3.5hp BMX .. Gearing help !! Please!!

jamtown unit

New Member
Local time
5:28 AM
May 25, 2008
I have a new BMX bike i am thinking of installing a 3.5 hp 148cc 4 stroke engine on it but this is where the problem. I have a 4.8:1 gear ratio. A 10 tooth gear on the engine and a 48 tooth on the rear wheel the wheels are 20 inch diameter. Will this be enough to move me at 175 pounds.
the law generally is 48cc max on a motor assisted bike, otherwise it becomes a moped or motorcycle which need licence and insurance.
It would move you but not in a particularly satisfactory fashion.With that kind of setup you would be doing close to 60mph at 5000 engine rpm !!,assuming that there is no internal reduction in the engine itself.You would not be likely to get there because you would have run out of torque.Hills would be a problem too. You need substantally more reduction to use your engine power effectively,as a minimum twice as much,like 10:1.Otherwise you will be running out of torque at higher speeds.It is not impossible, but does not make much sense.
This is not for daily use just fun for me and the boys to take it to the dirt tracks on weekends
So i should probally reduce the size of the tyre to 16 inch to increase torque out put on the motor
Yes that would help,if you want to do that kind of riding, the smaller the better,you'd like to have torque on tap to accelerate,My son had something like that with a B&S engine on a mini bike with 12" wheels,it went up in flames I recall.