Kids ebike

Hi guys

I am endeavouring to convert my daughter's byk e-350 with a front hub motor and either 18/36v (one or two drill batts)

How would this kit stack up? I need 100mm dropout on the front and my rim is a weird 355mm. I think i
This may be 18". Not sure what size rim to go with.

Lots of 12" scooter kits out there and I could adapt the front brake to suit. 16" kit will not fit

https://www.uumotor.com/ws/p/350w-freewheel-12-inch-compact-geared-motor-kit-for-scooter/


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I don't think you want to use a scooter motor/whl ASM.
Buy a motor like this;
https://bmsbattery.com/motor/1128-18545-q85-24v200w-250w-front-driving-disc-brake-hub-motor.html#/213-rpm-201
and lace it into a rim. You will save on shipping as well.
Make sure it's the low-speed range motor, the "201".

thanks for the link Motomech

is it possible to modify the throttle with resistors to adjust the span until she gets used to it?

and also any tips on estimating the spoke length? Or is it easier to buy long and cut and thread it myself?

18" is a rim size you can get; it's just uncommon.

Almost all hub motors are drilled for 36 spokes. Your daughter's bike seems to have a 20 spoke front wheel, which doesn't offer easy ways to reconcile that rim with a hub motor.

If you pair a 16 inch (305mm) hub motor wheel with a 20 inch kid bike fork, the overall ride height should be the same. If that makes the conversion simpler or cheaper, there would be no functional problem with it.

I don't know of any brushless motor controllers offhand that will work with both 18V and 36V. Most 36V controllers cut off at 30V to spare the battery.

I think using a very small bicycle hub motor laced to a rim, rather than a unit wheel intended for a scooter, would give you better fit and a more workable RPM range. BMS Battery offers hubs laced into wheels as small as 16", and controllers to match the motor's multi-pin plug. 18" might be a size they can furnish.

The very small and lightweight Q75 motor is offered in a 28 hole version, which makes finding a compatible rim easier in the wheel diameters you're looking at.
https://bmsbattery.com/home/887-17045-q75-28-spoke-holes-36v-front-driving-hub-motor.html#/213-rpm-201

Spoke length is best determined using a calculator like the one at Grin Technologies or http://leonard.io/edd . For a wheel the size you're working with, use radial (cross zero) lacing. Most likely, the spoke length you need won't be commercially available. Don't cut spokes to length yourself, unless you have the equipment and expertise to do it already. Buy them from an online seller or qualified bike shop that can do it for you.

Chalo knows bikes so we take his advice to heart.
BMS Battery offers "off the shelf solutions" to both your motor/whl and battery/controller problems.
The Q85 is available already laced in a 16" rim (suitable for V-brk);
https://bmsbattery.com/ebike/595-16237-q85-36v250w-front-driving-v-brake-e-bike-motor-wheel-ebike-kit.html#/213-rpm-201,
Aside from the controller/drill motor batt mis-match issues, those batt.s have issues in and of themselves. since you might make an order from BMS B., I would recommend taking advantage of their batteries, which over the years, have shown to be a gd. value. They offer a sm. "water bottle" batt with an intergrated controller;
ps://bmsbattery.com/ebike-battery/1094-18534-36v5ah7ah-haitu-bottle-battery-pack-with-usb-output.html#/510-inner_controller-with/542-cells_capacity-25ah_high_c_rate_5ah
This comes with most of the sm. stuff (ebrake levers, throttle, PAS, etc) and a programable display which would allow either 3 or 5 speed limit settings. There isn't much info on this new product, but it could be a sine-wave controller which allow at least 5 "power settings", better than just a speed limit.
The WB batt. comes in both 24V and 36V. A rough estimate is, w/ the sm. whl. and low speed motor, the 24V system would top out 12 to 14 mph and the 36V, around 15. Sorta a guess, as I don't usually deal with these sorts of parameters. I would recommend the 36V, as the speed limiting feature could be used the set speeds between 10 and 15 mph.
Having the whl. already laced will up the freight, as it is calculated by mass and volume and the freight can be almost the value of the gd.s. You can do a "dry run" order to see what the freight would be.
Not exactly what you intended I guess, but going this route would eliminate quite a few headaches and will make for a nice clean install. She will be the envy of the neighborhood.
If you decide you might want to go this route, let us know and we can go over how to order from BMS B and what other sm stuff you would need.

Thanks guys

Great info, I have had a good look at the bmsbatt kit which would fit nicely on the 16in rim.
Just wondering what you think the issue would be with putting 2x 18v in series for 36v?
Already having the batteries and them being smaller (4 or 5ah) makes them much easier to fit the bike and easier to swap out for charging. Not to mention the cost saving.

I couldn't find any dimensions on the battery offered, to reference to the small bike.

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I am not normally a fan of Lipo but if you want small and compact then you might also want to take a look at RC LiPos. They make them in 5S (18 Volt) and the rare 10S(36 Volt). The 5S would be easier to find a balance charger for.

Bikegirl said:

Already having the batteries ...

Click to expand...

oops .... missed that bit. What kind ?

LewTwo said:

I am not normally a fan of Lipo but if you want small and compact then you might also want to take a look at RC LiPos. They make them in 5S (18 Volt) and the rare 10S(36 Volt). The 5S would be easier to find a balance charger for.

Bikegirl said:

Already having the batteries ...

Click to expand...

oops .... missed that bit. What kind ?

Click to expand...

Milwaukee 18v some 4 some 5ah

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Seems that I may have to fit a couple of diodes but not sure what my discharge control FETs are rated for.

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How is this project shaping up?

I'm working on a range extending option for my child. I'd like the assist to be of the sort that melts into her own pedaling.
-smooth ramp up
-minimal but not negligible assist
-no throttle
-negligible off delay pedelec function (in technical off road, a few hundred milliseconds off delay can be terrifying)
- light weight

My custom tune on my BBSHD controller/36 pole pedelec achieves all this except the weight. Well over 10% of her body weight for just the BBS01 bare motor.

Cute Q100C looks like it is about 4.2 lb, offsetting a roughly 1 lb hub. But is there a compatible controller and pedelec sensor that will achieve my other goals? Or is the S06ST torque sensing controller any good?