New BMX build, advice needed!

[bignose]

Hi guys, this is my first post, i'm from the UK, thinking of doing an e-bike conversion. Havent sourced a suitible donor bike, but i'm thinking of going for BMX (have no experience with these bikes, but from what i know they are pretty damn sturdy and I wouldn't mind being a bit different and doing one of these instead of an MTB. (Also, you could put ur feet up on the front pegs and ride around without pedaling which sounds awesome! )

Anyways, i've got a few issues...I'm in the UK, where the law here is strict, at no more than 200w (which is completely useless IMO), I also live in bristol at the moment, and it's a hilly city! I've also got a tight budget and while I have a good understanding of the mechanical issues involved in mounting a non-hub motor drive, I've got no tools to manufacture the bits, so i'm afraid is got to be hub motor. So what i'm looking for is approx 1000w hub motor that's fairly descrete, quiet, and batterys to be mounted inside the triangle and hidden away as best as possible, just to avoid ever getting pulled (but i'll hopefully try to keep off-road)

I do however have some experience in the field of R/C planes, so i'm already pretty familiar with Lipo's and their chargers, which I will be using in this build. I'm planning on building up a suitable pack from R/C batterys and insulating them in Nomex fire-proof fabric for safety. To be descrete and to look cool, i'll probably cover the whole triangle with leather, that should waterproof to some extent too. Any potential issues with lack of battery / controller overheating if all the componants are fully insulated like this?

As for motors, I'm on such a tight budget, i'd love to use R/C motors as the power-to-weight / cost ratios are so high, but like I said, my complete lack of materials and tools at the moment makes that not an option, for the moment! So, front or rear hub motor? Now, i know most of you guys shoot down all the chinese kit available on ebay, but for the price, this looks good: http://stores.shop.ebay.co.uk/asiansources__W0QQ_armrsZ1

If these chinese motors really are shit, then what else could I get for maybe a little more (considering I wouldn't be apposed to using a cheap R/C based motor controller, which would potentially save money if i didn't get one of these chinese kits) Perhaps a geared hub motor for hill-climbing?

Really need some advice wrt hub motors as the americans on this forum are constantly refering to various US brands like cyrstalite and they don't appear to be cheap or easy to get hold of in the UK.

Want to keep costs down to an absolute minimum here, i.e. sub £500 including donor bike (about $750), kinda just a proof of concept thing as eventually i'd like to convert a scooter and get it properly regestered and liscenced, I wouldn't be able to use this ebike to communite every day cos i'd eventually get busted, and it seems like if you do get caught you can get a hefty fine and loose ur car liscence.

Cheers in advance,

Mark.

 

[JEB]

If I were faced with your 200 watt limit I would use a cyclone type drive (that allows you to use all the gears on the rear wheel) it works !

 

[icecube57]

The motor you are looking at is a Golden Motor. 48v 1000w. You would need a very very nice battery pack to feed it the 30A it wants. SLA is fine for the start but heavy. If you get LifePo4 battery you probably need 15AH minimum on a ductape pack. Or 10AH for a high quality pack.

A high speed bafang in a bmx wheel should be very nice. 15-18mph speed and nice torque for those hills. Kinda hard to recommend things for other countries. My state ebike laws supposedly limits me to 750w 20mph anything higher has to be registered. I have a GM that goes 30+ MPH and consumes every bit of 1700 to 1800w. Not registered. I ride in the road in traffic on motorized restricted trails. Never been question by the authorities. If they wanted to check I would be busted. My motor is laser etched with the voltage and wattage but just being responsible when I need to be has protected me. If cops begin carrying around meters to check stuff on my bike i would say fudge it and get out of the ebike scene.

 

[dogman dan]

Use enough lipoly and you won't need to worry about c rates. That can allow some real fast hubmotors. You don't say how far you expect to go, so that is what will determine the size of battery pack you need. A real screamer of a bike would be a bmx with a rear chrystalite 530x motor. They come in different speeds and the x number designates the winding count. at 72v riders tend to say 8) :lol:

 

[Drunkskunk]

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=8581

Thats the battery I use. $70 US, they ship all over the world fast and cheap. My orders from them take 3 days from hongkong to texas. I have 4 of them currently, and ordered 4 more yesterday, and understand they are on a plane in the air as of today.

On my bike, I can get 20 miles or more easily on 4 of those, but if I ride it like I stole it (i.e., All the time) I get 5 to 8 miles from them

I don't recomend Lipo to anyone who doesn't have experiance with it.

As for a motor, with the hills you have, you are going to need something geared. A BMC 400 watt would probably be good, a 600 watt even better. Nothing legal is going to climb hills very well.
I wouldn't recomend a golden motor. 1st, its not geared. 2nd, there are better motors for cheaper out there.

 

[bignose]

Okay, so I know these golden motors are direct drive, but it seems to me that most of the other (geared) motors out there are alot more expensive for the same power output.

According to this simulator here: http://ebikes.ca/simulator/ A 500w golden motor can put out 43Nm of torque, now I reckon my budget could actually stretch to getting two 1000w kits (with the newer controllers), now the 1000w kits presumably can put out a bit more torque, and doubling the motors will obviously double that torque output too. Lets say that conservatively, with combined torque output from both these motors is about 100Nm, putting that into 20" BMX wheels should give about 50kg (100lbs) of thrust (which would be PLENTY!)

If there are better motors for the money then please let me know, because getting a 2kw bike with that much hill-climbing potential (even on a non-geared motor) does seem pretty good for $673 (including independant controllers for each motor, wheels, throttles and international delivery)

I am a bit cynical tho...it's all a bit too good to be true, I figure these motors must not be that well made, but I guess since it's not a bike i'll be using every day, reliability isn't going to be my priority.

Thanks for the link to the 5AH Lipo packs, I saw some similar but quite a bit cheaper on the UK ebay site and that was what I was going for, i think 3 of them give the required voltage and would be easily be able to supply my 40A needed, however, at that max current, I'd get just 15 minutes endurance (15 minutes of 2kw fun tho!)

I've already got Lipo chargers, so donor bike and a few trivial materials (like the ones i mentioned above) would be the only additional cost.

Looks like my budget's just skyrocketed after added these lipos to the equation, but if none of you guys reckon it'll all be a huge waste of time and money going with these cheap motors, then in a few weeks time I might put an order in, (after my summer exam period)

 

[Drunkskunk]

I am a bit cynical tho...it's all a bit too good to be true, I figure these motors must not be that well made, but I guess since it's not a bike i'll be using every day, reliability isn't going to be my priority.

Thanks for the link to the 5AH Lipo packs, I saw some similar but quite a bit cheaper on the UK ebay site and that was what I was going for, i think 3 of them give the required voltage and would be easily be able to supply my 40A needed, however, at that max current, I'd get just 15 minutes endurance (15 minutes of 2kw fun tho!)

Yeah, it is too good to be true. check http://www.goldenmotor.com/ and you'll see they don't offer a 1000 watt bike motor. those ones you find out there are... well, they stretch the truth a little.

Motors are rated at there constant power, and conventionaly, at 36 volts. A 500 watt hub motor cruises at 500 watts at 36 volts. it can be used at many voltages, and is often sold at 48 volts, but it's still considered a 500 watt motor.
except a few companies now resell motors, and raise the rating on them for 48 volts (often called 750 watt motors), or try to rate them by the peak wattage (48 volt, 20 amp controller peaks at 960 watts, or close enough to call 1000 watts.) its still a 500 watt motor.

Nothing wrong with real golden motors, but there are better 500 watt motors out there in the same priice range.

Be carefull with those ebay packs, they are often old stock being liquidated, or inferior packs. More than a few horror stories around here of ebay lipos.

 

[bignose]

GM do have a 1000w motor available on their website. I don't know if its just the same 500w motor with the voltage upped to 48, but i interpret the site to mean the 1000w has 1000w rated, with 1200 (ish) peak at 27 amps (got those numbers from a chinese spec sheet on the site - cant read a word of it but you can see whats power, whats current and whats torque.) I think i might go for it, but get one motor at a time, that way i can upgrade the power if it's needed. it looks like, when ordering direct from the GM site that you pay postage per motor, not per order.

 

[swbluto]

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=8581

Thats the battery I use. $70 US, they ship all over the world fast and cheap. My orders from them take 3 days from hongkong to texas. I have 4 of them currently, and ordered 4 more yesterday, and understand they are on a plane in the air as of today.

Dang, that's a good price! I was eyeballing their 6S1P 5000 mAh Flightmax 6s, of which they are out, and it's "5s" cost is about $75 compared to this's $72. And these are available!

 

[icecube57]

I believe that GM markets the same motor at 500w and 1000w. They just play around with the voltage and amperage.My motor is laser etched 48v 1000w. Other GM variants have different codings laser etched on them. Internally I dont think they are different. It is the controller that changes the rating in my opinion. My 1000w GM comes with a 30A controller. Under full load/acceleration it peaks at 1800w. II honestly dont know where they got the number from.