Originally Posted by nohorjo
Hi im new to this site.
I built an ebike from a specialized hardrock 2000. Its chain driven and has a 250watt motor (MY8029), with 2 x 12v 7ah lead acid batteries in series.
attatched to the motor is a 5 speed free-wheel sprocket. its smallest gear is connected to the largest of the actual rear wheel.
the torque is really low it doesnt feel like i get much of a push. the motor on its own cant even push at 4mph.
im thinking of moving the motor to the middle of the frame and connecting it to the chainset so i have a higher gear ratio so hopefully more torque.
Im really disappointed at the motors performance. when the wheel is lifted it reaches speeds of ~600rpm. it ony gives me +2mph to my normal cycling speed.
Thanks everyone for your help!
ive dropped the sealed lead acid batteries a few times so im wondering if that might be causing a problem. i did order new ones that should arrive soon.
What you describe is exactly what's gonna happen....
A 24v 250w motor is gonna try to pull 10a from the batteries..
The are 7ah batteries and will only be able to supply 10a with no resistance.
The second you start adding a load to it, your gonna drop the available voltage that the batteries will be able to supply.
You're pulling 1.5C (10a) from a battery rated to provide C/20(.35a).
The voltage to that motor is gonna go from 25.2v (actual) to probably 18.0v or below while you're trying to run it on the bicycle, so you're gonna get very little torque.
i.e. The "harbor freight type" motors are either On or Off. There is no variable speed on them. So they are gonna try to pull full amperage which those size batteries won't be able to supply...
The ONLY way you're gonna get decent help from that motor is by using 2-12v 20AH batteries.
I know you were hoping that the simple (and relatively light) setup would help tremendously, but you won't get the desired results from such small batteries.
Last edited by SpecialX; 09-24-13 at 04:24 PM.