Topic: Frame mod cb 750

[Terry in Australia]

If you're knowledge exceeds my observation, please accept my apology and I stand corrected if this is not possible on the OP's frame.

Apology accepted, if it was a practical solution, it would have been done long ago.

My reply was targeted at "installing the new tube outside of the original cut ends" which WILL crush those ends and potentially fail unless a Gordon's style kit is used.

Nope, the external sliders bolted thru the frame isn't a perfect solution by a long shot, but unless you're using 3/4" diameter bolts and a ten foot bar on your socket handle, you definitely won't crush the frame tube.

Having said that, I disagree with Scott and his assumption that the side rails may or may not serve any worthwhile purpose. They were designed as support rails for the top frame tube, and you don't have to be a mechanical engineer to understand why they are there, and what they do. If you don't, you should not be allowed to own a hacksaw, for obvious reasons........

[timbo750]

In the days before CAD a lot of things were over engineered and I am betting that someone told the engineers back in the 60's make sure the frame has no issues. The crucial component is the triangle brace in the middle - look at any bridge made out of steel or timber. it would seem that the sole purpose of the rails is to support the triangle brace and as such would only have forces along the length of the braces. All together it would make the top frame support much more rigid, most likely under hard braking and when the suspension was working hard over bumps.

This may of been said before and I am no expect on these bikes, it would seem that these parts are important but not crucial as proven by people in the past who have ridden for years with out it.

[Terry in Australia]

You're close Timbo, but it's actually the other way around, the triangle brace ties the top main tube to the two side support rails. The frames were still capable of some serious twisting, particularly in racing conditions. (not drag racing, but real racing, where bikes were made to go around corners really fast)

Race bike builders would further brace the frame by tieing the steering head to both the top rail and the support rails, tieing the front downtubes together below the header pipes, tieing the top tube and the support rails to the front downtubes, tieing the support tubes to the rear downtubes, tieing the rear downtubes together behind the carbs, bracing the area around the swingarm pivot, etc etc.

Here's some old posts of mine from the CR thread which show some great pics of "Big Benly", the race bike built by Patrick Bodden from "Cycle World". (from memory) Take a look at the bracing on it's frame, this is the way to make an old CB750 frame as rigid as some of the bikes that were coming out of Europe "back in the day". Removing the factory bracing might be convenient for in-frame top end work, but it certainly won't improve the handling. Cheers, Terry.

Here you go mate, I might have to photocopy the A3 pages and reduce them so they'll fit in my A4 scanner, but at least here are some images you can drool over. Cheers, Terry.

Here you go mate, it's a bit grainy, but the best I can do. Cheers, Terry.

 

[754]

ALMOST DONE the Prototype parts...real soon now..

[timbo750]

ALMOST DONE the Prototype parts...real soon now..

Frank, any teaser photos for us?

[754]

 These will weigh around 1 lb for all 4 joints
 A bit more machining o go on these..

[BPellerine]

nice Frank what do you think the weight will be with the split steel joint that I will be using on the rear?bill

[754]

 The steel kits are a bit over 4 lbs, so 2 lbs.

[BPellerine]

didn't think a couple of pieces of bar weighed that much but it really dosent  matter to me.just want to be able to change jets on my webbers.bill

[MCRider]

Looking good, I understand now, but... Is there enough room under the tank for that sushi roll to fit?

[754]

Everywhere I checked, there was clearance.

[andrewk]

Neat idea Frank.  Forgive me if this is a repeated question, but what sort of price do you think you will want for a set?

[754]

They will probably be 125, or lower.

[timbo750]

These will weigh around 1 lb for all 4 joints
 A bit more machining o go on these..

Frank, love your work. Its great when someone thinks outside the square and comes up with what is effectively a simple answer and it also looks very strong. Count me in for 2 sets.

also how do you get accurate machine work with eyesight like that? lol.

[MCRider]

Everywhere I checked, there was clearance.

My APE kit with thin walled sleeves was also very close to the carburetors.

[754]

Actually i have been checking on a 77/78 frame with flattops. Will take a few days to run some checks on the forktop carbs. The flattop carbs are a bit better IMO. For buildability, less clearance issues.

 Now for the maybe cool part, the part with the flat side by the screws, it is desinged to take an additionan brace between the two mounts, but it may not work with forktops, that will certainly not hurt stiffness.

[MCRider]

That would be good.

BTW: Not trying to give you a hard time, just trying to anticipate problems.

Carry on.

[754]

It only needs 3/16 inch or less clearance.

[Black 750K8]

So will it take four of the parts you show? would we do away with the short angle iron brace behind the coils?

[Terry in Australia]

Looking good Frank, well done.

[754]

Thanks Terry, I am just checking the clearances for K1 - 6, i dont have one here so doing it by phone.
 It will take 4 of the rounder version, or two of each if you want more bracing or to tie into valve cover.
 Still working on the prototype sets. They will get more machining and look better than they do in the pic posted.

 Re the triangle brace, you will be able to use it but I want to make the flat bars out of alloy, and fit tight against the rolled edge...keep everything light.

 The best part will be..six cuts to install.   AND..you can always just put it back to stock, by welding it back in.

 They will look like this..

[KrautKoffin]

Can't wait! My k7 is in dire need!

[Terry in Australia]

Thanks Terry, I am just checking the clearances for K1 - 6, i dont have one here so doing it by phone.
 It will take 4 of the rounder version, or two of each if you want more bracing or to tie into valve cover.
 Still working on the prototype sets. They will get more machining and look better than they do in the pic posted.

 Re the triangle brace, you will be able to use it but I want to make the flat bars out of alloy, and fit tight against the rolled edge...keep everything light.

 The best part will be..six cuts to install.   AND..you can always just put it back to stock, by welding it back in.

 They will look like this..

That looks great Frank, beautiful work. Cheers, Terry.

[timbo750]

Hey Frank, got any updates on the kits? how did you go with the early carb clearance?

[754]

We checked clearance on an assembled bike, and it checked out fine. Going to take a close look at old style ignitionswitch bracket, i think that will fit, but have to check. , i will be back n them next week .

HERE IS HOW TO CHECK; roll a 3/16. Drill or rod around the frame tube, in any areod concern, ie carbs..and coils, and wires.... But for it clears, you are good to go.