BMX frame E-Bike discussion thread?

[wdwrkr51]

Hey guys,

There are many reasons for choosing a 20" BMX frame as the basis for an E-bike project, though not so many of us choose to go that route. In general, stronger frame and wheels are part of the appeal, especially if we choose the "freestyle" type frame over a lighter "race" type frame. 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 drawbacks to the concept; limited space overall for battery placement being one. Still, with good planning and execution, a really outstanding urban bike is possible. I know! I've built both the 26" and the 20" bikes, and in my opinion, for my purposes, the 20" bike is far superior.

I'd like to start a thread, for general discussion purposes, wherein people interested in the 20" BMX frame as a starting point for an e-bike project can get together and share ideas. Anybody else out there interested??

wdwrkr51

 

[Thud]

Wdwrkr,
You don't need to convince me of the benifits of the BMX platform. Its my favorite. Although I find the smaller diamond a bit constraining at times. I am really pleased though with the BMX cruiser (24" wheel) frames. then there is the added dillema of overall dimensions for a total "handling" package.

I have completed 3 builds thus far on the small frame bikes & can say they are totaly enjoyable. I have a few more in the "reclaim" pile currently that I will be used as the test frames for some goodies I am making as bolt-on power systems.
I have yet to do anything with hub motors in any E-bikes yet, & unless I an going to be comuting I doubt I will. one of the main atractions for me is the feather light feel of the BMXers.

I have been restraining myself from ordering a Walmart Bike that cought my eye a couple weeks ago that is beggind for a little chopping & streaching...prolly a very disapointing fork....but disc brake mounts are already there front & back & rear v brakes releases the rear disc to become the dive side sprocket.

http://www.walmart.com/ip/Mongoose-Girder-20-Boys-BMX-Bike/14956336
A couple of these in the garage guarantees some fun side yard action when friends drop by.

 

[wdwrkr51]

Hey, Thud:

I've gotta check out the Mongoose Girder bike that you recommended I look at. I've never seen one. Front Suspension and disc brakes are very attractive to me. Do you think the frame/wheel setup is stout enough for the extra duty of a 200 pound operator and speeds up to about 30 mph??? Or is this really a KIDS bike and not a bmx intended for teens, etc.?

Thanks for the tip.

 

[lcyclist]

Yeah, I love small, rigid frames. To me the best attribute to these frames are the dropouts; they are usually very beefy, horizontal dropouts. Small frames have challenges, like battery placements, but their toughness is attractive for ebikes. With a cromo frame, torque arms are optional.

Also, the dirt jumping frames in 26" or 24" wheel (to me, they are big BMX) share similar traits. Since these frames share components with 26" mountain bikes, parts are easily found and numerous---ie suspension forks, cranks, etc. Had an Azonic Steelhead---all steel, jump frame with 1/4" horizontal dropouts. Very confident at high speed.... I guess this is due to their long toptube (long wheelbase), low center of gravity and stiffness.

My first ebike platform was the Redline Monocog---a big BMX, all steel, rigid forks. Ran a front hub without torque arms.

 

[dogman dan]

I started getting interested in fixie, or single speed frames. They often have very similar rear dropouts to the BMX frames. So if you want the drops, but not the short wheelbase, look at that approach.

Personally, I seem to be heading in the opposite direction lately.

 

[Thud]

There is a lot to be said for the larger frames also.
Mostly the 135mm rear spacing that allows for a multitude of rear hub selections, I think the standarg bmx is 120mm.
Nearly all the newer alluminum frames have bolt on drop-outs these days...way easy to make them steel & suitable as tourque arms.

the real apeal for the bmx is the stronger 20" wheel set. & a lot of the larger frames dont lend themselves to the small wheels (bottom bracket clearnce)

Here is the exact frame my death race build was based on:
http://cgi.ebay.com/BLOODLINE-ARCHETYPE-BMX-24-BLACK-FRAME-AND-FORK-/380303446622?pt=BMX_Bikes&hash=item588bda125e

from that to this:
http://endless-sphere.com/forums/viewtopic.php?f=6&t=26747

It has front & rear disc brake options, came with a fork. & to my suprise, a very favorable steering geometry that complimented the speeds It is capable of.
I have ridden a lot of 24" bmxers that are very unforgiving in steering performance.

I love the chop your doing on that build Dogman. I think your going to love the way that cadilac rolls down the road.

 

[wdwrkr51]

First off, I'd like to thank everybody for reading this thread. Second, I'd like to thank those who took the time to write a reply. Now, I want to say I'm a bit disappointed that (1) we got away from the BMX (20" wheel) frame concept so quickly, and (2) that so few seemed interested enough to participate. There was very little real information exhanged that was on point.

If we forget about the image of an overgrown male child recklessly risking life and limb to perform stunts on his BMX, and think about the "Urban Bike" concept instead, maybe the idea becomes more universally viable?? It took me a while to accept the frame as a starting point for my project, and the negative associations are exactly the reason why. However, once thought through and then assembled according to plan, what I got is a very practical, enjoyable transportation device. If your MTB conversion is the F150, and your beach cruiser conversion is the Cadillac, then the good BMX conversion is the Mazda Miata of E-Bikes. You might not want to take a thousand mile cross-country trip on one, but your day-to-day driving would be very enjoyable. Get it?

FYI: my BMX conversion features a Trek TR10 steel frame, a GT steel front fork, an AMPEDBIKES/9c front-mount direct-drive motor, stock 22-amp 36-60volt controller, and a hand assembled Turnigy 10 amp-hour 55.5 volt nominal LiPo battery (high of 63 volts, low of 48 volts). Measured range when ridden for conservative use of available power is 30+ miles. I have a two-speed derailleur on my front sprocket (40/48 teeth) and a 15-tooth rear sprocket. I use standard V-brakes. Top speed is about 28 mph on the flats right off the charger. There seems to be enough rake and trail with the fork that the steering is not at all twitchy. 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 huge and heavy rear dropouts made it simple to drill and tap holes for mounting the rack support legs. I run Maxxis "Miracle" ribbed 20 x 2.1 tires (85 to 110 lbs. pressure rating) at about 90 lbs. pressure. A big, wide, coil spring suspension saddle takes care of the bumps on my old back.

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.

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. That also was not the case with my old 36-volt 26" lead-acid MTB. BTW: just because I can ride almost 30 mph doesn't mean that I always do. I am careful about when and where.

Does all this spark your interest? I am strongly considering conversion to rear drive, and 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 and fit it between the rear drop outs with a single speed freewheel. I'm willing to spread the rear "fork" a bit if need be. I am also considering ordering an Edward Lyen 72 volt controller and adding one battery pack to my assembly, just to see what a difference that makes. 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 the upgrade.

I wonder if anyone else wants to jump in here...... I'D like to read about your experiences.

I'm going to edit this post and move it to the ebike reviews and testing page, hoping to get more interest generated.

Thanks for reading - wdwrkr51

 

[izeman]

@wdwrkr51: did you make the change to rear hub motor? any pictures? build thread?

 

[LI-ghtcycle]

This is a very interesting thread as I am building up a couple 20 inch BMX bikes myself. One is a bit of an extra challenge because it lacks even the small frame triangle found with the average BMX:

This is a pic I found, not the same one I have, mine is gold colored, I'll get a pic of it later today.

 

[jdmfreek]

what about a 26 inch bmx ebike?

 

[izeman]

What's the rational behind that?

 

[Deafcat]

Totally agree with the topic here... I'm also a big fan of "BMX Cruiser" format, my favourite bike when I was a teenager was essentially that format.

The project I'm working on right now is inspired by this... It's a very low frame with a 20" fat tire in the rear, 26" front wheel in a carbon fork, short cranks for greater clearance, reversed thompson setback post and sprung saddle to deliver the correct posture. Still playing around with handlebar selection... could go cruiser or BMX style, currently have mild sweep $20 bars on it for testing. next big step is tossing a bigger geared hub in the rear wheel

photo of current config:

my theory behind the setup: 20" fat tire in the back to deliver maximum thrust with a small hub, 26" in front for reduced rolling resistance and smoother over bumps, overall size of the bike is small enough to drop into the trunk of my little BMW with the front wheel removed, which is pretty amazing.

(the stem config is temporary... have a scope ring keeping the bearings locked at correct tension allowing me to move the stem up and down or swap stem/bars to find the sweetspot)