Just finished the final touches of my China girl two stroke 60cc mounted to a generic mountain bike with Shimano gears and brakes. I intentionally got a 52 teeth rear gear to mount w/ the motor's gear drive to give me a better pulling power during acceleration noting that my 260lbs body frame would certainly need just that. And it has worked well. Since I really don't speed on the bike, I am not bothered by the lower top speed. It just cruises at 25mph with the engine sounding just right. I must also include the fact that I use standard road bicycle tires (not the balloon tires commonly partnered with MBs) with tire pressure of up to 45psi. (the tire inscription says max of 60lbs.) to lighten the load. My real concern is that with the present configuration, my shimano gear set is rendered inutile. I learned that w/ an installed jack shaft, I could at least shift gears from the rear. This would mean I could increase my max speed and even lower the load on the motor by shifting to higher gear when traveling uphill. Some of you might have some experience already on using jack shafts. What are the advantages and disadvantages compared with an ordinary single gear rear drive? Not to mention, the jack shaft would cost me probably the same as buying a new motor. Is it worthwhile? Thanks for your comments.