Motor solutions for fat bike frames with 170mm dropouts...

[Supertux1]

I am investigating making my next electric bike creation a 'fun' and ridiculously powered fat bike (as opposed to my all-business commuter in the signature.)

Fat framed and a gear motor on each wheel, maybe use one of cell_man's new 10C 17.5Ah or 3C 25.0Ah triangle packs.

I can't really find any information on motors with axles long enough to accommodate 170mm dropouts.

I'm thinking that has to be a length of about 240mm to accommodate torque arms, washers, bolts and spacers etc...

Ideally this would be a torque wound BMC or MAC. 8t or 10t.

Then again I could go the other way and make it mid drive, which would mean mid drive BB spindles of 100mm, which seems also non-existent.

Should I wait for fat bikes in China? How easy is it to have a custom axle made?

 

[Supertux1]

Well no sooner had I post this when I came across an interesting article:

http://fat-bike.com/2013/03/fat-bike-101-frame-types/

So there are two choices:

1. 170mm dropouts with a symmetrical frame, wheel dish offset to the right as per usual.

2. 135mm dropouts with an asymmetrical frame, the drive side chainstay is pulled 17.5mm to the right to accommodate the gear cluster and the left side (brake) side chainstay curves around the tire. (Most Surly frames.) Wheel dish is offset to the LEFT...

This seems do-able, but with current motors, only offset frames seem to work.

I found a post here of someone who had an AWD fat bike but it seems they got around the 170mm dropout with by not using torque arms...

 

[Drunkskunk]

One advantage to the offset frames is that with the extra wide rims, you can use an offset spoke hole and lace your wheel sith no dish, but still be off set. Several wheels are made jus6t for that, but it's easy enough to drill your own holes on these rims. with anything from 60mm to 110mm wide rims, getting the hole exactly right isn't mission critical. get it a few mm off and it's still fine, so this is an average joe with hand tools type job.

How much power are you looking to run in this thing?

 

[Samd]

What about 10mm torque plates on the inside of each dropout? That leaves 150mm, perfect for the cheaper scooter motors / cromotor clones popping up everywhere.

 

[Supertux1]

One advantage to the offset frames is that with the extra wide rims, you can use an offset spoke hole and lace your wheel sith no dish, but still be off set. Several wheels are made jus6t for that, but it's easy enough to drill your own holes on these rims. with anything from 60mm to 110mm wide rims, getting the hole exactly right isn't mission critical. get it a few mm off and it's still fine, so this is an average joe with hand tools type job.

How much power are you looking to run in this thing?

I was looking at perhaps a Surly Moonlander with 100mm clownshoe rims, a total of 2 to 3 kW (1 to 1.5kW each) if going with MAC/BMC motors.

The clownshoe rims have two rows of holes offset from center 20mm on each side (to dish to left or right etc), the rear frame dropout is offset 28mm to the right as well, the flanges on a MAC/BMC motor are about 14mm offset to the left.

Another issue is that I find most fat bike rims are 32 holes whereas the average hub motor is 36.

I can see leaving two holes empty on each side, eg every 8th one, but then we're talking about four different spoke lengths per wheel to do it right.
I know how to calculate for a normal dish, and have all the tools/stand etc... for lacing, but this would take some thinking.
Maybe time to break out the construction paper and figure out the effective offset.

 

[Supertux1]

What about 10mm torque plates on the inside of each dropout? That leaves 150mm, perfect for the cheaper scooter motors / cromotor clones popping up everywhere.

Yeah that might cause issues with derailers and disc brakes, may need spacers on the frame mounts etc... definitely worth looking into.

So here's what I found, with a 205mm axle in 170mm drop outs, that leaves 205-170mm = 35mm of threads left, assuming the torque arms and various spacers are placed on the inside between the dropouts. Maybe a big 10 gear freewheel. The frame is going to be about 7.5mm wide on either side (measured from my existing bike), which leaves about 20mm of usable threads for bolts and washers on the outside, or 10mm each side. The bolt that comes with the MAC is about 12.5mm wide and then I use about 4mm of nordlock washer under that. So assuming the problem of the disc brake spacing and derailleur spacing can be solved, there's not much left to hold the wheel on securely on the outside.

 

[Drunkskunk]

The clown shoe isn't a good option. its a singlewalled, seriously drilled out 32 hole only rim meant for light weight at pedal power only.

What you want is a double walled rim without all the lightening holes. Something like this: http://www.choppersus.com/store/product/612/Rim-Only-26-x-4-Black/

 

[Supertux1]

Oh awesome! Those are much better! Much Cheaper too!

I did some more digging and found this too:

http://forums.mtbr.com/fat-bikes/comprehensive-list-fat-bike-rims-746811.html

Which has some good info on 'chopper' rims.

 

[teklektik]

MAC axles are replaceable and cost $12. You might order one up and have a shop make you up a longer version. With a new original in hand for measurements and comparisons, the shop should be able to get you a proper fit without difficulty. Kind of a PITA to install and pull wires, but it's all low-risk sweat equity stuff - no new custom adapters to alter load/shear points, etc.

 

[Bluefang]

Just go to the local bolt shop and buy some of those long joint bolts for rigging. Screw then onto each end of the axle and then use a clamp style torque arm to hold it in place on the wire side and bolt into the other side. Or as others said get a new shaft machined or use a bigger motor

For rims if you really want to go proper fat bike use alloy motorbike stuff You can just drop into the local tire shop and pick up the old used tires for free. Then you are not limited to 4.25" and dont have to pay some stupidly high price and as a bonus dont have to worry about punctures or pinch flats ever again. Unless you are seriously nutz and jump it off a cliff.