View Full Version : Advice before sprocket change

01 May 2013, 1220
Konstantin is my name and I need an advice from you more experienced guys about my electric motorcycle build.

I use a frame from 1982 Kawasaki GPZ750, with ME1003 motor, Alltrax 4844 controller and 4 large gel lead-acid batteries 85Ah each from Sonnenschein. So my system voltage is 48 V nominal.


I know those weight a lot, but I get them for free. My plan is to build a electric bike with those and register it, so I can use it on road and later when it approved, upgrade batteries to something more lighter and more expensive.

My gearing right now at test drive is 12 in front and 33 in rear. I know it a bit low, but it is a reason why I need an advice. I need to check it before I order a larger sprocket in rear.

I think total weight for everything is around 350 kg with me as driver.

Here is picture of everything. I know it look like a mess, but it just a test drive.


Last week I did some test drive with my built, and it works well, but I seen that I cannot go over 50 km/h. I have good acceleration from start, but after 30 km/h everything slow down and after 40 km/h i need to keep full gas to keep my speed.

As I remember theory, electric motors have a lot of torque in small RPM, and lose it in high RPM. So when I cruise in 40 km/h and my RPM is around 1000, I loose torque. Is it correct?

Is this behaviour normal for my gearing? Or is something wrong?

I see that you guys often use a 72T sprocket in rear in your builds, according to gear ration calculator at http://thunderstruck-ev.com/speed-calc.htm my top speed will be 43 km/h with it, and it is bit too low. (motor RPM 2200 and wheel diameter 25”).

If I want cruise speed to be about 70 km/h my rear sprocket should be 43 according to calculator. Will I have more power after 30 with 43 sprocket?

Should I order a larger rear sprocket than 43? maybe 72? Or will 43 work for fine for me?

02 May 2013, 0907
Hi Konstantin,

It's probably a good idea to order a larger rear sprocket, bearing in mind that with only 48 volts and (?) 100A current draw you only make ~4.5kW. So how fast can you go with 5kw? It should be possible to cruise at 70km/h on level ground and I would calculate gear ratio based on the maximum motor speed. You want to turn the motor fairly quickly to maintain good flow of cooling air, but not so quickly that you exceed mechanical limits due to overspeeding.

Good luck and please post what you decide to do.

02 May 2013, 1225
hi Frank.

Thank you for your answer.

I dont realy understand how i can calculate with motor maximum speed.

With voltage constant 0,0207 V/RPM, my systems 48 V should be around 2300 RPM without load. With load it should be lower. And according to gear calculator it is not enough for 70 km/h.

what is I missunderstand?

02 May 2013, 1235
and you also have to remember voltage drop. so your 48v will not stay 48v with a load on the cells.