Will this become a design flaw?

Posted this on my build thread! But wanted to get it out there too there who might not be following the build.

Does anyone see any catastrophic design flaw here with the rear section?
I really like the dropped look, and it allows the rear hard tail section bar to match the angle of the backbone, which is most appealing.



My biggest concern with this is I'm drastically limiting triangulation of the rear end, and we all know triangles are strong.

Thinking of rolling a section of tubing to a radius and utilizing that as a support.
Like this.



My other concern is the force of the backbone on the center hoop.
Normally, I would assume the rear section would provide support at the top of the hoop, but with the rear section dropped a few inches, it would allow the top half of the hoop to want to force back.
Of course it would take some serious force to move it, but rathar be safe than sorry.

All meterial used is 1" .125 wall DOM

Aaaaannnnnd discuss.......

It is already a design flaw if you are worried about it. Peace of mind is the key grasshopper. Make the upright section into an "A", and bring a lower bar in above the motor back to the crossbar on the "A".

Bonehead said:

It is already a design flaw if you are worried about it. Peace of mind is the key grasshopper. Make the upright section into an "A", and bring a lower bar in above the motor back to the crossbar on the "A".

Click to expand...

To be honest, my buddy really likes the drop bar, but isn't too keen on the additional support.
I told him I was unconfortable with not having the extra support. He asked if it was really needed, and your comment is exactly what I told him.
I would much rather have this additional support since it makes me more comfortable.

In my opinion if you add in that support piece that you showed in the second pic that will add a lot of strength to those members. You are then seperating that section into two smaller triangles.

As far as the center hoop goes i would take a piece of steel plate and cut it out to match the inner profile of the hoop. The plate doesent have to cover the entire inner area. I would go down to where the rear "hard tail section bar" connects to the hoop. You could cut the bottom of the plate in a half circle shape to make it look nicer.

If you are still concerned with the strength at the point where the back bone ties into the top of the hoop. You could take some more steel plate and make triangular shaped filler pieces, one for the left and one for the right side. These would tie the back bone and hoop together on both sides. This would give it much more strength against torsion and the forces it will see going down the road. Since it is a rigid rear end the impulse forces that it will see can be pretty large.

I hope that my explanations are clear enough and make some kinda sense. I'll try and make a couple sketches of what im talking about and post the pics when i get home from work. As they say pics are worth a thousand words.

Disclaimer: These are my opinions ive never built a bike before. Im just looking at it from an engineering stand point.

Good Luck,

-Chris

EDIT: I've tried uploading a picture showing the gussets that i was talking about but i cant upload pics for some reason [S

What about an "S" bend off the main tube down to the rear tube? It would be hidden, but connect the two as you would have liked to see?

The admins fixed the picture uploading issue... so heres my sketch that i made in paint of the gussets i was talking about in my earlier post. They are the objects in red. I think these would really tie everything together and add a lot of strength, just an idea. Good Luck.

-Chris

This is what I had in mind: