Topic: 1970 Honda CB750 Restromod

[scrapvalue]

If you guys could put your bongs down for a moment and bring back the subject of frame failure, what is going to fail with this mod?

Is the frame going to twist, break forward or backward?

The top tube is obviously not going to stretch or compress.

And I can't see where it could hardly twist.

So if there was a failure, what would it be?

If there was a failure, the lower frame tubes would have to fail also.

Plus once the motor was in an "all" the motor mounts were installed it would tie it all together also.

[scrapvalue]

O.K. put the bong back down.

It's not my frame, I am just trying to figure out where the "failure" would actually take place.




 

[754]

Alright you frame DOOMsayers..
 Please explain how Blue Turbo manages to keep his bike in one piece and going down the road with at least 50 percent more horsepower than this guy....

What is up with frames in general..

It has to meet or exceed the requirements of the rider..not the rest of the Universe.

[scrapvalue]

[/quote]


I only covered about 1/5 of the actual problems that can occur with a motorcycle frame. Acceleration, braking, passenger load, etc all are additional factors in frame stress. Static load also needs to calculate the weight of the motor, live load includes rider, motor, gear and passengers, kinetic force adds even more issues to the neck/frame.


[/quote]

Believe me, I understand where all the forces are coming from.

I have built a couple of stock cars from the frame up.   Had to take into consideration all the things you mentioned and the probability of very high impacts at all different angles and speeds.
The cars were built to with stand all the forces and then some. But also the consideration of weight.  Pounds cost horse power.

I am just not seeing where or how a major malfunction could occur with this modification.  Although I would reinforce what was left there a bit with a few gussets.

Inquiring minds want to know.

[754]

Blue turbos bike has the two lower frame rails cut out..like this one..
 I have had one with the rails cut out,toured two up a bit with it.
Had another buddy had a cut one, dragraced it a lot.. Rode it to Sturgis and raced it there..over 1100 miles each.way.. He was riding it at speeds as high as 125 mph on that trip..

[Retro Rocket]

Oh Frank, the last person in the world i would ask for handling recommendations is a drag racer, if you don't understand why the frame was built the way it was in the first place then why persist with the , "it won't make any difference" mantra. I could introduce you to many people that would laugh at your assertions and they would be 100% correct. The 750's weren't the best handling bikes stock as it is, this frame will flex like a #$%* and nothing you or anyone else says will change that. The thing that pisses me off with your answers is that you are smart enough to actually know this, you just choose to answer based around your straight line back ground. I know many guys that can do amazing things on just about any bike, ride around their inherent problems as if they don't exist, we all know people like that, they are knowledgeable enough to know what to do and the consequences of it if it all goes to sh1t, on the contrary, here we have a guy that really doesn't know and has been given some good advice, Look at the style of bike he's building..  In Australia that bike, with those mods, would be deemed unroadworthy, due to the weakening of the frame, those top rails are structural, whether you like it or not, and you know that... Your mate that rode his at "125MPH" is lucky....   The funniest thing is that you make frame rail kits that actually give access to the engine while keeping the frame safe...?? You trying to do yourself out of business..?   

[754]

Maybe you didn't read the last line of my second last post...Retro please re read that.
 To (last line)which I perhaps should have added..you may or may not be able to pass local inspection.

I never said it was a good idea,nor recommended it. I simply pointed out that despite the gloomy picture some posters were painting, that some folks have ridden their bikes like that..past and present..and some continue to do so.

 Far as I know we are not all bound by OZ rules,nor do I believe everyone in OZ thinks the same.
 His frame is cut back to far, now it has to be welded, isit fixable,  yes it is, but not without welding.

 Am I trying to put myself out of business,  no I am not ....have already done so..

[Retro Rocket]

Maybe you didn't read the last line of my second last post...Retro please re read that.
 To (last line)which I perhaps should have added..you may or may not be able to pass local inspection.

I never said it was a good idea,nor recommended it. I simply pointed out that despite the gloomy picture some posters were painting, that some folks have ridden their bikes like that..past and present..and some continue to do so.

 Far as I know we are not all bound by OZ rules,nor do I believe everyone in OZ thinks the same.
 His frame is cut back to far, now it has to be welded, isit fixable,  yes it is, but not without welding.

 Am I trying to put myself out of business,  no I am not ....have already done so..

I addressed your first part, Oz rules or not mate, its illegal here, so it doesn't matter what anyone else here thinks, I'm talking about the frame generally, yes its fixable, but now it needs someone who knows what they are doing to do so, this could have been avoided with a question first, you could have even sold another one of your well made kits... ...

[754]

The only way he can use my kit is to sleeve and weld a new tube front to back.
rear needs internal sleeve and or at least welded  all around..front could be tacked,  or clamped...then the rear part could be cut again..further ahead of seat mount area.

Probably other solutions,including swapping frame..

[754]

I did not say anything about safety, I simply reported. That people ride with them that way , and are neither complaining,  nor bothering to fix things.
Make your bike the way you need it to do what you are going go do with it.

[Retro Rocket]

The only way he can use my kit is to sleeve and weld a new tube front to back.
rear needs internal sleeve and or at least welded  all around..front could be tacked,  or clamped...then the rear part could be cut again..further ahead of seat mount area.

Probably other solutions,including swapping frame..

Swapping the frame would be cheaper and easier...

[754]

I suspect. That frames are harder to find over there..
 And the lower rails fix is far less work than installing a weld in frame kit..

 I went without a welder for so long, that it became a big pita to get anything welded which to an extent held me back.. But I know own a small Miller mig, and may finally get on to some frame mods I have been thinking of for a few years.... Soon as I find somewhere to work..
 Got a plan to take the fugly out of a CB 750 frame for customizers.

[SOHC4 Cafe Racer Fan]

Got a plan to take the fugly out of a CB 750 frame for customizers.

Which portions of the frame, Frank?

[754]

Mainly. Front of the seat area and tank mount area..

[Retro Rocket]

I suspect. That frames are harder to find over there..
 And the lower rails fix is far less work than installing a weld in frame kit..

 I went without a welder for so long, that it became a big pita to get anything welded which to an extent held me back.. But I know own a small Miller mig, and may finally get on to some frame mods I have been thinking of for a few years.... Soon as I find somewhere to work..
 Got a plan to take the fugly out of a CB 750 frame for customizers.

I have an idea for a good mod for a Honda frame too Frank, for a custom build that is... It also involves a single center section of frame as well , but not a hack job, it also involves the area you mentioned, as well as replacing the center rail with something more rigid, or adding some strength to it....

[754]

I would like to get..(this is not what I mentioned  above) a different approach. For a bike with Webers.. Take out two lower rails under the tank, and the side braces from mototmount to seat area.
 Then run across the top of motormount connecting the two mototmounts up top.
 Then using a second backbone under the original (hopefully with room for coils) then running back to rear tank mount area but curving down to meet up with the Centre of the new horizontal tying the top of mototmount.. together.

 Mainly for unlimited aircleaner selection for Webers, and to be able to run an alloy tank...like in the picture.

[Retro Rocket]

I would like to get..(this is not what I mentioned  above) a different approach. For a bike with Webers.. Take out two lower rails under the tank, and the side braces from mototmount to seat area.
 Then run across the top of motormount connecting the two mototmounts up top.
 Then using a second backbone under the original (hopefully with room for coils) then running back to rear tank mount area but curving down to meet up with the Centre of the new horizontal tying the top of mototmount.. together.

 Mainly for unlimited aircleaner selection for Webers, and to be able to run an alloy tank...like in the picture.

I was thinking about the same for the side sections up from the rear engine mounts but making a new center rail, I would remove the side and center rails and make the new section out of folded and boxed steel sheet, straight at top and curved down  {underneath} to a full height section at the front meeting the steering head, at the rear it would open out to accommodate the new side frame sections , it would look similar to the front of this frame, except made out of folded sheet...Am i making any sense..?

[Duke McDukiedook]

If you guys could put your bongs down for a moment and bring back the subject of frame failure, what is going to fail with this mod?

Is the frame going to twist, break forward or backward?

The top tube is obviously not going to stretch or compress.

And I can't see where it could hardly twist.

So if there was a failure, what would it be?

If there was a failure, the lower frame tubes would have to fail also.

Plus once the motor was in an "all" the motor mounts were installed it would tie it all together also.

OK, I am an engineer and have had some structures classes so I can speak with some authority until a mechanical or structural engineer comes along to correct me.

The problem with removing those two top tubes is you are removing the space fame structure that helps to stabilize torsional forces on the overall frame. This type of twisting can weaken the connection of the top tube connections and result in shear failure at those connections.

Those top tubes are not there for redundancy, I can tell you that. That would mean an overall localized reduction of resistance to moment, shear and torsion forces by at least one half.   

[70CB750]

I say the frame will develop cracks where down tubes enter the head - how soon and how serious is hard to say.

But I did not achieve more than B in my Pružnost a pevnost/Strength of Materials college class 

[drkush]

Sorry I have been gone so long, and yes I do have the kit and No I'm not the one that removed the bars, I received the bike that way, course I did get the bike for FREE, But Yes I have the kit


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[eigenvector]

If you guys could put your bongs down for a moment and bring back the subject of frame failure, what is going to fail with this mod?

Is the frame going to twist, break forward or backward?

The top tube is obviously not going to stretch or compress.

And I can't see where it could hardly twist.

So if there was a failure, what would it be?

If there was a failure, the lower frame tubes would have to fail also.

Plus once the motor was in an "all" the motor mounts were installed it would tie it all together also.

OK, I am an engineer and have had some structures classes so I can speak with some authority until a mechanical or structural engineer comes along to correct me.

The problem with removing those two top tubes is you are removing the space fame structure that helps to stabilize torsional forces on the overall frame. This type of twisting can weaken the connection of the top tube connections and result in shear failure at those connections.

Those top tubes are not there for redundancy, I can tell you that. That would mean an overall localized reduction of resistance to moment, shear and torsion forces by at least one half.   

This structural engineer can agree that without those additional connections, yes indeed - the top tube is essentially useless.
I would argue that the center tube isn't particularly valuable, and if you relocated the side tube connections to the top of the steering head, you could do without it.  You'd still need the lateral braces.

[Don R]

 As I suspected, everyone jumped on the OP assuming he cut this frame. I've bought a couple like that, it was common once.

[Don R]

 Agreed, I fixed both of mine one with welded tubes, one with a kit, but on the other hand I've still never heard of one breaking and there were a lot of them hacked up judging by how many of them I've ended up with.

[bathcollector]

Has anyone heard or seen a frame fail from this mod?   Just wondering.

Yes. October 1971 HotBike magazine have an article with Action 4,s guys, Bill Hahn and Fred Stepp  modding a frame for Drag racing just like this. Maybe the original guy was going racing ?

[Retro Rocket]

Has anyone heard or seen a frame fail from this mod?   Just wondering.

Yes. October 1971 HotBike magazine have an article with Action 4,s guys, Bill Hahn and Fred Stepp  modding a frame for Drag racing just like this. Maybe the original guy was going racing ?

He said frame "FAIL" mod, not frame rail mod, I have one like it too but I wouldn't ride it like that on the street..