Qs 4kw mid drive gear reduction

General Discussion about large electric scooters and motorcycles and other things with no pedals.
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Sattva Ram   10 W

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Qs 4kw mid drive gear reduction

Post by Sattva Ram » Dec 11 2020 1:06pm

Can someone tell me what gearing should I use for this motor? 130/70 R17 is the wheel approximately 2 meter circumference. 21S lion pack. The aim is 120 kph top speed. I have the 14T sprocket.

Thanks

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Skaiwerd   100 W

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Re: Qs 4kw mid drive gear reduction

Post by Skaiwerd » Dec 20 2020 3:02pm

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Sattva Ram   10 W

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Re: Qs 4kw mid drive gear reduction

Post by Sattva Ram » Dec 23 2020 11:16am

As if it was this easy. It's not an ICE. The rpm is dependent on the amp draw. I'd need real advice from someone who actually has this motor...

caza   10 mW

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Re: Qs 4kw mid drive gear reduction

Post by caza » Dec 24 2020 12:09pm

There are a few variables but a calculator is what's going to get you in the ballpark.

Your RPM is going to depend on your pack voltage, controller, settings, flux weakening, etc. But you can get a pretty good idea from the datasheet.

You didn't specify exactly which motor you're using but I'm assuming its the QS 138 90H. It says it runs at 4000RPM but with flux weakening up to 6500RPM. We can assume these are speeds with no load, and under load RPM will be somewhat lower. I reached out to Electrobraap who used this motor in his pit bike conversion kit, and he said he gets between 5000-5500rpm depending on voltage/current/load.

You can use those numbers and the calculator skaiwerd linked to to gear pretty close to the top speed you're looking for. But you won't know exactly until you build the bike and test it.

Sattva Ram   10 W

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Re: Qs 4kw mid drive gear reduction

Post by Sattva Ram » Dec 30 2020 1:42am

Well so far I only tested the motor with 50 volts and it's 2500 no load so at 72 volts it's the rated 4000 it seems. 5000 seems like field weakening to me. 1:4 is the manufacturers recommendation for 100kph so the ballpark is already known but I want more than ballpark. It is a very sleek sportsbike so the top speed with a good tuck has definitely not the same power requirement as a pitbike where the rider literally forms a stopping parachute with their body. So it's very hard to calculate as to how much is the requirement without real experience.

Anyway it doesnt matter now I only asked this because I didnt know what sprocket to buy. I have the 53 tooth stock sprocket that came with the bike which is 420 and the chain has to be 428, but for the test I can put the 428 chain on the 420 sprocket so I will know the data and then I can buy the proper 428 sprocket once I tested it. So my question is kinda moot at this point. Anyway thanks everyone!

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