bambikeforum

Need help - I have a fat tire bike with coaster brakes that I want to convert to an electric fat tire bike. I need to dismantle the wheel then rebuild it adding new shorter spokes extending from the rim to the new no hubmotor. Concerns: 1.) Will the rim be strong enough for the motors torque, 2.) If the answer to 1. is no can I reinforce the rim?, 3.) Can I get spokes the right length, 4.) I want to ride this in sand/ dirt - I've read the motor should be in the rear only but several completed fat tire electric bikes I've seen have the motor in the front?, 5.) Helpful tips appreciated - how to do it, or a have a diy link. Please. Thanks

mrrabbit 

You have a lot of issues to deal with here...

1. Most electric hubs have very narrow flange spacing - and horrible offsets where 5/6/7 speed gearing is concerned.

2. If the rim is extra wide with extra wide drilling - the holes on the rim will reside on or outside the hub flanges offset locations.

You'll have to decide whether to lace all elbows in, or all elbows out - or even consider lacing spokes from one flange to the rim holes on the other side.

3. If this will be a single speed rear motor hub - worth trying. If this will be a multi-speed rear - suggest going with a front motor instead.

4. Strongly recommend going with single-butted 13g/14g spokes at a minimum. Doesn't matter if Pillar, CN, DT, Sapim, etc. 2.34mm at the elbow is a lot of beef.



Most importantly - make sure you order a hub that is referenced correctly. Most hub motors axle ends are NOT defined by the end knurls - but rather by the inverted tanged washer that sits on the end of the knurls.

For example, a lot of folks will order what they think is a 135mm rear 7-speed - build the wheel - install without the tanged washers - only to have the freewheel jam against the drive side stay. Install the tanged washers only to discover they actually have a 144mm - 145mm TANDEM rear 7-speed motor.

Many sellers of hub motors don't read the documentation properly for the hubs they order and sell and don't realize what they think is a 135mm MTB rear is actually a 145mm TANDEM rear. They inadvertently end up screwing their customer...

=8-)

zukahn1 

Actualy if you want to go this way which I advise against. You need to change the entire rear wheel.

bambikeforum

Thank you for your time. I lack much bike knowledge but I'll look up/ figure out what I don't understand in your message. Sounds doable, and sounds like rim should hold up if I do it right.

dabac

To what?

Unless the OP can source a motorized wheel already laced to the kind of extra wide rim needed to be able to run the extra wide tires he wants, the options are limited:
1) forget about it entirely
2) get a "standard" motorwheel, but forget about the fat tire option
3) buy a motorhub, buy spokes, buy a fat bike rim and build a new wheel from scratch
4) buy a motorhub, buy spokes, disassemble the wheel he has, reuse the rim and lace it to the motorhub

Option 3 would leave him with his SS wheel intact, which may be of use if he ever wants to return the bike to stock configuration, but that's the only obvious advantage from option 4. And it would probably have him using different rims front and rear which may not fit with how the OP wanted the bike to look like.

To the OP: what are you planning to do about brakes? Are there motor hubs with coaster brakes? Or will you be relying on a front brake alone?

bambikeforum

I was thinking option "4) buy a motorhub, buy spokes, disassemble the wheel he has, reuse the rim and lace it to the motorhub" The pic above is the actual bike w wheel
as to brakes I guess I could either put the motor in front wheel & rely on coaster brakes at rear or put in hand brakes front. & rear

dabac

Although I used to find them OK for casual riding, I'm guessing quite a few people here would see the phrase "relying on coaster brakes" as an oxymoron. They haven't got a particular good reputation.

Not to rain on your parade, but that would most likely take some doing.
With fat tires, caliper brakes are out of the question.
Do your fork have brake mounts? Which kind?
If it hasn't your options are limited.
Retrofitting rim brake mounts is possible, but takes some metal working skills and associated tools.
And some repainting.
And will be a pointless exercise unless your rims have brake tracks.
The option of rebuilding the front wheel around a drum/roller brake hub is probably out, due to "true" fat bikes often using/needing a wider front hub.
It might be possible to substitute the front axle with a rear axle, and add a hefty stack of spacers on both sides.
But if the fork width of your bike is still the normal 100 mm, then it'd be readily doable.
Still doesn't mean you're home safe though.
Hub brakes put a different kind of load on the fork, and it has happened that forks not made for the purpose have bent at the reaction arm/ brake mount.
At this stage, sourcing a fat bike fork with disc brake mount, and rebuilding the front wheel around a "real" fat bike disc brake hub might be both simpler and safer.
Is your bike perhaps of the beach cruiser type?
If it is, it might not be such a suitable candidate for motorising in the first place.
Cruisers are generally not intended for or expected to be used for energetic riding and may not hold up to the extra strain.
OTOH weight isn't generally much a concern for a cruiser, so it may survive on sheer chunkiness alone.

Dan Burkhart 

How about a picture of the entire bike? If it's a chopper style with a long slack fork, a front hub motor would have serious traction issues.
On the whole, having done a few electric conversions, I generally favor front hub motors. I like the idea of having the electric assist independent of the drive train.
I posted one of my electric builds in the electric bike forum, and they actually made it a sticky. I posted a bunch of videos showing the process and results . https://www.bikeforums.net/showthread...ic-assist-bike

bambikeforum

dabac

So you have NO front brake, and you want motorize it and rely on a coaster brake?
Sounds sketchy to me.
But you do have rims with a brake track.
I'd recommend getting a disc-brake ready fatbike fork, and and a disc brake fatbike front wheel.
I don't recognize the stated 146 mm fork dropout width so you may not be able to use a regular fat bike front hub.
Or have the fork modded to allow the use of front rim brakes.

Kimmo

I think cable coaster hubs might be the simplest way to get more braking...

Re-spacing them (spokes may need to be laterally crossed; I'd run a drill through the holes in the rim to allow it, maybe use nipple washers) and tweaking the reaction arm has to be easier than messing around modifying the frameset.