a project that's been taking me few days to resolve was my dads bike where the chain kept falling off the big front pedal sprocket when in low gear (offset between the large front & rear sprockets too large).
I lathed this shaft to hold the cogs from a rear derailer and use it as an idler gear to keep the chain moving straight to/from the pedal gear
idler cogs mounted to a stiff L-bar bolted to the frame.
also notice I welded up a new pedal sprocket closer to the center to reduce the offset between front/rear gears
(welded the big sprocket onto an old steel cutting disc)
but this didn't work. the steep angle of the chain simply made it skip off the cogs, especially when changing down-gear and the chain rattles up/down.
also the sprocket wasn't welded perfectly straight & concentric on the flat disc.
so made these flanged idler wheels to hold the sides of the chain
this kinda worked (flange wasn't high enough to enclose the chain) but due to the oscillating shape of the chain on the smooth wheel, it vibrates too much and very noisy.
so rather than use this scrapheap welded off-center pedal sprocket, decided to use the original straight crank sprocket and rather than lathe & retap a new length of rod, I lathed a threaded rod to hold the cog
the straight round pedal sprocket helped but the angled chain still skips off
instead of using wheels to guide the chain, I decided to fully enclose the chain with a tube so it has no chance of skipping off the pedal sprocket.
wanted to flare this tube to smoothly guide the chain from all angles, so made this cone forming tool
welded them to threaded rod for adjustments
mounted to the L-bar, it now works soo well
other than the slight chatter of the chain running along the tubes, the chain no longer falls off
christ that was quite a distraction and made a mess in the garage.
now I can begin fixing the loose driveshaft