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AmpedBikes/9C 60volt BMX conversion Urban Bike


100 mW
Feb 7, 2011
Snellville, Georgia, USA
Hey guys,

If an MTB conversion is the F150, and a beach cruiser conversion is the Cadillac, then the good BMX 20-inch "Urban Bike" conversion is the Mazda Miata of E-Bikes. We might not want to take a thousand mile cross-country trip on one, but our day-to-day driving will be very enjoyable. My experience bears this out! I'd like to persuade you to consider this format for your next e-bike project.

In general, the stronger frame and wheels are part of the appeal, especially if we choose the "freestyle" type frame over a lighter "race" type frame. Precise handling is a great benefit. Stronger thrust from any motor, as compared to a 26" or 700c bike is also a real plus. It is also nice for us older riders to be able to place both feet on the ground at a stop, making mounting and dismounting easier.

There are also certain real drawbacks inherent to the concept; limited space overall for battery placement being one. The narrow rear dropout width (110mm) makes a rear drive motor problematic. Still, with good planning and execution, a really outstanding urban bike is possible.

FYI: my BMX conversion features: a Trek TR10 steel frame(20" top bar and 14mm dropouts), a GT steel front fork (3/8"  dropouts), a stock Trek 48-spoke rear wheel, and Maxxis "Miracle" ribbed 20" x 2.1" tires (85 to 110 lbs. pressure rating) at about 90 lbs. pressure. I have a two-speed derailleur on my front sprockets (40/48 teeth) and a 15-tooth rear sprocket. I use standard V-brakes. (A big, wide, coil spring suspension saddle from cloud nine takes care of the bumps for my old back.)

I have an AMPEDBIKES/9c (2807?) front-mount direct-drive motor, with stock 35-amp (max) 36-60volt controller, stock thumb throttle, etc. as supplied by AmpedBikes and Doug Daut at Electric Bike Kit Solutions (gocarlite.com.), and a hand assembled Turnigy RC 10 amp-hour (5S2P x 6) - 55.5 volt nominal LiPo battery (high of 63 volts, low of 48 volts).

My battery pack weighs a tiny bit more than ten pounds when wired and bagged for riding, and is mounted on a TopPeak rear touring rack capable of 50 pounds load. My controller is mounted underneath the rack. The extremely heavy rear dropouts made it simple to drill and tap holes for mounting the rack support legs.

I also operate two LED battery operated headlights, two ebike.ca halogen 10 watt headlights that run off the main battery, and a Planet Bike LED battery operated taillight.

This complete package weighs in at slightly less than 69 pounds on the bathroom scale. Measured range when ridden for conservative use of available power is 30+ miles. Top speed is about 28 mph on the flats @ 63 volts right off the charger. That doesn't last long! More realistically, top speed is a nominal 25 mph. AVERAGE speed in my subdivision, up hill and down, maxes out at a measured 21 mph.

This bike tackles the hills with verve, yet has plenty of speed for the flats, and I only pedal when I want to. Rarely is the hill so steep that I feel like I HAVE to pedal. (That was not the case with my old 36-volt 26" lead-acid MTB.)

There seems to be enough rake and trail with the GT fork that the steering is not at all twitchy. Potholes are handled with aplomb due to the strong small wheels.

Interestingly enough, the "magnetic drag" issue is less of a factor on this bike with 20 inch wheels than it was on the MTB with 26 inch wheels. I don't know why, but it is not nearly so apparent when coasting down steep hills. I would have thought the opposite would prove true.

For the future: I am strongly considering conversion to rear drive. I'm about at the limit of front-wheel drive traction at this voltage, on wet pavement, so I believe I'd need rear wheel drive before making any further upgrades. I have ordered a "spare 9c side cover, threaded for freewheel" from ebikes.ca just to see if it is possible to adapt one to the other using the front drive motor with the short axle. If I can, I want to fit the motor between the rear drop outs with a single speed freewheel. I'm willing to spread the rear "fork" a tiny bit if need be. 110mm does not leave a lot of room to play.

Then I will consider ordering an Edward Lyen 72 volt 45-amp controller, and adding one battery pack assembly to my power pack, just to see what a difference that makes. (high of 84 volts right off the charger, low of 64 volts - this is higher than my HIGH voltage now)

IF I can successfully adapt the front mount DD motor to rear drive, complete with single-speed freewheel, I will post the results here. I will also strongly consider adding an RST Capa T10 Suspension front fork to the bike, and front disc brakes to boot.

Hopefully, this narrative will encourage more of you to take on the BMX-20-inch build for your next e-bike. I'D like to read about your BMX frame build experiences.

wdwrkr51 - Greater Atlanta area, State of Georgia
with my rear hub bmx build I had to move the batteries from the rear rack to the front to keep it from being a complete wheelie machine.

I just mounted them to the front handle bars (over the wheel, not inboard).

And it has the cheapest zoom suspension fork, which really helps moderate the turbulance.

in the winter it gets the studded tires for snowy / icy conditions

definitely the most fun ride i have
Tell me a little more about your rear-drive BMX. Do you have the ability to pedal as well as use the motor? What brand of motor, and how did you go about installing it?
What voltage are you running?

It's a 9c 2805 on 48vnom.

16t freewheel in the rear

and I changed the crankset to 52t available here:

but that meant i needed to convert the bottom bracket:

So it's better than stock, but I certainly can't keep up with it @48v.

Maybe if I spent another $100-$120 on a new chainring:

I'm actually thinking of going down to 36v, or maybe swapping the 2805 for a 2807 wind motor. Or even a geared hub for the torquy fun factor.
I've other bikes for speed, and my boy rides this one.
Joe, I don't understand yet how you fitted the 9C/2805 motor into the rear dropouts, (and secured it there) while retaining the ability to pedal. Can you educate me, and/or maybe include a photo? I understand you have a single rear sprocket/freewheel, but the rest of it is beyond my experience.

I'm just guessing that you have a rear-mount DD with threaded right-hand cover for installing the freewheel or freewheel cluster. Did installing the single sprocket freewheel leave you enough extra space to get it between the rear dropouts, or did you have to spread the frame some? Doesn't that leave you with too much axle on the right side? How did you handle all this?

BTW: my ampedbike/9C is almost certainly a plain-jane 2807, and the torque/thrust is very good. I can't keep up with it either, using my 48/15 gear combination. (3.2 ratio to your 3.25.)

Ah yes!

Completely forgot about that.

Here is where I mention spreading the rear stays with the use of an Oxy-ACt torch and some threaded rod.

Still havn't posted any pictures of that. Sorry.

I would value your experience and your advice as I go about my next project. I'm conducting an experiment. I have ordered a spare right-side threaded cover for a rear-drive 9C DD motor. I want to see if the bearings and mounting holes line up to the front mount ampedbikes/9C motor I already have. I couldn't figure out a way to check this out without actually ordering the side cover and making the experiment. I don't know anyone else who has done this. From the photos, technical drawings and from examination of my own motor, I decided it was a good possibility and worth checking out.

If it works, this would give us BMX frame users a way to adapt the front drive motor with it's short axle to the rear drive dropouts on a BMX frame, while still retaining the ability to pedal. The cost is less than $50 plus shipping. It MIGHT mean spreading the rear dropouts wider than 110mm but less than the 135mm demanded by the rear mount motors available now. Then we'd have to add spacers to the axle.

What do you think? In the course of your project building, do you have any direct experience that might contribute to my education?? :D

I'm posting this in the hope that anyone else who can offer direct experience would feel free to reply. Y'all keep those cards and letters coming in, now, hear?

Thanks for posting this, I'm doing a BMX style build myself right now and think it's going to be a ton of fun.

I'm doing a BMX sized build with a kids full suspension mountain bike. The rear drop outs are bigger because they have/had gear clusters. It looks like my 2806 motor will fit but haven't been able to try yet since it's still in a 26" wheel. Should be a blast on 67V LiPo. I'm going with a single gear and keeping a 3 chainring front with the largest being 48T.
I just spread the rear dropouts on a 20" folding steel bike using a car scissor jack. It was quick and easy. I made a few attempts before it was spread enough. You have to go way wider than needed but once the drop outs spring back into position they are wider. This technique seems to have spread the frame evenly on the left and right side.
I am a little nervous about trying to spread the rear dropouts a full inch over what they are now, as the frame structure is very heavy. (That is one of the reasons I chose it for this build. I tend to "overkill" every problem. :lol:) I do have a scissors jack. I am willing to give it a try, especially if the needed expansion is a good bit less than one inch. If I have to expand the rear a full inch, I should have gone with the rear mount motor in the first place! :oops:

Well, the "spare 9C right cover plate, threaded for freewheel" arrived today from ebike.ca (Grin Technologies) , and my initial impression is that it looks quite viable as a replacement for the right side cover on my front wheel mount motor. The two covers appear identical, except that the rear mount cover plate has threading that takes up the entire outside shoulder on that side.

P6050012.JPGView attachment 2View attachment 1P6050016.JPG

The BMX freewheels thread right on! My sixteen tooth single-speed freewheels will add 6mm to the right side, and my fifteen tooth freewheel (different design) will probably add a full 10 mm on the right. I don't know if I will be able use it and still have enough axle on that side to secure the motor. Now my concern is that the axle will prove TOO short. The adaptive steel plates needed to secure the motor in the 14mm dropouts will take up another 10mm or so. This should be interesting, and I hope it works. Maybe I will get motivated enough to attempt this next week. I'm enjoying the riding and hate to have to "do without" for any length of time.

I wonder if I will have to change my frame for one with 3/8" dropouts to make this work......

Wouldn't it be nice if 9C would produce an axle length just right for the rear dropout spacing on the 20" BMX frame? Wouldn't it be nice if one of the importers would request this small modification?

"The best-laid plans of mice and men often go astray" (Robert Burns? Shakespeare??) My front derailleur is unexpectedly giving me some problems. I will try adjustments and more tension on the chain, but I am wondering if this is going to work out. I might have to keep the dual front chain rings and use the 40-tooth gear as a limp-home gear, shifting the chain manually.

I'm going to have to get a new frame if I want to move this motor to rear wheel drive. AT 63 volts / 35 amps maximum, the front wheel will spin if it hits sand or debris on the pavement, when starting out or at very low speeds. Wet pavement is a tiny bit dicey, too. Off pavement, forget it. 84 volts / 40 - 45 amps maximum, and another 20 pounds+ of thrust, or so, (ebike.ca simulator) and the problem might be dangerous. It does steer well at speed on the pavement with fwd, however.

There just isn't enough axle to secure the motor in the dropouts on this frame, (with 14mm rear dropouts) what with the adapter plates, spacer, etc.

BTW, my Edward Lyen 12 MOSFET controller and Turnigy 18.5 volt (5S/1P) batteries are on order. One day I will have everything worked out - and then what will I do with my time? :lol:

Still, this setup has been reliable for 300 miles+ of testing and riding. I know it works, and I can always go back to it it need be.

BMX Urban bikes rule!

(hey - you there! Stop laughing at me!) :)

Well, here is how matters stand. I have purchased a new(ish) cro-moly 20" BMX frame off ebay, with 3/8" rear dropouts, but it hasn't arrived yet. I've ordered the suspension front fork and steerer tube, disc-brake compatible front wheel hub, spokes and rim. I've ordered an Avid BB7 front disc brake assembly and disc. I've already received the Edward Lyen 72 volt 12 Mosfet controller and am waiting on delivery of two more 18.5 volt 5000 MAH turnigy battery packs (one 18.5 volt 10 amp-hour battery assembly). When all arrives, I will begin again.

I plan to assemble an outstanding BMX/Urban bike, using some parts and the AMPEDBIKES/9C from the Trek frame pictured above. In the end, it will feature a 74 volts nominal RC Lipo battery pack (about 12 pounds ready to ride) and rear-wheel drive. Front suspension and disc brake will only improve on the first build. The ebikes.ca simulator shows that this build should offer nearly 100 pounds of thrust and nearly 35 mph top speed on this set-up, and the 20" rearwheel drive will better handle the increased "oomph". :shock:

I believe I will work on the front fork, build the wheel, assemble and install the disc brakes first. Then I will install new bottom bracket bearings, chain ring and cranks. This is also a good time to do any adjusting and painting of the frame.

Until I am absolutely ready, I plan to hold off removing and modifying the AB/9C direct drive motor. That way I will be able to "keep on ridin' " until the last minute, and will be back "on the road again" as soon as possible. 8)

"I'll be back!"