Cut frame to accept layback brace?

[KD5ZXG]

If no obvious problems demand forewarnings, here's my rough plan:
Want to cut the frame to a stub just tall enough for seatpost clamp.
Then want to cut a long U down the back to clear the layback brace.
Already a small thin U cut there, else the post clamp wouldn't work.
Fortunately lots of "RatRod" gusset on the front side of this area.

Also grind and narrow sides of the brace weld where it must now slide
into the frame. Maybe braze some extra fillet top and bottom instead?
Picture doesn't do a great job of showing the weld interference problem.
Finally, clamp the frame-top stub through the eye of the layback brace.

Problem here seems the slight difference between 25mm and 1inch.
Tempted to abuse a beer can instead of masking tape for the shim.
Maybe a can of tin plated steel would be less galvanic vs my frame?

Turn and stop lights now have to go somewhere else. Back of seat or rear fender?
Ammo box battery paneer (not yet installed) another place light cluster might go.

 

[RustySprockets]

If nothing else, I'd say first make sure the post clamp can fit into the 'eyelet' before you commit to any cutting. I predict the brace will obstruct that in some way.

 

[toro1978]

Why not just cut the brace off?
I did on mine and it hasn't bent even a milimeter, I weigh around 200 pounds.

 

[KD5ZXG]

yeah, but you added a stem or something to brace the back end. I do have a pair of struts leftover from the original Electra front fork that could be abused underseat as sissybars. Still, I see no great difficulty with keeping the brace.

 

[toro1978]

Oh, you have seen that monstrosity? That stem is only used there because I cut off the seat clamp and anyone could pull the seat out - its actual purpose is theft prevention and not support. Originally there was no stem, but a chain lock preventing stealing the seat. Since you are keeping your seat clamp you don't need that stem "solution".
I only suggested cutting the brace off because it seemed less trouble than cutting the frame.

 

[KD5ZXG]

Test fit of the clamp:
Opens just far enough that getting it past the brace and into the eye was a nondestructive process.
However, the brace is in the way of tightening. Moving up till the clamp fully clears the brace is not
an option, cause the post is becoming narrow. But the clamp can be turned backwards or sideways
and still function. Worst case, I got a box of heavy hose clamps. Same variety as the motormount.

Keep in mind this clamp's new function will mostly be to keep the post from bending backward.
U shaped groove would not let this brace turn but a few degrees, nor could the brace ever slide
down further than the groove. Familiar seatpost concerns of twist and drop don't seem to apply.

I've got a new problem to solve with seat coming off the end of the post! Sitting force tries to
slide backward and come apart, not downwards to hold together. A nub inside the underseat
clamp keeps a normal upward oriented post from skewering the rider. But in this orientation is
preventing secure assembly. Half of the underseat clamp can't get a bite on the round end of
the post. If I can just slide it forward onto the straight part, should clamp better. So that nub
probably needs to go. Problem merits a photograph, tomorrow morning maybe...

 

[SSG]

I personally think it would be better to just find/fabricate a seat post that works better rather than hacking up that frame.

 

[KD5ZXG]

Its a "RatRod", not hacking this frame would be a grave sacrilege.
Nightcheese sais 'go for it.' Not going to argue with Nightcheese...

Underseat clamp also demands some minor work.

Nub might have to go. I'll try bending before cutting.
Perhaps notch or nub the post to match? But simply
won't do without some southern engineering to fix
what wasn't broke before I made it so.

 

[RustySprockets]

Affixing the clamp collar with its "fastener forward" wouldn't be the end of the world. Like you said, it only has to prevent the post from sliding out, as opposed to dropping or twisting.