get the "then" out of the link....
clever. hate forums full of these dead links or "user has removed picture" nonsense.
post the pics right here for all to see
anyways....
23... yeah, thats old enough...
unanswered (may seem irrelevant even...) questions...
previous experience with silly things like these? equipment available and/or funds available?
things to be wary of...chain pitches. finer is better, just for space and weight and smoothness and reasonably small sprockets...
your 4:1 X2 is just on the money.
covers... chains do serious damage when snapped... something a bit heavier than the tin foil used on that pic you linked
especially on the primary redux. (the fast one!)
the engine will always try to pull itself to the jack axle,twsit its mounts off, vibrate them off, etc etc. there is a LOT of force. mount everything securely, and dont think because its rigid when you try bending it by hand, that it wont break... ie, if you can isolate the whole thing from the frame BUT still have it held firmly and squarely...(3point clamping, etc) thats ideal. hose clamps and tek screws are not the way! nor are welders, if you can avoid them...
an outrunner bearing for the primary pinion on the crankshaft is almost mandatory... huge distance between the crank bearing and the actual clutch/sprocket makes for huge leverage on a small shaft... remember that on a grass trimmer the engine is NOT loaded like a sprocket will do, to the side... they only are designed for torsional(twisting) loads really...
personally, i was churning over the idea of running a flat pulley off the flywheel, removing the clutch and starter so i could "just" squeeze the engine in between the cranks, and using a slip-belt clutch to start and idle etc... (to get really fancy, one would remove the flywheel as well, and make a new one as like rc planes... but to suit this application)
if i was to run solely chain and a centri clutch, it would be on the jackshaft, with some fiddling to get the spring tension right for the lower speed.
(just like in your pic i now notice! which, to my mind, would buzz like a tuning fork being 1/4 inch(? if not its close!) alloy plate...any support up near the sprockets? and what exactly is being used for bearings because it looks like they are very close together! the sprocket facing us gets pulled towards the engine under load... on the top side. the clutch pinion gets reefed down towards the wheel...also on the top side... what happens if you get a bar pivoted in the middle and apply equal but opposite forces to the ends? yep. it twists.ie, a tap handle? and if the bearings arent going to flex...the mount will...that flimsy alloy plate? the more i look at that design the more i see wrong with it...
you want two bearings with a fair distance between them and as close to the sprockets as possible.)
that takes me to the last real thought... only my opinion...
sticking an engine between the cranks makes for a nice ride...IF the cranks stay standard. any oddly bent or offset cranks etc make it awkward to pedal. most engines are simply too wide for anything except the schwinn stingray type bottom brackets... once you factor in a clutch and sprocket and all that, the things are 300mm wide (long?) at least!
rack mounts may ride nice and make it easy to build but really are simply frustrating at all other times...im talking about leaving them against fences and stuff like that... too top heavy and at some point they will always fall over and scrape your shin despite not even riding the freaking things! by the third time its gone beyond a nuisance and you would rather stomp on the things once its down.
anyways, boil it all down... there will always be compromises and steep learning curves... cant have everything. especially on a bike!
yep. flange bearings are fine. ideal even. find the pressed steel carriers...much cheaper