mud's RC motor bmx build

Hi all, I'm starting a new build with a bmx bike. My last build was an RC motor moutain bike that I never fully finished but had it quite ridable and learned alot about what I really want in an ebike.
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=7791&hilit=astroflight+turned+brushless
On the last bike I didnt like the twist throttle, the 11lbs of battery over the back wheel, and how overall it just felt kinda flimsy.
to fix these issues Im going to use a thumb throttle, smaller batteries and a stronger frame bike.
I found a specialized fuse 1 bmx on craigslist that was barely ridden for $225 so I bought it thinking it had a flip-flop rear hub. turns out the left side has left hand threads for people who want to ride with their chain on the left lucky find.
I added a new seatpost, seat, chainwheel, and tires. For tires I got the Primo Comet brand slicks that go up to 110psi.
next I bought a 14t southpaw freewheel and an 80t #25 sprocket from SDP. I found a guy with a garage machine shop who bored the sprocket out and put in bolt holes for only $20.
I custom ordered an astroflight 8120 10 turn motor and got an email today saying they shipped it it runs 135Kv so Im going with 16t on the motor to 80t on the wheel so its geared to go over 40mph. Im not sure if thats going to be too high but I'll find out soon
I have a 24volt 20ah ping battery from my last build, but I want to save weight this time. I cant decide between a ping 24volt 10ah version 3 battery or trying some lipos. Ive been thinking alot about it and its really a tough decision.
anyway heres some early pics, next week I'll have the motor and hopefully a new lighter battery View attachment 1

Super clean sprocket! I like the simplicity of this build, one reduction and done. Just curious how the #25 chain will handle the load. I even like the colors on that Specialized, very "green".

Man, if one more BMX build shows up, we will have to start a new section. :wink:

You have to get Lipo to keep that frame clean. IMHO

Oh, Man, another BMX bike with an Astro motor!

SWEET!

If you decide to go with Lipo, Zippy packs from Hobby city are the best price. You can get a 24 volt 10ah setup for $160! Fo course, you will need a lipo charger too. That will be around $200 or so.

Anyway, if you have any lipo questions, or any other questions on the build, let me know.

Matt

thanks guys, I'm also wondering how long the #25 chain will last under load. I can't wait to find out if it breaks or the batteries die this thing should be light enough for easy pedaling around. I'm thinking about drilling some holes in the sprocket to save a bit of weight, but I'm worried I'll ugly it up. I really want to get another sprocket made and send it to a water jet cutter and get this pattern below, but its going to take time.
As far as lipos go, I do have a question for you Matt, is it true you run lipos with no bms and havent had any problems? I had read your post in another thread and thats what it sounded like you were saying, but I wanted to confirm before buying anything.
I was looking at the 15c zippy lipolys 22v 5000ah and using 2 packs in parrallel. can I charge them like that? whats the best charger for this size pack?
I'm leaning toward getting some lipos for a nice lightweight bike, but I need to figure out a few things first.

That is correct. Lipos do not need a BMS. I have never had a pack (and I have had over 100 packs) go out of ballance. I check them with a ballance device periodically. But, none have ever been out of ballance.

Yes, you can parrallel them. No problem there.

You can get any decent name brand charger that will support 6S. Remember, RC chargers are designed as field chargers. As such, they are designed for a 12 volt input (so they could be used off a charge battery at the RC flying field). So, you will need a 12 volt power supply for the charger.

Matt

got the motor in the mail the other day it looks great can't wait to try it. Also drilled some holes in the sprocket to lighten it, went perfectly
I made an image in "the gimp" and printed it then cut it out and taped it onto the sprocket, and then just used a punch to mark the spots to drill the holes.
Also I ordered a battery from ping, I went with the v3 24volt 10ah pack at 5 lbs. I would have got some lipos, but I just can't get past the possibility of fire issue.
I work at the airport and I would be putting my bike inside while I'm working and I wouldn't be near it. So if my pack spontaneously bursts into flames the genius security guards are going to think its a bomb or something. Anyway I just want a pack I'm 100% sure won't catch on fire.

Looking awesome! Can't wait to see how this one turns out.
Are you going to use a CC HV110? Where / how is the motor going to be mounted? Details!

That looks really nice. The sprocket turned out great.

That V3 Ping should be fine. There are only two issues with this;

#1 Size. A BMX bike needs everything to be small.
#2 Performance. LiPos are generally far higher C rated. However, the V3 you ordered should work out fine and, like you said, they are a worry free chemistry.

Matt

Yeah, I wouldn't race the PK with this bike you definitely have more power and less weight. I'm having to make some compromises for safety and cost.
On my last bike I got >20mph on 20 amps at 24volt, so the performance should be acceptable at 3c.
I got the dimensions of the pack 75x250x85mm. ~(3x3.4x9.9inches) which is a lot smaller than my last pack, but the bmx is a smaller bike.
Hopefully I can find a good place to put that size battery, if not then I will get some lipos and lots of fireproof bags :|

Are you going to use a CC HV110? Where / how is the motor going to be mounted? Details!

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yes on the CC HV110, and I should have pics of the motor mounted very soon.

I made a bracket to mount the motor out of 3/8" aluminum, here's some more pics

Brilliantly simple.
I don't know how much those Astro motors weigh, but I'd be a bit worried about it being mounted there and hitting some unexpected potholes under acceleration or something. I would also make a second brace, maybe connect it to the left side brake if you can squeeze something else in there?

+1 on more support... specifically to suppress rotation of the flat-stock. It will be trying to swing opposite the wheel rotation, just like the axle of an X5.

motor weighs 2 lbs.

It will be trying to swing opposite the wheel rotation, just like the axle of an X5.

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It can try but it won't be able to, the aluminum bracket butts up against the frame tubes at the dropout, keeping it from rotating.

I'd be a bit worried about it being mounted there and hitting some unexpected potholes under acceleration or something.

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why? if you mean the weight of the motor will bend the bracket, no worries. on my last build I did a similar mount and only used 1/4" aluminum and it didn't bend at all hitting bumps, but I could get it to flex under extreme acceleration. The 3/8" aluminum should be strong enough to eliminate the flex issue.

I think what people are recommending is increasing the mechanical advantage against rotational torque on the motor bracket through the bracing of the motor bracket further outward toward the motor (greater leverage).
I think you might have misunderstood the earlier remark as the suggestion to rotate the motor bracket until it aligns with the rear dropout like the torque arm of a hub motor - which as you say cannot be done as the flat axle bracket welded to the rear dropouts can only accomodate a flush mount with the motor bracket throughout an arc anticlockwise (from the photos point of view) of its current angle.

something like this....

I think what people are recommending is increasing the mechanical advantage against rotational torque on the motor bracket through the bracing of the motor bracket further outward toward the motor (greater leverage).
I think you might have misunderstood the earlier remark as the suggestion to rotate the motor bracket until it aligns with the rear dropout like the torque arm of a hub motor - which as you say cannot be done as the flat axle bracket welded to the rear dropouts can only accomodate a flush mount with the motor bracket throughout an arc anticlockwise (from the photos point of view) of its current angle.

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no, I understood the concern over rotational torque on the motor bracket. At first glance it looks like the bracket will rotate under torque but if I had better pictures you could see the bracket edge sits flush against the frame tube welds and cannot rotate at all.

wow, that's so simple. I love it. I hope it works. When you get it running, can you do a video of it in action?

yes on the video I have a friend who offered his help (and cam) making a video.

That is only a 5 to 1 reduction. Most guys doing RC stuff are reducing much more than that and running higher volts for more RPM. 40mph would be about right under no load, will be interesting to see what it does loaded. I'm guessing that at the very least, there will be much pedal assist at startup needed.

Dude, I wouldn't even *think* of trying 24V 10Ah ping with a set-up like that. At 2C, that's a measly 480 watts and 3C is barely 1HP and I can guarantee you won't be getting anywhere near 40 mph with that. I'd be surprised if you'd hit 25. Also, let's say you do get a 24V 10AH which has an internal resistance near .4 ohms...

My ebike calculator suggests 24V of 10ah of pings would get you to 20 mph at 25.7 amps(2.5C is a bit much) and you'd accelerate to 10 mph, with no pedaling, in about 10 seconds. By contrast, at 24V, zippys would take you to 30 mph at 47.6 amps and they wouldn't even get warm(At 9C of 5 AH versions 20C versions). Also, you'd get to 10 mph in about 3 seconds and you'd near the top speed in about 10 seconds.

I assumed a 20 inch wheel and 250 pounds of mass(mass only affects acceleration, mostly).

Every number is plus or minus 15%.

well my battery pack choice is based on experience from my last build which used the same motor except is was a 5-turn. I used a 24volt 20ah ping, it weighed 11lbs and delivered 2c continuous. It was geared to go 23mph and went that speed usually using 15-20 amps. I rarely used 30 amps (did I mention I only weigh ~125lbs) I don't really want to go 40mph all the time, thats just the setup I'm trying first, I can always change the gearing.
If the new ping pack can do 30 amps continuous, I can almost guarantee 25mph will be no prob. A measly 480 watts on an efficient lightweight setup is not as measly as you might think, but mostly in my case its my light body weight that makes this possible. If I were a bit shorter I would have had an excellent career as a jockey.

well you asked for video, I took this with my digital camera and the video quality is bad and the sound quality worse but you get the idea
Its just a quick test to see if everything works. I rev up the motor to full speed (40+ mph) hold it for a few secs the let go of the throttle and let it spin.
It freewheels for 30 secs, and it sounds awful on the vid, nothing like the real sound. really choppy sound too, goes in and out. this camera is old.
anyway, you've been warned, here's the vid:

can't wait for my new battery to do testing so I zip tied and masking taped my old battery to the bike and took it for a ride
It's very quiet with the 10 turn motor , but the acceleration was poor as you guys predicted. I'm going to look into different ways to get a higher ratio of gearing
I cant go too much bigger than 80T on the sprocket because of frame clearance, and I cant go much smaller than 16T on the motor for obvious reasons
If I go 15T to 90T that will be 6:1, but I'd rather do 16T to 125T. the sprocket would be huge but I'd get 7.8:1 or 26.7mph top speed which would be perfect.
the only way to do this would be to find a frame with more clearance, but it would also have to have the same size bb and everything or my components from the old bike wont fit
maybe I'll look into adding the gearbox from my other build, but I hoped to avoid that.

swbluto said:

Dude, I wouldn't even *think* of trying 24V 10Ah ping with a set-up like that. At 2C, that's a measly 480 watts and 3C is barely 1HP and I can guarantee you won't be getting anywhere near 40 mph with that. I'd be surprised if you'd hit 25. Also, let's say you do get a 24V 10AH which has an internal resistance near .4 ohms...

My ebike calculator suggests 24V of 10ah of pings would get you to 20 mph at 25.7 amps(2.5C is a bit much) and you'd accelerate to 10 mph, with no pedaling, in about 10 seconds. By contrast, at 24V, zippys would take you to 30 mph at 47.6 amps and they wouldn't even get warm(At 9C of 5 AH versions 20C versions). Also, you'd get to 10 mph in about 3 seconds and you'd near the top speed in about 10 seconds.

I assumed a 20 inch wheel and 250 pounds of mass(mass only affects acceleration, mostly).

Every number is plus or minus 15%.

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Sorry, disregard this post. I was taking stupid pills that day. My 48V 10Ah ping battery had an internal resistance near 250 mOhm so a 24V10Ah ping battery would have something near .125 ohms(the version 1) and not .4. That would mean you'd probably have a higher top speed like 23-25. Which now makes me lead to believe... my hub motor kind of sucks. It's sipping away at 900 watts but yet only goes 27, which made me think 700 watts could only get you something like 22-23.

Which now makes me lead to believe... my hub motor kind of sucks. It's sipping away at 900 watts but yet only goes 27

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no way, I just went for another test with 27mph as my target speed. I couldn't even get to 27mph without my eyes watering so bad I could barely read the watt meter. The wind resistance above 22mph gets really strong, so if you're doing 27 at 900 watts your hub motor rocks.
Anyway I couldn't really get an accurate reading but I have a feeling I would draw more than 900 watts to go that fast.