Silver Bullet - A 1998 Hoffman "George" BMX conversion

[teslanv]

I wish I was 8 years old again.

Fortunately for my son, Harrison, who just turned 8, his dad is so addicted to ebikes, that he got every young boy's dream realized today. Powered transportation.

This bike was impressively simple to build, once I had developed my solid game plan.

The biggest challenge to this build was fitting a rear drive (MXUS 9X7T 750W DD Hub Motor, radially laced in a 20" rim) while keeping the singe 16T rear sprocket drive.

As you may know, all Rear hub motors are built for 135mm and larger drop-outs.

A BMX bike typically has only 110mm drop-outs. The good news with a fixed gear BMX, and particularly with this Hoffman "George" BMX is that the horizontal drop-outs are very stout.



1/4" thick steel plates with 10mm slots. Perfect without the need for torque arms, except for a rear hub motor is too wide.

However a FRONT hub motor has only 100mm clear between the flats. So the challenge ultimately came down to figuring out how to mount a 16T sprocket to a front drive motor. And the answer is simple indeed. I ordered a spare threaded cover plate for a rear motor, and just swapped cover plates. The Disc brake side of the front hub motor came off, and the threaded cover went back on. Direct swap with minimal hassle.

So now I had a hub motor with 100mm between the flats, and a threaded cover with 16T rear sprocket mounted.


Just one more issue to resolve:

The motor spun backwards. :?

But not to worry. The phase & hall wires can simply be swapped until the correct combination is found to make the motor spin in "reverse" which, in my case, propelled the bike in the forward direction.

The controller is a Lyen 6X4110 controller, customized for 24V-36V OR up to 36V-84V supply voltage. - The kid can ride slower (about 15 mph), at lower voltages, and when I want to have some stupid fun, I can mount up to 20S LiPo for speeds approaching 35 mph. Regen is enabled, and the bike stops quite well, even on 33V.

The battery I set up for him is a pair of 4S Turnigy hardcase packs, ran in series for 8S (33.6V Hot) And Harry knows not to touch the battery. That's dad's job to make sure it is charged and connected properly.

A quick test ride demonstrated that the bike has plenty of starting torque, but not so much that it will pop a wheelie with a 50-lb. kid on it. PERFECT.

I have to give thanks to Ypedal for his inspiring BMX conversions and advice, as well as Dogman.

These are easy conversions, as long as you order the proper parts. I will definitely be building more. You should too.

Before Pic:

 

[The fingers]

That's awesome. 8) When I was 8 years old all there was were minibikes, and I never got one (sometimes nicknamed a doodlebug or a skat kitty). They had a gas lawnmower engine, two little go cart wheels and a centrifical clutch with chain drive. Kids regularly got killed riding them on residential streets to avoid the cops. They were so very low to the ground that drivers couldn't see them if they darted into the street from behind a parked car. So I'm glad I'm not 8 again, unless I was 8 in this present day and had the ebmx! What a great dad!

 

[UBMag]

Lucky kid!!!
(You have probably allready thought it yourself: but at some point in time, that kid is going to take initiative to impress dad, by connecting up his own battery.... I hope you have given him the safety-lesson... anyway. Awsome dad!!

 

[melodious]

I'd protect the axle wires more. Kids drop down their bikes all the time, with no regard to bike protection. Can save the hassle of rewiring new phase & halls.

Its nice that you give your little ones the pleasure of riding and having as much fun as their bigger counterparts. :wink:

 

[spinningmagnets]

He will now become the most popular kid in the neighborhood! This is awesome. I can envision him eventually meeting an adult who tells him that electric vehicles don't have good performance, and then the kid offers him a ride...

 

[teslanv]

Lucky kid!!!
(You have probably allready thought it yourself: but at some point in time, that kid is going to take initiative to impress dad, by connecting up his own battery.... I hope you have given him the safety-lesson... anyway. Awsome dad!!

We had a brief "incident" with the batteries when we were building the bike. It involved a short period of crying (but no one was injured, luckily). The kid is quite aware now of how dangerous batteries are, and has agreed to let Dad handle them.

 

[TSAR]

Awesome build, nice and simple. Your kid is lucky!

In the pics, it looks like the power wires make a loop, that may get a bit melty at 20s as the wires make an induction coil. I would straighten those out along the frame or shorten them. The phase wires aren't really coiled, but could lay along the stays more closely.

I love your build, not trying to troll, just want everyone to be safe.http://endless-sphere.com/forums/posting.php?mode=reply&f=6&t=65044#

 

[teslanv]

Battery leads do not make a full loop.
Of course for 20S, I would have to mount the batteries a bit differently.
Went for a short ride with my son today. The eBMX is a solid runner. Now my daughter wants her BMX converted. Twist my arm. :wink: