Topic: CB750 Fours using altered OEM frames

[kos]

Frames breaking from racing loads. YES, we have experienced that on more than one occasion. We broke the down tubes just where the frame down tube is attached to the upper lugs that come down from steering head. This was due in part to high engine revs, as this was a roadracer and very high loading of the frame tubes from extreme high braking forces with Lockheed dual brake calipers and World Superbike compound rubber on Dunlop tyres we were using. The fork tubes would be  to the point of bending and thus, and steering head was taken all these loads, along with frame downtubes.

We found a way to ad material to frame where lugs intersect the frame tubes... and problem never came back.


Happened twice, lower frame tube completely broken .
KOS
M3 Racing USA

[wannabridin]

this doesn't seem to be a problem in a road bike though, so i would guess that what MC stated would be more than ample for a street machine? 

[Big Bob]

Anyone know for fact the frame is chromed?

 Almost all plated racing frames are nickle plated, not chromed...for a good reason..

If you're referring to Mike Hodgson's bike (in the pics I posted) the frame is nickle plated.

[Big Bob]

this doesn't seem to be a problem in a road bike though, so i would guess that what MC stated would be more than ample for a street machine? 

More than ample?  Yes.

For the last five years I've been racing a 1970 CB 750 in AHRMA's Production Heavyweight class.  Stock displacement, Webcam, ported head with stainless valves.  Big jets, needles all the way up, Dyna S.  Stock frame.  Roller bearings in the swingarm, rollers in the steering stem.  Progressive springs up front with a lot of preload, spent some money on good shocks.  Dunlop GT501's front and back on straight rims with heavy duty spokes and new bearings.

I weigh 320 pounds. 6'2".  Scored wins at Mid-Ohio, Road America, Roebling Road, and Daytona.  Got on the podium at those tracks and at Grattan, Gingerman, and Barber plenty of times.

I beat that bike like a rented mule and it did not wobble, chatter or skip one bit.

[MCRider]

Frames breaking from racing loads. YES, we have experienced that on more than one occasion. We broke the down tubes just where the frame down tube is attached to the upper lugs that come down from steering head. This was due in part to high engine revs, as this was a roadracer and very high loading of the frame tubes from extreme high braking forces with Lockheed dual brake calipers and World Superbike compound rubber on Dunlop tyres we were using. The fork tubes would be  to the point of bending and thus, and steering head was taken all these loads, along with frame downtubes.

We found a way to ad material to frame where lugs intersect the frame tubes... and problem never came back.


Happened twice, lower frame tube completely broken .
KOS
M3 Racing USA

Thanks I love those back stories. I'll never ride that hard.

[MCRider]

Big Bob: Did you do any gusseting to your frame?

Oh I just read "stock frame" so I guess not?

[kos]

Big Bob: Did you do any gusseting to your frame?

Oh I just read "stock frame" so I guess not?

Shoot, I know Bob...he just ran it like a RENTAL!


KOS

[kos]

Anyone know for fact the frame is chromed?

 Almost all plated racing frames are nickle plated, not chromed...for a good reason..

If you're referring to Mike Hodgson's bike (in the pics I posted) the frame is nickle plated.

That's the shiniest nickel I have ever seen!!!

KOS

[kos]

this doesn't seem to be a problem in a road bike though, so i would guess that what MC stated would be more than ample for a street machine? 

WE raced at the highest level at the time and pushed the limits of every component. Street bike, production race bike would not be be a problem...read Big Bob's comments. The only ting I would worry about is is the usage if the bike was wheelied a lt. Dropping that heavy front end down many times, might make it break or al least start to crack frame tubes?


KOS

[Doctor_D]

That's the shiniest nickel I have ever seen!!!

KOS

Because nickel is soft and prone to scratches and scuffing, many platers recommend a clear powercoat over the top of the nickel.

[wannabridin]

well then, i guess i won't worry about any gussets before i strip the frame and send it to get powder coated!!  i'm 6'3 and 200 lbs, and this old girl won't wheelie, nor will i try, so that's comforting!!  i WILL be helping the suspension out and use Rob's motor mounts though, i don't see what's wrong with that!

[754]

Nickle frame can tarnish fairly quickly in certain climates.. my buddy had/has a nickle plated K2 750.
 Where we live it would stay shiny, in LA it would tarnish pretty quick..

 Be OK if you keep it indoors most of the time

[Big Bob]

Anyone know for fact the frame is chromed?

 Almost all plated racing frames are nickle plated, not chromed...for a good reason..

If you're referring to Mike Hodgson's bike (in the pics I posted) the frame is nickle plated.

That's the shiniest nickel I have ever seen!!!

Dude likes to live large.

[Big Bob]

Big Bob: Did you do any gusseting to your frame?

Oh I just read "stock frame" so I guess not?

Nope.  No gusseting.  Just replaced all the wear components with high quality items.

[voxonda]

This is how I started:


Grinded and rewelded some of the welds, added some minor gussets and stronger engine mounts and stronger, longer swingarm.

[wannabridin]

i like what you did for your rearset support, def. something to think about.  what material did you use for the gussets Rob?

[voxonda]

Hey G,

Some sheet metal from the shop, nothing fancy. The steel from the frames is crap, so no need to use other materials. Still these 'crap' frames excist for over 40 years. The Japs surely must have done something right.
Think I said it before, there is not so much wrong with the frame. It is more the pivot area's that need attention, and in my experience a longer swing (box or F2 item) and triples from the DOHC's which have a offset of 60 m/m instead of 50 m/m. Makes it stable and still likes to turn in. For all out racing there are other options.

Rob