Hello,
This is my first post in this forum, so I apologize in advance for any mistakes I might make.
I built a motorized bicycle (not a mini bike, an engine attached to a regular bicycle) a few months ago using a lightly modified Non-Hemi Predator 212cc (governor internally removed, upgraded air filter, tuned carburetor, free-flowing exhaust, removed oil sensor) and was able to hit 46mph with a CVT/torque converter. I've had a blast riding it and I also learned a lot about engines and mechanics in general. Now, before I continue, I know that going 46mph on a bicycle is dangerous and so I take safety seriously. I use full motorcycle gear when riding, inspect the bike carefully after each ride, and only hit top speed on long straightaways with no other vehicles in sight.
The engine has been working amazingly well for a $100 clone of a Honda. I haven't had any issues with it. Currently, I have a CVT/torque converter setup which allows me to use a small rear sprocket. Unfortunately, I'll have to get rid of it and use a centrifugal clutch instead since since the chain rubs badly against the frame and I have no easy way of correcting/aligning it. Since I will no longer have the 3:1 gear reduction of the CVT when using the centrifugal clutch, I ordered a 56T rear sprocket and a 10T centrifugal clutch to get a reasonable gearing ratio with with a calculated top speed of 66mph at 5,000 RPM (I'm using 26" wheels but the actual diameter of the wheels is 25"). Realistically, I will probably reach 53mph at 4,000 RPM. I have looked at dyno charts of a Predator 212 with my same setup and the power starts to flatten out at 4,000 RPM; there is practically no power increase after 4,000.
I want to make the engine more powerful but cannot allow the it to rev much beyond 5,000 RPM because of two factors: the gearing and vibrations. Finding a sprocket with more than 56T that is designed for a bicycle wheel is impossible so I would have to modify a blank sprocket to fit my mount. I calculated that I would need an 80T rear sprocket to give me a realistic top speed of 65mph at 7,000 RPM. The Stage 2/3 kits for Predators all allow the engine to rev to 7,000+ RPM to produce maximum power and I don't think they would help much at 4,200 RPM or so. Secondly, at high RPM, the vibrations become very severe in the engine. The single-cylinder design doesn't help, and using a billet rod/piston would further throw the crank off-balance. On a go-kart, the large wheels and frame can dampen a lot of the vibrations, but when using a bicycle frame that weighs less than the engine and where the engine is between your legs, you can feel every single vibration from the engine. High RPM = uncomfortable ride.
What modifications could improve the engine's performance for my application? I already have the governor internally removed, an upgraded air filter, free-flowing exhaust, and removed oil sensor. Everything else is stock. I already ordered a genuine Mikuni vm22 carburetor. I was thinking about 1.3 ratio rockers, a better camshaft, increasing compression by using a flat-top piston, and a thinner head gasket. Thanks!
This is my first post in this forum, so I apologize in advance for any mistakes I might make.
I built a motorized bicycle (not a mini bike, an engine attached to a regular bicycle) a few months ago using a lightly modified Non-Hemi Predator 212cc (governor internally removed, upgraded air filter, tuned carburetor, free-flowing exhaust, removed oil sensor) and was able to hit 46mph with a CVT/torque converter. I've had a blast riding it and I also learned a lot about engines and mechanics in general. Now, before I continue, I know that going 46mph on a bicycle is dangerous and so I take safety seriously. I use full motorcycle gear when riding, inspect the bike carefully after each ride, and only hit top speed on long straightaways with no other vehicles in sight.
The engine has been working amazingly well for a $100 clone of a Honda. I haven't had any issues with it. Currently, I have a CVT/torque converter setup which allows me to use a small rear sprocket. Unfortunately, I'll have to get rid of it and use a centrifugal clutch instead since since the chain rubs badly against the frame and I have no easy way of correcting/aligning it. Since I will no longer have the 3:1 gear reduction of the CVT when using the centrifugal clutch, I ordered a 56T rear sprocket and a 10T centrifugal clutch to get a reasonable gearing ratio with with a calculated top speed of 66mph at 5,000 RPM (I'm using 26" wheels but the actual diameter of the wheels is 25"). Realistically, I will probably reach 53mph at 4,000 RPM. I have looked at dyno charts of a Predator 212 with my same setup and the power starts to flatten out at 4,000 RPM; there is practically no power increase after 4,000.
I want to make the engine more powerful but cannot allow the it to rev much beyond 5,000 RPM because of two factors: the gearing and vibrations. Finding a sprocket with more than 56T that is designed for a bicycle wheel is impossible so I would have to modify a blank sprocket to fit my mount. I calculated that I would need an 80T rear sprocket to give me a realistic top speed of 65mph at 7,000 RPM. The Stage 2/3 kits for Predators all allow the engine to rev to 7,000+ RPM to produce maximum power and I don't think they would help much at 4,200 RPM or so. Secondly, at high RPM, the vibrations become very severe in the engine. The single-cylinder design doesn't help, and using a billet rod/piston would further throw the crank off-balance. On a go-kart, the large wheels and frame can dampen a lot of the vibrations, but when using a bicycle frame that weighs less than the engine and where the engine is between your legs, you can feel every single vibration from the engine. High RPM = uncomfortable ride.
What modifications could improve the engine's performance for my application? I already have the governor internally removed, an upgraded air filter, free-flowing exhaust, and removed oil sensor. Everything else is stock. I already ordered a genuine Mikuni vm22 carburetor. I was thinking about 1.3 ratio rockers, a better camshaft, increasing compression by using a flat-top piston, and a thinner head gasket. Thanks!
