24” kids e-mtb rear drive conversion finished build with pictures

[raggertje]

First post after spending a lot of evenings reading this forum. It's a great source and feels like going to school again

I’m about to place an order @bmsbattery.com. Parts will be for my 10 years old son’s 24” mountainbike which we build together last year. I’m looking for a light and clean build like the https://www.ben-e-bike.net/ben-e-bike-twentyfour-e-power-pro_140_1076 Maximum speed has to be 25km/h (15.5mph) with a range of more then 40km. We are located in the Netherlands with a huge mountainrange of a stunning 75 meters around the corner Bike will be used for road and off road.



I’m thinking about to order following parts:

Q100C CST 36V350W 32-hole Rear Driving Hub Motor, https://bmsbattery.com/motor/651-16348-q100c-cst-36v350w-32-hole-rear-driving-hub-motor-ebike-kit.html#/213-rpm-201 (we are gonna lace the wheel oureselves)
S06S 250Watts Torque Simulation Sine Wave Controller, https://bmsbattery.com/home/1006-s06s-250watts-torque-simulation-sine-wave-controller.html
S-LCD5 LCD Meter for S-Series Controlers, https://bmsbattery.com/home/740-16673-s-lcd5-lcd-meter-for-s-series-controlers-ebike-kit.html#/137-color-black
A Pair of EBike Torque Arm, https://bmsbattery.com/parts/450-15871-a-pair-of-ebike-torque-arm-parts.html#/301-size-m12
Dual Hall Sensor 12 Signals Easy Assembling PAS, https://bmsbattery.com/parts/705-dual-hall-sensor-12-signals-easy-assembling-pas-ebike-kit.html
Eight poles PAS--Pulse Padel Assistant Sensor, https://bmsbattery.com/parts/570-eight-poles-pas-pulse-padel-assistant-sensor-parts.html (Probably both PAS sensors won’t fit the external bearing BSA cranks but at least I can give it a try...)
THUMB LEVEL THROTTLE WITH WATERPROOF PLUG, https://bmsbattery.com/parts/904-thumb-level-throttle-with-waterproof-plug.html
Left Handed Half Twist Throttle, https://bmsbattery.com/parts/611-left-handed-half-twist-throttle-parts.html (Then he can try both…)

The triangle of the frame is quite small so bmsbattery BOTTLEBATTERY PACK will not fit. The alternative I found is the 36V, 7Ah, 240mm high bottle battery from aliexpress: https://nl.aliexpress.com/item/4000348300702.html?spm=a2g0o.productlist.0.0.7ad631bcbKHSJ2&algo_pvid=7d456a63-6332-45f0-b7c9-2995b9daac38&algo_expid=7d456a63-6332-45f0-b7c9-2995b9daac38-1&btsid=0b0a187b15892308689067895ed237&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_ which will be a snug fit.

Motor will be 201 RPM so according to https://www.ebikes.ca/tools/simulator.html?motor=MXUS_XF07&wheel=24i&batt=cust_36_0.2_7&cont=cust_35_70_0.03_V&mass=45&hp=25 topspeed will be around 23.7km/h with a range of 46km which sounds good to me.

So I have the following questions:

• Did I picked the right parts for this build?

• Is the aliexpress battery a good choice?

• If the bike will befaster then 25km/h, can I reduce speed in the S06S Controler? Also if there is to much torque?

• Any ideas about the PAS controller?

• How do I now all the parts will fit with al the connectors? (I know... I need to learn a lot about electrics

• Did I forgot something? Maybe I need the Wires With Four Waterproof Connectors, https://bmsbattery.com/parts/912-wires-with-four-waterproof-connectors.html

Thanks and so stoked to build my first electric bike!

 

[motomech]

That's a pretty good list, but I'm not sure about the Aliex. batt. One of the good things about using BMS Battery is that are such a lg. vendor, that they have a lot of turn over on batt.s so they are fresh.
Actually, A "water bottle" or "shark" style battery will fit on the frt. of that frt. down-tube. The wheel doesn't go up and down and it won't hit. Any bike shop can install the mounting lugs (it takes a kit, so it's just easier to let them do it). I'm not sure what capacities those batt.s come in these days, but figure about 2.4 Km per Ah on the road if you pedal along and half that trail-riding. Those packs don't quite hold what they say, so you might have to scale back your range expectations a bit.
Speaking of trails, the Q100C is not going to cut it, even on light trails. The stator is 30% narrower than a standard Cute and they are weak sisters. I've killed two of them over the years and have given up on them. Even a Q100H is not a heavy climber, but can work ok on easy walking trails. Personaly, if I was going to build another mini hub motored trail bike, I would do the same as w/ my road bike here in Mexico(the roads here are like off road), I would put a Q100H 201 of the frt. and run 52 Volts and around 20 Amps. At some point, if the trail is steep enough the tire will slip and spin, but that's a good thing. By spinning, it keeps it turning and the last thing you want to happen is for it to "lug" to a stop, because that will smoke things. In fact, if the frt. spinning tire stops you, you should get off and push and a mini in the rear is not going to help and the result could be worse.
On a steel fork, you just to use one of BMS Batt's simple TA's and it leaves the rear wheel and shifting stuff intact and you can mount the controller up frt. with the batt. and motor and have everything right there together for nice short cable runs. And speaking of keeping things intact, the Left-hand twist throttle is a good idea. You can take off the chain-ring shifter (that you will never use) and put it there, leaving the rear shifter alone.
52 Volts will get a 201 in a 24" whl. up to about 19 mph, maybe 20, fast enough to be fun. 15 Mph is flat boring. 48 Volts will do 17 - 18 mph, not a bad compromise.
The KT sine wave, while nice on the street, is rather weak. It is a "soft start" controller which is nice on the road, but exactly what you don't want on the trail. You need something with a little snap. I would just use a simple square wave in the 17 to 20 Amp range, preferably a 9 FET (to keep it cool). It will only have a 3-speed PAS, but that's all that's need on a bike that only goes to 20 mph.
This is the controller I use;
https://www.ebay.com/itm/48V-350W-9MOSFET-ebike-Electric-Bicycle-Brushless-Motor-Controller-with-LED/223971278277?hash=item3425ba7dc5:m:mgxcwDHTuIgK4sxrLPhfyrQ&var=522736001534
It comes w/ the LED display and PAS stuff.
Don't worry about mounting the PAS, there are some tricks I can go over if you decide to go ahead w/ this approach.
Off road on ebikes is murder on the tires, so figure on spending some on flat resistant tires and extra thick tubes, and you probably should think about wider rims so you can mount 2.4" heavy side-wall tires (like the Crazy Bobs I run). And you will need a real seat, and the right pedal gearing, and....the list goes on and on

 

[raggertje]

Thanks motomech for your detailed answer!

The trails around here are pretty light and the build should also be light for my son. The motor has to replace my occasional 'push in the back' to him on the short singletrack climbs and on longer flat distances were we ride max. speeds of 16mph (also the restrictions here for normal e-bikes). Holland is sooooo flat, trails have an elevation gain of max. 30meters. I'm also looking for a simple setup of parts, so I can convert to a normal mountain bike quite easily and I don't have to swap parts like wider rims, tires, seat etc. when using a bigger powered motor.

The idea of ​​the rear wheel motor seems better to me, although more stress on the motor, it's better to control the bike, especially for a light kid of 10y, which I read on different forums.

Maybe is it an idea to pick the Q100H 36V350W https://bmsbattery.com/motor/631-16312-q100h-36v350w-rear-driving-ebike-hub-motor-ebike-kit.html#/213-rpm-201 with a single speed cassette like 8 SPEED 11-40T FREEWHEEL https://bmsbattery.com/home/1045-8-speed-11-40t-freewheel.html
And the pick 24/10.5AH BOTTLE BATTERY PACK https://bmsbattery.com/ebike-battery/1093-18527-24v75ah105ah-haitu-bottle-battery-pack-with-usb-output.html#/510-inner_controller-with/543-cells_capacity-samsung_35ah_105ah
and mount it on the downtube. That would also lower the center of gravity
The idea of a soft starter controler is maybe better for him, even off-road, I don't want him to be lazy
But then again, I don't know how te integrated controller in the bottle battery pack will react.

 

[motomech]

Oh, I read your first post as a "10 year old bike", missed the "son" part.
That list looks good for him, but I would get a 36V battery. You can still limit the speed w/ the controller and 36 Volts will give him room to grow into it.
And I would still look into some quality tires with a flat resistant tread. Even a 10 year old can have a branch or something go right thru the side wall of a cheap tire. When you take the original tires off, I think you will find they are wafer thin.
Like motor bikes, if you are going to spend any extra money on an ebike, put it in the tires.

 

[motomech]

If you don't want to remove the frt. chain whl. to install the PAS disc, you can cut the plastic disc into 3 pieces and epoxy it on the inside of the gear. Then it's only a matter of either bending the supplied sensor bracket to position the sensor to the disc and epoxying it the the bottom bracket or making your own bracket. Turn the bike upside down and try everything first. The sensor needs to very close to the disc and you may need to mount it backwards to make it work.

 

[raggertje]

Thanks motomech, I'll stick to the plan

I only doubt whether I should take a 201 or 328 RPM motor. Maybe the 201 is just not fast enough. But if I take the 328 the range will drop to 21km instead of 46km for the 328 (according to the MOTOR SIMULATOR).

 

[raggertje]

I'm still in doubt which motor to order

According to https://www.ebikes.ca/tools/simulator.html?motor=MCUTE36V_328&wheel=24i&batt=cust_36_0.2_7&cont=cust_35_70_0.03_V&mass=45&hp=25

Q100C CST 201 RPM, 36V 7aH, 23.7kph, Range 46 km
Q100C CST 328 RPM, 36V 7aH, 32.6kph, Range 21 km

What do you guys think? 23.7kph is less then 25kph but a range of 21km is less then 50km what I'm looking for...
Is it true that a higher RPM motor is that less efficient or should I not take the values of the simulator too seriously?

 

[raggertje]

I finished the build!

Finally I picked the 201 RPM motor for this build and together with the other parts it really fits the bill. Range wise we have to check it out though. Now it is time for (hopefully) some bigger mountainbike adventures together

 

[2old]

Great conversion, and hope your son's ass can accommodate that seat. Mine couldn't, and forwarded a "please return to WTB" message.