12 inch hub motor

[justinr9111]

Hi guys
Im trying to build a 12 inch wheeled electric folding bike.
BMS sell a complete wheel with 36v front hub motor at bms, another option is to build my own wheel with a small hub motor.I was planning to use 36v 10ah limnco bottle battery. My prerequisite is that it must be capable of travelling at 20km/h but also be legal to ride in australia (same laws as euro ie 250w). My problem is that these wheels are rated at 270rpm, which only gets me to 15km/h. What can I do to get that extra speed out of it while staying within the law? Is it even possible? my understanding of electronics isnt the greatest!

Many thanks
Justin

 

[mlt34]

The first thing to know is that to an engineer, power in watts =amps x volts. To a cop, power in watts equals whatever the sticker says. That's why I keep a 250W sticker on my 1500W nine continent hub...

To answer your question seriously though (or more seriously, since I wasn't joking above) just over volt that motor with a 48v battery. You'll get about 33% more speed, though you'll somewhat shorten the useful life of the gears in the motor.

 

[dogman dan]

Definitely, a 250w sticker is the starting point. They can't know your power level really, but it gets obvious your are using more than 250w when you exceed 30 kph, which tends to take about 300w. By 40-50 kph, any cop can be smart enough to know that aint 250w.

You should actually be able to reach 20 kph on the flats, on 250w easy, unless you weigh 300 pounds. The trick is to match the motor rpm you need with the voltage. Each motor has a "winding speed" At a given voltage, say 36v, the motor goes a given max rpm with no load. Then slower with a load of course.

Your very small 12" wheel means you need to seek out a faster than most motor no load rpm to start with. Then find out what voltage turns it fast enough to go 20 kph under load. I would think look for 30 kph ish no load. Likely you will need at least 48v, but might need 72v.

It won't matter too much how much over 250w you end up. The motor will very quickly drop to 250w or less as soon as you are moving, if it's max speed under load is 20 kph. 99.9% of the time, you will be riding legal if you limit speed to 20 kph. However, you might end up having to go easy on the throttle starting up, with all that torque you will get with a 12" wheel. Don't wheelie in front of cops.

 

[mlt34]

By 40-50 kph, any cop can be smart enough to know that aint 250w.

Your cops must be smarter than mine. From what I've seen, they know their toaster is 500W but don't really know how that compares to an ebike. Though I guess in some countries where the ebike laws are enforced more, they are a little more educated.

 

[justinr9111]

The first thing to know is that to an engineer, power in watts =amps x volts. To a cop, power in watts equals whatever the sticker says. That's why I keep a 250W sticker on my 1500W nine continent hub...

To answer your question seriously though (or more seriously, since I wasn't joking above) just over volt that motor with a 48v battery. You'll get about 33% more speed, though you'll somewhat shorten the useful life of the gears in the motor.

Hey Thanks for the answers guys.
So, using the equation for watts, if I increase the volts to 48v but decrease the amps to around 5 amps, then I would get more speed and stay under power output? Ok so is 5 amps going to be enough and how can I control the amps? Would a cycle analyst do this?

IF (and its a big 'if') I could wind the motor to be, say a 400rpm at 36v as opposed to a 36v 270rpm,would this be another way of getting the speed I need? Is that in the realms of possibility?
Please note that the motor will be pedal assisted.

 

[dogman dan]

I wouldn't worry so much about the amps. Cops just won't know.

Re prhasing. If the cops expect you to go 15-20 kph, they MIGHT be paying attention enough to notice you going 50 kph. But at whatever power level you have, 50w or 5000, they won't be likely to notice you if you still ride 15-20 kph.

You can speed limit with a DP CA. So build your EV, then just enable a speed limiter.

Or just use a controller with a eruo mode jumper. It will limit speed, yet still allow some amps at start up.

Cops won't hassle you if you keep the speed low. That is, unless they know you, and you always deserve hassling.

 

[mlt34]

5A is going to be pretty weak, plus its hard to find a controller that low unless you get a programmable one.

Dogman said it right, just build your bike with a decent controller and don't worry about the amps too much. 20 should be fine. Unless the cop pulls you over and then whips out his multimeter, he'll never know the amps of your controller and thus how many watts you're packing. BMSbattery's controllers, among others, have a jumper that limits to about 25 km/hr so you don't even need a cycle analyst for that feature (though i still think the CA is a great gadget to use).

 

[justinr9111]

Ok, so at the moment im thinking I want either a 24v or 36v q85 motor high speed wind, with a 36-48v battery.
As I want to keep it as neat as possible, I want to try the 24v motor with the 36v 10ah bottle battery.

Q1. If this isnt delivering the speed/power I need I would then try the 36v motor and rather than buy a whole new battery could I somehow add an extra 12v battery to the circuit (just to see if there are any gains)?

Q2.If there is an advantage can I convert the 36v 10ah battery to a 48v 6ah battery, ie rewire internals? This bottle battery is the biggest size that will fit on the frame ie 340mm, hence buying a 48 10ah battery isnt an option. Has anyone done this?, would it be relatively straightforward? (Trip is only 5 miles a day so range isnt an issue I think.)
Q3. If I do end up with a 48v battery what controller would work best for q85.

Thanks for the help guys, really appreciate it. Cant wait to get this project together.

 

[mlt34]

First off, the 36V bottle battery is pretty weak. I've had a few. The 10.4 AH is better than the 8.8 AH, but still its not the best battery, besides having a great price. Will the 36V frog style battery fit your frame? I like that battery.

Next, you should contact BMSbattery.com and ask for the exact motor speeds of the 12" Q85's they have. Sure, it says 270 on their site, but they are rarely the figures given, and I don't know why they do it that way. They should be able to tell you like this: The 12" Q85 24V is XXX RPM actual, the 12" Q85 36V is XXX RPM actual, etc.

Next, you'll find a lot more 36V controllers that are appropriate for you than 48V controllers, plus it sounds like a 48V battery will be a problem on your bike, so overvolting a 24V motor to 36V seems like a better option than a 36V motor at 48V, if the RPM value works out to something reasonable. The KU-63 or KU-65 36V controllers should be fine for you. I don't think they come in 48V, so 36 is your limit with those.