Driveline, Sprockets, and Gear Ratios

72 Tooth Rear Sprocket

I bet by now that you want to make the cycle go!

So, it’s time to talk about the driveline.

This motorcycle is about as simple and efficient as you can get. It’s more or less the same as a single-speed bicycle.
The motor has a drive sprocket, which connects to a chain, which turns the back wheel. That’s it!

Drive Sprocket
The motorcycle uses a standard machine sprocket. I simply went to a farm store, which had a decent tractor repair aisle and located parts for the sprocket and chain. I bought a 14-tooth sprocket and a hub with a 7/8″ center hole to match the motor’s drive shaft. These are two parts, bought separately, which allows greatest flexibility in driveshaft diameter and sprocket tooth count.

14 Tooth Drive Sprocket

The sprocket and hub had to be welded together. In the earliest version of my cycle, that was the only welding done on the entire project. It was only later, when I had some welding experience that I tackled the welded battery rack. On the original sprocket, I just had somebody else weld those two pieces together for me. They were inexpensive – under $20 for both parts. If I want to change the gearing on the cycle, all I need to do is spend another $20 at the farm store and get a sprocket with a different number of teeth. These same parts could also be mail-ordered from a dealer such as Grainger or other industrial supplier.

The sprocket slides onto the end of the motor driveshaft, and is held in place by a keyway, square key, and set screws.

Chain
The chain is #40 chain from the tractor aisle. Cost about $10 for ten feet, and a few dollars for a master link. It is a popular size chain, so there is a wide variety of sprockets that match.

Driven (Rear Wheel) Sprocket
I did not use the stock sprocket on the back wheel of the motorcycle. Electric motors tend to work best spinning faster than gasoline engines, and geared down a bit more. This gives you plenty of power, without constantly running high current through the motor.
There are many on-line motorsports companies that will make custom rear sprockets. I used one called Sprocket Specialists. You simply tell them what motorcycle you have, what chain you want to use with it, and how many teeth you want on it. They custom make them on CNC equipment and send it to you in the mail.

I got an aluminum sprocket for a Kawasaki KZ440 for #40 chain and 72 teeth. It has a black protective finish. The larger aluminum sprocket weighs less than the stock steel one did. (Saving weight is always a good thing for electric vehicles.) I removed the rear wheel, unbolted the stock sprocket, and replaced it with the custom one. Consult the cycle’s repair manual to make sure the bolts are torqued correctly, and that the back wheel is re-installed correctly.

(Somebody asked about the sprocket being aluminum, and that this is a high-wear item. The black finish on this sprocket is a wear-resistant coating. The sprocket manufacturer highly recommend at least that for protecting the sprocket. I’ve been very happy with it, and wear on the sprocket has been minimal overall.)

Gear Ratios
After all of my riding, I believe that I COULD have kept the original stock rear spocket. It would have given me a higher top speed, poorer acceleration, and cause the motor to draw more amps. Most of my riding is in the city, so I would gladly have a lower top speed in exchange for better acceleration and less amp draw. By having the larger rear sprocket, I can always change out the inexpensive front sprocket to change gear ratios. If I kept the smaller stock rear sprocket, I wouldn’t have had that flexibility.

My current setup is a 14-tooth front drive sprocket and a 72-tooth rear driven sprocket for a 5.14:1 gear ratio. On my cycle, I’m very happy with the combination of range, acceleration, and top speed. On a fresh charge, I have just enough power to do a minor burn-out. Acceleration away from a stop for city use is very nice. There’s no clutch to slip or engine to rev, so the cycle just GOES the moment you twist the throttle.

Tweak the Driveline
Once I had the sprockets on, I wrapped the chain, checked it for length, cut it to length, wrapped it on to both sprockets, and closed it up with a brand new master link. (Make sure the clip on the master link faces the right direction. It can work its way off if you put it on backwards.)

The original chain guard still fit over the new (larger) rear sprocket, but just barely. I simply bent it a tad to make sure it had clearance.

On the front end, the transmission would normally have an integral cover over the chain and drive sprocket. Without the tranny, it meant I had to make a custom chain cover. It could have been made from almost anything – metal, plastic, wood, but I wanted to show off how the cycle works, so I went with plexiglass. I roughed out the shape required with some cardboard and a pencil, and then cut the plexiglass to fit the space. A straight piece of plexiglass covers  the top of the chain. I used a scrap of an aluminum rail as a spacer between the motor mounting plate and the plexiglass to hold it in position.

With everything in position. The chain needs to be tightened and aligned as per the user manual.

While I had the rear wheel off, I also used the opportunity to put on new tires. (Bought on sale during a close-out sale!)

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