[24KtC]

Hello! I'm new to EV conversions, and am by no means a bike mechanic, but I've wrenched on a few classic cars and have experience building battery packs for a solar system. I've wanted to make an electric motorcycle conversion, and a project 1979 Kawasaki Z650 found its way into my possession and so here I am.

The bike is heavy, according to what I could find, the rolling frame is ~300lbs. I'm hoping to reach at least 60 mph (95km) and with a range of at least 60 miles, hopefully a bit more. Before we get into specifics of what can physically fit on my bike and what I can comfortably afford, I wanted to know what sort of battery capacity/motor combination I would need to reach this goal. After doing research without fully understanding how it would work out, I settled on a custom 72v, 200ah battery bank with 200a continuous draw, and a goldenmotor's HPM-10KW 72v motor. I was hoping this would be overkill, but honestly don't know enough to tell.

Also, I'm not wanting insane acceleration, but I figured that can be toned down using the motor controller.

Please let me know if I'm super far off with my plan, or if there's a better way of going about it. I really appreciate the help!

P.S. I feel it's necessary to include a few pictures of the bike:

 

[brian_]

After doing research without fully understanding how it would work out, I settled on a custom 72v, 200ah battery bank with 200a continuous draw, and a goldenmotor's HPM-10KW 72v motor. I was hoping this would be overkill, but honestly don't know enough to tell.

Also, I'm not wanting insane acceleration, but I figured that can be toned down using the motor controller.

Fortunately, you won't get insane anything with a 10 kW motor.
If you're concerned about excessive torque from a standstill (perhaps with a larger motor) causing a risk of loss of control if you're not careful with the "throttle", you just need to set the controller's current limit to a reasonable value - torque is proportional to current.

 

[brian_]

95 km/h with 120/90-18 tires would be about 770 RPM. The Golden Motor HPM-10KW (air cooled or liquid cooled) can go to 6,000 RPM so you could use as much as 7.8:1 ratio in the drive system. It can't actually produce enough power at that rotational speed to drive the bike at that road speed, so a top motor speed of about 4,000 RPM and therefore reduction ratio of about 5:1 would be more workable.

With a peak torque of only 30 Nm, and 5:1 reduction making that 150 Nm at the rear wheel, and 330 mm tire radius, that's 450 newtons of drive force or 0.2 g acceleration with a 200 kg bike... which should be no problem for anyone to control. This combination is not going to do wheelies.

 

[brian_]

The bike is heavy, according to what I could find, the rolling frame is ~300lbs.

That sounded like a lot without the engine, but I'm not a motorcycle guy, so I looked it up... and apparently the bike weighs about 465 pounds complete (and dry), with the engine and transmission being 175 pounds of that, so 300 pounds without engine and transmission looks about right.

 

[24KtC]

95 km/h with 120/90-18 tires would be about 770 RPM. The Golden Motor HPM-10KW (air cooled or liquid cooled) can go to 6,000 RPM so you could use as much as 7.8:1 ratio in the drive system. It can't actually produce enough power at that rotational speed to drive the bike at that road speed, so a top motor speed of about 4,000 RPM and therefore reduction ratio of about 5:1 would be more workable.

With a peak torque of only 30 Nm, and 5:1 reduction making that 150 Nm at the rear wheel, and 330 mm tire radius, that's 450 newtons of drive force or 0.2 g acceleration with a 200 kg bike... which should be no problem for anyone to control. This combination is not going to do wheelies.

Okay, awesome, your comment has been very helpful. Sounds like I picked the right motor for what I'm wanting. In regards to battery capacity, I understand that in a perfect world, if going that 95km/h were to pull 200a continuous, and I have a 200ah capacity, then I'll be able to get 95km/h... In reality, I won't be driving at freeway speeds the whole time, so do you have any guess for what sort of range I'll be able to pull?

 

[brian_]

In regards to battery capacity, I understand that in a perfect world, if going that 95km/h were to pull 200a continuous, and I have a 200ah capacity, then I'll be able to get 95km/h... In reality, I won't be driving at freeway speeds the whole time, so do you have any guess for what sort of range I'll be able to pull?

The capacity in amp-hours isn't necessarily equal to the allowable maximum current in amps. The discharge current (in amps) divided by the capacity (in amp-hours) is called the "C rate" and is the inverse of the number of hours it would take to fully discharge the battery at that discharge rate. Assuming that a 200 Ah battery can provide 200 A continuously means assuming an allowed continuous discharge rate of 1C. Usually EV batteries can discharge for at least a significant period at a higher rate, but 1C is a reasonable conservative assumption.

The maximum current is needed to produce maximum torque. The motor will not be able to produce this much torque (and won't use this much torque) at high motor speed. If you look at the 72 V performance data published by Golden Motor, at 4000 RPM it can produce 19 Nm and takes 121 A to do that; the peak of almost 200 A is reached at 3783 RPM in their data, and although their data doesn't cover lower speed, it is reasonable to assume that current should be limited to 200 A (into the controller) from that speed down, and that it can produce the same 30 Nm from that speed down to zero.

The Golden Motor test data is typical for cheap motors, which are tested by turning a brake (just like a car's brake at the wheel) with the motor, starting as fast as the motor will go with the available voltage, then applying the brake harder and harder until the peak torque is reached... then they stop because the motor would just stall (stop turning) if they brake any harder.

In reality, I won't be driving at freeway speeds the whole time, so do you have any guess for what sort of range I'll be able to pull?

200 Ah at 72 V is 200*72 = 14.4 kWh. That's enormous for a motorcycle - can you really fit that much battery in the bike, along with a motor and controller? That's twice the energy capacity of the battery in one model from Zero Motorcycles that I checked.

Someone with more motorcycle experience could fill in how many watt-hours per kilometre or mile they think a bike that size might use, so you can calculate how far you can go on your battery capacity. I checked the specs for one model from Zero Motorcycles (the "S"), and its range varies from 45 miles (72 km) to 89 miles (143 km) - depending on how fast it's going - on a 7.2 kWh battery.

If by some chance the 8 kW that this motor can produce at 4,000 RPM is enough to get the bike to 95 km/h, then it will be using 121 amps, so a 200 Ah battery would last (theoretically) for one hour and 40 minutes, or long enough to go 157 km. Ride more slowly, and it would go further... if you can really carry that much battery.

Sounds like I picked the right motor for what I'm wanting.

I don't know if the motorcycle can move at 95 km/h on only 8 kW (which is what the motor performance data says it can produce at 4,000 RPM). A midsize car needs typically about 16 kW to maintain that speed, and I think it's likely that motorcycles - due to their poor aerodynamics - need more than half of the power of a car at highway speeds despite their comparatively light weight and small frontal area.

 

[Functional Artist]

Howdy,
I converted a Kawasaki 440 to electric a while back
...& it's still zoomin' along

FYI: Here is the beginning of the build thread
El Moto (Electric Motorcycle) | DIY Electric Car Forums

Here is a video of the beginning of the conversion