It is because I use the Comet 20 torque-a-verter and they run a secondary chain to the jack shaft and then the jack shaft runs a 'driver' sprocket to a 'driven' sprocket. It's really cool and I believe the shorter the chain runs the less apt you will have a chain fall off. I will download some more photos and I hope you buy a Jack-shaft kit. You can use it with any engine or motor you chose because you can choose any sprocket you want.
I will help you along here and I suggest a good welder for the job and you won't be disappointed. It also allows you to use a wide range of engines and placement cause you can slide the sprocket around on the jack-shaft before tightening the 'allen' set screws that hold it in place..
I will post the Jack-Shaft kits and the photos I took of my jack-shaft removed. See photos and enjoy. IF you need more, let me know. Also it allows you to have the rear wheel sprocket on either side of the frame or wheel.You can use a pulley system if you choose but I don't recommend it as it's easier to get a chain tight than a belt. Also, I recommend INDUSTRIAL PURPLE for your all around degreaser and oil removal from anything BUT, it says NOT to use on aluminum, but I have with no problem and I rinse quickly. I think they mean shiny pretty wheels as it might tarnish them. I leave my chains it this over night and when I pull them out the next day, they look like new!
I give a light brushing anyway with an old tooth brush. I then blow dry and let hang in the hot sun( some use their oven). Then I let the chain soak for a few days in a pail with 80w gear oil while I'm working on the vehicle so it gets in and around the rollers. THEN, I remove the chain and let it drip dry above the pail for a day or two. I then install, BUT, start slow because there is still oil inside the rollers and it does like to fling itself at you if it starts too fast. IF you have a chain guard, then don't worry other than where it will drip down onto when you put your vehicle away for the day.OK, enjoy and thanks! Scott