Should I upgrade to 15 or 20 tooth sprocket?

[Emm2849]

Hi everybody, my names is Emmanuil and I am new to this forum. I am currently upgrading a to razor pocket mod from a 24 volts to 48 volts, with a 48 volt 2000 watt motor and 48 volt 20 amp battery. The motor runs at 4200 rpm with maximum of 5200rpm. The stock pocket mod comes with a 11 tooth sprocket and 80 tooth wheel sprocket. I’m trying to figure out if I could use a 20 tooth sprocket without damaging the new motor and/or motor. I read in some other forums that it would work running it with 20/80, it would make it faster and it would also run cooler because of the reduced load on the motor . I’m new to gear ratios and do not know much about them, so please correct me if I’m wrong. Thank You!

 

[Hummina Shadeeba]

Can u jump from 24v to 48v without breaking the controller?

 

[Emm2849]

Sorry, I forgot to mention that I also got a 48 volt controller that came with the motor kit

 

[Chalo]

The stock pocket mod comes with a 11 tooth sprocket and 80 tooth wheel sprocket. I’m trying to figure out if I could use a 20 tooth sprocket without damaging the new motor and/or motor. I read in some other forums that it would work running it with 20/80, it would make it faster and it would also run cooler because of the reduced load on the motor .

I think you have it wrong. 20/80 increases the torque load on the motor by 82% versus 11/80. To put it another way, it decreases wheel torque by 45% versus stock gearing. The chain will be happier, but the motor will cook itself. You might also end up with lower torque at the wheel than it requires to sustain the implied road speeds, in which case the motor will never get to an efficient cruising speed and will struggle in a low efficiency (hot) motor speed range all the time.

11/90 would increase wheel torque and let the motor run in a sweet efficiency range more of the time, but lower maximum speed by a little. 12/80 or 13/80 would raise top speed at the expense of some climbing, acceleration, and efficiency. Chopping the gear reduction ratio by 45% with a 20T motor sprocket will gut both acceleration and hill climbing, and make the motor run hot, while maybe or maybe not giving you some extra top speed in return. I think it would be a mistake.

Are you going to use a controller with at least 60 amps capacity? Because if not, it won't be a 2000W motor. Most 20Ah batteries can't support 60A loads.

Unless I'm mistaken, that's a kid's toy scooter with 12" baby bike wheels. It would be a grievous error to ride one at the speeds that 2000W implies.

 

[Emm2849]

Would 15 tooth work?

 

[Emm2849]

Or should I get something smaller, the motor says 2000 watts but it actually runs at 48 volt 33 amps and my battery can handle 35

 

[Gruesome]

So, 4200rpm at the motor with 11/80 gear ratio is about 600 rpm or 10 revs per second at the wheel. If the wheel really is only 12" diameter, that's a circumference of 1m, so the bike speed would only be 10m/s or 36 km/h or 22 mph. Is that already too fast for those little wheels?
With 20/80 gear ratio, the bike speed at 4200rpm would be 63 km/h or 40 mph. If I read the bike power calculator correctly (power goes with speed^3), you could go 47 mph with 2000W.

 

[Emm2849]

So what upgrade should I get then, 20 tooth is out of the picture

 

[Gruesome]

That entirely depends on your risk tolerance. 30 mph is plenty fast to kill yourself if you are hitting an immovable object. But if all your friends are going 40 mph, do you really want to die alone of old age?

Personally, even though I do go 30 mph on the occasional downhill stretch, if I travelled regularly at that speed I'd want brakes like on a small motorbike, and motorbike wheels and a motorbike frame, and motorbike protective clothing.

But the majority here will probably think nothing of going 30 mph with just bicycle gear on all the time. So then it's just the wobbliness of those 12" wheels that Chalo alluded to, and I guess you'll find out how fast they can go before the whole thing throws you off or wobbles itself to pieces. 15 teeth sounds like a good compromise, running the motor at 600W (third of maximum power) at 30 mph. Power vs speed calculator: An interactive model-based calculator of cycling power vs. speed .

 

[Emm2849]

It can handle 30-40 easy. I have seen someone do a build with a 72 volt hub motor setup and got 73 miles an hour with the stock frame. He did get better breaks though

 

[MxusMick]

You are new to ebikes and don't realize what the proposed speeds feel like on a regular bicycle, much less a pocket bike with a wheelbase half that size. I have the knobbies you are describing, they have a very aggressive tread, much better in the dirt. Those tiny chains won't last, so convert to belt drive and find a big frame, it will only be a novelty for a grown adult to ride a Razor.