whit3rd wrote:

John Doe wrote:

Using 6061 3/8" thick aluminum to hold the front sprocket on a
bicycle.

A 1/2 inch square hole through 3/8 inch thick aluminum fits over the
bottom bracket square tapered spindle.

It's gonna fail.

What's gonna fail?

Why can't you use a proper crank with spider that takes replaceable
ring gears?

Because I'm using a 450 max RPM motor to turn the sprocket/chainring.
The chainring must be extraordinarily small, like 14 to 18 teeth. There
are no crankssets with chainrings that small. That, plus I wouldn't
want/need the extra hardware.

The thing is, even if it took chain tension, any wobble at all from
the rotation axis is going to loosen its contact with the square
spindle.

I've already made a chainring holder that I'm using on my current
electric bike. You can see some of it, the white wooden block, in this
picture.

https://www.flickr.com/photos/275322...in/photostream

The motor is weaker but it's max RPM is 600 RPM (33% faster). The
chainring holder failed once but it was an easy repair. Instead of a
thick aluminum slab like the current effort, that block of wood is
superglued to the outside of the socket shown in this picture.

https://www.flickr.com/photos/275322...in/photostream

There will be very little side to side wobble as the aluminum slab
rotates. The drive end of the socket shown in that picture will be
buttressed up against the aluminum slab (sprocket/chainring holding
piece).