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Author Topic: New CB350 Project - Back Breaker  (Read 31473 times)

Offline motofiaccone

  • Posts: 1946
Re: New CB350 Project - Ball Breaker
« Reply #50 on: Feb 08, 2011, 20:33:49 »

Where and why? 
 
 

Seriously? Look at the load paths on this thing. As the neck gets loaded vertically, the bending moment is exactly where the backbone is preloaded--out of sheet metal. That's going to force those tiny upper engine mounts down and will ultimately fail. One pothole or a rough railroad crossing will ensure that will happen. But that's nothing...

In the font, at least there is a dampening mechanism to, well, dampen the load on the neck. In the rear, the wheel will take infinitely variable shock loads as the wheel bounces down the road. Introduce a pothole or some other traumatic bump--which we see everyday whilst riding--and that upper mount gets jammed into the backbone, again at the bending moment. That backbone? .090" sheet metal which is trying to buckle in it's stock form. We're not talking massive speeds or overly traumatic loads here. A 250lbs man just sitting on the bike could do the trick.

Really, I don't care. The more I look at it, the more I realize the catastrophic event will happen close to home and at very low speeds. My hope is that we at least get video or photo evidence of the bike and no one gets hurt.

http://i230.photobucket.com/albums/ee5/mk25diver/backbreaker7.jpg

--Chris

Offline KeninIowa

  • Posts: 63
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Re: New CB350 Project - Ball Breaker
« Reply #51 on: Feb 08, 2011, 22:34:45 »
All that sounds serious. 

What's a load path?

Can they be fixed?

Seems unkind to not care about another motorcyclist experiencing catastrophic failure, albeit close to home and at hopefully a slow speed.

WWCFD?


Offline VonYinzer

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Re: New CB350 Project - Ball Breaker
« Reply #52 on: Feb 08, 2011, 23:08:03 »
People have stated concern and were ignored...
Like a river that don't know where it's flowin'
I took a wrong turn and just kept goin'

Offline dynamitedave

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Re: New CB350 Project - Ball Breaker
« Reply #53 on: Feb 09, 2011, 00:36:14 »
I'm not even going to bother arguing with you guys.  The frame is good, the bike isn't going to break in half.  Shut up and enjoy the pictures.

Offline VonYinzer

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Re: New CB350 Project - Ball Breaker
« Reply #54 on: Feb 09, 2011, 11:24:54 »
Hahaha... Ok man.
Like a river that don't know where it's flowin'
I took a wrong turn and just kept goin'

Offline Flugtechnik

  • Posts: 3034
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Re: New CB350 Project - Ball Breaker
« Reply #55 on: Feb 09, 2011, 12:07:05 »
Chris, with all this fancy talk about loadpaths, it sounds like you should be in my line of work.  We could use more people that actually understand loadpaths.  I agree with your assessment.  The piece under the "seat" punching into the middle of the downtube will bring about a quick fatigue failure.  But it is his bike.  As my mentor said, "You can't push a rope."

I'm not even going to bother arguing with you guys.  The frame is good, the bike isn't going to break in half.  Shut up and enjoy the pictures.

Well said Dave.  We will enjoy the pics.  Keep them coming.

Offline dynamitedave

  • Posts: 135
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Re: New CB350 Project - Ball Breaker
« Reply #56 on: Feb 09, 2011, 14:28:28 »
I usually don't post very little thing I do on forums. I post a little text and some pictures and let you guys fill in the blanks with the hope that you guys will just assume the fabrication is good.  Clearly some of you have more time on your hands and  need more hand holding than I would generally provide.

Here is a picture I colored on to show what I feel are the load paths, dynamic and equalizing.  It shows gussets and load distributing triangles.  It shows the dynamic load (green arrows) going up the frame from the start point of the load at the rear tire assuming the pivot point of the load would be the location of the stock swing arm support.  It also shows the equalizing load path (orange arrows) that will travel through the gussets and supports to distribute the load and prevent failure.  In the picture below "A" is assumed to be the failure point if failure was to occur.  What you can't see and what motofiaccone assumed wasn't there is a plate that runs above and below the point of attachment to distribute the load.  Not that the load at point A would be that significant since the load is being distributed along the path of the orange arrows.



That backbone? .090" sheet metal which is trying to buckle in it's stock form.
The backbone has sheet metal in the construction (area "B" above) of it but unlike a stock CB350 there is a 1.25" steel tube (3/16" wall thickness) that is welded into the neck and is fully welded through the remaining stock lower backbone (where the top engine mount is) and then welded to the down tube at the rear of the engine.  There is also extra support connecting the pipe to the neck that was not shown on this thread.  The sheet metal piece I welded in is 16 gauge (approx. 1/16" or 0.06" thick which it comparable or better than stock) and is welded to the neck and along the backbone that I put in.  The angle of the triangle that the sheet metal makes is appropriate to distribute the load along the backbone of the bike.  If the angle was too steep it would not provide enough rigidity to support the load.  The shape made is almost a perfect geometrical triangle.

A 250lbs man just sitting on the bike could do the trick.
This thing isn't cardboard, its steel.  Do you really think a 250lbs man just sitting on the bike would cause the frame to break?  Seriously?  I'm only 170lbs so I guess I'm good to go right?

Feel free to get your crayons out and color on a picture of the bike if you want but ultimately without seeing what I have done in person and the steps I have taken to ensure the rigidity of the frame you have no sold platform to base your unfounded opinions on now do you?

Offline motofiaccone

  • Posts: 1946
Re: New CB350 Project - Ball Breaker
« Reply #57 on: Feb 09, 2011, 14:46:36 »
I certainly appreciate some quick research, but you're off on so many different fronts. I don't even know where to begin. You don't really get this stuff (chopping up a swing arm is a good example). And that's OK. Really.

Again, I don't want to get into a pissing contest, but I've seen pictures of fat girls. I don't need to see them in person to know they're fat. A photo does just fine. Same with you bike. A photo suffices.

You've got 27 posts on this forum and knowledgeable and long-time contributors are offering constructive criticism. This is a pretty positive forum with people who sincerely want to help others. Some people get it and others don't. And that's OK too.

--Chris

Offline KeninIowa

  • Posts: 63
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Re: New CB350 Project - Ball Breaker
« Reply #58 on: Feb 11, 2011, 21:36:58 »
but you're off on so many different fronts. I don't even know where to begin.

--Chris

Pick one, I'd like to learn

Offline MotorbikeBruno

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Re: New CB350 Project - Ball Breaker
« Reply #59 on: Feb 20, 2011, 17:37:54 »
I've never chopped anything too crazily so I'd like to hear more too!  Motofiaccone definitely has his stripes when it comes to motorbikes, so don't take his words as arrows or bullets...he's actually concerned for your well being, as am I and probably the entire site.  I'll offer this up to Moto....can it be fixed at this point?