Want to build a BMX bike

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bikester   1 µW

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Want to build a BMX bike

Post by bikester » Apr 05 2008 3:20am

I'm hoping someone can point me in the right direction to build a bike. I want to build a lightweight simple cheap bike (I know those requirements are not synonymous :D ). My roundtrip commute to work is about 4.4 miles so I only need a range of about 5 miles give or take, and there's one or two short small hills on the way that I can pedal on.

My idea is to buy a used lightweight racing BMX bike and put a lightweight hub motor on the rear. From what I've read A123 cells are the way to go. I'd like to custom mount them and the controller in the frame triangle and cover the sides with some kind of covering so it wouldn't really look like an e-bike and it would be lightweight. I think I probably want to go around 20-30mph and have a range of 5 miles and I think I can get by with the stock brake(s). What should I be looking at as far as how many cells volts and amps, and cells etc? I saw somewhere a while back that someone had a lightweight motor. Can someone point me in the right direction? I am fairly familiar with electronics and have had several small model aircraft with lipos but am new to the e-bike thing. What kind of dollars am I looking at? Thanks.

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knoxie   10 MW

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Re: Want to build a BMX bike

Post by knoxie » Apr 05 2008 5:54am

Hi

It depends on how much you have to spend, the BMX is a good platform for a conversion, its not an ideal assist bike though ha ha infact you wont ever peddle. I would go with a front mounted Puma (you wont get one in the rear) and some duct tape life batts in the back, you dont need any more than 40V in a 20 inch wheel to have a lot of fun, 72V is hilarious but is expensive and may get you pulled as you will be doing 30mph and you wont stop the wheel spinning.

I take it you have seen my BMX?

http://tinyurl.com/5k9czx

Is that the sort of thing that you are after?

Cheers

Knoxie
Electric KMX 72V Lipo, USPD drive unit. BMX 72V, Puma BMC,
Raleigh 26 inch MTB Puma 50V Turnigy 25C Lipo 10AH, Trek MTB, X5 48V NIMH. Electric BMX
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Drunkskunk   100 GW

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Re: Want to build a BMX bike

Post by Drunkskunk » Apr 05 2008 10:57am

Welcome to Endless Sphere!

Sounds like fun. Should be doable, with a few revisions. A123 aren't cheap, or compact. I doubt you could fit 30mph @5 miles range worth of A123 And the controller inside the frame's triangle on a BMX. Roughly figuring, you need atleast 4 amps at 72V to handle that speed and distance, meaning 4 dewalt packs. you could fit it to the sides, but it would look like an electric.

Since you're familure with RC planes, and Lipo batteries, that may be the way to go. Are you familure with TrueRC.com? check out there custom Lipo packs. If you already have the charger, then you'll be saving some cash here.

As for the motor, there are some 250-300 watt hub motors, but none of them can get you up to 30mph. a Puma would do, but they make noise and being on the front wheel, they aren't very stealth. The answer may be the Crystalyte 40X series. A 406 or 407 should do it. You realy only need the 20A controller, and that will save you some space.
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Ypedal   100 GW

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Re: Want to build a BMX bike

Post by Ypedal » Apr 05 2008 2:17pm

The BMX is a riot.. but it's a white knuckle ride at high-speed... sharp steering, and short wheelbase make for a fun ride..

This is my X5'erized 72v 40 amp GT !
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bikester   1 µW

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Re: Want to build a BMX bike

Post by bikester » Apr 05 2008 3:53pm

Thanks for the advice all.

Knoxie, your bike is the one I saw several months ago that made me get interested in doing this and I couldn’t remember where I saw it. I think that’s more than what I’m looking for though! :shock:

So after watching the video and looking at power requirements I think I may be interested in maybe 20-25mph at 5mile range as light and stealth as possible. I weigh about 76kg. I like the BionX because of the lower weight now but I wouldn’t mind piecing my own components and it looks difficult to do with the BionX.

I’m not an expert by any means I just know enough to respect lipos and that VxA=W. That Truerc.com looks like a good site, I used to order stuff from radicalrc.com but I don’t have a charger anymore. I’m up in the air on all this stuff. I didn’t know the A123s were expensive, I thought they were cheaper than lipos. I’d like some batteries that would fit or that I could take a part and stack in the triangle and cover it instead of having a battery “box”.

So a 250 watt motor on the rear with a 20 amp controller should do the trick? Will 24 volts work or do I need to go to 36? So with my lower speed requirements what type of batteries would have the best chance of fitting in there?

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Link   100 MW

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Re: Want to build a BMX bike

Post by Link » Apr 05 2008 9:39pm

Wiggy. You're doing pretty much EXACTLY what I intend to do. My intention really is not so much speed (I'd even be happy with a little over 25mph), but I want killer acceleration. Like, "touch throttle for wheelie" acceleration.

Anyway, a custom Ping Pack might do it. If you keep the current levels relatively low, they should be able to do it. A 36V 10Ah pack is made of 5Ah cells in parallel. If you're up to it, you could switch those to series and get the speed you're after. The battery would only be rated for 10A draw continuous if you do it like this, but that shouldn't be too much of a problem in a 20".

Fitting them in the frame is my problem, too. Originally, I didn't want to alter the profile of the bike at all (e.g. the pack wouldn't be any thicker than the frame itself), but I've conceded that that wouldn't be possible to do and still have decent range. Since you only want 5 miles of range, though, you shouldn't have too much of a problem at all. You may even be able to fit the controller in there, too.
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Re: Want to build a BMX bike

Post by Bill Best » Apr 06 2008 9:00am

I'm looking for a good 20" tire with lots of traction on paved surfaces. I've got a 72V 50A system waiting to be installed on my trike in the front, so I'm going to need as much traction as possible.

Someone suggested Kenda Kontact or Maxxis Hookworm, but I'm looking for other suggestions.

Sorry about posting in this thread, but since most 20" tires are used on BMX bikes I thought it would be appropriate. :)

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Re: Want to build a BMX bike

Post by jdbauman » Apr 09 2008 5:52pm

Hi Ypedal,

I love the BMX Gt that you have pictured in this thread. I really don't need a whole lot of power however and would like to get about a 20 mile range. My thoughts are:

1. BMX Frame (maybe a diamondback)
2. 36-72V (35Amp) controller
3. Rear 405 Crystalinte motor
4. 8 - Emoli V28 batteries

I really want a rear motor, but my understanding was that you couldn't fit a 4XX or a 5XX Crystalyte motor on the rear since most BMX bikes have about 100 - 110 mm dropouts. How did you get yours to fit? Does your BMX have wider rear dropouts?

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Re: Want to build a BMX bike

Post by jdbauman » Apr 09 2008 6:00pm

I was also wondering if you think this connector from powerwerx.com would work for connecting all of my emoli packs. I am currently using the "connector blocks", and then have crimped all the negative leads together with the negative lead to the controller. Same with the positives. I want to add more batteries to my pack however and was wondering if you guys think this would work...

Image

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Ypedal   100 GW

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Re: Want to build a BMX bike

Post by Ypedal » Apr 09 2008 6:59pm

Thanks.

Well.. for a 20 mile commute.. i don't recomend a BMX. Not that you can't do it if you really want to.. but the BMX is more of a short trip joy ride type bike.

a 405 is good for a 20" at up to 48v, if you want to run 72v get a 408 or 407.

Edit : Oh, about the rear dropouts... notice the lack of chain in the picture !! hehe... it's a front 5304 on the rear side of the frame ! :D
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Re: Want to build a BMX bike

Post by jdbauman » Apr 10 2008 12:05am

Very clever Ypedal... I was wondering just how the heck you could fit a 5xxx into a rear BMX Hub.

My commute is actually about 10 miles each way, so it wouldn't be a full 20 miles at a time. I just would like to have a full 20 mile range so that I could make it back home without having to recharge at work. I would also put a thudbuster on it to make things a bit more bearable.

Okay, I'm confused. Is the 405 motor not able to utilize the voltage above 48 Volts? I was thinking that in order to go about 25 MPH on a 20" wheel that I would need either a 404 or a 405 and increase the voltage. Not true?

Also, what do think about the Powerwerx connector in the previous post?

Jer

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Ypedal   100 GW

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Re: Want to build a BMX bike

Post by Ypedal » Apr 10 2008 6:26am

Oh. you " Can " run a 405 at 72v.. but the no-load speed gets rediculous and you will never go fast enough to allow the amp draw to go down.

A 409 at 72v in a 20" does 25 mph at roughly 10 amps ( depends on many factors ) even if you use a 40 amp controller.. the only time 40 amps will show up is from a dead stop or major hills..

A 405 will draw 40 amps way into 35 + mph at 72v..

http://www.ebikes.ca/simulator/

Great tool !!!

About the powerex.. i'll have to look up what it does.. looks like a straight 4 x 12v disconect but i'm not sure what the purpose is exactly. will need to read up on that later today..
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Re: Want to build a BMX bike

Post by kbarrett » Apr 16 2008 3:39pm

Bill Best wrote:I'm looking for a good 20" tire with lots of traction on paved surfaces. I've got a 72V 50A system waiting to be installed on my trike in the front, so I'm going to need as much traction as possible.
For bikes, most traction on pavement equals no little or no tread. Bikes don't go fast enough to hydroplane, so tread pattern is fairly ornamental ... except for trail tires, which have knobs for traction in loose stuff ... but have less traction on pavement.

I would suggest Schwalbe Big Apples .

Or maybe a Demolition Monaco if you can stand a completely bald tire.
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Electricdirtbiker   100 mW

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Re: Want to build a BMX bike

Post by Electricdirtbiker » Apr 17 2008 12:09pm

Trying to include a picture here. Hope this works. My bike is made from a full suspension bmx bike with 20" wheels. It uses a magmotor, 24v hawker batteries, alltrax controller. The rear derailler system still works, but its a wide ratio 3 speed cluster, home brewed. In low gear, the bike can nearly climb a wall or wheele up. The suspension is really a must for trail riding. The high gear can reach 23mph, gearing limited. I can get 5 miles off road and 7miles on road. 23mph is a little scary on this bike size. At 75lbs theres not much stability. 30mph would be for flat smooth black top and white knuckles. Borrow a bike this size and try it at 30 mph. Have fun.
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Link   100 MW

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Re: Want to build a BMX bike

Post by Link » Apr 17 2008 3:02pm

Electricdirtbiker wrote:Trying to include a picture here. Hope this works. My bike is made from a full suspension bmx bike with 20" wheels. It uses a magmotor, 24v hawker batteries, alltrax controller.
Alltrax? Overkill much? :o
Electricdirtbiker wrote:Borrow a bike this size and try it at 30 mph.
Oh, I intend to. 8)
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Electricdirtbiker   100 mW

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Re: Want to build a BMX bike

Post by Electricdirtbiker » Apr 18 2008 1:11pm

The alltrax is an over kill. Its really for another project. I have some a123 cells to try next. Maybe a 8s 10p pack. This would save 10lbs and 3-5x my range. Or increase the voltage and gearing to increase the top end. The mid size motors running in the 20-30mph range will get fairly hot. When I trail ride several good hills in a roll, or a few good runs over 20mph, the motor is too warm to touch.
If you do much riding on trails, the multi gears are a must. The hub motors add considerable unsprung weight to the end its mounted. Traction will not be so good, especially for a nonsuspension bike. If you ride mostly black top, the hub motors are far simpler to setup.
Good riding.

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Link   100 MW

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Re: Want to build a BMX bike

Post by Link » Apr 18 2008 3:52pm

Electricdirtbiker wrote:The alltrax is an over kill. Its really for another project. I have some a123 cells to try next. Maybe a 8s 10p pack. This would save 10lbs and 3-5x my range. Or increase the voltage and gearing to increase the top end. The mid size motors running in the 20-30mph range will get fairly hot. When I trail ride several good hills in a roll, or a few good runs over 20mph, the motor is too warm to touch.
Sweet.

Yeah, they will. A long run at inefficient speeds with my BD36 will get it pretty hot. It won't burn you instantly, but it gets uncomfortable to touch after a few seconds.
Electricdirtbiker wrote:If you do much riding on trails, the multi gears are a must. The hub motors add considerable unsprung weight to the end its mounted. Traction will not be so good, especially for a nonsuspension bike. If you ride mostly black top, the hub motors are far simpler to setup.
It's all street here. But even then I notice the extra weight and gyroscopics on even my front hub equipped S40 mountainbike. I like motors that are able to use gears, but hubs are easier and stealthier to setup.
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