Build pack onto and around frame tube, rather than inside?

[fatty]

That might actually work for me ....

Nice. LFP is safe and heavy enough already -- no need to make it worse with a heavy mount. And even 40s would easily clear the crank arms.
Are you planning to use short threaded rod to get the cell ends flush?

 

[LewTwo]

Nice. LFP is safe and heavy enough already -- no need to make it worse with a heavy mount. And even 40s would easily clear the crank arms.

I am going to take a bit of a closer look. Crank arms are about 7 inches apart. I found a dimensioned drawing of the Headway 40152 and they are actually a bit longer: 165.5mm threaded end to threaded end. That 'tightens' things up a bit :? However if I leave out the top two front cells it will still likely work for this frame and I like the numbers: 15 Ahrs at 52 volts (nominal) and 17 pounds. It also keeps the CG low and centered. They are a bit pricey.

Are you planning to use short threaded rod to get the cell ends flush?

Yes but I would also include a brass washer between the cells with a wire for a BMS.

I do not want to hijack the thread. I will open another if I decide to follow through with this.

 

[fatty]

They are a bit pricey.

BatterySpace lists them for a silly $40/ea, but I've seen other listings in the low $20s, and a package deal at $20/ea would be ~2.4Wh/$ -- that's maybe double what even cheap LiPo or Samsung INR21700-50E -- super cheap. In fact, the only higher Wh/$ I know offhand is Queen QB26800.

I do not want to hijack the thread. I will open another if I decide to follow through with this.

It's okay -- I think threaded LFPs are ideal for this idea.