More information as requested. Perhaps more than you wanted....
My current plan is to use sixthreezero's Evryjourney as my frame (
https://www.sixthreezero.com/customizer/mens-evryjourney-steel-touring-hybrid-bike). My goal for the bike is to have a solid all-a-rounder. Ideally the bike is solid on trails and roads, and has a cruising speed of 40-45 mph, top speed around 50 (low end is more important than anything over that - I dont plan on ever going over 60). Torque isn't all that important to me as I'm only 120 lbs. and don't mind pedaling on start up and don't plan on towing anything, but a dead stop start would be nice. Right now I have a flying horse engine (BBR tuning), but I plan on using the Phantom 85 for this next build.
Everything I have read about jackshaft kits so far has been very mixed. Some people have found lots of success with them and promote them, while others claim that there the power loss that they have makes them useless.
Before I get too far gone I'd like to make sure I actually understand jackshafts correctly. My understanding is that jackshafts are basically a transmission, allowing you to run you engine at lower RPMs while at higher speeds. It does this by using a chain to connect the 10 tooth clutch sprocket (that would normally power your rear wheel directly), to your jackshaft input sprocket (usually a 17 tooth sprocket). This is transferred through the actual jackshaft to the other side of the bike where there is another sprocket (usually a 10 tooth sprocket) that connects to the actual free wheel down by the cranks. This free wheel lets you pedal the bike to start without having to pedal at the same speed as the engine. The power goes into the freewheel at a 44 tooth sprocket and exits at a 36 tooth sprocket. At this point the power is transferred through the standard rear cassette which you can shift like normal.
10 tooth -> 17 tooth in (jackshaft here) 10 tooth out -> 44 tooth free wheel (connected via free wheel) 36 tooth out -> cassette.
Is this understanding correct? If so how does one find the best gearing for what they need (balancing torque and top speed)? Can the actual bike chain (that connects the free wheel to the cassette) withstand the torque and abuse associated with a jackshaft?
Thanks for your patience in explaining this to me!