Kid's 24" Mountain Bike Rear Hub Motor Build - Help Needed

[MoutainTime]

Hi, I am getting ready to build my daughter an electric assisted mountain bike. I have a budget of under $200 for the kit. I want the bike to be as light as possible, and just have enough power to help her along on grades from 5% to 10%. This will be offroad as well. Top speed isn't important. I feel I have researched as much as I can, so I would appreciate some feedback.

Here is the kit I am looking at: http://www.ebay.com/itm/36V-250W-26-Rear-Wheel-Electric-Bicycle-Light-Motor-Kit-Cycling-Hub-Conversion-/231821412407?hash=item35f9a20837:g:5wQAAOSwYIhWl-f9

I will have to lace the hub up to a 24" rim obviously. It would be ideal to buy a kit that didn't come with the rim and tire, but I don't think I can purchase the parts separately for anywhere near this price. I have read that hub motors don't like grades, but since this kit is 221 rpm with a top speed of 13.67 mph on a 26" tire, I am hoping that on a 24" rim the gearing will be low enough to assist my daughter in pedaling up the grades. We would like to average 4 to 5 mph on the steepest grades with her pedaling. She only weighs about 65 lbs, and the bike is about 26 lbs.

Are there any better options than this kit for the money? Do you see any red flags on the project?

I am only looking for a maximum of 1 1/2 hours run time from the battery, so I am hoping a small and light pack will be sufficient. I may have more questions about that later.

I look forward to sharing my build as it progresses. I have another great build that is actually started that I will share in another thread.
Thanks!

 

[Drunkskunk]

That should be [strike]good[/strike] functional for a 65lbs rider up a 5% grade without pedaling. I don't think you're going to find a cheaper option. bare motors cost that much with shipping. Figure another $50 for 24" spokes. If her bike uses a 36 hole rim, you can probably reuse it, otherwise, figure another $50 for a rim with shipping. So budget ~$250 for that kit into a 24"

Another ~$100 would get you something that would be a whole lot more functional, like this one already in a 24" wheel: first-fleebay-find-better

 

[MoutainTime]

That should be [strike]good[/strike] functional for a 65lbs rider up a 5% grade without pedaling. I don't think you're going to find a cheaper option. bare motors cost that much with shipping. Figure another $50 for 24" spokes. If her bike uses a 36 hole rim, you can probably reuse it, otherwise, figure another $50 for a rim with shipping. So budget ~$250 for that kit into a 24"

Another ~$100 would get you something that would be a whole lot more functional, like this one already in a 24" wheel: first-fleebay-find-better

Hi, Thanks for the response. I do like the idea of a kit that comes with the 24" wheel already laced. I am really trying to keep weight to a minimum so that the bike will behave off road in a manner that such a light weight kid can handle. Are you saying the other kit is a lot more functional because of the 500 watts versus 250? I really just want this to provide a little assist. I don't want my daughter to rely on it, but have it just encourage her. I am hoping she will only be drawing about 100 watts maximum.

What do you think of the battery plan? I want to go super light. I was thinking of using 2 Turnigy 5S x 5500 mah batteries. This would be a very light set up but only 5500 capacity. I would hope that would be adequate for an hour and a half of riding.

Thanks

 

[motomech]

That's just a bid, not necessarly the going price(although no other bids yet).
The pics show a free wheel, but it's not on the parts list.
Using Lipo requires a hard and fast Low Voltage Cutoff, no exceptions!
What is the LVC of that controller? Most sm genaric controllers have a LVC @ 31 or 32V.
31V doesn't match 10S of Lipo, but it does match 9S well(I've done it).
I hate these sellers on Ebay, they just don't give enough info.

Ckeck out this seller;
ww.ebay.com/sch/diyebike/m.html?item=321940385383&rt=nc&_trksid=p2047675.l2562
and look at the Q85 or Q100 and the 6-FET controller.
You would have to make sure it's a low-speed motor, BUT this seller is really Elifebikes, the second most popular seller in China. They are ok folks and you can get more info at their website and they will even give you real answers.

 

[motomech]

I really just want this to provide a little assist. I don't want my daughter to rely on it, but have it just encourage her. I am hoping she will only be drawing about 100 watts maximum.

Hard to do with the controller in the kit you listed. you would have to shave the shunt(Not advised)
The 6-FET available from Elifebike can be had with the 810 display which allows speed setting. On 9S in a 24" wheel would be;
16 to 17 mph-high
14 to 15 mph-mid
12 to 13 mph-low.
You would have to fine a way to "lock" it in a lower speed, because after she gets used to the bike, she surely would up the speed. "Encourage" her to use the motor? You're kidding, right? She will use it all the time.

You should spend a little more and get everything from Elifebike.

EDIT:

I just checked their site. This would be the kit;

http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2011-43-GQQR.234DR

It's only avail. for frt. mount, but it's avail. as a 24V /201, so it would come with a 24V controller and run less Voltage lipo. Then You could drop my speed est. my a couple mph. Like top speed around 15 mph.
Less than $100 and shipping should be around a $100.
You can "dry run" the order to see exactly what shipping would be.
OR, BMS Battery has a rear Q100 24V kit;
https://bmsbattery.com/ebike-kit/601-q100-24v350w-rear-driving-e-bike-motor-wheel-ebike-kit.html

24V would be around 6S Lipo, very easy to charge and if you ran 6S/2P, you would double the range. Use Turnegy 6S, 20 C, 5000 mAh for best value Lipo.

 

[MoutainTime]

I really appreciate your help. Thanks for taking the time to look all that up. I will really delve into everything you wrote. Thanks again!

I really just want this to provide a little assist. I don't want my daughter to rely on it, but have it just encourage her. I am hoping she will only be drawing about 100 watts maximum.

Hard to do with the controller in the kit you listed. you would have to shave the shunt(Not advised)
The 6-FET available from Elifebike can be had with the 810 display which allows speed setting. On 9S in a 24" wheel would be;
16 to 17 mph-high
14 to 15 mph-mid
12 to 13 mph-low.
You would have to fine a way to "lock" it in a lower speed, because after she gets used to the bike, she surely would up the speed. "Encourage" her to use the motor? You're kidding, right? She will use it all the time.

You should spend a little more and get everything from Elifebike.

EDIT:

I just checked their site. This would be the kit;

http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2011-43-GQQR.234DR

It's only avail. for frt. mount, but it's avail. as a 24V /201, so it would come with a 24V controller and run less Voltage lipo. Then You could drop my speed est. my a couple mph. Like top speed around 15 mph.
Less than $100 and shipping should be around a $100.
You can "dry run" the order to see exactly what shipping would be.
OR, BMS Battery has a rear Q100 24V kit;
https://bmsbattery.com/ebike-kit/601-q100-24v350w-rear-driving-e-bike-motor-wheel-ebike-kit.html

24V would be around 6S Lipo, very easy to charge and if you ran 6S/2P, you would double the range. Use Turnegy 6S, 20 C, 5000 mAh for best value Lipo.