the holes on the foot pegs too.

All this is for bragging rights. And some might argue it is detrimental. But that's what I've done. Should be stiff.

[Tintop]

Frame looks very nice Brent, Denis' work?  Given that about the only thing remaining of the stock frame / swingarm is the engine cradle not sure how 'production' it still is.   Probably OK though, as all the rest of the class is use to seeing is the backend.

[JohnN]

Brent:
 Nice work on that backbone replacement! I agree with your comment about placing the x as close to the swingarm pivot as possible. On the 750 the frame actually compresses slightly when you tighten the pivot bolt until it contacts the collar. Thus I was hesitant to add too much bracing at that point that could have affected the clearances.

Do you have any details regarding the head stay you mentioned? Was the valve cover modified to accept the attachment point?

John

[turboguzzi]

Clearly see your points. I've often wondered if the rear engine mounting flanges themselves weren't underperformers already. I've seen many who upgrade the hangar bolts, double side the bracket on the right side, but the flange on the left always struck as me as very weak.

Would removing and replacing that with higher grade steel violate class rules in your situation? Not suggesting anything, only inquiring.

No, I have higher grade bolts. I had envisioned another possible solution after reading what others have tried. The engine would be more solid support for the S/A pivot if the hanger bolts were a "reamed fit".

back in the early superbike days, pierre des roches, the tuner of the Vetter Kawasakis seemingly used to do just that. I thought it'd give me real headaches to mount the engine if it's a real tight fit.
BUT,I upgraded the rear bolts in both the CB and GPZ racers to a bigger size sourced from newer models.  What was interesting in many of the modern motor mount bolts i found in swap meets is that they had at one end a long cylindrical surface that is only gripped form the side, hence you dont feed any twisting forces to the frame when nipping then up.... a bit hard to explain, but got the idea? 

[bwaller]

Tintop I have a picture from 74?? at Laconia where someone made these similar changes. It's a "modified stock" frame, here's what was replaced.



John, sorry I couldn't find a picture but there was definitely welding on the top of the valve cover. I didn't want to appear too interested and is was tough to see what they had done! There were two 750's pitted together this summer and both were done, although not exactly the same.

What I did around the swingarm was 1st, torque the engine bolts, then the arm pivot bolt, then weld the brace in place. When undone, the frame relaxes enough to remove the swingarm, I just had to grind part of any excess weld from the frame to slip the arm out of the frame.

[Tintop]

Just pulling your chain bw.

Bob Hansen use to do something similar with the 350 frames.  Added horizontal tubes from the backbone/seat area forward to the neck.  They overlapped the down tubes in a similar fashion.